Chapter 8: Heredity
Variation and Inheritance
Reproduction results in the formation of new individuals that resemble their parents but also show variations. These differences arise from small inaccuracies in DNA copying and become more prominent in sexual reproduction, which allows greater diversity. Variations can be beneficial, especially when environmental conditions change, as they may help some individuals survive and pass on traits that improve the chances of survival of the species.
Mendel’s Laws of Inheritance
Gregor Mendel’s experiments with pea plants established the basic rules of heredity. He discovered that traits are inherited as discrete units (now called genes), with dominant traits masking recessive ones. For example, tallness is dominant over shortness. When plants with two different traits (e.g., tall/short or round/wrinkled seeds) are crossed, the traits assort independently, producing new combinations in the next generation. This independent inheritance is due to genes being carried on different chromosomes, which segregate during gamete formation.
Genes, Chromosomes, and Sex Determination
Genes are segments of DNA that code for proteins, which influence traits. Each individual inherits one copy of each gene from each parent. During gamete formation, cells reduce their chromosome number by half so that the zygote formed after fertilisation has a complete set. In humans, the 23rd pair of chromosomes determines sex. Females have XX chromosomes, and males have XY. The child’s sex is determined by whether the sperm carries an X or Y chromosome, since the egg always contributes an X.