Unit 5 connects what happens to chromosomes during cell division with the patterns we observe in offspring. Meiosis shuffles and halves the genetic material, producing gametes that are genetically unique. Fertilization then restores the diploid number and combines alleles from two parents. The rules Gregor Mendel deduced — segregation and independent assortment — are simply the predictable consequences of how homologous chromosomes line up and separate. This unit asks you to reason from molecular and cellular events up to probabilistic predictions about phenotypes, and to use the chi-square test to decide whether observed data fit a genetic model.
Meiosis is a two-stage division that converts one diploid (2n) cell into four haploid (n) gametes, each with half the chromosome number. DNA is replicated once, but the cell divides twice (meiosis I and meiosis II). Meiosis I is the reduction division: homologous chromosomes pair as tetrads and separate, lowering the chromosome number from 2n to n. Meiosis II resembles mitosis — sister chromatids separate.
Three processes create genetic variation. Crossing over during prophase I exchanges segments between homologous chromosomes, producing recombinant chromatids. Independent assortment in metaphase I means each homologous pair orients randomly relative to others, yielding 2n possible gamete combinations (over 8 million in humans). Finally, random fertilization combines any sperm with any egg.
Mendel's law of segregation states that the two alleles for a trait separate during gamete formation, so each gamete carries only one allele. His law of independent assortment states that alleles of different genes assort independently — provided the genes are on different chromosomes (or far apart on the same chromosome).
A Punnett square predicts genotype and phenotype ratios. A monohybrid cross between two heterozygotes (Aa × Aa) yields a 3:1 phenotypic ratio and a 1:2:1 genotypic ratio. A test cross (crossing an unknown dominant phenotype with a homozygous recessive, aa) reveals whether the unknown is homozygous or heterozygous based on the offspring.
A dihybrid cross tracks two genes at once. Crossing two double heterozygotes (AaBb × AaBb) produces the classic 9:3:3:1 phenotypic ratio when the genes assort independently. Rather than drawing a 16-box square, use the rules of probability: the multiplication rule (probability of two independent events both occurring is the product of their separate probabilities) and the addition rule (probability of either of two mutually exclusive events is the sum of their probabilities).
For example, the chance of an aabb offspring from AaBb × AaBb is (1/4 aa) × (1/4 bb) = 1/16. Breaking each gene into its own monohybrid probability is faster and less error-prone for crosses with several genes.
Not all alleles show simple complete dominance. In incomplete dominance, the heterozygote shows an intermediate, blended phenotype — a red and white flower cross producing pink offspring. In codominance, both alleles are fully and separately expressed in the heterozygote, such as the AB blood type, where both A and B surface antigens appear.
Multiple alleles exist when a gene has more than two allele versions in a population (any individual still carries only two). The ABO blood group has three alleles: IA and IB are codominant to each other and both dominant over i. These dominance variations alter the expected ratios but do not break Mendel's law of segregation.
Polygenic inheritance occurs when many genes each contribute additively to a single trait, producing continuous variation along a spectrum (height, skin color). Such traits typically form a bell-shaped distribution. Pleiotropy is the reverse situation: a single gene affects multiple, often unrelated, phenotypic traits — for instance, the sickle-cell allele influences red blood cell shape, oxygen transport, pain, and malaria resistance.
Epistasis occurs when one gene's expression masks or modifies the phenotypic effect of a different gene. In Labrador retrievers, one gene determines pigment color (black vs. brown) while a second gene controls whether pigment is deposited at all; a recessive genotype at the second gene produces a yellow dog regardless of the first gene. Epistasis can modify the standard 9:3:3:1 ratio (e.g., to 9:3:4 or 12:3:1).
In humans, females are XX and males are XY. Genes located on the sex chromosomes show sex-linked inheritance. Because the X chromosome carries many genes absent from the small Y, X-linked recessive traits (red-green color blindness, hemophilia) appear far more often in males, who need only one recessive allele to express the trait. Females must inherit two recessive alleles, so they are more often unaffected carriers.
A diagnostic pattern of X-linked recessive inheritance is that affected fathers pass the allele to all daughters (as carriers) but to no sons, since sons receive the father's Y. Pedigrees help track these patterns across generations.
The genotype sets potential, but the phenotype is the product of genotype and environment. Identical genotypes can produce different phenotypes under different conditions. The coat color of Himalayan rabbits and Siamese cats depends on temperature: a heat-sensitive enzyme produces dark pigment only in cooler body extremities. Hydrangea flower color shifts with soil pH, and human height depends partly on nutrition.
This concept reinforces that traits are not always rigidly determined by DNA alone. Phenotypic plasticity — the ability of one genotype to express different phenotypes — is an important source of variation that selection can act on.
The chromosomal theory of inheritance holds that genes are located on chromosomes, whose behavior in meiosis accounts for Mendel's laws. Genes physically close together on the same chromosome are linked and tend to be inherited together, violating independent assortment. The frequency of recombination (crossing over) between linked genes is proportional to the distance separating them and is used to build genetic maps; closer genes recombine less often.
Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during meiosis, producing gametes with an abnormal chromosome number. This leads to aneuploidy, such as trisomy 21 (Down syndrome). Nondisjunction in meiosis I differs from meiosis II in which chromosomes are affected.
The chi-square (χ2) test determines whether observed genetic ratios differ significantly from those expected under a hypothesis. The formula is χ2 = Σ (observed − expected)2 / expected, summed across all phenotypic categories.
Compare the calculated value to a critical value from a chi-square distribution table at the appropriate degrees of freedom (number of categories minus one) and a significance level of p = 0.05. If χ2 is less than the critical value, the deviation is attributable to chance and you fail to reject the null hypothesis (data fit the expected ratio). If χ2 exceeds the critical value, the deviation is statistically significant and you reject the null hypothesis.
Practise exam-style questions on this topic.