Sex-linked genes; Errors in chromosomal dynamics
Campbell Chapter 15:
Ed. pp. 272; 276-282
Ed. pp. 276, 282-285; 287.
Ed. pp 288-292,
PART I: Sex linkage
Chromosomes in homologous pairs are called autosomes, while the sex chromosomes are not in
In humans and many other species (including Drosophila), females have 2 X
chromosomes, while males have one X and one Y chromosome in each somatic cell.
may differ, as shown in Fig. 15.8/15.9 in Campbell. For instance, grasshoppers have an X-O
system, where males have only 1 X chromosome, and no other sex chromosome.
Chickens have a
ZW system; that is, females have 2 different sex chromosomes, Z and W, while males have two Z
The Y chromosome contains few genes, and we won’t talk about it any more.
However, many important genes are on the X chromosome.
Genes on the X chromosome are sex-
We say that males are hemizygous
for these genes, meaning that they have only one copy.
During meiosis and gamete formation, the sex chromosomes segregate.
That means that each egg
gets one X chromosome, while each sperm can get either an X or a Y chromosome.
Since all eggs
contain an X chromosome, fertilization of an egg with a sperm containing an X chromosome gives a
female (XX), while fertilization of an egg with a sperm containing a Y chromosome gives a male
Genes on the X chromosome are transmitted according to Mendel’s laws, but with a twist;
phenotypes that reflect sex-linked traits are transmitted with different frequencies to male and
female offspring, reflecting the fact that females are XX and males XY.
In his early studies of fruit fly genetics, Morgan studied a gene for eye color.
Wild type flies have
red eyes, while mutants have white eyes.
Red is dominant, and white recessive.
Morgan did a
similar experiment to the one Mendel did with peas.
He first made true-breeding stocks of flies
with either red or white eyes and used these flies as his P generation.
He crossed a red-eyed female
with a white-eyed male.
As expected, all the offspring in this F1 generation had red eyes.
crossed these individuals of the F1 generation with each other.
As expected from Mendelian
genetics, the progeny in the resulting F2 generation had a 3:1 ratio of red:white eye color.
However, all the females had red eyes, while half of the males had red eyes and half white eyes. It
turns out that the eye color gene is sex-linked; that is, it's on the X chromosome.
You can follow
through the genotypes of the P, F1 and F2 generations to explain the observed results.
color gene is called w for the mutant allele and w+ for the wild type allele.