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Sex Determination
Of the 46 chromosomes, 2 are designated sex chromosomes (X & Y) the rest are
designated autosomal.
Sex Determining Chromosomes
Female
XX
X
Y
X
XX
XY
X
XX
XY
Male
XY
A punnett square shows that offspring will be 1/2 male,
1/2 female
Since females are XX, every egg must contain an X chromosome. Since males are XY, a
sperm cell will contain either an X chromosome of a Y chromosome. The sex of a
fertilized egg is therefore determined by the sperm! (King Henry VIII & Ann Boleyn)
Question
Is everything on these chromosomes just for sex determination or is their other stuff on them?
SRY gene development of testes
X chromosomes codes for
1500 genes
Y chromosomes codes for
78 genes
Sex-linked Traits
Men have an X and a Y combination of sex chromosomes, while women have two X's. Since only men
inherit Y chromosomes, they are the only ones to inherit Y-linked traits. Men and women can get the Xlinked ones since both inherit X chromosomes.
If a
gene is found only on the X chromosome and not the Y chromosome, it is said to be a sex-linked trait
The result is that females will have two copies of the sex-linked gene while males will only have one
copy of this gene.
If the gene is recessive, then males only need one such recessive gene to have a sex-linked trait
rather than 2 recessive genes for traits that are not sex-linked. This is why males exhibit some
traits more frequently than females.
Examples of Sex-linked Traits
Colour blindness
Hemophilia
Duchenne Muscular Dystrophy
Male Pattern Baldness
X Chromosome Recessive?
Hemophilia
NO Hemophilia
Hemophilia
Hemophilia
Infrequent Mutuation?
Who is more likely to have hemophilia, male or female?
1/7000 frequency of Xh
1/7000 X 1/7000 =
1/49000000
This is why males exhibit some traits more frequently than females
Sex linked Traits Worksheet
Homework
pg. 151 #8,9