Heredity
CBSE Class 10 · Science · Notes, formulas and practice questions
This chapter explains how traits are inherited from parents, Mendel's monohybrid and dihybrid crosses, dominant and recessive characters, and human sex determination. It also clarifies the important difference between acquired and inherited traits.
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Heredity is the passing on of traits from parents to offspring through genes. Every feature like height, seed colour or flower position is a trait. A key starting point is the difference between inherited traits, which are coded in DNA and actually transferred through gametes, and acquired traits, which arise during an organism's life as a result of use, injury or the environment and are not passed on. This distinction is essential before analysing Mendel's experiments. Mendel used garden pea plants because they have clearly contrasting forms of traits, and their pollination can be controlled.
Mendel first crossed pure-breeding tall pea plants with pure-breeding dwarf ones. All the F₁ plants were tall, so tallness was described as dominant and dwarfness as recessive. When these F₁ plants were self-pollinated, the F₂ generation contained both tall and dwarf plants in the ratio of three tall to one dwarf. To account for this, Mendel proposed that each plant carries a pair of hereditary factors for a trait and that the two factors separate into different gametes. The recessive factor is not lost in the F₁ generation; it reappears whenever two recessive factors meet in a zygote. This is the essence of the law of segregation.
Mendel also studied inheritance of two traits together, such as seed shape and seed colour. In a dihybrid cross between round-yellow and wrinkled-green peas, all F₁ offspring were round-yellow. Self-pollination of the F₁ plants gave four types of seeds in the F₂ generation in the ratio 9 round-yellow : 3 round-green : 3 wrinkled-yellow : 1 wrinkled-green. This ratio arises when alleles of one gene segregate independently of alleles of another gene while gametes form. That statement is the law of independent assortment. Along with dominance and the law of segregation, it forms the foundation for rules of inheritance.
Traits are expressed through proteins whose synthesis is controlled by genes. A recessive allele may remain unexpressed in a heterozygote yet be passed to offspring, explaining how hidden traits reappear. Sex is also an inherited trait: in humans, females have two X chromosomes and males have one X and one Y. All eggs carry an X, but a sperm can be X or Y. An XX zygote develops into a girl and an XY zygote into a boy, so the father's gamete is responsible for the child's sex.
Key terms
- Inherited trait
- A characteristic that is encoded by genes and passed from parents to offspring through gametes, such as seed shape or eye colour.
- Acquired trait
- A change that happens in an organism's body during its lifetime from use, injury, or environment, and is not caused by a change in gamete genes, so it is not passed to offspring.
- Gene and allele
- A gene is a unit of inheritance located on a chromosome that controls a trait. An allele is a specific form of a gene; an organism receives one allele from each parent, and the two may be identical or different.
- Dominant and recessive traits
- A dominant trait is expressed even when only one copy of its allele is present. A recessive trait is expressed only when both alleles are the recessive form. For example, in peas the allele for tallness is dominant over the allele for dwarfness.
- Homozygous and heterozygous
- An organism is homozygous for a gene when it has two identical alleles, such as TT or tt. It is heterozygous when the two alleles differ, such as Tt. Heterozygotes normally show the dominant trait.
- Genotype and phenotype
- The genotype is the pair of alleles an organism possesses for a trait, such as TT, Tt or tt. The phenotype is the observable or measurable effect of that pair, such as tall or dwarf. Different genotypes can produce the same phenotype when dominance is involved.
- Monohybrid cross
- A cross between two parents that differ in one trait, such as height. When one allele shows complete dominance, the F₂ generation has a 3 : 1 phenotypic ratio.
- Dihybrid cross
- A cross between parents that differ in two traits. When the genes are on different chromosomes, the F₂ generation shows a 9 : 3 : 3 : 1 phenotypic ratio.
- Sex chromosomes
- Chromosomes that determine the sex of an organism. In humans, a female is XX and a male is XY; the mother's egg always provides an X, while the father's sperm provides either X or Y.
