Sex-Linked Inheritance XY XX

Autosomal chromosomes Chromosome pairs 1 to 22 Responsible for determining nonsexual characteristics (e.g. eye and hair colour)

Sex chromosomes The 23rd pair of chromosomes Responsible for determining sex (male or female) and sex-related characteristics (e.g. facial hair)

Simple Punnett’s Square showing how gender is determined

Sex-linked Inheritance Many nonsexual traits appear to be inherited along with sex (and are more common in one sex than in the other). Why are some traits more common in one sex vs the other? Y chromosome is small ➔ most gene locations determine sexual characteristics (e.g. facial hair) X chromosome is larger ➔Nearly 100 genes that control nonsexual characteristics

Sex-linked Inheritance con’t For example: the X chromosome carries the gene for colour-blindness B = normal vision b = colour-blind XBXB = female with normal colour vision XBXb = female who carries gene for colour-blindness (but has normal colour vision) XbXb = colour-blind female XBY = male with normal colour vision XbY = colour-blind male *Note* males cannot be a carrier, only females can be a carrier.

Practice Questions XB Xb XBXB XBXb Y XBY XbY 1. A woman who carries the gene for colour-blindness has a child with a man who has normal colour vision. What are the possible genotpyes and phenotypes of their child? 25% female with normal colour vision 25% female with normal colour vision (carrier for colour-blindness) 25% colour-blind male 25% male with normal colour vision XB Xb XBXB XBXb Y XBY XbY

2. A colour-blind man has children with a woman who has normal colour vision (and no history of colour-blindness in her family). What is the possibility that their children will be colour-blind? XB Xb XBXb Y XBY There is a 0% chance that their children will be colour-blind. However, their daughters will be carriers of colour-blindness.

3. A man and his wife have normal vision, but the daughter is colour-blind. The man sues his wife for divorce on grounds of infidelity. You are his lawyer. What evidence will you provide to the judge? XB Xb XBXB XBXb Y XBY XbY Even if mother was a carrier of colour-blindness, there is a 0% chance of having a colour-blind daughter if the father is not colour-blind.