How do Mendel's laws predict the inheritance of traits?
Topic 5.3 Mendelian Genetics: apply the laws of segregation and independent assortment to predict genotype and phenotype ratios.
A focused answer to AP Biology Topic 5.3, covering the laws of segregation and independent assortment, Punnett squares, monohybrid and dihybrid crosses, and the chi-square test for goodness of fit, with worked calculations.
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What this topic is asking
The College Board (Topic 5.3) wants you to apply Mendel's law of segregation and law of independent assortment to predict the genotype and phenotype ratios of crosses, using Punnett squares, and to test observed data against expected ratios with the chi-square test.
The two laws
These laws are the genetic consequence of what physically happens in meiosis.
Monohybrid and dihybrid crosses
For a single gene, a cross of two heterozygotes (Aa x Aa) gives offspring in a 3:1 phenotype ratio (dominant to recessive) and a 1:2:1 genotype ratio (AA : Aa : aa).
The chi-square test
Degrees of freedom equal the number of categories minus one. For a monohybrid cross with two phenotype categories, that is degree of freedom; for a dihybrid cross with four categories it is . A larger sample size makes the test more sensitive, so a real ratio that is slightly off from the expected one is more likely to be detected as a significant deviation. This is why exam data sets give specific counts: the test turns a vague impression of "close enough" into a defensible statistical conclusion.
Try this
Q1. State the phenotype ratio expected from a dihybrid cross of two double heterozygotes. [1 point]
- Cue. 9:3:3:1.
Q2. Explain how the law of segregation relates to meiosis. [2 points]
- Cue. The two alleles of a gene are on homologous chromosomes; when homologues separate in anaphase I, the alleles segregate into different gametes.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2019 (style)4 marksSection II (long FRQ excerpt, statistics). A monohybrid cross of two heterozygotes (Aa x Aa) for a dominant trait gives 90 dominant and 30 recessive offspring out of 120. (a) State the expected ratio and calculate the expected numbers. (b) Use a chi-square test to decide whether the data fit the expected 3:1 ratio (critical value at 1 degree of freedom, p = 0.05, is 3.84).Show worked answer →
A 4-point statistics FRQ using the chi-square test.
(a) Expected (1 point): a 3:1 ratio of 120 offspring is 90 dominant and 30 recessive.
(b) Chi-square (3 points): use . (1 point) for dominant: ; for recessive: . (1 point) . (1 point) , so we fail to reject the null hypothesis; the data fit the expected 3:1 ratio.
Markers reward the correct expected values, the chi-square calculation using the provided formula, and comparing to the critical value to draw a conclusion.
AP 2018 (style)1 marksSection I (multiple choice). A cross between a homozygous dominant (TT) and a homozygous recessive (tt) plant produces offspring that are all: (A) TT. (B) tt. (C) Tt and show the dominant phenotype. (D) a 3:1 ratio of dominant to recessive.Show worked answer →
The correct answer is (C).
Each parent contributes one allele: T from one and t from the other, so every offspring is Tt (heterozygous) and shows the dominant phenotype. A 3:1 ratio would come from crossing two heterozygotes (Tt x Tt), not a homozygous-dominant by homozygous-recessive cross.
Related dot points
- Topic 5.4 Non-Mendelian Genetics: explain inheritance patterns that depart from simple dominance, including incomplete dominance, codominance, sex linkage, polygenic traits and linkage.
A focused answer to AP Biology Topic 5.4, covering incomplete dominance, codominance, multiple alleles, sex-linked traits, polygenic inheritance and gene linkage, with a worked sex-linkage cross.
- Topic 5.5 Environmental Effects on Phenotype: explain how environmental factors can affect the phenotype produced by a given genotype.
A focused answer to AP Biology Topic 5.5, covering how temperature, nutrients, pH and other environmental factors influence phenotype, the genotype-by-environment interaction, and norms of reaction, with a worked example.
- Topic 5.2 Meiosis and Genetic Diversity: explain how crossing over, independent assortment and random fertilization produce genetic variation.
A focused answer to AP Biology Topic 5.2, covering crossing over, independent assortment and random fertilization as the three sources of genetic variation, with a worked calculation of gamete combinations.
- Topic 5.6 Chromosomal Inheritance: explain the chromosomal basis of inheritance, including sex determination and the consequences of nondisjunction.
A focused answer to AP Biology Topic 5.6, covering the chromosome theory of inheritance, sex determination, linkage, nondisjunction and aneuploidy, with a worked example of nondisjunction.
- Topic 7.5 Hardy-Weinberg Equilibrium: use the Hardy-Weinberg equations to calculate allele and genotype frequencies and test whether a population is evolving.
A focused answer to AP Biology Topic 7.5, covering the Hardy-Weinberg conditions, the equations p + q = 1 and p squared plus 2pq plus q squared = 1, and how to calculate and interpret allele and genotype frequencies, with worked calculations.
Sources & how we know this
- AP Biology Course and Exam Description — College Board (2020)