Principles of Inheritance & Variation — Revision Notes

1. Mendel & His Laws

Gregor Mendel worked on the garden pea (Pisum sativum) and studied 7 pairs of contrasting traits. His success came from choosing true-breeding lines, clearly contrasting characters and analysing large numbers statistically.

Law of Dominance

• Characters are controlled by discrete units called factors (now genes), occurring in pairs (alleles).
• In a dissimilar pair, one allele (dominant) expresses over the other (recessive).

Law of Segregation

• The two alleles of a pair separate during gamete formation, so each gamete carries only one allele. Alleles do not blend.

Law of Independent Assortment

• Alleles of different genes assort independently of one another during gamete formation (seen in a dihybrid cross).

2. Deviations from Mendel

Incomplete dominance

Heterozygote shows an intermediate phenotype — e.g. pink (Rr) flowers in Mirabilis jalapa. F₂ ratio becomes 1 : 2 : 1 for both genotype and phenotype.

Co-dominance

Both alleles express fully and independently in the heterozygote — e.g. AB blood group (IᴬIᴮ).

Multiple alleles

A gene with more than two alleles in the population — e.g. ABO blood group (Iᴬ, Iᴮ, i).

Pleiotropy

A single gene affecting multiple traits — e.g. the gene for phenylketonuria or for sickle-cell anaemia.

Polygenic inheritance

A trait controlled by many genes, each adding a small effect — e.g. human skin colour, height. Shows continuous variation.

3. Chromosomal Theory & Linkage

• Sutton and Boveri proposed the chromosomal theory of inheritance — genes are located on chromosomes.
• T. H. Morgan worked on Drosophila melanogaster (fruit fly) and demonstrated linkage.
• Linkage: genes on the same chromosome tend to be inherited together. Closely-placed genes are tightly linked (low recombination); distant genes show more recombination.
• Recombination frequency is used to map the relative position of genes on a chromosome.

4. Sex Determination

• XY type (humans, Drosophila): female XX, male XY. The sperm decides the sex of the child.
• XO type (grasshopper): female XX, male XO.
• ZW type (birds): female ZW, male ZZ.
• In humans, 44 autosomes + XX = female (46, XX); 44 autosomes + XY = male (46, XY).

5. Genetic Disorders

Mendelian disorders (single gene)

• Haemophilia — X-linked recessive; blood fails to clot.
• Colour blindness — X-linked recessive; red–green defect.
• Sickle-cell anaemia — autosomal recessive; point mutation (glutamic acid → valine at 6th position of β-globin).
• Thalassemia — autosomal recessive; reduced globin chain synthesis.
• Phenylketonuria — autosomal recessive; enzyme defect causing phenylalanine build-up.

Chromosomal disorders (number/structure)

• Down syndrome — trisomy 21 (47 chromosomes).
• Klinefelter's syndrome — 47, XXY (male with some feminine traits).
• Turner's syndrome — 45, X0 (sterile female).