Formula sheet
| What | Formula | Notes |
|---|---|---|
| Monohybrid cross, F₂ phenotypic ratio | 3 : 1 | Two parents differing in ONE trait, with one allele completely dominant. Three offspring show the dominant trait for every one showing the recessive trait. |
| Dihybrid cross, F₂ phenotypic ratio | 9 : 3 : 3 : 1 | Two parents differing in TWO traits whose genes are on different chromosomes. The four classes are both dominant, each dominant with the other recessive, and both recessive. |
| Test cross with a heterozygote, offspring ratio | 1 : 1 | Crossing a heterozygote (Tt) with a homozygous recessive (tt) gives Tt and tt offspring in equal numbers, so half show the recessive trait. |
Practice questions with answers
1. Differentiate between inherited traits and acquired traits, with one example of each.
An inherited trait is coded by genes and is passed from parents to offspring through the gametes; seed shape in pea plants is an example. An acquired trait develops in an individual's lifetime because of use, injury or environment, and it does not change the genes present in the gametes; the increased muscle size of a bodybuilder is an example. Therefore, acquired traits are not inherited by children.
2. Why did Mendel choose the garden pea for his experiments on inheritance?
Mendel selected the garden pea because it had many clearly contrasting characters, such as tall versus dwarf and round versus wrinkled seeds. It is naturally self-pollinating but can also be cross-pollinated artificially, and it produces many offspring in a short time, so numerical ratios could be counted accurately.
3. In a monohybrid cross between pure tall (TT) and pure dwarf (tt) pea plants, write the genotype and phenotype ratios in the F₂ generation.
F₁ plants are all heterozygous tall (Tt). When Tt plants self-pollinate, the possible gametes are T and t, so the F₂ genotypes are TT, Tt, Tt and tt, giving a genotypic ratio of 1 TT : 2 Tt : 1 tt. Since both TT and Tt look tall and only tt is dwarf, the phenotypic ratio is 3 tall : 1 dwarf.
4. In Mendel's dihybrid cross with round-yellow and wrinkled-green pea seeds, what was the F₂ phenotypic ratio and which law does it illustrate?
The F₂ generation produced round-yellow, round-green, wrinkled-yellow and wrinkled-green seeds in the ratio 9 : 3 : 3 : 1. This result shows that alleles of different genes separate independently during gamete formation, and is explained by the law of independent assortment.
5. Explain sex determination in human beings.
Females have two X chromosomes (XX) and males have one X and one Y chromosome (XY). Eggs always carry an X, but sperm may carry either X or Y. If an X-bearing sperm fertilises the egg, the child is XX or female; if a Y-bearing sperm fertilises it, the child is XY or male. Therefore the father's sperm decides the sex of the child.
6. A heterozygous tall pea plant (Tt) is crossed with a dwarf pea plant (tt). What proportion of the offspring will be dwarf?
The tall plant produces gametes T and t in equal numbers, while the dwarf plant produces only t gametes. The offspring genotypes are therefore Tt and tt in a 1 : 1 ratio. Fifty percent of the offspring, the tt plants, will be dwarf.
7. What is the difference between genotype and phenotype? Give an example where two genotypes produce the same phenotype.
Genotype is the combination of alleles an organism has for a trait, such as TT, Tt or tt; phenotype is the observable trait such as tall or dwarf. Because the allele for tallness is dominant, both TT and Tt plants show the same tall phenotype, so these are two different genotypes giving the same phenotype.
8. State the law of segregation and the law of independent assortment.
The law of segregation says every organism has two alleles for a trait and these alleles separate during gamete formation, so each gamete receives only one allele. The law of independent assortment says that alleles of different genes separate independently of one another when gametes form, which is why dihybrid F₂ crosses give a 9 : 3 : 3 : 1 ratio.
9. In a monohybrid cross, the recessive trait disappears in F₁ but reappears in F₂. Why?
The recessive allele is not destroyed; it is simply masked by the dominant allele in the heterozygous F₁ plants. During gamete formation in F₁ plants, the dominant and recessive alleles separate, so when a male gamete carrying the recessive allele fuses with a female gamete carrying the same allele, a homozygous recessive zygote is formed. This zygote develops into an individual showing the recessive trait, which is why it reappears in F₂.
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