Mendelian Genetics

Hybridization and Generations

Mendel's experiments with pea plants established foundational principles of inheritance. He mated two contrasting, true-breeding varieties, a process known as hybridization.

• P Generation: The true-breeding parental generation.

• F1 Generation: The hybrid offspring of the P generation.

• F2 Generation: Produced when F1 individuals self-pollinate or cross-pollinate with other hybrids.

Genes, Alleles, and Chromosomes

Genes are units of heredity, and their alternative versions are called alleles. Each gene resides at a specific locus on a chromosome.

• Alleles: Alternative versions of a gene that account for variations in inherited characters.

• Locus: The specific location of a gene on a chromosome.

Dominance and Segregation

When two alleles at a locus differ, the dominant allele determines the organism’s appearance, while the recessive allele has no noticeable effect.

• Law of Segregation: The two alleles for a heritable character separate during gamete formation and end up in different gametes.

• This segregation corresponds to the distribution of homologous chromosomes during meiosis.

Punnett Squares and Genetic Terminology

Punnett squares are used to predict possible combinations of sperm and egg.

• Homozygote: An organism with two identical alleles for a gene (homozygous).

• Heterozygote: An organism with two different alleles for a gene (heterozygous).

• Heterozygotes are not true-breeding.

Phenotype vs. Genotype

An organism’s traits (phenotype) do not always reveal its genetic composition (genotype).

• Phenotype: Physical appearance.

• Genotype: Genetic makeup.

• Example: PP and Pp pea plants both have purple flowers (same phenotype) but different genotypes.

Testcross

A testcross is used to determine the genotype of an individual with the dominant phenotype by breeding it with a homozygous recessive individual.

• If any offspring display the recessive phenotype, the mystery parent must be heterozygous.

Monohybrid and Dihybrid Crosses

Monohybrids are heterozygous for one character. A cross between monohybrids is a monohybrid cross. Mendel also used dihybrid crosses to develop the law of independent assortment.

• Law of Independent Assortment: Each pair of alleles segregates independently during gamete formation.

• This law applies only to genes on different, nonhomologous chromosomes or those far apart on the same chromosome.

• Genes located near each other on the same chromosome tend to be inherited together.

Probability Rules in Genetics

Probability rules help predict genetic outcomes.

• Multiplication Rule: The probability that two or more independent events will occur together is the product of their individual probabilities.

• Addition Rule: The probability that any one of two or more mutually exclusive events will occur is the sum of their individual probabilities.

Types of Dominance

Dominance relationships between alleles affect phenotypes.

• Complete Dominance: Phenotypes of the heterozygote and dominant homozygote are identical.

• Incomplete Dominance: The phenotype of the heterozygote is intermediate between the two parental varieties.

• Codominance: Two dominant alleles affect the phenotype in separate, distinguishable ways.

Pleiotropy and Epistasis

Some genes affect multiple traits, and some genes can influence the expression of others.

• Pleiotropy: A single gene has multiple phenotypic effects. Example: alleles responsible for cystic fibrosis and sickle-cell disease.

• Epistasis: Expression of a gene at one locus alters the phenotypic expression of a gene at a second locus.

• Example: In Labrador retrievers, coat color depends on two genes—one for pigment color (B/b) and one for pigment deposition (E/e).

Polygenic Inheritance

Traits controlled by multiple genes show quantitative variation.

• Quantitative Characters: Traits that vary along a continuum, usually indicating polygenic inheritance.

• Example: Over 180 genes affect height; skin pigmentation is controlled by many genes.

Pedigrees and Human Genetics

Geneticists use pedigrees to analyze inheritance patterns in humans.

• Pedigree: A family tree describing the inheritance of a trait across generations.

• Recessively Inherited Disorders: Show up only in individuals homozygous for the allele.

• Carriers: Heterozygous individuals who carry the recessive allele but are phenotypically normal.

• Most individuals with recessive disorders are born to carrier parents.

• Albinism: A recessive condition characterized by lack of pigmentation in skin and hair.

Summary Table: Types of Dominance

Summary Table: Genetic Terms