Sex-linked Inheritance: Criss-cross Genes on the X Chromosome

In the paper

Unit 5 carries 6 of the 40 botany marks; with five chapters sharing them, this one averages just over one (MEC publishes weights by unit, not by chapter). The MEC scope line: Concepts and patterns of sex-linked inheritance; Colour blindness in humans; Eye colour in Drosophila melanogaster. Three ways it is asked:

• Recall: which chromosome carries the colour-vision genes; who found the white-eyed fly; what "hemizygous" means.
• Understanding: why X-linked recessive traits are commoner in men; why reciprocal crosses differ; why a colour-blind man never passes the trait to his son.
• Application: a colour-blind father and a carrier mother — what fraction of sons and of daughters are affected; a white-eyed female crossed with a red-eyed male; an affected boy with two normal parents.