Children resemble their parents, yet they are not identical to them. This similarity in the transmission of characteristics from parents to offspring is called heredity, and the differences between parents and offspring are called variation. The science that studies heredity and variation is called genetics. Every organism carries traits such as eye colour, height, blood group and the shape of leaves, and the way these traits are passed on follows specific rules that were first worked out by Gregor Mendel.
Mendel, an Austrian monk, performed experiments on pea plants (Pisum sativum) and formulated the laws of inheritance. He chose pea plants because they have many contrasting traits, are self-pollinating, grow quickly, and produce many offspring. His experiments established that heredity is determined by discrete units called genes, which are present on chromosomes and are inherited from parents.
Variation is the raw material of evolution. Over many generations, the accumulation of heritable variations, along with natural selection, leads to the evolution of new species. Evolution is the gradual change of living organisms over long periods of time, leading to the diversity of life we observe today. In this chapter, we will study Mendel's experiments, sex determination, the basics of evolution, and the evidence that supports the theory of evolution.
Mendel studied traits that had two contrasting forms, for example tall (TT) and short (tt) plants, round (RR) and wrinkled (rr) seeds, and green (GG) and yellow (gg) seeds. He crossed pure-bred tall plants with pure-bred short plants. All the plants in the first generation (F1) were tall. This showed that the tall trait dominated over the short trait.
When the F1 plants were self-pollinated, the second generation (F2) showed both tall and short plants in the ratio of 3:1. The trait that appeared in the F1 generation is called the dominant trait, and the trait that was hidden but reappeared in F2 is called the recessive trait.
The height of a plant is controlled by the gene pair Tt. Each parent contributes one copy of the gene. The F1 plants have genotype Tt (one T from the tall parent and one t from the short parent). Since T dominates over t, the plants are tall. The observable characteristic is the phenotype, and the genetic constitution is the genotype.
A monohybrid cross considers one trait, for example plant height, and gives a 3:1 phenotypic ratio in F2. A dihybrid cross considers two traits simultaneously, for example seed shape (round vs wrinkled) and seed colour (yellow vs green). The F2 generation of a dihybrid cross shows the ratio 9:3:3:1, demonstrating the independent inheritance of traits. This is Mendel's law of independent assortment.
Each cell contains chromosomes, which carry genes in pairs. Humans have 23 pairs of chromosomes (46 in total). One copy of each gene comes from the mother and one from the father. During reproduction, the gametes (sperm and egg) carry one copy of each gene, so the offspring has a combination of traits from both parents.
Sex determination is the mechanism by which the sex of an offspring is fixed. In humans, females have two X chromosomes (XX) and males have one X and one Y chromosome (XY). The mother's egg always carries an X chromosome, while the father's sperm can carry either an X or a Y chromosome.
If a sperm carrying X fertilises the egg, the child is female (XX). If a sperm carrying Y fertilises the egg, the child is male (XY). Therefore, the sex of the child is determined by the father's sperm. In some other organisms, sex is determined differently; for example, in many insects, the female is XX and the male is XO.
Variation is the difference in traits among individuals of the same species. Variations arise due to:
Variations are important because:
Evolution is the gradual change in the characteristics of organisms over successive generations. The process by which organisms better adapted to their environment survive and reproduce is called natural selection, as proposed by Charles Darwin. Organisms that are better adapted have a higher chance of survival and pass on their favourable traits.
Complex organs evolve through a series of gradual steps. For example, the eye has evolved independently in different organisms through intermediate stages, with each stage being useful for survival. Evolution is not a simple, straight process; it can happen by the accumulation of small changes or even sudden changes.
Speciation is the process by which new species are formed. It occurs when a group of individuals becomes reproductively isolated from the rest of the population, usually due to geographical isolation (a river, mountain or other barrier) or because of changes in behaviour or genetics. Over time, the isolated population accumulates enough variations to form a new species.
Evolutionary relationships can be traced by comparing the DNA sequences of organisms. Organisms with more similar DNA are more closely related. Human evolution can be traced through fossil records, which show that modern humans (Homo sapiens) evolved from ape-like ancestors over millions of years.
| Cross | Considered Trait | F2 Phenotypic Ratio |
|---|---|---|
| Monohybrid cross | One trait (e.g. height) | 3:1 |
| Dihybrid cross | Two traits (e.g. seed shape and colour) | 9:3:3:1 |
| Type of Organ | Structure | Function | Example | Conclusion |
|---|---|---|---|---|
| Homologous | Same basic structure | Different functions | Forelimbs of frog, bird, human | Common ancestor, divergent evolution |
| Analogous | Different structure | Same function | Wings of bird and butterfly | Convergent evolution |
Heredity and evolution together explain both the similarity of children to their parents and the enormous diversity of life on Earth. Mendel's laws, established through careful experimentation with pea plants, laid the foundation of modern genetics and showed that traits are inherited in predictable patterns. The mechanisms of sex determination reveal that the father's chromosome determines the sex of the child. Variation, generated through reproduction, mutation and DNA copying, provides the raw material upon which natural selection acts. The evidence for evolution, drawn from homologous and analogous organs, fossils, embryology and vestigial organs, paints a coherent picture of life changing over millions of years. Understanding these concepts connects the biology of inheritance to the grand history of life and prepares students for deeper studies in genetics and evolutionary biology.