Table of Contents
Chapter 04. Chromosomes and sex linkage
Until now our analysis of genesplugin-autotooltip__default plugin-autotooltip_bigGene: read Chapters 02, 03, 04, 05, and 06 for a definition of gene :-) has focused on defining genesplugin-autotooltip__default plugin-autotooltip_bigGene: read Chapters 02, 03, 04, 05, and 06 for a definition of gene :-) based on phenotypicplugin-autotooltip__default plugin-autotooltip_bigPhenotype: an observable feature or property of an organism. differences brought about by different allelesplugin-autotooltip__default plugin-autotooltip_bigAllele: a version of a gene. Alleles of a gene are different if they have differences in their DNA sequence. or by a direct test of function – the complementation testplugin-autotooltip__default plugin-autotooltip_bigComplementation test: a genetic experiment that answers the question: how many different genes are represented within a collection of mutants?. In Chapters 4 and 5, our analysis will be concerned with tests of geneplugin-autotooltip__default plugin-autotooltip_bigGene: read Chapters 02, 03, 04, 05, and 06 for a definition of gene :-) position.
So far in this book we’ve taken it for granted that genesplugin-autotooltip__default plugin-autotooltip_bigGene: read Chapters 02, 03, 04, 05, and 06 for a definition of gene :-) reside on chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins., but how do we know this? The relationship between genesplugin-autotooltip__default plugin-autotooltip_bigGene: read Chapters 02, 03, 04, 05, and 06 for a definition of gene :-) and chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. was discovered by Thomas Morgan in 1910.
Chromosome theory and sex determination
As discussed at the end of Chapter 03, we saw (retrospectively) the relationship between what Mendel observed and the chromosomeplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. mechanics of meiosisplugin-autotooltip__default plugin-autotooltip_bigMeiosis: a process involving two sequential cell divisions that usually produces four gametes (reproductive cells such as sperm or eggs).. This immediately and strongly suggested that genesplugin-autotooltip__default plugin-autotooltip_bigGene: read Chapters 02, 03, 04, 05, and 06 for a definition of gene :-) are physically located on chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins.. But for scientists in the early 20th century, what was needed to more definitively demonstrate that genesplugin-autotooltip__default plugin-autotooltip_bigGene: read Chapters 02, 03, 04, 05, and 06 for a definition of gene :-) are on chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. (the idea known as “chromosome theoryplugin-autotooltip__default plugin-autotooltip_bigChromosome theory: the theory that genes, the basic unit of inheritance, are located on chromosomes. First demonstrated by Thomas Hunt Morgan in 1911 using Drosophila and sex-linked mutants.”) was a chromosomeplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. that could be identified in the microscope and that carried an alleleplugin-autotooltip__default plugin-autotooltip_bigAllele: a version of a gene. Alleles of a gene are different if they have differences in their DNA sequence. for a phenotypeplugin-autotooltip__default plugin-autotooltip_bigPhenotype: an observable feature or property of an organism. that could also be easily observed. The evidence for chromosome theoryplugin-autotooltip__default plugin-autotooltip_bigChromosome theory: the theory that genes, the basic unit of inheritance, are located on chromosomes. First demonstrated by Thomas Hunt Morgan in 1911 using Drosophila and sex-linked mutants. would then depend on correlating the segregation of the trait with segregation of the chromosomeplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins..
