WJEC Biology for A2: Student Bk

Variation between the individuals of a population is essential if a species is to survive in a constantly changing environment. Meiosis, as a prelude to sexual reproduction, is a major source of genetic variation. The way in which characteristics are passed from one generation to the next was demonstrated by the work of Gregor Mendel (1822–84). He was the first person to work out the ways in which characteristics are inherited. This was an extraordinary intellectual achievement because at that time, no one knew of DNA, genes or chromosomes. More recent work has shown exceptions to the rules Mendel described, including co-dominance, incomplete dominance, linkage, sex linkage, mutation and epigenetic modification. By the end of this topic you will be able to: ▪ Define genetic terms. ▪ Predict the outcome of monohybrid crosses, including the test cross and incomplete and co-dominance. ▪ Predict the outcome of dihybrid crosses, including the test cross. ▪ Understand the concept of gene linkage and its consequences for genetic ratios. ▪ Carry out a chi 2 test. ▪ Explain mechanisms of sex determination. ▪ Describe the origins of mutations. ▪ Describe sex linkage using haemophilia as an example. ▪ Describe gene mutations using sickle cell anaemia as an example. ▪ Describe chromosome mutations using Down’s syndrome as an example. ▪ Describe the relationship between carcinogens and genes. ▪ Explain the control of gene expression in epigenetics. Topic contents 195 Genetic terms 196 Monohybrid inheritance 200 Dihybrid inheritance 202 Linkage 203 The chi 2 test 205 Sex determination 207 Sex linkage 210 Mutation 215 Control of gene expression CH 11 Inheritance 194