Genetic diagrams and crosses
Genetic crosses
- A genetic diagram shows how alleles pass to offspring.
- The handiest tool is the Punnett square — a grid of all gamete combinations.
- Then we meet the special cases.
The Punnett square
- A monohybrid cross follows one gene; a dihybrid cross follows two at once.
- Some genes have multiple alleles (more than two versions in a population), like human blood groups.
Practice
In a Tt × Tt cross (T dominant), the offspring ratio is:
The Punnett square gives TT, Tt, Tt, tt — three with a dominant T (tall) to one tt (short).
Practice
A dihybrid cross follows:
A monohybrid cross follows one gene; a dihybrid cross follows two genes together.
Special inheritance patterns
- sex linkage: the gene is on the X chromosome, so results differ for males and females.
- linkage: genes on the same chromosome tend to be inherited together.
- epistasis: one gene affects how another gene is shown.
Practice
Sex linkage means that:
A sex-linked gene sits on the X chromosome, so inheritance differs between the sexes.
The chi-squared test
- The chi-squared test compares the numbers you actually counted with those you expected.
- It tells you whether the difference is small enough to be chance, or large enough to mean something else is going on.
Practice
The chi-squared test is used to:
Chi-squared tests whether the gap between observed and expected counts is small enough to be chance.
You've got it
Key idea
- a Punnett square shows all gamete combinations (Tt × Tt → 3 : 1)
- monohybrid (one gene) vs dihybrid (two genes); multiple alleles = >2 versions
- sex linkage (gene on X), linkage (same chromosome, inherited together), epistasis (one gene affects another)
- chi-squared compares observed vs expected — is the difference just chance?