It is important to know that during this period of history, despite being able to see chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. under microscopes, scientists did not know what chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. were made of. It's also useful to know what scientists in Morgan's day knew about chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. and their role in sex determination (this winds up being important in our story). Drosophilaplugin-autotooltip__default plugin-autotooltip_bigDrosophila melanogaster: a fruit fly species used in genetics research., like humans, are obligate diploidsplugin-autotooltip__default plugin-autotooltip_bigObligate diploid: a species wherein all cells except gametes are diploid. (2n). While humans have 46 chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins., Drosophilaplugin-autotooltip__default plugin-autotooltip_bigDrosophila melanogaster: a fruit fly species used in genetics research. have 8 chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins., or 4 pairs of chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. (2n=8); this includes one pair that are different in males and females. In female flies this pair looks like a regular pair of chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins., but males only have one of these chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins.; instead of a second homologous chromosomeplugin-autotooltip__default plugin-autotooltip_bigHomologous chromosomes: highly similar but non-identical chromosomes that are the same size and contain the same genes. as a partner, this chromosomeplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. pairs with a much smaller and different looking chromosomeplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. during meiosisplugin-autotooltip__default plugin-autotooltip_bigMeiosis: a process involving two sequential cell divisions that usually produces four gametes (reproductive cells such as sperm or eggs). in males. These chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. are named X and Y and also called sex chromosomesplugin-autotooltip__default plugin-autotooltip_bigSex chromosomes: in some species, these are a set of chromosomes that determine sex. In some species, females are homogametic (XX) and males are heterogametic (XY), whereas in other species it is the other way around (homogametic ZZ males and heterogametic ZW females). Not all species use sex. Female flies have two X chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins., while male flies just have one X chromosomeplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. that pairs with the small Y chromosomeplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. during meiosisplugin-autotooltip__default plugin-autotooltip_bigMeiosis: a process involving two sequential cell divisions that usually produces four gametes (reproductive cells such as sperm or eggs).. The three other chromosomeplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. pairs in Drosophilaplugin-autotooltip__default plugin-autotooltip_bigDrosophila melanogaster: a fruit fly species used in genetics research. are called II, III, and IV. Around the time of Morgan, scientists studying mealworms and butterflies already had data that strongly suggested sex determination was controlled by whether an individual carried XX vs XY pairs, although genetic evidence to support this was not discovered until after the sex-linkedplugin-autotooltip__default plugin-autotooltip_bigSex linkage: a gene is said to be sex-linked if it maps to a sex chromosome. studies we discuss in this chapter. Humans also have an XY chromosomeplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. pair that determines sex, but the details of how sex determination works is very different between flies and mammals. While we focus on the XX/XY system in this book, not all species with sexual dimorphism have XX/XY chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. (birds use a system called ZZ (male)/ZW (female)), and some species don't use chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. to determine sex (yeastplugin-autotooltip__default plugin-autotooltip_bigYeast: in this book, refers to Saccharomyces cerevisiae, a single-celled eukaryotic microbe used as a model genetic organism. See Chapter 02 is one example of this; but fish and reptiles do not use sex chromosomesplugin-autotooltip__default plugin-autotooltip_bigSex chromosomes: in some species, these are a set of chromosomes that determine sex. In some species, females are homogametic (XX) and males are heterogametic (XY), whereas in other species it is the other way around (homogametic ZZ males and heterogametic ZW females). Not all species use sex either).
The $white$ mutation has unusual segregation patterns
Thomas Hunt Morgan “proved”1) chromosome theoryplugin-autotooltip__default plugin-autotooltip_bigChromosome theory: the theory that genes, the basic unit of inheritance, are located on chromosomes. First demonstrated by Thomas Hunt Morgan in 1911 using Drosophila and sex-linked mutants. in 1910 using Drosophilaplugin-autotooltip__default plugin-autotooltip_bigDrosophila melanogaster: a fruit fly species used in genetics research.. Wildtypeplugin-autotooltip__default plugin-autotooltip_bigWildtype: a reference strain of an organism that scientists operationally define as “normal” to which mutants are compared. Not to be confused with wild organisms. (normal) flies have brick-red eyes. The first laboratory mutantplugin-autotooltip__default plugin-autotooltip_bigMutant: an individual that has a different phenotype than wildtype and likely contains one more mutations that cause this difference. for Drosophilaplugin-autotooltip__default plugin-autotooltip_bigDrosophila melanogaster: a fruit fly species used in genetics research. was found by Morgan’s wife, Lillian, who worked in his lab at Columbia University in New York City. Compared to wildtypeplugin-autotooltip__default plugin-autotooltip_bigWildtype: a reference strain of an organism that scientists operationally define as “normal” to which mutants are compared. Not to be confused with wild organisms., these mutantsplugin-autotooltip__default plugin-autotooltip_bigMutant: an individual that has a different phenotype than wildtype and likely contains one more mutations that cause this difference. had white eyes and were therefore named $white$ (or $w$ for short). In fact, what they initially found by sheer luck was a single male fly with white eyes.
