Principles of Inheritance and Variation — Short Notes
Genetics = study of inheritance & variation. Founded by Gregor Mendel (1856–1863) on pea plants (Pisum sativum).
Mendel's Experimental Approach
- Chose 7 contrasting characters — seed shape, seed colour, pod shape, pod colour, flower colour, flower position, stem length.
- Statistical (numerical) analysis of large offspring populations.
- Used true-breeding lines through self-pollination for generations.
Terminology
- Gene: unit of inheritance (a portion of DNA that codes for a trait).
- Allele: alternative forms of a gene (e.g. T for tall, t for dwarf).
- Homozygous (TT, tt) / Heterozygous (Tt).
- Genotype (genetic make-up) vs Phenotype (observable trait).
- Dominant (expressed in heterozygote) / Recessive.
Mendel's Laws
- Law of Dominance — in a heterozygote, only the dominant allele is expressed.
- Law of Segregation — alleles separate during gamete formation; each gamete receives one allele.
- Law of Independent Assortment — alleles of different genes assort independently during gamete formation (only for genes on different chromosomes, or far apart on the same one).
Monohybrid Cross (Tt × Tt)
Ratios — Genotypic 1 : 2 : 1 (TT : Tt : tt) — Phenotypic 3 : 1 (Tall : Dwarf).
Test cross — cross with homozygous recessive (tt) to reveal genotype of tall plant. Ratio 1:1 if Tt; all tall if TT.
Dihybrid Cross (RrYy × RrYy)
- Phenotypic 9 : 3 : 3 : 1 (Round-Yellow : Round-Green : Wrinkled-Yellow : Wrinkled-Green).
- Genotypic 16 combinations.
Deviations from Mendel
- Incomplete dominance — heterozygote shows intermediate phenotype. E.g. Antirrhinum (snapdragon): RR (red) × rr (white) → Rr (pink). Ratio: 1 red : 2 pink : 1 white.
- Co-dominance — both alleles equally expressed. Example: ABO blood group — I^A and I^B are both expressed in I^AI^B (AB blood). i is recessive.
- Genotypes: I^AI^A / I^Ai (A), I^BI^B / I^Bi (B), I^AI^B (AB), ii (O).
- Multiple alleles — more than 2 alleles for a gene in the population (ABO has 3: I^A, I^B, i).
- Pleiotropy — one gene affects multiple traits (e.g. phenylketonuria, sickle-cell anaemia).
- Polygenic inheritance — multiple genes control one trait. Traits show a quantitative range with continuous variation (e.g. skin colour, height).
Chromosomal Theory of Inheritance
- Proposed by Sutton and Boveri (1902).
- Chromosomes carry genes; behaviour of chromosomes in meiosis parallels behaviour of Mendel's factors.
- Morgan verified using Drosophila.
Linkage & Recombination
- Linkage — genes on same chromosome tend to be inherited together.
- Recombination — new combinations from crossing over during meiosis.
- Distance between genes on the chromosome ∝ recombination frequency.
Sex Determination
| Organism | System | Male | Female |
|---|---|---|---|
| Humans | XY | XY | XX |
| Drosophila | XY | XY | XX |
| Grasshopper | XO | XO | XX |
| Birds, moths, butterflies | ZW | ZZ | ZW |
| Bees | Haplo-diploidy | Haploid (n, from unfertilised egg) | Diploid (2n) |
Human males decide the sex of offspring (X or Y sperm).
Mutations
- Sudden heritable change in DNA.
- Chromosomal: deletion, duplication, inversion, translocation.
- Point mutations: single-base change; e.g. sickle-cell anaemia — GAG → GUG in HBB → Glu → Val.
- Frame-shift: insertion/deletion of non-triplet bases shifts reading frame.
Genetic Disorders
Mendelian disorders (single-gene)
| Disorder | Type | Notes |
|---|---|---|
| Haemophilia | X-linked recessive | Blood doesn't clot; carrier mothers → 50% affected sons |
| Colour blindness | X-linked recessive | Red-green |
| Sickle-cell anaemia | Autosomal recessive | HbS/HbS shows disease; HbA/HbS is carrier |
| Phenylketonuria (PKU) | Autosomal recessive | Lacks phenylalanine hydroxylase → mental retardation |
| Thalassaemia | Autosomal recessive | Faulty globin production → anaemia |
| Cystic fibrosis | Autosomal recessive | Mucous thickens |
Chromosomal disorders (numerical)
| Disorder | Chromosome | Karyotype | Features |
|---|---|---|---|
| Down's syndrome | Trisomy of 21 | 45+XX or 45+XY | Short stature, palm crease, mental retardation |
| Klinefelter's syndrome | XXY | 47 | Male, gynaecomastia, sterile |
| Turner's syndrome | XO | 45 | Female, sterile, short stature |
Take-aways
- Mendel's discoveries were remarkable because he had no knowledge of chromosomes or DNA.
- Later deviations (incomplete dominance, co-dominance, polygenic inheritance) enriched, but did not invalidate, his laws.
- Modern genetics ties inheritance to chromosome behaviour and DNA sequence.