CBSE Class 10 Biology Heredity and Evolution Notes Set 07

Heredity and Variation

Heredity is the transmission of characters from parents to offspring through genes carried on chromosomes. Variation is the difference between individuals of a species. Gregor Mendel worked out the basic laws using garden pea plants and seven pairs of contrasting characters such as tall and short plants.

Terms Used

1. Gene: a unit of heredity; alleles are alternative forms of a gene.
2. Genotype: the genetic make-up (TT, Tt, tt); phenotype: the outward appearance.
3. Homozygous: two identical alleles; heterozygous: two different alleles.
4. Dominant allele shows in the hybrid; recessive allele is masked.

Mendel's Laws

1. Law of dominance: in a cross between pure parents with contrasting characters, only the dominant character appears in F₁.
2. Law of segregation: the two alleles of a pair separate during gamete formation, so each gamete has one allele. Monohybrid cross Tt × Tt gives 3 tall : 1 short in F₂ (genotypes 1 TT : 2 Tt : 1 tt).
3. Law of independent assortment: alleles of different genes separate independently. A dihybrid cross RrYy × RrYy gives 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green. The double-recessive class is 1 in 16.
A test cross (with a homozygous recessive) shows whether a dominant individual is pure or hybrid; a back cross is a cross of the hybrid with either parent.

Human Genetics

Sex determination: females are XX and males XY. All eggs carry X; half the sperm carry X and half Y, so the father's sperm decides the sex of the child.
Blood groups: controlled by alleles I^A, I^B (co-dominant) and i (recessive). Group A (I^AI^A or I^Ai), B (I^BI^B or I^Bi), AB (I^AI^B) and O (ii). The Rh factor is a second system; erythroblastosis foetalis can occur when an Rh-negative mother carries an Rh-positive baby.
Sex-linked disease: haemophilia and colour blindness are carried on the X chromosome and appear mostly in males. Down's syndrome is trisomy 21 (an extra chromosome 21).

Mutations and Applications

A mutation is a sudden heritable change in the genetic material, caused naturally or by radiation and chemicals. Polyploidy means extra sets of chromosomes (euploidy), whereas gain or loss of a single chromosome is aneuploidy. In agriculture, hybridisation, mutation breeding and genetic engineering give high-yielding and disease-resistant crops such as hybrid maize and Bt cotton; in animal husbandry selective breeding raises milk yield. Genetic engineering transfers genes between organisms (for example human insulin made by bacteria). Eugenics aims at improving the human gene pool; today genetic counselling is its accepted form.