Flies with the $white$ mutationplugin-autotooltip__default plugin-autotooltip_bigMutation: a change in the DNA of a gene that results in a change of phenotype compared to a reference wildtype allele. See also: mutant. gave strange outcomes in various crosses. Note that in Chapter 03, we ignored the fact that to actually breed flies, you must cross male and female flies. From here on out, we also consider sex in our crosses, and use the male ♂ and female ♀ symbols to indicate sex:
$$ \begin{aligned} P: white\text{ ♂} &\times \text{red eyes ♀ (wildtype)}\\&\downarrow\\F1: \text{all} &\text{ red} \text{ (both ♂ and ♀)}\\ &\downarrow\\F2: \text{red}&:white = 3:1 \text{ (but only ♂ had white eyes)} \end{aligned}$$
The fact that all the F1 progenyplugin-autotooltip__defaultProgeny: a synonym for offspring. were normal (i.e., had red eyes) and the F2 progenyplugin-autotooltip__defaultProgeny: a synonym for offspring. had a 3:1 ratio for red vs. white eyes indicated that the $white$ mutationplugin-autotooltip__default plugin-autotooltip_bigMutation: a change in the DNA of a gene that results in a change of phenotype compared to a reference wildtype allele. See also: mutant. behaves like a recessiveplugin-autotooltip__default plugin-autotooltip_bigRecessive: used to describe an allele, usually in comparison to wildtype. Recessive alleles do not exhibit their phenotype when combined with a wildtype allele. mutationplugin-autotooltip__default plugin-autotooltip_bigMutation: a change in the DNA of a gene that results in a change of phenotype compared to a reference wildtype allele. See also: mutant. in a single geneplugin-autotooltip__default plugin-autotooltip_bigGene: read Chapters 02, 03, 04, 05, and 06 for a definition of gene :-) (similar to what we saw in Chapter 03 for $shibire$). But there was something unusual about the $white$ mutationplugin-autotooltip__default plugin-autotooltip_bigMutation: a change in the DNA of a gene that results in a change of phenotype compared to a reference wildtype allele. See also: mutant. because only the male flies in the F2 had white eyes. In fact, half the F2 males had white eyes, while the other half had red eyes. All of the F2 females had red eyes.
Morgan then took the red-eyed F1 females from Figure 2 and crossed them to white males:
$$\begin{aligned} white\text{ ♂} &\times \text{red ♀ (F1 females from Fig. 2)}\\ &\downarrow\\ \text{red}&:white = 1:1 \text{ (both ♂ and ♀)} \end{aligned}$$
Since the monohybrid crossplugin-autotooltip__default plugin-autotooltip_bigMonohybrid cross: a cross, developed by Gregor Mendel, that illustrates Mendel's First Law. See Chapter 03. in Figure 2 suggests that $white$ is recessiveplugin-autotooltip__default plugin-autotooltip_bigRecessive: used to describe an allele, usually in comparison to wildtype. Recessive alleles do not exhibit their phenotype when combined with a wildtype allele., the cross in Figure 3 is therefore roughly equivalent to using $white$ males as testers to test crossplugin-autotooltip__default plugin-autotooltip_bigTest cross: a genetic cross devised by Gregor Mendel that allows a researcher to easily determine the genotype of an individual that appears wildtype but has an unknown genotype. This assumes you have a tester strain readily available. See Chapter 03. the F1 red-eyed females. The results indicated that those red F1 females were heterozygousplugin-autotooltip__default plugin-autotooltip_bigHeterozygous: a state for a diploid organism wherein the two alleles for a gene are different from each other. and further confirmed that $white$ is a recessiveplugin-autotooltip__default plugin-autotooltip_bigRecessive: used to describe an allele, usually in comparison to wildtype. Recessive alleles do not exhibit their phenotype when combined with a wildtype allele. trait.
