Reproduction and Inheritance

Inheritance explains why offspring resemble but do not exactly copy their parents.

Genes and Alleles

Chromosomes are made of DNA. A gene is a section of DNA coding for a trait. Different versions of a gene are alleles; an organism has two copies of each allele.

Dominant and Recessive

A dominant allele shows even when paired with a different allele (shown with a capital letter, e.g. T). A recessive allele shows only when both copies are the same (e.g. tt).

Punnett Squares

A Punnett square shows the possible allele combinations from two parents. A cross between Tt and Tt gives TT, Tt, Tt, tt — a 3:1 ratio of dominant to recessive traits.

Practice

1. Define gene and allele.
2. Complete a cross between TT and tt.
3. Two brown-eyed parents have a blue-eyed child. Explain how this is possible.

Recap

Genes come in alleles. Dominant alleles mask recessive ones, and Punnett squares predict offspring ratios.