THE LIVING WORLD | Master Biology

Chapter focus: heredity and variation

Genetics deals with inheritance of characters from parents to offspring and variation among offspring. Mendelian analysis, chromosome theory, sex determination, mutation and genetic disorders explain this continuity and diversity.

Genes & alleles→Mendel's laws→Chromosomes→Variation→Disorders

4.1 Mendel's laws of inheritance

Gregor Mendel used true-breeding pea plants and carefully selected seven pairs of contrasting traits. He proposed that discrete units, now called genes, occur in pairs as alleles.

In a monohybrid cross, tall (TT) × dwarf (tt) gives all tall F1 (Tt). Selfing F1 produces F2 phenotypic ratio 3:1 and genotypic ratio 1:2:1.
Law of dominance: one allele can express in heterozygous condition.
Law of segregation: allele pairs separate during gamete formation.
In a dihybrid cross, independently assorting genes yield F2 ratio 9:3:3:1.

Test crossCross an F1 hybrid with its recessive parent. A monohybrid test cross gives 1:1 offspring if the tested parent is heterozygous.

4.2 Inheritance of one gene

Complete dominance is not universal. In incomplete dominance, the heterozygote has an intermediate phenotype; in snapdragon, red × white produces pink F1 and 1 red : 2 pink : 1 white F2.

Codominance means both alleles express in a heterozygote. The ABO system shows both codominance and multiple allelism: IA and IB are codominant and both dominate i.

Many traits are influenced by several genes. In polygenic inheritance, the additive effect of genes produces continuous variation, as in human skin colour.

4.3 Inheritance of two genes: linkage and recombination

Chromosome theory of inheritance, proposed by Sutton and Boveri, connects Mendel's factors with chromosomes. Genes are arranged linearly on chromosomes.

Genes close together on the same chromosome are linked and tend to be inherited together.
Crossing over during meiosis produces recombination. Recombination frequency is proportional to distance between genes.
Morgan's Drosophila experiments established linkage and recombination; Sturtevant used recombination frequency to prepare genetic maps.

MutationA mutation is a sudden heritable change in genetic material. It may involve a single base pair (point mutation) or a change in chromosome number/structure.

4.4 Sex determination

In humans, females are homogametic (XX) and males are heterogametic (XY). All ova carry X chromosome, whereas half the sperm carry X and half carry Y. Fertilisation by X-bearing sperm produces XX female and by Y-bearing sperm produces XY male.

In birds, females are heterogametic (ZW) and males are homogametic (ZZ).
In honey bees, sex follows haplodiploidy: fertilised eggs develop into diploid females; unfertilised eggs develop into haploid males.
The SRY gene on the Y chromosome initiates male development in humans.

ImportantThe male parent determines the sex of a human child; the mother always contributes an X chromosome.

4.5 Mutation and genetic disorders

Gene mutations can cause Mendelian disorders. Haemophilia and colour blindness are X-linked recessive disorders; sickle-cell anaemia is caused by a point mutation in the beta-globin gene, changing glutamic acid to valine.

Chromosomal disorders commonly result from nondisjunction during meiosis. Down's syndrome is trisomy 21; Klinefelter's syndrome is XXY male; Turner's syndrome is XO female.

Pedigree analysisPedigrees trace inheritance of a trait through generations and help distinguish dominant, recessive, autosomal and sex-linked patterns.

NCERT summary: rapid recall

Mendel's laws explain dominance, segregation and independent assortment.
Incomplete dominance, codominance, multiple alleles and polygenes extend simple Mendelian inheritance.
Linkage and crossing over determine recombination and chromosome maps.
Humans follow XX-XY sex determination; the Y chromosome carries SRY.
Mutations and chromosomal abnormalities lead to inherited disorders.