The most informative cross wound up being the reciprocal (switching the sex/traits of the parents) of Morgan's original cross shown in Figure 2:
$$ \begin{aligned} \text{red ♂} &\times white \text{ ♀}\\ &\downarrow\\ \text{all ♀ are normal and} &\text{ all ♂ have white eyes} \end{aligned}$$
This is effectively using the $white$ females as a testerplugin-autotooltip__default plugin-autotooltip_bigTester: a strain that is homozygous mutant at $n$ different loci that is used in a test cross. See Chapter 03. to test crossplugin-autotooltip__default plugin-autotooltip_bigTest cross: a genetic cross devised by Gregor Mendel that allows a researcher to easily determine the genotype of an individual that appears wildtype but has an unknown genotype. This assumes you have a tester strain readily available. See Chapter 03. normal red-eyed males. Drosophilaplugin-autotooltip__default plugin-autotooltip_bigDrosophila melanogaster: a fruit fly species used in genetics research. crosses produce roughly 50% male and 50% female; therefore, ignoring the sex of the flies, it looks as if the red-eyed parent male is heterozygousplugin-autotooltip__default plugin-autotooltip_bigHeterozygous: a state for a diploid organism wherein the two alleles for a gene are different from each other. in this cross - but this cannot be correct, as we used males from true-breedingplugin-autotooltip__default plugin-autotooltip_bigTrue breeding: a true breeding strain is one that has been inbred for multiple generations. We assume that the vast majority of loci are homozygous in a true breeding strain. wildtypeplugin-autotooltip__default plugin-autotooltip_bigWildtype: a reference strain of an organism that scientists operationally define as “normal” to which mutants are compared. Not to be confused with wild organisms. stock. Furthermore, in Figure 4 the wildtypeplugin-autotooltip__default plugin-autotooltip_bigWildtype: a reference strain of an organism that scientists operationally define as “normal” to which mutants are compared. Not to be confused with wild organisms. alleleplugin-autotooltip__default plugin-autotooltip_bigAllele: a version of a gene. Alleles of a gene are different if they have differences in their DNA sequence. is only passed on to the daughters and the $white$ alleleplugin-autotooltip__default plugin-autotooltip_bigAllele: a version of a gene. Alleles of a gene are different if they have differences in their DNA sequence. is only passed on to sons. If the $white$ mutationplugin-autotooltip__default plugin-autotooltip_bigMutation: a change in the DNA of a gene that results in a change of phenotype compared to a reference wildtype allele. See also: mutant. “behaved” the way Mendel might have expected it to, you would expect that the white eye phenotypeplugin-autotooltip__default plugin-autotooltip_bigPhenotype: an observable feature or property of an organism. would be split equally between males and females, which is not what Morgan observed. Clearly something unusual and “un-Mendelian” is going on here; whatever is going on with $white$, it is associated with sex.
The $white$ mutation is sex-linked
Morgan explained these unusual results by hypothesizing that the eye color geneplugin-autotooltip__default plugin-autotooltip_bigGene: read Chapters 02, 03, 04, 05, and 06 for a definition of gene :-) $white$ ($w$) is physically located on the sex chromosomeplugin-autotooltip__default plugin-autotooltip_bigSex chromosomes: in some species, these are a set of chromosomes that determine sex. In some species, females are homogametic (XX) and males are heterogametic (XY), whereas in other species it is the other way around (homogametic ZZ males and heterogametic ZW females). Not all species use sex $X$. Males only have one copy of the $X$ chromosomeplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. and females always get one copy of $X$ from their mother and one copy from their father. We can use modified symbols to show the genotypesplugin-autotooltip__default plugin-autotooltip_bigGenotype: the combination of alleles within an organism or strain. When used as a verb, it means to determine the genotype experimentally. that are directly associated, or linkedplugin-autotooltip__default plugin-autotooltip_bigLinkage: two loci are linked to each other if they are less than 50 m.u. apart. Two loci are unlinked if they are either (1) greater than 50 m.u. apart on the same chromosome, or; (2) are on separate chromosomes., with the $X$ chromosomeplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins., and re-write Figure 4 as follows:
$$\begin{aligned} \frac{X^+}{Y}\text{ ♂} &\times \frac{X^w}{X^w}\text{ ♀}\\ &\downarrow\\ \frac{X^w}{Y} &\text{ ♂, } \frac{X^w}{X^+} \text{ ♀} \end{aligned}$$
In Figure 5, each parent contributes one alleleplugin-autotooltip__default plugin-autotooltip_bigAllele: a version of a gene. Alleles of a gene are different if they have differences in their DNA sequence. to the progenyplugin-autotooltip__defaultProgeny: a synonym for offspring.; therefore, the wildtypeplugin-autotooltip__default plugin-autotooltip_bigWildtype: a reference strain of an organism that scientists operationally define as “normal” to which mutants are compared. Not to be confused with wild organisms. alleleplugin-autotooltip__default plugin-autotooltip_bigAllele: a version of a gene. Alleles of a gene are different if they have differences in their DNA sequence. for red eyes in female progenyplugin-autotooltip__defaultProgeny: a synonym for offspring. is always inherited along with the $X^+$ chromosomeplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. from the father that carries the wildplugin-autotooltip__default plugin-autotooltip_bigWild: refers to organisms that grow in wild populations. Not to be confused with wildtype. type $w^+$ alleleplugin-autotooltip__default plugin-autotooltip_bigAllele: a version of a gene. Alleles of a gene are different if they have differences in their DNA sequence. (abbreviated as simply +), and the mutantplugin-autotooltip__default plugin-autotooltip_bigMutant: an individual that has a different phenotype than wildtype and likely contains one more mutations that cause this difference. $w$ alleleplugin-autotooltip__default plugin-autotooltip_bigAllele: a version of a gene. Alleles of a gene are different if they have differences in their DNA sequence. is inherited from the $X^w$ chromosomeplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. from the mother. The fact that these individuals get two $X$ chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. (one from each parent) is what makes them females. Male progenyplugin-autotooltip__defaultProgeny: a synonym for offspring., on the other hand, are male because they get a $Y$ chromosomeplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins., and the only way to get a $Y$ chromosomeplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. is from their father; this means that they must get their $X$ chromosomeplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. from their mother, and their mother is homozygousplugin-autotooltip__default plugin-autotooltip_bigHomozygous: a state for a diploid organism wherein the two alleles for a gene are identical to each other. $\frac{X^w}{X^w}$. Morgan named this phenomena sex linkageplugin-autotooltip__default plugin-autotooltip_bigSex linkage: a gene is said to be sex-linked if it maps to a sex chromosome..
