3.15 Gender Determination in Humans

1. 3.15 Gender Determination in Humans (Sec 4.4 pg 123)

2. Recall how in mitosis we were able to see the chromosomes during metaphase. If you had a powerful microscope and took a picture of the chromosomes you could sort them into homologous pairs.

3. The arrangement of an organism’s chromosomes is called its karyotype. • The human karyotype has 46 chromosomes (23 pairs)

4. The first 22 pairs are called autosomes. • The final pair arecalled the sex chromosomes. They are the pair that determines what gender a person becomes. There are two possibilities for this chromosome, the ‘X’ (long), or the ‘Y’ (short). • Females have 2 ‘X’ chromosomes (‘XX’). • Males have an ‘X’ and a ‘Y’ chromosome (‘XY’).

5. During meiosis, the maleproduces 2 ‘X’ chromosome sperm and 2 ‘Y’ chromosome sperm.

6. Note the parent female (whose cells are ‘XX’) can only produce ‘X’ eggs!

7. The gender of the baby is always determined by the male’s sperm. The mother always gives an ‘X’ chromosome, and the male gives either an ‘X’ or ‘Y’.

8. There are some genetic traits that are only carried on the ‘X’ chromosome. Since males only have one copy of the X chromosome, if there is a mutation on their mother’s copy, they will inherit that mutation. • For this reason, males are more likely to express sex-linked traits such as colour blindness, have baldness, haemophilia, etc.