Cells 20

What is a sex-linked trait?

A trait controlled by a gene located on a sex chromosome (X or Y).

What are autosomes?

All chromosomes other than the sex chromosomes.

What does hemizygous mean?

Having only one copy of a gene, as males do for most X-linked genes.

Why are X-linked recessive disorders more common in males?

Males have only one X chromosome, so a recessive allele is always expressed.

Can a father pass an X-linked allele directly to his sons?

No, fathers pass their Y chromosome to sons.

What are the offspring of a normal female (XAXA) and a colour-blind male (XaY)?

All daughters are carriers (XAXa); all sons are normal (XAY).

What are the offspring of a carrier female (XAXa) and a normal male (XAY)?

Half daughters are carriers, half normal; half sons are colour blind, half normal.

What are linked genes?

Genes located on the same chromosome that tend to be inherited together.

Why do linked genes violate Mendel's Second Law?

They do not assort independently.

What are parental types?

Offspring with the same allele combinations as the parents.

What are recombinant types?

Offspring with new allele combinations produced by crossing over.

What is crossing over?

The exchange of DNA between non-sister chromatids of homologous chromosomes.

What is a chiasma?

The physical site where crossing over occurs.

How is recombination frequency calculated?

Recombinant offspring / total offspring

What does a high recombination frequency indicate?

The genes are far apart on the chromosome (or effectively unlinked).