Mendelian Patterns

Objective: Distinguish allele combinations from observed traits.

In a diploid organism, two homologous chromosomes carry corresponding loci, often with two alleles of a gene. Genotype denotes the allele combination at a stated locus; phenotype describes a measurable feature influenced by genotype and often environment. Homozygous means two copies of the same allele at that locus, whereas heterozygous means two different alleles. A dominant allele produces its characteristic effect in the specified heterozygote under a simple complete-dominance model; recessive does not mean rare, harmful, or weaker in every biological sense. Gene symbols are shorthand for a particular model, not universal properties of every trait. Many real traits are more complex than one locus and two alleles.

Objective: Connect meiosis to one allele per gamete in a standard diploid model.

Mendel’s law of segregation states that the two alleles at a diploid locus separate into different gametes during meiosis, so each gamete ordinarily carries one allele at that locus. Homologous chromosomes separate in meiosis I; sister chromatids separate in meiosis II. For an Aa individual under equal segregation and no viability effects, half of its gametes are expected to carry A and half a. This is a probability expectation over many gametes, not a promise that every small sample has a perfect half-and-half split. Errors in segregation and atypical reproductive systems require different models.

Objective: State when a simple two-locus model applies.

Mendel’s independent-assortment model says that alleles at different loci can be inherited independently when the loci are on different chromosomes or behave as effectively unlinked. An AaBb individual then has four gamete types AB, Ab, aB, and ab with probability 1/4 each under equal segregation. Genes near each other on one chromosome may be linked and inherited together more often, although crossing over can recombine their alleles. Independent assortment concerns probabilities of allele combinations; it does not assert that all genes are unlinked or that every offspring outcome is equally likely. Trait expression may also depend on interactions between genes.