3.15 Gender Determination in Humans (Sec 4.4 pg 123)

 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.

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

 The first 22 pairs are called autosomes.  The final pair are called 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’).

 During meiosis, the male produces 2 ‘X’ chromosome sperm and 2 ‘Y’ chromosome sperm.

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

 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’.

 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.