Genes, inheritance and selection Study Guide

Sexual reproduction involves the fusion of two specialised cells called gametes (sperm and egg). Each gamete must carry only half the normal number of chromosomes found in other body cells. This halving of the chromosome number is crucial because when a sperm and an egg fuse during fertilisation, the resulting zygote will then have the full, correct number of chromosomes for that species. If gametes had the full set of chromosomes, the chromosome number would double with each generation, leading to genetic abnormalities that are usually fatal. Meiosis is the specific type of cell division responsible for producing these gametes. It involves two rounds of cell division, but only one round of DNA replication. This unique process ensures that a diploid parent cell (containing two sets of chromosomes) produces four haploid daughter cells (each containing a single set of chromosomes). In humans, body cells have 46 chromosomes (23 pairs), so meiotic division reduces this to 23 chromosomes in each gamete. This reduction in chromosome number is essential for maintaining a stable chromosome count across generations and introduces genetic variation through the independent assortment of chromosomes and crossing over.

Evolution is a fundamental process in biology that describes how the inherited characteristics of biological populations change over successive generations. These changes occur over long periods, leading to the diversity of life we see today. The primary mechanism driving evolution is natural selection. Natural selection occurs when individuals within a population have variations in their characteristics, some of which make them better suited to their environment. These advantageous characteristics increase an individual's chances of survival and reproduction. Consequently, individuals with favourable traits are more likely to pass these traits on to their offspring. Over time, as these advantageous traits become more common in the population, the overall characteristics of the population change. This cumulative change can eventually lead to the formation of new species if populations become isolated and diverge significantly enough that they can no longer interbreed to produce fertile offspring. This process highlights that evolution is not about individual organisms changing during their lifetime, but rather about changes in the proportion of different characteristics within a population across many generations.