Inheritance: Class 10 Biology

Key points

Inheritance is the passing of characters from parents to offspring through genes. Genetics is the study of inheritance and variation. Genes are sections of DNA found on chromosomes in the nucleus. Human body cells have 46 chromosomes (23 pairs), and gametes have 23.

Gregor Mendel, called the father of genetics, worked on garden pea plants in the nineteenth century. Peas were suitable because they have clear contrasting characters, such as tall and short stems, grow quickly, give many seeds, and can be self or cross pollinated by hand. He studied one character at a time and counted the offspring.

Basic terms used in genetics:

• Gene: a unit of heredity that controls a character.
• Allele: one of the alternative forms of a gene, such as T for tall and t for short.
• Dominant allele: shows its effect even when only one copy is present (written as a capital letter).
• Recessive allele: shows its effect only when two copies are present (small letter).
• Homozygous: two identical alleles (TT or tt). Heterozygous: two different alleles (Tt).
• Genotype: the genetic makeup. Phenotype: the visible character.

In a monohybrid cross one pair of contrasting characters is studied. Mendel crossed pure tall plants (TT) with pure short plants (tt). All plants of the first filial generation (F1) were tall (Tt). When the F1 plants were self pollinated, the second generation (F2) had tall and short plants in the ratio 3 : 1.

The F2 generation is worked out with a Punnett square. The F1 plants (Tt) make gametes T and t in equal numbers. Random fusion gives TT, Tt, Tt and tt. So the genotype ratio is 1 TT : 2 Tt : 1 tt, and the phenotype ratio is 3 tall : 1 short. In a test cross, an individual with the dominant phenotype is crossed with a homozygous recessive one to find out whether it is TT or Tt.

Mendel's law of segregation says that the two alleles of a gene separate during gamete formation, so each gamete gets only one. His law of independent assortment says that in a dihybrid cross the alleles of different genes are distributed into gametes independently. A dihybrid cross of round yellow and wrinkled green peas gives a 9 : 3 : 3 : 1 ratio in F2.

Sex is determined by sex chromosomes. Humans have 22 pairs of autosomes and one pair of sex chromosomes. Females are XX and males are XY. All eggs carry X, while half of the sperms carry X and half carry Y. So the sex of a child depends on the sperm. The chance of a boy or girl in each birth is 50 percent.

Not all characters show complete dominance. In incomplete dominance the heterozygote shows a blend of the two parents. For example, crossing red flowered and white flowered four o'clock plants gives pink flowers in F1, and in F2 the ratio is 1 red : 2 pink : 1 white. In codominance both alleles show fully, as in the AB blood group.

ABO blood groups are controlled by three alleles: IA, IB and i. IA and IB are codominant, and both are dominant to i. Group A can be IAIA or IAi, group B can be IBIB or IBi, group AB is IAIB and group O is ii.

Variation is the difference between individuals of a species. It may be continuous, like height, or discontinuous, like blood groups. It arises from mutation, from the random mixing of genes in gametes and from the effect of the environment. A mutation is a sudden change in the genetic material. It may be caused by X rays, ultraviolet light, some chemicals and radiation. Most mutations are harmful, but some can be useful and give material for evolution.

Some genetic disorders in humans:

• Haemophilia: a sex linked recessive disorder in which blood does not clot properly. The gene is on the X chromosome, so it affects males more often.
• Colour blindness: also X linked and recessive, and it is the failure to tell some colours, often red and green.
• Down syndrome: caused by an extra copy of chromosome 21, giving 47 chromosomes.
• Thalassaemia: an inherited disease of haemoglobin that causes severe anaemia.

Study tip: Practise Punnett squares until you can write them quickly and always show the parents, gametes, offspring and both ratios. Use the same letter in capital and small form and state clearly which allele is dominant.

Definitions and flashcards

Say the meaning first, then open the card to check.

Short questions with model answers

These are the kind of questions asked for 2, 3 and 5 marks. Try each one before opening the answer.

Common mistakes

• Writing that dominant alleles are always more common. Dominant only means that the allele shows in the heterozygote.
• Writing the F2 ratio as 3 : 1 for genotypes. The genotype ratio is 1 : 2 : 1.
• Saying that the mother decides the sex of the baby. The sperm decides it.
• Mixing up homozygous and heterozygous.
• Using different letters for the two alleles of one gene, instead of capital and small forms of the same letter.

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Practice questions and timed test Printable worksheet