Download Sex Linked Traits

Sex Linked Traits
Sex Linked Traits
• X and Y chromosomes behave as a
homologous pair at meiosis.
• However, there are differences between them.
The larger X carries many genes not present
on the smaller Y.
• These genes located only on the X
chromosome are said to be sex-linked.
Sex Linked Traits
• When X and Y chromosomes meet at
fertilization, each sex-linked gene on the X
chromosome (whether recessive or dominant)
becomes expressed in the phenotype.
• This is because the Y chromosome does not
possess alleles of any of these genes and
cannot offer dominance to them.
Sample Problem #1
Hemophilia is a sex-linked recessive trait. A hemophiliac man marries a woman
without hemophilia (homozygous). Show the phenotypic ratios of their offspring.
Solution:
Let N represent the allele for normal
Let n represent the allele for hemophilia.
Father genotype:
Punnett square:
Genotype:
Phenotype:
Mother genotype:
Sample Problem #2
A woman who is a carrier for hemophilia and has blood type AB marries a man without hemophilia,
who is heterozygous for blood type A. Show all possible phenotypes of their sons.
Solutions:
Let N represent the allele for normal
Let n represent the allele for hemophilia.
Let IA, IB, i represent the alleles for blood types A, B and O, respectively.
Father genotype:
Punnett square:
Phenotype of Sons:
Mother genotype:
Conclusions
• If a male has even 1 copy of the disordered
gene (recessive or dominant) on the X
chromosome, he suffers from the disorder.
• This occurs because there is no opposing gene
on the Y chromosome to mask the disorder on
the X chromosome.
Conclusions …cont’d
• A female can carry 1 disordered gene on one
X chromosome but still have a normal gene to
mask it on the other X chromosome.
• A “carrier female” usually doesn’t suffer from
the disorder; she just carries it and can pass it
on.
Homework
• Genetics Word Problems:
– Sex-Linkage #1-4