Inherited disorders and sex determination

The idea

Polydactyly (extra fingers or toes) is caused by a dominant allele; cystic fibrosis, a disorder of cell membranes, is caused by a recessive allele. Embryos can be screened for these disorders, which raises ethical and economic issues. Human body cells have 23 pairs of chromosomes; one pair determines sex: females are XX and males XY. Each pregnancy has an expected 50% chance of a boy or a girl.

1. Say whether the allele is dominant or recessive.
2. Use a Punnett square to find probabilities.
3. For sex, cross XX with XY.

Watch out A carrier of cystic fibrosis (Ff) has the allele but not the disorder.