Lesson 9: Biology: Genetics and Evolution
Genetics is the most logical part of Biology: a cross, once set up correctly, gives its answer by arithmetic. Evolution then explains why those patterns of inheritance, variation and selection produce the diversity of life.
This lesson covers Mendelian inheritance, chromosomal inheritance and linkage, molecular genetics (DNA, replication, transcription, translation), and the evidence and mechanisms of evolution. The method for every cross is the same: write the parents' genotypes, write their gametes, then combine them.
Keep the standard numbers ready: humans have 46 chromosomes (23 pairs); a monohybrid cross of heterozygotes gives a 3:1 phenotypic ratio; a dihybrid cross of heterozygotes gives 9:3:3:1; and DNA base pairing is A–T and G–C.
Mendelian inheritance and linkage
Mendel's law of segregation: the two alleles of a gene separate into different gametes. The law of independent assortment: alleles of different genes assort independently — unless the genes are linked on the same chromosome, in which case parental combinations predominate and recombinants are rarer, with the frequency rising with the distance between genes.
Aa × Aa with A dominant: genotypes AA 1/4, Aa 1/2, aa 1/4; phenotypes dominant 3/4, recessive 1/4 — ratio 3:1.
AaBb × AaBb (both genes dominant, unlinked): phenotypic ratio 9:3:3:1.
A test cross Aa × aa gives Aa and aa in equal numbers: phenotypic ratio 1:1.
A DNA sample with 25% adenine has 25% thymine (A–T pairing), leaving 50% for G + C, so guanine is 25% and cytosine is 25%.
Incomplete dominance (for example, snapdragon flower colour) gives a 1:2:1 phenotypic ratio in the F₂ because the heterozygote shows an intermediate phenotype. Co-dominance (for example, AB blood group) shows both alleles fully in the heterozygote. Multiple allelism (ABO blood groups) and polygenic inheritance (for example, human skin colour) extend the same logic to more alleles or more genes.
Molecular genetics: from DNA to protein
DNA replication is semi-conservative: each new double helix keeps one old strand and one new strand. Transcription copies a gene's template strand into messenger RNA (with uracil replacing thymine); translation reads mRNA in codons of three bases, each specifying an amino acid, starting at AUG (methionine) and stopping at UAA, UAG or UGA.
A coding sequence of 300 nucleotides contains 300/3 = 100 codons.
If one of those is a stop codon, the polypeptide has 99 amino acids (the stop codon specifies no amino acid).
A gene's expression is regulated at several levels; in bacteria, the lac operon is the classic example: lactose (as allolactose) inactivates the repressor, switching the operon on so the cell can use lactose. Mutations — changes in DNA sequence — are the raw material of variation; most are neutral or harmful, and a few are useful.
Evolution: evidence and mechanisms
Evidence for evolution includes fossils (including transitional forms), comparative anatomy (homologous structures with a common origin but different functions, versus analogous structures with similar functions but different origins), embryology, and molecular evidence (similar DNA and protein sequences in related species).
Natural selection acts on existing variation: individuals better adapted to their environment survive and reproduce more, so favourable alleles increase over generations. Genetic drift changes allele frequencies by chance, most strongly in small populations; the Hardy–Weinberg principle describes the equilibrium (no evolution) when there is no selection, mutation, migration or drift and mating is random.
If the frequency of a recessive phenotype (aa) is q² = 0.04, then q = √0.04 = 0.2 and p = 1 − 0.2 = 0.8.
Carrier frequency is 2pq = 2 × 0.8 × 0.2 = 0.32, or 32% of the population.
Common pitfalls
Do not write a phenotype ratio without writing gametes and genotypes first: most genetics errors come from skipping the gamete step.
Do not confuse homologous with analogous structures: homologous share origin but may differ in function; analogous share function but not origin.
Further study
- NCERT Class 12 Biology, chapters on principles of inheritance and variation, molecular basis of inheritance and evolution.
📝 Quiz
1. In Aa × Aa with A dominant, what fraction shows the recessive phenotype?
Show answer
b) Only aa (1/4) shows the recessive phenotype; the ratio is 3:1.
2. A DNA sample has 25% adenine. What percentage is guanine?
Show answer
b) A = T = 25%, so G + C = 50%, giving G = 25%.
3. If q² = 0.04 for a recessive phenotype, what is the carrier frequency 2pq?
Show answer
b) q = √0.04 = 0.2, p = 0.8, so 2pq = 2 × 0.8 × 0.2 = 0.32.
4. A 300-nucleotide coding sequence contains how many codons?
Show answer
b) 300/3 = 100 codons.