Based on this discussion, we now have a clear definition of sex linkageplugin-autotooltip__default plugin-autotooltip_bigSex linkage: a gene is said to be sex-linked if it maps to a sex chromosome. in Drosophilaplugin-autotooltip__default plugin-autotooltip_bigDrosophila melanogaster: a fruit fly species used in genetics research.: a geneplugin-autotooltip__default plugin-autotooltip_bigGene: read Chapters 02, 03, 04, 05, and 06 for a definition of gene :-) is sex-linkedplugin-autotooltip__default plugin-autotooltip_bigSex linkage: a gene is said to be sex-linked if it maps to a sex chromosome. if it is physically associated with a sex chromosomeplugin-autotooltip__default plugin-autotooltip_bigSex chromosomes: in some species, these are a set of chromosomes that determine sex. In some species, females are homogametic (XX) and males are heterogametic (XY), whereas in other species it is the other way around (homogametic ZZ males and heterogametic ZW females). Not all species use sex. The way a geneticist would say it is, “A geneplugin-autotooltip__default plugin-autotooltip_bigGene: read Chapters 02, 03, 04, 05, and 06 for a definition of gene :-) is sex-linkedplugin-autotooltip__default plugin-autotooltip_bigSex linkage: a gene is said to be sex-linked if it maps to a sex chromosome. if it maps to a sex chromosomeplugin-autotooltip__default plugin-autotooltip_bigSex chromosomes: in some species, these are a set of chromosomes that determine sex. In some species, females are homogametic (XX) and males are heterogametic (XY), whereas in other species it is the other way around (homogametic ZZ males and heterogametic ZW females). Not all species use sex”. The relevance of sex linkageplugin-autotooltip__default plugin-autotooltip_bigSex linkage: a gene is said to be sex-linked if it maps to a sex chromosome. is that this phenomena is what first allowed scientists to show that genesplugin-autotooltip__default plugin-autotooltip_bigGene: read Chapters 02, 03, 04, 05, and 06 for a definition of gene :-) are associated with chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins.. Morgan's experiments showed that at least one geneplugin-autotooltip__default plugin-autotooltip_bigGene: read Chapters 02, 03, 04, 05, and 06 for a definition of gene :-) ($w$) is physically linkedplugin-autotooltip__default plugin-autotooltip_bigLinkage: two loci are linked to each other if they are less than 50 m.u. apart. Two loci are unlinked if they are either (1) greater than 50 m.u. apart on the same chromosome, or; (2) are on separate chromosomes. to the $X$ chromosomeplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins.. In the next chapter, we will see that there are other Drosophilaplugin-autotooltip__default plugin-autotooltip_bigDrosophila melanogaster: a fruit fly species used in genetics research. genesplugin-autotooltip__default plugin-autotooltip_bigGene: read Chapters 02, 03, 04, 05, and 06 for a definition of gene :-) that are also sex linkedplugin-autotooltip__default plugin-autotooltip_bigSex linkage: a gene is said to be sex-linked if it maps to a sex chromosome.. Subsequent experiments using Drosophilaplugin-autotooltip__default plugin-autotooltip_bigDrosophila melanogaster: a fruit fly species used in genetics research. and other organisms showed that other genesplugin-autotooltip__default plugin-autotooltip_bigGene: read Chapters 02, 03, 04, 05, and 06 for a definition of gene :-) are physically located on autosomesplugin-autotooltip__default plugin-autotooltip_bigAutosome: any chromosome that is not a sex chromosome. (any chromosomeplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. that is not a sex chromosomeplugin-autotooltip__default plugin-autotooltip_bigSex chromosomes: in some species, these are a set of chromosomes that determine sex. In some species, females are homogametic (XX) and males are heterogametic (XY), whereas in other species it is the other way around (homogametic ZZ males and heterogametic ZW females). Not all species use sex). Genesplugin-autotooltip__default plugin-autotooltip_bigGene: read Chapters 02, 03, 04, 05, and 06 for a definition of gene :-) that are not sex-linkedplugin-autotooltip__default plugin-autotooltip_bigSex linkage: a gene is said to be sex-linked if it maps to a sex chromosome. are located on autosomesplugin-autotooltip__default plugin-autotooltip_bigAutosome: any chromosome that is not a sex chromosome. and are also called autosomalplugin-autotooltip__default plugin-autotooltip_bigAutosome: any chromosome that is not a sex chromosome. genesplugin-autotooltip__default plugin-autotooltip_bigGene: read Chapters 02, 03, 04, 05, and 06 for a definition of gene :-). In the next chapter, we will also see that not only are genesplugin-autotooltip__default plugin-autotooltip_bigGene: read Chapters 02, 03, 04, 05, and 06 for a definition of gene :-) physically linkedplugin-autotooltip__default plugin-autotooltip_bigLinkage: two loci are linked to each other if they are less than 50 m.u. apart. Two loci are unlinked if they are either (1) greater than 50 m.u. apart on the same chromosome, or; (2) are on separate chromosomes. to chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins., they also have defined physical positions on chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins..
