Sex Determination and Sex Linkage

Question 1

What is karyotype?

Answer 1

Karyotype is simply the number and shape of chromosomes in a species. The term is also used to describe a complete set of chromosomes in a species or individual.

Question 2

What are autosomes?

Answer 2

Autosomes are chromosomes that are not sex chromosomes, they often appear in pairs.

Question 3

What are the two types of abnormalities that can arise in human gametes?

Answer 3

• XX

- O (no X chromosome)

Question 4

What happens if these abnormalities are fertilized?

Answer 4

• XXX Sterile female
• XXY Klinefellar’s syndrome. Sterile male exhibiting some female characteristics. 1 in 1000 males affected.
• XO Turner’s sydrome. Sterile female. 1 in 2000 female births affected.
• OY Not viable

Question 5

How is it possible for a small number of males to have an XX genotype and for a small number of females to have XY genotype?

Answer 5

The SRY gene.

• Due to the transfer of a small region of the Y chromosome to the X
• The region contains a single gene that has been shown to confer maleness. The gene is known as the SRY gene.

Question 6

Although sex chromosomes determine sex, they do not carry all the genes necessary for determining the sexual phenotype.

Answer 6

• They also carry many other genes which have nothing to do with sexual development.
• Genes carried on sex chromosomes are said to be sex linked and show unusual patterns of inheritance

Question 7

What are the consequences of sex linkage?

Answer 7

• In mammals, the X and Y chromosomes only share a short region of homology
• Hence there are gene loci on the X that are not present on the Y
• Males inherit their Y chromosome from their father and their X from their mother therefore they are hemizygous for X linked genes
• This means that mutant alleles will always show a phenotype in males

Question 8

Examples of sex linked disorders in humans

Answer 8

• Red/Green colour blindness ( 8% in Males, 0.7% in Females)
• Duchenne Muscular Distrophy (Affects 1 in 3500 Males)
• Haemophilia (Affects 1 in 10,000 males)

Question 9

Describe Dosage Compensation

Answer 9

• In female mammals only one X chromosome is active in each cell
• The inactive X chromosome is highly condensed
• Called a ‘Barr Body’