15.2

Question 1

What is the pattern of inheritance in the offspring of fathers with X-linked alleles?

Answer 1

• Fathers pass the alleles to all of their daughters.
• Fathers pass the alleles to none of their sons.

Question 2

What is the pattern of inheritance in the offspring of mothers with X-linked alleles?

Answer 2

Mothers can pass alleles to both sons and daughters.

Question 3

If an X-linked trait is due to a recessive allele, when will a female express the phenotype?

Answer 3

She is homozygous for that allele.

Question 4

If an X-linked trait is due to a recessive allele, when will a male express the phenotype?

Answer 4

He will always express the trait.

Question 5

Why will males always express a recessive X-linked trait?

Answer 5

Males have only one locus (hemizygous).

Question 6

X Inactivation

Answer 6

Each cell “switches off” one of its X chromosomes at random.

Question 7

Why is X Inactivation important?

Answer 7

It is a type of gene dosage compensation (for the extra X chromosome). It ensures that the correct number of genes are expressed and prevents abnormal development.

Question 8

Barr Body

Answer 8

The inactive X in each cell condenses into this compact object.