Genetics Study Guide

Human sex is determined at the moment of fertilisation by the sex chromosomes carried by the sperm. Females have two X chromosomes (XX), and males have one X and one Y chromosome (XY). During meiosis, females produce eggs, all of which carry a single X chromosome. Males produce sperm, approximately half of which carry an X chromosome and half carry a Y chromosome. If an egg (X) is fertilised by a sperm carrying an X chromosome, the resulting zygote will be XX, developing into a female. If an egg (X) is fertilised by a sperm carrying a Y chromosome, the zygote will be XY, developing into a male. This means the father's sperm determines the sex of the offspring, with roughly an equal probability of conceiving a male or female child.

Meiotic cell division is a specialized type of cell division that plays a crucial role in sexual reproduction. Its primary function is to produce gametes (sex cells), such as sperm and egg cells, which are genetically distinct from each other and from the parent organism. During meiosis, a single diploid parent cell undergoes two successive divisions to produce four daughter cells. Each of these daughter cells contains half the number of chromosomes of the original parent cell, making them haploid. This reduction in chromosome number is essential so that when two gametes fuse during fertilisation, the resulting zygote has the correct, full complement of chromosomes for the species. Meiosis also introduces genetic variation, ensuring that offspring are not identical to their parents or siblings.