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--- chapter_04 [2024/08/28 14:42] – [The $white$ mutation is sex-linked] mike
+++ chapter_04 [2024/09/01 23:28] (current) – mike
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-<typo fs:x-large>Chapter 04. Chromosomes and sex linkage</typo>
+<-chapter_03|Chapter 03^table_of_contents|Table of Contents^chapter_05|Chapter 05->
+<typo fs:x-large>Chapter 04. %%Chromosomes and sex linkage%%</typo>
 Until now our analysis of genes has focused on defining genes based on phenotypic differences brought about by different alleles or by a direct test of function – the complementation test. In Chapters 4 and 5, our analysis will be concerned with tests of gene position.
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-Morgan explained these unusual results by hypothesizing that the eye color gene white ($w$) is physically located on the sex chromosome $X$. Males only have one copy of the $X$ chromosome 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 genotypes that are directly associated, or linked, with the $X$ chromosome, and re-write Figure {{ref>Fig4}} as follows:
+Morgan explained these unusual results by hypothesizing that the eye color gene $white$ ($w$) is physically located on the sex chromosome $X$. Males only have one copy of the $X$ chromosome 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 genotypes that are directly associated, or linked, with the $X$ chromosome, and re-write Figure {{ref>Fig4}} as follows:
 <figure Fig5>
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-A final note on sex linkage: many students are confused by sex linkage - 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 linkage more as a general case of genetics. What sex linkage historically taught us is that genes are physically associated with chromosomes. It just so happens that some genes are on sex chromosomes, and those sex chromosomes also happen to determine sex. Since sex is easy to observe, it means that this fact that genes are associated with chromosomes just happened to be first discovered for genes on sex chromosomes. But the general statement that "genes are physically associated with chromosomes" is true for all genes and for all chromosomes. When thinking about sex linked genes and their inheritance, it can be easier to think about how these genes segregate with the $X$ and $Y$ chromosomes first, since chromosomes always follow the rules of meiosis. Then think about the sex of the offspring as a secondary thing. Consider chromosomes and genotypes and how they segregate first, then ask what the resulting phenotypes come from those genotypes second. Use this approach to think about all genes and chromosomes, including sex linked genes and sex chromosomes.
+A final note on sex linkage: many students are confused by sex linkage - 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 linkage more as a general case of genetics. What sex linkage historically taught us is that genes are physically associated with chromosomes. It just so happens that some genes are on sex chromosomes, and those sex chromosomes also happen to determine sex((Also note that just because a gene is sex-linked doesn't necessarily mean that this gene is involved in sex determination. There are lots of "regular" genes on the X chromosome that have nothing to do with sex determination.)). Since sex is easy to observe, it means that this fact that genes are associated with chromosomes just happened to be first discovered for genes on sex chromosomes. But the general statement that "genes are physically associated with chromosomes" is true for all genes and for all chromosomes. When thinking about sex linked genes and their inheritance, it can be easier to think about how these genes segregate with the $X$ and $Y$ chromosomes first, since chromosomes always follow the rules of meiosis. Then think about the sex of the offspring as a secondary thing. Consider chromosomes and genotypes and how they segregate first, then ask what the resulting phenotypes come from those genotypes second. Use this approach to think about all genes and chromosomes, including sex linked genes and sex chromosomes.
 ===== Questions and exercises =====