Lesson #11Sex-Linked genes

Sex-linked genesInheritance linked to the X chromosome

Sex-Linked genes Women Men

X

X

X

Y

Male gametes

Fem

ale

gam

etes

Inheritance of genes in the X chromosome has some pecularities:

• Women carry 2 alleles for genes in the X chromosome, therefore they can be homozygous or heterozygous for those genes

• Men can only carry one allele for the genes in the X chromosome, therefore they will express the phenotype of that single allele they carryfor genes on the X chromosome.

We represent the allele on the sex chromosome, the Y chromosomo wouldnot carry an allele, because it has different genes.

1 12 2 3 3. .. .. .X X

1 12 2 3 3. .. .. .X Y

X X X Y

Haemophilia (Sex-Linked trait)

Normal Factor VIII Healthy (H)

Deficient Factor VIIIHaemophilia (h)

Healthy > HaemophiliaAlleles

Gene: Coagulation Factor VIII

Linked to the X chromosome

Disorder mainly caused by a deficient coagulation Factor VIII which limits the ability of the blood to clot or coagulate. This deficiency is due to genetic reasons. The gene that codifies for this coagulation Factor VIII is on the X chromosome.

Females Males

Genotype XHXH XHXh XhXh XHY XhY

Phenotype Healthy Healthy Haemophilic Healthy Haemophilic

Sex-Linked

XHXh

Problem solving exerciseA healthy woman who carries an allele for a deficient Factor VIII has offspring with a healthy man. What are the chances of this couple of having a hemophilic child?

P: Healthy carrier woman Healthy man

XHY

x

x

Gametes: p(XH)= p(Xh) = ½

Punnet F1:

Answer: There is a 25% probability of having a hemophilic child.

F1 Genotype F1 Phenotype

p(XHXH) ¼ 25% Healthy girl

p(XHXh ) ¼ 25% Carrier healthy girl

p(XHY) ¼ 25% Healthy boy

p(XhY) ¼ 25% Hemophilic boy

p(XH) = p(Y) = ½