BIOL 1500 Lecture Notes - X-Inactivation, Sex Linkage, Y Chromosome

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Published on 20 Mar 2013
Heena Loomba
Chapter 12: Genes, Chromosomes, and Human Genetics
12. 2 Sex-Linked Genes
Sex-linked genes: genes located on sex chromosomes, they are inherited differently
in males and females
Autosome: chromosomes other than sex chromosomes, the genes have the same
patterns of inheritance in both sexes
Females have two copies of the X chromosome, a fully homologous XX pair
Males have one X chromosome and one Y chromosome
The Y chromosome has a short region of homology with the X chromosome
that allows them to pair during meiosis
When a sperm cell carrying an X chromosome fertilizes an X-bearing egg cell, the
new individual develops into an XX female
When a sperm cell carrying a Y chromosome fertilizes an X-bearing cell, the
combination produces an XY male
SRY gene carried on the Y chromosome is the “master switch” that directs
developments toward maleness during embryonic development
After 6-8 weeks of embryonic development, the SRY gene becomes active in
XY embryos, producing a protein that regulates the expression of other
genes which stimulates the structures to develop as testes, penis, and
For the first month of embryonic development, the structures that give rise to
reproductive organs and tissues are the same in XX or XY embryos
In XX embryos, development proceed toward female reproductive structures
because rudimentary male structures degenerate because the hormones released
by the developing XY embryos are not present
Since males and females have different sets of sex chromosomes, the genes carried
on these chromosomes can be inherited by sex linkage
Sex linkage arises from two differences between males and females:
Males have one X chromosome, so one allele for each gene on this
chromosome. While females have two X chromosomes, so two alleles for all
genes on the X chromosome
Males have one copy of Y chromosome and one allele for each gene on this
chromosome. While females do not have a Y chromosome, so no Y alleles.
Sex linkage was discovered when Morgan crossed a white-eyed male fly with a
true-breeding female with red eyes. He observed that all the F1 flies had red eyes.
Next, he allowed the F1 flies to interbreed. He found that all the F2 females had red
eyes and half of the F2 males had red eyes and half had white eyes
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