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: Mother genotype: Punnett square: Genotype: Phenotype:

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: Mother genotype: Punnett square: Phenotype of Sons:

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