Unit 5 Heredity: Learning Mendelian Genetics from First Principles

Mendelian genetics

A set of inheritance rules that predicts how traits are passed on when a trait is mainly controlled by a single gene with alleles that follow simple patterns during sexual reproduction.

Gene

A region of DNA that influences a trait, often by coding for a protein or a functional RNA.

Locus

The specific physical location of a gene on a chromosome.

Homologous chromosomes

A matching pair of chromosomes (one from each parent) that carry the same genes at the same loci.

Allele

A version of a gene; diploid organisms typically carry two alleles for each gene (one on each homolog).

Homozygous

Having two identical alleles for a gene (e.g., AA or aa).

Heterozygous

Having two different alleles for a gene (e.g., Aa).

Genotype

The allele combination an organism carries for a gene (e.g., AA, Aa, or aa).

Phenotype

The observable expression of a trait (physical or physiological), such as flower color.

Dominant allele

An allele whose trait is expressed in the phenotype of a heterozygote.

Recessive allele

An allele that is masked in a heterozygote and is expressed in the phenotype only when two recessive alleles are present.

Meiosis

Cell division that produces haploid gametes and explains Mendelian ratios by separating homologous chromosomes (and thus allele pairs).

Diploid

Having two copies of each chromosome (and typically two alleles per gene), as in most body cells.

Haploid

Having one copy of each chromosome (and one allele per gene), as in gametes (sperm/eggs).

Law of Segregation

Mendel’s law stating that an organism’s two alleles for a gene separate during gamete formation, so each gamete receives only one allele.

Law of Independent Assortment

Mendel’s law stating that alleles of different genes assort independently into gametes, especially when the genes are on different chromosomes (or far apart on the same chromosome).

Punnett square

A grid used to list parental gametes and predict expected offspring genotype/phenotype proportions.

Monohybrid cross

A genetic cross that tracks inheritance of one gene.

Testcross

A cross between an individual with a dominant phenotype (unknown genotype) and a homozygous recessive individual to determine whether the unknown is homozygous dominant or heterozygous.

Dihybrid cross

A genetic cross that tracks two genes at once; under independent assortment and complete dominance, AaBb × AaBb yields a 9:3:3:1 phenotypic ratio.

Product rule (AND rule)

A probability rule: if two independent events must both occur, multiply their probabilities (e.g., P(aabb)=P(aa)×P(bb)).

Sum rule (OR rule)

A probability rule: if an outcome can occur in multiple different ways, add the probabilities of each way (e.g., P(Aa)=P(A from mom and a from dad)+P(a from mom and A from dad)).

Pedigree

A family tree diagram used to track a trait across generations and infer genotypes/inheritance patterns (especially in humans).

Chi-square goodness-of-fit test

A statistical test used to evaluate whether deviations between observed and expected counts (e.g., Mendelian ratios) are likely due to chance, using χ² = Σ((O−E)²/E).

Genetic linkage

The tendency of genes close together on the same chromosome to be inherited together, which can cause inheritance patterns to deviate from independent assortment predictions.