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Credit Sprint's Guide to Biology Unit 5: Heredity

AP® weight 8–11%CLEP® weight 10–14%

Meiosis makes genetically different gametes, and Mendel's laws predict how traits pass from parents to offspring. Many traits break the simple rules through linkage, sex linkage, incomplete dominance or the environment.

Punnett square Aa × AaAAAaAaaaAaAaParent 2 gametesParent 1 gametesGenotypes 1 AA : 2 Aa : 1 aaPhenotypes 3 dominant : 1 recessiveEach box is a 1/4 chance

What you need to know

  • Meiosis makes four haploid gametes from one diploid cell, with two divisions and one round of DNA replication.
  • Variation comes from crossing over (prophase I), independent assortment of homologous pairs (metaphase I) and random fertilization.
  • Law of segregation: the two alleles for a gene separate into different gametes. Law of independent assortment: genes on different chromosomes sort independently.
  • Monohybrid cross Aa × Aa gives 3:1 phenotypes and 1:2:1 genotypes; a dihybrid cross AaBb × AaBb gives 9:3:3:1.
  • Use the multiplication rule for 'and' (independent events) and the addition rule for 'or' (mutually exclusive outcomes).
  • Incomplete dominance blends (red × white → pink); codominance shows both (AB blood type).
  • Sex-linked (X-linked) recessive traits show up more in males, who have only one X.
  • Genes close together on the same chromosome are linked and are inherited together more often; recombination frequency estimates map distance.
  • Nondisjunction makes gametes with an extra or missing chromosome (for example, trisomy 21).
  • The environment can change phenotype (hydrangea flower color with soil pH, Himalayan rabbit fur with temperature).

Equations and rules

Probability of A and B = P(A) × P(B)
Probability of A or B = P(A) + P(B)
Chi-square: χ² = Σ (observed − expected)² ÷ expected; degrees of freedom = categories − 1
Map distance (map units) = recombination frequency × 100

Exam traps

Watch forIf χ² is LESS than the critical value, you fail to reject the null hypothesis: the difference is due to chance.
Watch forMitosis ≠ meiosis. Only meiosis halves the chromosome number and shuffles alleles.
Watch forA carrier female (XᴬXᵃ) × normal male gives a 1 in 2 chance that each SON is affected, not 1 in 4 children.

Key terms

  • Allele: A version of a gene.
  • Homozygous: Two identical alleles for a gene (AA or aa).
  • Heterozygous: Two different alleles for a gene (Aa).
  • Crossing over: Homologous chromosomes swap segments in prophase I, making new allele combinations.
  • Independent assortment: Homologous pairs line up randomly in metaphase I, so genes on different chromosomes sort independently.
  • Incomplete dominance: The heterozygote shows an in-between phenotype.
  • Codominance: Both alleles are fully expressed, as in AB blood type.
  • Sex-linked trait: A trait whose gene is on the X (or Y) chromosome.
  • Linked genes: Genes close together on one chromosome that tend to be inherited together.
  • Nondisjunction: Chromosomes fail to separate in meiosis, making gametes with too many or too few.
  • Chi-square test: Compares observed and expected results to judge whether differences are due to chance.

Practice questions

Five of the 19 Unit 5 questions in Credit Sprint. Every choice is explained in the app.

1. Crossing over occurs during
  1. S phase
  2. prophase of mitosis
  3. prophase I of meiosis
  4. anaphase II
Show answer

prophase I of meiosis. Homologous chromosomes exchange segments.

2. For two independent genes, what fraction of the offspring of AaBb × AaBb will be AaBb?
  1. 1/16
  2. 1/4
  3. 1/2
  4. 9/16
Show answer

1/4. P(Aa) × P(Bb) = 1/2 × 1/2.

3. A chi-square test gives χ² = 2.1 with a critical value of 3.84. What should you conclude?
  1. Reject the null hypothesis
  2. The data are exactly as expected
  3. Fail to reject the null hypothesis; the deviation is likely due to chance
  4. The genes must be linked
Show answer

Fail to reject the null hypothesis; the deviation is likely due to chance. χ² is below the critical value.

4. A pedigree shows a trait that skips generations and appears in children of two unaffected parents. The trait is most likely
  1. recessive
  2. dominant
  3. Y-linked
  4. caused by the environment
Show answer

recessive. Unaffected carriers pass it on.