1. Sex-Linked InheritanceXY XX

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

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

5. Sex-linked Inheritance
Many nonsexual traits appear to be inherited along with sex (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

6. Sex-linked Inheritance con’t
X chromosome is larger
Nearly 100 genes that control nonsexual characteristics
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

7. 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

8. 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.

9. 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.