MDCAT Biology · Biology · Inheritance. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
Mendel crossed pea plants and counted the offspring. Each character is controlled by a gene that has two copies, called alleles, in every body cell. A dominant allele shows even when only one copy is present. A recessive allele shows only when both copies are recessive.
The law of segregation: the two alleles of a gene separate when gametes form, so each gamete carries only one. The law of independent assortment: alleles of different genes are distributed to gametes independently, provided the genes lie on different chromosomes.
Both laws follow from the behaviour of homologous chromosomes in meiosis. Use them to predict ratios.
Among the dominant-phenotype offspring of Aa × Aa, the chance of being a carrier is 2/3, not 1/2, because the aa class has been excluded.
Answer: 27/64 of the offspring.
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