How traits pass between generations and species change over time
Heredity is the passing of traits from parents to offspring, and variation is the small differences between individuals. Sexual reproduction creates more variation than asexual reproduction because it combines genes from two parents. Mendel crossed tall and dwarf pea plants and found that all F₁ plants were tall, but in the F₂ generation tall and dwarf plants appeared in a ratio of about 3 : 1. He concluded that traits are controlled by pairs of factors, now called genes, one inherited from each parent, and that one trait can be dominant over another, which is recessive.
In humans, sex is determined by chromosomes. Mothers always pass an X chromosome, while fathers pass either X or Y; a child with XY is a boy and XX is a girl. Over many generations, variations that help survival become more common through natural selection, leading to evolution. Isolated populations can become separate species through speciation. Fossils and homologous organs, such as the similar forelimbs of humans, bats and whales, provide evidence of evolution.
Why it matters
Heredity explains family resemblances and why some disorders run in families. It shows that the father’s chromosome decides a baby’s sex, an important fact against unfair blame on mothers. Understanding evolution explains how bacteria become resistant to antibiotics. This chapter leads to genetics and evolution in Class 12.
Key terms
- Heredity
- The transmission of traits from parents to their offspring.
- Dominant trait
- A trait that appears in the offspring even when only one parent passes it on.
- Speciation
- The formation of new species from existing ones.
- Fossil
- The preserved remains or traces of organisms that lived long ago.