Finally, let's have some further discussion on genetic notation. Note that writing $X^w$ is redundant. Since the $X$ chromosomeplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. always pairs with the $Y$ chromosomeplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. during meiosisplugin-autotooltip__default plugin-autotooltip_bigMeiosis: a process involving two sequential cell divisions that usually produces four gametes (reproductive cells such as sperm or eggs)., the presence of the $Y$ chromosomeplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. automatically implies that any genesplugin-autotooltip__default plugin-autotooltip_bigGene: read Chapters 02, 03, 04, 05, and 06 for a definition of gene :-) written together with the $Y$ chromosomeplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. in fractional notationplugin-autotooltip__default plugin-autotooltip_bigFractional notation: a style of genotype notation that uses “fractions”, e.g., $\frac{unc\text{-}4}{+}$. We strongly encourage this notational style as it generally is preferred by genetics researchers. must be on chromosomeplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. $X$. Furthermore, the male and female symbols are also redundant, since the presence of a $Y$ chromosomeplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. tells you everything you need to know about the sexes of the individuals in a cross where sex-linkedplugin-autotooltip__default plugin-autotooltip_bigSex linkage: a gene is said to be sex-linked if it maps to a sex chromosome. genesplugin-autotooltip__default plugin-autotooltip_bigGene: read Chapters 02, 03, 04, 05, and 06 for a definition of gene :-) are involved. Drosophilaplugin-autotooltip__default plugin-autotooltip_bigDrosophila melanogaster: a fruit fly species used in genetics research. geneticists also use the $\rightharpoondown$ symbol to represent the $Y$ chromosomeplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins.. We can therefore re-write Figure 5 as:
$$ \frac{+}{\rightharpoondown} \times \frac{w}{w}\\ \downarrow\\ \frac{w}{\rightharpoondown}\text{, } \frac{w}{+}$$
Sex linkage in humans - an example
Sex linkageplugin-autotooltip__default plugin-autotooltip_bigSex linkage: a gene is said to be sex-linked if it maps to a sex chromosome. is not unique to Drosophilaplugin-autotooltip__default plugin-autotooltip_bigDrosophila melanogaster: a fruit fly species used in genetics research.. Some forms of color blindness in humans are also sex-linkedplugin-autotooltip__default plugin-autotooltip_bigSex linkage: a gene is said to be sex-linked if it maps to a sex chromosome.. If there is historical information on phenotypesplugin-autotooltip__default plugin-autotooltip_bigPhenotype: an observable feature or property of an organism. in a family tree, a pedigree analysisplugin-autotooltip__default plugin-autotooltip_bigPedigree analysis: a method used to examine and analyze genetic inheritance using a family tree. can be used to observe this. Figure 7 shows an example of a human pedigree that contains individuals affected by red-green color blindness. In the general population, red-green color blindness is somewhat common (8% of males and 0.5% in females) but is particularly prevalent in this pedigree. In just three generations, 5 males are affected (to just one female). The underlined $X$ chromosomeplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. is a model to explain the inheritance pattern. It makes sense that individuals 1 and 2 in generation IV, who are both males, are colorblind if their mother is homozygousplugin-autotooltip__default plugin-autotooltip_bigHomozygous: a state for a diploid organism wherein the two alleles for a gene are identical to each other. for a sex-linkedplugin-autotooltip__default plugin-autotooltip_bigSex linkage: a gene is said to be sex-linked if it maps to a sex chromosome. geneplugin-autotooltip__default plugin-autotooltip_bigGene: read Chapters 02, 03, 04, 05, and 06 for a definition of gene :-) that confers the color blind phenotypeplugin-autotooltip__default