UnofficialAP BiologyUnit 5 of 8

AP Biology Unit 5: Heredity

Meiosis, Mendelian genetics and chi-square analysis.

Unit overview

Heredity explains how traits pass between generations. Meiosis produces haploid gametes and creates variation through crossing over, independent assortment and random fertilisation. Errors such as nondisjunction can produce gametes with the wrong number of chromosomes.

Mendel's laws of segregation and independent assortment let you predict offspring ratios with Punnett squares or the product rule. Many traits do not follow simple Mendelian patterns: incomplete dominance, codominance, sex linkage, linked genes and environmental effects all change the ratios. The chi-square test checks whether observed offspring counts fit the expected ratio closely enough.

Key topics

Meiosis: Two divisions, crossing over and independent assortment.
Mendelian genetics: Segregation, independent assortment and the product rule.
Non-Mendelian inheritance: Sex-linked traits, linked genes, incomplete dominance and codominance.
Chi-square analysis: Comparing observed and expected counts with $\chi^2 = \sum (O-E)^2/E$ .

Worked example

Two heterozygous parents ( $Aa \times Aa$ ) have offspring. What fraction are expected to show the recessive phenotype?

Each parent passes $a$ with probability $\tfrac{1}{2}$ .
$P(aa) = \tfrac{1}{2} \cdot \tfrac{1}{2} = \tfrac{1}{4}$ .

Answer: $\frac{1}{4}$

Common mistakes

Confusing alleles with genes, or chromatids with chromosomes.
Applying independent assortment to genes that are close together on the same chromosome.
Using the wrong degrees of freedom in chi-square; it is the number of phenotype classes minus one.

Exam tips

For multi-gene crosses, multiply probabilities for each gene instead of drawing a large Punnett square.
State the null hypothesis and compare $\chi^2$ with the critical value at $p = 0.05$ explicitly.

See the AP Biology exam format and every unit on the course page.