Genetics, Heredity and Evolution: Inheritance and Change
What this domain tests
This unit examines how characteristics are inherited and how allele frequencies change over time. MATSEC IM questions typically ask you to use genetic diagrams, interpret pedigrees, explain monohybrid and dihybrid ratios, and apply natural selection to explain adaptation and speciation.
Core rules
- A gene is a length of DNA that codes for a polypeptide; alleles are alternative versions at the same locus. Genotype is the allele combination, phenotype is the observable characteristic.
- In a monohybrid cross between two heterozygotes (Aa × Aa), the expected offspring ratio is 3 dominant : 1 recessive, with genotype ratio 1 AA : 2 Aa : 1 aa.
- The test cross identifies an unknown dominant genotype: cross it with a homozygous recessive. A 1:1 ratio indicates the unknown was heterozygous.
- Codominance and incomplete dominance alter ratios; ABO blood groups show multiple alleles and codominance.
- Sex-linked genes are carried on sex chromosomes; in humans, X-linked recessive conditions affect males more often because males have one X.
- Evolution by natural selection requires variation, heritability, differential survival and reproduction, and change in allele frequency over generations.
Worked example
In a dihybrid cross AaBb × AaBb, assuming independent assortment, each parent produces four gamete types (AB, Ab, aB, ab) in equal proportions. The phenotypic ratio is 9 A_B_ : 3 A_bb : 3 aaB_ : 1 aabb. The probability of an offspring showing both recessive traits is 1/4 × 1/4 = 1/16. Show the gametes and the Punnett square; do not simply quote 9:3:3:1 without demonstrating independent assortment.
Traps
Dominant does not mean more common. Ratios are expected probabilities, not guaranteed results in small families. Do not confuse a mutation with its selection; mutations create variation, selection changes its frequency. Linkage breaks independent assortment.