Meiosis is the process of cell division that produces gametes, or sex cells, with half the number of chromosomes as normal body cells. This ensures that when a male and female gamete fuse during fertilization, the resulting offspring has the full number of chromosomes. Meiosis occurs in two divisions, first separating the paired chromosomes and then dividing the cells again, resulting in four gamete cells each with a single set of chromosomes. Errors during meiosis where a gamete retains an extra chromosome can lead to genetic disorders if that gamete is involved in fertilization.

1. Meiosis Cell division – ‘ reduction division’ Production of sex cells – gametes Meiosis errors dr shabeel pn

4. Meiosis Parent cell – chromosome pair Chromosomes copied 1 st division - pairs split 2 nd division – produces 4 gamete cells with ½ the original no. of chromosomes

5. Meiosis – mouse testes Parent cell 4 gametes 1 st division 2 nd division

6. Meiosis – division error Chromosome pair

7. Meiosis error - fertilisation Should the gamete with the chromosome pair be fertilised then the offspring will not be ‘normal’. In humans this often occurs with the 21 st pair – producing a child with Downs Syndrome

8. 21 trisomy – Downs Syndrome Can you see the extra 21 st chromosome? Is this person male or female?