plugin-autotooltip_bigPhenotype: an observable feature or property of an organism.. We now know that this kind of heritable red-green colorblindness is caused by a mutationplugin-autotooltip__default plugin-autotooltip_bigMutation: a change in the DNA of a gene that results in a change of phenotype compared to a reference wildtype allele. See also: mutant. in the geneplugin-autotooltip__default plugin-autotooltip_bigGene: read Chapters 02, 03, 04, 05, and 06 for a definition of gene :-) that codes for an opsin, a proteinplugin-autotooltip__default plugin-autotooltip_bigProtein: a molecule that is formed by the translation of messenger RNAs (mRNAs). Functions that proteins provide are what usually give organisms their phenotypes. that is sensitive to specific colors of light in the retina of the eye. We also know that this opsin geneplugin-autotooltip__default plugin-autotooltip_bigGene: read Chapters 02, 03, 04, 05, and 06 for a definition of gene :-) is located on chromosomeplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. $X$ in humans.
Why don't we do pedigree analysisplugin-autotooltip__default plugin-autotooltip_bigPedigree analysis: a method used to examine and analyze genetic inheritance using a family tree. for Drosophilaplugin-autotooltip__default plugin-autotooltip_bigDrosophila melanogaster: a fruit fly species used in genetics research.? Why is it only necessary for humans?
Closing thoughts
A final note on sex linkageplugin-autotooltip__default plugin-autotooltip_bigSex linkage: a gene is said to be sex-linked if it maps to a sex chromosome.: many students are confused by sex linkageplugin-autotooltip__default plugin-autotooltip_bigSex linkage: a gene is said to be sex-linked if it maps to a sex chromosome. - they think of it as some kind of special case in genetics. Technically, it is indeed a special case. But it is best to think of sex linkageplugin-autotooltip__default plugin-autotooltip_bigSex linkage: a gene is said to be sex-linked if it maps to a sex chromosome. more as a general case of genetics. What sex linkageplugin-autotooltip__default plugin-autotooltip_bigSex linkage: a gene is said to be sex-linked if it maps to a sex chromosome. historically taught us is that genesplugin-autotooltip__default plugin-autotooltip_bigGene: read Chapters 02, 03, 04, 05, and 06 for a definition of gene :-) are physically associated with chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins.. It just so happens that some genesplugin-autotooltip__default plugin-autotooltip_bigGene: read Chapters 02, 03, 04, 05, and 06 for a definition of gene :-) are on sex chromosomesplugin-autotooltip__default plugin-autotooltip_bigSex chromosomes: in some species, these are a set of chromosomes that determine sex. In some species, females are homogametic (XX) and males are heterogametic (XY), whereas in other species it is the other way around (homogametic ZZ males and heterogametic ZW females). Not all species use sex, and those sex chromosomesplugin-autotooltip__default plugin-autotooltip_bigSex chromosomes: in some species, these are a set of chromosomes that determine sex. In some species, females are homogametic (XX) and males are heterogametic (XY), whereas in other species it is the other way around (homogametic ZZ males and heterogametic ZW females). Not all species use sex also happen to determine sex2). Since sex is easy to observe, it means that this fact that genesplugin-autotooltip__default plugin-autotooltip_bigGene: read Chapters 02, 03, 04, 05, and 06 for a definition of gene :-) are associated with chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. just happened to be first discovered for genesplugin-autotooltip__default plugin-autotooltip_bigGene: read Chapters 02, 03, 04, 05, and 06 for a definition of gene :-) on sex chromosomesplugin-autotooltip__default plugin-autotooltip_bigSex chromosomes: in some species, these are a set of chromosomes that determine sex. In some species, females are homogametic (XX) and males are heterogametic (XY), whereas in other species it is the other way around (homogametic ZZ males and heterogametic ZW females). Not all species use sex. But the general statement that “genesplugin-autotooltip__default plugin-autotooltip_bigGene: read Chapters 02, 03, 04, 05, and 06 for a definition of gene :-) are physically associated with chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins.” is true for all genesplugin-autotooltip__default plugin-autotooltip_bigGene: read Chapters 02, 03, 04, 05, and 06 for a definition of gene :-) and for all chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins.. When thinking about sex linkedplugin-autotooltip__default