Lecture 5: Mendel, Heredity, and Inheritance Patterns

Vocabulary flashcards covering Mendelian genetics concepts, inheritance patterns, and associated terminology from Lecture 5.

Monohybrid cross

Cross between two true-breeding individuals differing in a single trait; F1 is all dominant, and F2 typically shows a 3:1 phenotype ratio.

Dihybrid cross

Cross tracking two traits; offspring show all combinations and the F2 phenotype ratio is typically 9:3:3:1, demonstrating independent assortment.

P generation

Parental generation; true-breeding individuals used to start a cross.

F1 generation

First filial generation; offspring of the P cross, usually showing the dominant trait in phenotype.

F2 generation

Offspring of the F1 cross; used to observe genotype and phenotype ratios (e.g., 3:1 for monohybrid crosses).

True breeding

Organisms that, when crossed, produce offspring identical to themselves for a trait.

Hybrids

Offspring resulting from crossing two true-breeding parents with different traits.

Allele

Alternative form of a gene; alleles can be dominant or recessive.

Gene

Basic unit of heredity; Mendel’s term for the factor that transmits traits.

Homozygous

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

Heterozygous

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

Dominant allele

Allele that is expressed in the phenotype when present in a genotype; usually indicated with uppercase letters.

Recessive allele

Allele that is masked by a dominant allele in a heterozygote; usually indicated with lowercase letters.

Genotype

The two alleles present for a given gene in an individual.

Phenotype

The observable trait or physical appearance resulting from the genotype.

Punnett square

A diagram used to predict the genetic outcomes of a particular cross by tabulating possible gametes and their offspring.

Mendel's First Law (Law of Segregation)

Parental alleles segregate into gametes and reunite randomly during fertilization, producing offspring with those allele combinations.

Mendel's Second Law (Independent Assortment)

Alleles of different genes assort independently during gamete formation, leading to various phenotypic combinations (e.g., 9:3:3:1 in dihybrid crosses).

Linked genes

Genes located close together on a chromosome that tend to be inherited together, violating independent assortment.

Polygenic inheritance

A trait controlled by many genes, producing a continuous distribution of phenotypes (e.g., human height).

Epistasis

Interaction between two or more genes where one gene’s alleles mask or modify the expression of another gene (e.g., Labrador coat color).

Pleiotropy

A single gene affecting multiple, seemingly unrelated traits (e.g., sickle cell gene affecting blood chemistry and malaria resistance).

Incomplete dominance

Heterozygote phenotype is intermediate between the two homozygotes (e.g., pink snapdragons from red x white).

Codominance

Heterozygote displays both parental phenotypes fully (e.g., human blood type AB shows A and B antigens).

Multiple alleles

More than two alleles for a gene in a population (e.g., ABO blood groups IA, IB, i).

Environmental effects on phenotype

Environment can influence trait expression (e.g., temperature-sensitive fur color in Siamese cats).

Sickle cell anemia (pleiotropy example)

Single gene mutation causes multiple effects: sickling of red blood cells and malaria resistance in heterozygotes.

Blood group codominance (ABO system)

Heterozygotes express both A and B antigens; type O results from a separate recessive allele and has no A/B antigens.