Inheritance, variation and applied genetics
Inheritance, variation and natural selection
This domain tests genetic terms, monohybrid crosses, sex determination, variation, natural selection, selective breeding and genetic engineering.
Core rules
- DNA carries genes; alleles are different versions of a gene. Genotype is the allele combination; phenotype is the observable characteristic.
- Dominant alleles are expressed when present; recessive alleles only when homozygous. Homozygous means two identical alleles; heterozygous means different.
- A monohybrid cross predicts offspring ratios. Use a Punnett square with parental gametes.
- Sex determination: XX female, XY male; the father determines sex because he can pass X or Y.
- Variation can be genetic or environmental. Natural selection: variation exists; some variants are better adapted; they survive and reproduce; allele frequencies change.
- Selective breeding chooses parents with desired traits; genetic engineering transfers a gene between organisms.
Worked example
Cross two heterozygous tall pea plants, Tt × Tt. Gametes are T or t from each parent. Punnett square gives TT, Tt, Tt, tt. Genotype ratio 1 TT : 2 Tt : 1 tt; phenotype ratio 3 tall : 1 short if T is dominant. Reasoning: each parent has a 1/2 chance of passing T and 1/2 of passing t; the short phenotype appears only when both recessive alleles combine.
Traps
- Writing phenotype ratio as 1:2:1 when the question asks phenotype.
- Saying a dominant allele is more common; it is not necessarily.
- Confusing genotype and phenotype.
- Claiming acquired characteristics are inherited.
- Saying natural selection gives organisms what they need.