Heredity and Evolution: Passing Down the Recipe of Life
Explore how traits pass from parents to offspring through genes, Mendel's dominant and recessive rules with a monohybrid cross, human sex determination, and the basics of evolution.
By the PadhoDost Team · 📖 7 min read · Updated 4 August 2026
Part of Class 10 (CBSE) prep🧠 Why you have your grandmother's eyes
Ever noticed how a dimple, a certain nose shape, or curly hair shows up again and again across a family - sometimes even skipping a generation? It is as if parents hand down a recipe book to their children. Heredity is that recipe book, and genes are the individual recipes that decide many of your features.
What is heredity?
Heredity is the passing of characters, or traits, from parents to their offspring. The instructions travel inside genes, which sit on chromosomes in every cell. Because a child receives one set of genes from each parent, they resemble both - yet are never identical to either. Small differences, called variations, make each individual unique and provide the raw material for evolution.
Mendel's rules of inheritance
Gregor Mendel, working with pea plants, discovered how traits pass on. Each trait is controlled by a pair of alleles (versions of a gene). Some alleles are dominant - written with a capital letter, e.g. T for tall - and mask the recessive ones, written with a small letter, e.g. t for short. A dominant trait appears even if only one dominant allele is present, while a recessive trait shows only when both alleles are recessive (tt).
📝 Monohybrid cross: tall x short pea plants
Parents: a pure tall plant (TT) is crossed with a pure short plant (tt).
F1 generation: every plant gets one T and one t, so all are Tt - and all look TALL, because T is dominant.
Now cross two F1 plants (Tt x Tt). The Punnett square gives TT, Tt, Tt, and tt.
Phenotype (appearance): 3 tall : 1 short - the famous 3:1 ratio.
Genotype (gene make-up): 1 TT : 2 Tt : 1 tt, that is a 1:2:1 ratio.
| Term | Meaning |
|---|---|
| Gene | Unit of heredity that controls a trait |
| Allele | One of the alternative forms of a gene (e.g. T or t) |
| Dominant | Allele that shows its effect even when single (T) |
| Recessive | Allele that shows only when present as a pair (tt) |
| Genotype | The actual gene combination (TT, Tt, or tt) |
| Phenotype | The visible trait (tall or short) |
Sex determination and a glimpse of evolution
In humans, sex is decided by the sex chromosomes. Females have two X chromosomes (XX); males have one X and one Y (XY). The mother always contributes an X. The father's sperm decides the child's sex: an X-carrying sperm gives a girl (XX), while a Y-carrying sperm gives a boy (XY). So, scientifically, it is the father - not the mother - who determines the baby's sex.
Over many generations, useful variations that help organisms survive and reproduce become more common. This is natural selection, the engine of evolution first explained by Charles Darwin. Evidence includes homologous organs (same basic structure, different functions - like the forelimbs of a human, a cat, and a bird), analogous organs (different structure, same function - like the wings of a bird and an insect), and fossils, which reveal life forms of the past. Importantly, only variations in the germ cells (inherited traits) pass on; traits you acquire in your lifetime, such as a muscular body from exercise, are not inherited.
Quick revision
- ✓Heredity passes traits from parents to offspring through genes located on chromosomes.
- ✓Dominant alleles mask recessive ones; a recessive trait needs a pair (tt) to appear.
- ✓Mendel's monohybrid cross gives a 3:1 phenotypic and a 1:2:1 genotypic ratio in F2.
- ✓In humans XX = female and XY = male; the father determines the child's sex.
- ✓Evolution works through variation and natural selection; homologous and analogous organs and fossils are key evidence.
⚡ Quick check
In a monohybrid cross between pure tall (TT) and pure short (tt) pea plants, what is the phenotypic ratio of the F2 generation?
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