Heredity — AP Biology Unit 5 practice.
This unit covers meiosis, Mendelian genetics and non-Mendelian genetics — essential concepts for AP Biology. Use our interactive study games to test your understanding, or review questions in traditional format below.
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This unit covers meiosis, Mendelian genetics and non-Mendelian genetics — essential concepts for AP Biology. Use our interactive study games to test your understanding, or review questions in traditional format below.
Key Concepts Breakdown
1 Meiosis
Meiosis is the two-stage cell division process that produces haploid gametes from diploid parent cells, reducing chromosome number by half. Students must understand how crossing over in prophase I and independent assortment in metaphase I generate genetic variation. Know the differences between meiosis I (homologs separate) and meiosis II (sister chromatids separate), and how errors like nondisjunction produce aneuploid gametes.
Key Points
- Crossing over (recombination) occurs between homologous chromosomes during prophase I, creating new allele combinations on chromosomes
- Independent assortment occurs at metaphase I: homolog pairs align randomly, producing 2^n possible gamete combinations (n = haploid number)
- Nondisjunction during meiosis I or II produces gametes with n+1 or n-1 chromosomes, leading to trisomy or monosomy after fertilization
- Meiosis II is identical to mitosis but starts with haploid cells; sister chromatids separate at anaphase II
A diploid organism (2n = 4) has two pairs of homologous chromosomes: pair 1 (A/a) and pair 2 (B/b). How many genetically distinct gamete types can it produce through independent assortment alone?
With n = 2 chromosome pairs, independent assortment gives 2^2 = 4 possible gamete combinations: AB, Ab, aB, and ab. Each combination arises from one of the two possible orientations of homologs at metaphase I for each pair. Crossing over would increase this number further by creating recombinant chromosomes, but independent assortment alone accounts for these four types.
2 Mendelian Genetics
Mendelian genetics describes inheritance patterns governed by the laws of segregation (alleles separate during gamete formation) and independent assortment (genes on different chromosomes sort independently). Students must be able to set up and solve monohybrid and dihybrid crosses using Punnett squares and calculate predicted phenotypic and genotypic ratios. Know how to determine dominance relationships and deduce parental genotypes from offspring ratios.
Key Points
- Monohybrid cross of two heterozygotes (Aa × Aa) yields a 3:1 phenotypic ratio and a 1:2:1 genotypic ratio
- Dihybrid cross of two double heterozygotes (AaBb × AaBb) yields a 9:3:3:1 phenotypic ratio when genes assort independently
- Test cross (unknown × homozygous recessive) reveals the unknown genotype: all dominant phenotype offspring → homozygous dominant; 1:1 ratio → heterozygous
- Law of independent assortment applies only to genes on nonhomologous (different) chromosomes or genes far apart on the same chromosome
In pea plants, round seed (R) is dominant over wrinkled (r), and yellow seed (Y) is dominant over green (y). Two plants with round yellow seeds are crossed and produce 315 round yellow, 108 round green, 101 wrinkled yellow, and 32 wrinkled green offspring. What are the genotypes of the parents?
The approximate 9:3:3:1 ratio among offspring indicates both parents are heterozygous for both traits. Converting the observed counts: 315:108:101:32 ≈ 9:3:3:1, confirming a dihybrid cross. Therefore, both parents must be RrYy × RrYy.
3 Non-Mendelian Genetics
Non-Mendelian inheritance includes patterns that deviate from simple dominant/recessive ratios: incomplete dominance, codominance, multiple alleles, pleiotropy, polygenic inheritance, and epistasis. Students must recognize which pattern is operating from a given phenotypic ratio or cross result and explain why the Mendelian ratios are modified. Sex-linked inheritance (especially X-linked recessive) is also heavily tested.
Key Points
- Incomplete dominance: heterozygote shows intermediate phenotype (e.g., red × white → pink); F2 ratio is still 1:2:1 but phenotypic ratio matches genotypic ratio
- Codominance: both alleles fully expressed in heterozygote (e.g., MN blood type, sickle cell trait); use superscripts or I^A, I^B notation for ABO system
- Epistasis modifies the 9:3:3:1 dihybrid ratio (e.g., 9:3:4, 12:3:1, or 9:7) because one gene masks the expression of another
- X-linked recessive traits appear more often in males (XY) because they only need one copy of the recessive allele; carrier females (X^A X^a) are phenotypically normal
A woman with normal vision (whose father was color-blind) marries a man with normal vision. What is the probability their son will be color-blind? Color blindness is X-linked recessive.
Because the woman's father was color-blind (X^b Y), he passed his X^b to her, making her a carrier: X^B X^b. Her husband has normal vision, so his genotype is X^B Y. The cross X^B X^b × X^B Y produces sons who are either X^B Y (normal) or X^b Y (color-blind) with equal frequency. Therefore, there is a 1/2 (50%) probability their son will be color-blind.
Questions, answered.
What is Heredity?
Heredity is Unit 5 of AP Biology, covering meiosis, Mendelian genetics and non-Mendelian genetics.
How to study for AP Biology Unit 5?
Start with the Quick Summary above, review the Key Concepts, then test yourself with our interactive study games. Aim for 80%+ accuracy before moving on.
How many questions are in this unit?
This unit has 30+ review questions across 5 different game modes.