plugin-autotooltip_bigSex linkage: a gene is said to be sex-linked if it maps to a sex chromosome. genesplugin-autotooltip__default plugin-autotooltip_bigGene: read Chapters 02, 03, 04, 05, and 06 for a definition of gene :-) and their inheritance, it can be easier to think about how these genesplugin-autotooltip__default plugin-autotooltip_bigGene: read Chapters 02, 03, 04, 05, and 06 for a definition of gene :-) segregate with the $X$ and $Y$ chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. first, since chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. always follow the rules of meiosisplugin-autotooltip__default plugin-autotooltip_bigMeiosis: a process involving two sequential cell divisions that usually produces four gametes (reproductive cells such as sperm or eggs).. Then think about the sex of the offspring as a secondary thing. Consider chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins. and genotypesplugin-autotooltip__default plugin-autotooltip_bigGenotype: the combination of alleles within an organism or strain. When used as a verb, it means to determine the genotype experimentally. and how they segregate first, then ask what the resulting phenotypesplugin-autotooltip__default plugin-autotooltip_bigPhenotype: an observable feature or property of an organism. come from those genotypesplugin-autotooltip__default plugin-autotooltip_bigGenotype: the combination of alleles within an organism or strain. When used as a verb, it means to determine the genotype experimentally. second. Use this approach to think about all genesplugin-autotooltip__default plugin-autotooltip_bigGene: read Chapters 02, 03, 04, 05, and 06 for a definition of gene :-) and chromosomesplugin-autotooltip__default plugin-autotooltip_bigChromosome: a structure that organizes dsDNA in a cell through interactions with various DNA binding proteins., including sex linkedplugin-autotooltip__default plugin-autotooltip_bigSex linkage: a gene is said to be sex-linked if it maps to a sex chromosome. genesplugin-autotooltip__default plugin-autotooltip_bigGene: read Chapters 02, 03, 04, 05, and 06 for a definition of gene :-) and sex chromosomesplugin-autotooltip__default plugin-autotooltip_bigSex chromosomes: in some species, these are a set of chromosomes that determine sex. In some species, females are homogametic (XX) and males are heterogametic (XY), whereas in other species it is the other way around (homogametic ZZ males and heterogametic ZW females). Not all species use sex.
Questions and exercises
Exercise 1: While working in a Drosophilaplugin-autotooltip__default plugin-autotooltip_bigDrosophila melanogaster: a fruit fly species used in genetics research. lab, you discover a mutantplugin-autotooltip__default plugin-autotooltip_bigMutant: an individual that has a different phenotype than wildtype and likely contains one more mutations that cause this difference. male fly with miniature wings that are much smaller than wildtypeplugin-autotooltip__default plugin-autotooltip_bigWildtype: a reference strain of an organism that scientists operationally define as “normal” to which mutants are compared. Not to be confused with wild organisms. wings. You temporarily name this mutantplugin-autotooltip__default plugin-autotooltip_bigMutant: an individual that has a different phenotype than wildtype and likely contains one more mutations that cause this difference. $mini$. What experiments could you do to answer the question: is $mini$ sex linkedplugin-autotooltip__default plugin-autotooltip_bigSex linkage: a gene is said to be sex-linked if it maps to a sex chromosome.? What would be the outcome if $mini$ was sex linkedplugin-autotooltip__default plugin-autotooltip_bigSex linkage: a gene is said to be sex-linked if it maps to a sex chromosome.? What would be the outcome if $mini$ was autosomalplugin-autotooltip__default plugin-autotooltip_bigAutosome: any chromosome that is not a sex chromosome.? Write out your crosses in fractional notationplugin-autotooltip__default plugin-autotooltip_bigFractional notation: a style of genotype notation that uses “fractions”, e.g., $\frac{unc\text{-}4}{+}$. We strongly encourage this notational style as it generally is preferred by genetics researchers..
Conceptual question: You are a pediatrician working in a remote rural community where there is an unusually high frequency of children with polydactyly - they have extra fingers or toes. What could you do to try to determine if this was a genetically inherited trait? What could you do to try and determine if this was a sex linkedplugin-autotooltip__default plugin-autotooltip_bigSex linkage: a gene is said to be sex-linked if it maps to a sex chromosome. trait?