Most fruit flies have red eyes, although there is a white-eyed variant. Fig. 7.1 presents the red-eyed and white-eyed fruit fly variants. The gene for eye colour is on the X chromosome. Using suitable symbols, draw a genetic diagram to show the possible offspring from a cross between a heterozygous red-eyed female fruit fly and a white-eyed male fruit fly.
One gene involved in blood clotting in humans is also found on the X chromosome. A rare gene variant, a recessive allele for haemophilia, can cause the blood to fail to clot properly. State why a man who has haemophilia cannot pass the condition to his son.
Queen Victoria of Great Britain in the 19th century was a carrier of haemophilia, but she did not have the condition. State the term used to describe the genotype of a carrier.
Neither of Queen Victoria’s parents carried the haemophilia allele. Suggest how Queen Victoria might have become a carrier.