Heredity and Evolution
What this domain tests
Heredity and Evolution connects inheritance to changes in populations over time. Expect Punnett squares, probability, DNA and protein synthesis, mutation, natural selection, genetic drift, gene flow, and speciation. Items often ask you to distinguish what happens to an individual from what happens to a population.
Core rules and concepts
- Mendelian inheritance. Alleles are alternative forms of a gene. Dominant alleles mask recessive alleles in heterozygotes. Genotype is the allele combination; phenotype is the observable trait. The law of segregation separates alleles during meiosis; independent assortment applies to genes on different chromosomes.
- Probability. A monohybrid cross of two heterozygotes (Bb × Bb) gives a 1:2:1 genotype ratio (BB:Bb:bb) and a 3:1 phenotype ratio when B is completely dominant.
- Molecular basis. DNA replication is semiconservative. Transcription makes mRNA from DNA; translation uses mRNA codons and tRNA anticodons to build proteins. Mutations can be silent, missense, or nonsense.
- Evolution. Natural selection requires heritable variation and differential reproductive success. Allele frequencies change through mutation, gene flow, genetic drift, and selection. Speciation often follows reproductive isolation. Individuals do not evolve; populations do.
Worked example
In rabbits, brown fur (B) is dominant to white fur (b). Cross two heterozygous brown rabbits (Bb × Bb). What fraction of offspring is expected to be white? Reasoning: Each parent makes B and b gametes. The Punnett square gives BB, Bb, Bb, and bb. Only bb is white, so 1/4, or 25%, of offspring are expected to be white; 3/4 are brown.
Traps that cost marks
- Saying a dominant allele is “better” or more common; dominance does not imply frequency.
- Claiming an individual evolves during its lifetime.
- Treating acquired traits as inherited.
- Confusing genetic drift (random change, especially in small populations) with natural selection (nonrandom survival and reproduction).
- Forgetting that meiosis, not mitosis, produces gametes with half the chromosome number.