Mendelian Genetics: Key Concepts and Punnett Square Practice

Pea plants

The model organism used by Mendel in his studies; they have many traits, short generations, and can self- or cross-pollinate.

Self-pollination

Pollen from a flower fertilizes eggs on the same plant.

True-breeding

Producing offspring identical to the parent for a trait.

F1 offspring phenotype in monohybrid cross

All purple flowers.

Dominant and recessive traits

Dominant alleles mask recessive alleles; recessive traits reappear in the F2.

F2 plants in monohybrid cross

Obtained by allowing F1 plants to self-pollinate.

F2 phenotypic ratio in monohybrid cross

3:1 dominant to recessive.

Inheritance not blending

Reappearance of the recessive trait (white flowers) in the F2.

Four parts of Mendel's model

Traits come from alleles; organisms have two alleles; dominance; alleles separate in gametes.

Allele

A version of a gene.

Law of Segregation

Alleles separate during gamete formation so each gamete gets one allele.

Punnett square

A diagram used to predict offspring genotypes.

Completing a Punnett square

List parent gametes and fill the grid with allele combinations.

Phenotype

Observable traits.

Genotype

Genetic makeup for a trait.

Homozygous

Having two identical alleles.

Heterozygous

Having two different alleles.

F1 results in RR × rr cross

All Rr and all show the dominant phenotype.

F2 genotype and phenotype ratios in Rr × Rr cross

Genotype 1:2:1; phenotype 3:1.

Genotype and phenotype ratios not identical

Dominance hides recessive alleles in heterozygotes.

F1 genotype and phenotype in dihybrid cross RRYY × rryy

All RrYy with round yellow seeds.

Gametes produced by RrYy plant

Four: RY, Ry, rY, ry.

F2 phenotypic ratio in dihybrid cross

9:3:3:1.

Law of Independent Assortment

Alleles of different genes sort independently during gamete formation.

Meiosis and independent assortment

Chromosome pairs align randomly during metaphase I.

Segregation vs independent assortment

Segregation = alleles of one gene separate; independent assortment = genes separate independently.

Multiplication rule in inheritance

Multiply probabilities of independent events.

F1 results in RR × rr (red × green leaves)

All red leaves.

F2 leaf color percentages in F1 (Rr) × F1 (Rr) cross

75% red, 25% green.

Percent of F1 that are RrPp in RRPP × rrpp

100%.

Proportion with red leaves and purple flowers in RrPp × RrPp

9/16.

Proportion with green leaves and white flowers in RrPp × RrPp

1/16.

Proportion with genotype RrPp in RrPp × RrPp

1/4.

Trait

A characteristic of an organism.

Cross-pollination

Fertilization between different plants.

P generation

Parental generation.

F1 generation

Offspring of the P generation.

F2 generation

Offspring of the F1 generation.

Dominant trait

A trait expressed when at least one dominant allele is present.

Recessive trait

A trait expressed only when both alleles are recessive.

Multiple alleles

More than two allele forms exist in a population.

Codominance

Both alleles are fully expressed in the phenotype.

Incomplete dominance

The heterozygote shows an intermediate phenotype.