Sex Linkage and Sex Determination

What are the Male Sex Chromosomes?

XY

What are the Female Sex Chromosomes?

XX

What was Morgan’s Hypothesis about the eye colour gene?

Eye colour gene is on X chromosome

Where is Sex Linkage relevant?

Normal and disease-related phenotypes

What is Hemizygous Exposure?

Recessive alleles on X chromosome become exposed in males

Describe the X Chromosomes

Large, many genes for biological functions, X-linked genes

Describe the Y Chromosomes

Small, few genes, most relate to male fertility, Y-linked genes

Who is affected most by X-linked disorders in humans?

Males

How are X-linked disorders in humans inherited?

From mothers who are carriers

What is the cause of Haemophilia A?

Lack of clotting factor 8

What is the cause of Haemophilia B?

Lack of clotting factor 9

Which is more common, Haemophilia A or B?

A, 1 in 10K vs 50K

What are the side effects of Haemophilia?

Incomplete blood clotting, continued bleeding, easy bruising, internal bleeding around joints

What does the severity of Haemophilia depend on?

Amount of remaining clotting factors

What was Mary Lyon’s discovery?

In females, 1 X chromosome is randomly inactivated in every cell

How does X-Inactivation relate to Gene Dosage?

Gene dosage mechanism

Does X-Inactivation occur in all organisms?

Only mammals, other species have different strategis

How does X-Inactivation occur?

Early embryo randomly inactivates 1 X chromosome in each cell

How is X-Inactivation inherited?

Passed on to daughter cells during mitosis, stable once established

What are Chimaeras?

Female carriers of X-linked abnormalities

How does Inactivation work in Chimaeras?

½ their cells inactivate chromosome w/ normal allele

What is the effect of the loss of an X Chromosome?

Survivable, only 1 is necessary for life

What is the effect of the gain of X Chromosomes?

Survivable, X inactivation can handle multiple Xs

What is the effect of the loss of a Y Chromosome?

Survivable, Y can’t be essential for life (females don’t have)

What is the effect of the gain of Y Chromosomes?

Survivable, Y only has a small number of genes

Describe the evolution of Sex Chromosomes

Rapid

What are the Sex Chromosomes of most Insects?

Female - XX, Male - X

What are the Sex Chromosomes of Chickens?

Female - ZW, Male - ZZ

What are the Sex Chromosomes of Ants and Bees?

Female - Diploid (32 Chromosomes), Male - Haploid (16)