Genetics Key Terms and Concepts

Overview

This lecture provides definitions of key terms related to Mendel's experiments and heredity, focusing on patterns of inheritance and genetic terminology from Chapter 12 of Biology 2e.

Mendelian Genetics and Inheritance Patterns

• An allele is a gene variant occurring at the same location on homologous chromosomes.
• Autosomes are non-sex chromosomes that carry genetic information unrelated to sex determination.
• The blending theory of inheritance is the outdated idea that parental traits mix to produce intermediate offspring.
• Codominance occurs when both alleles in a heterozygote are fully expressed.
• Continuous variation describes traits showing a range of values without large gaps.
• Discontinuous variation involves distinct traits transmitted independently.
• Dominant traits produce the same phenotype with one or two copies of the allele.
• Dominant lethal alleles are fatal in both homozygotes and heterozygotes but may be passed on if lethality occurs after reproduction.
• Epistasis is when one gene masks or interferes with another gene’s expression.

Genetic Crosses and Probability

• F₁ is the first filial generation, the direct offspring of the parental generation.
• F₂ is the generation produced by crossing or self-crossing F₁ individuals.
• Dihybrid crosses involve two characteristics, while monohybrid crosses involve one.
• Hybridization is mating of two different individuals to achieve desired traits.
• The law of dominance states that one trait in a heterozygote conceals another.
• The law of independent assortment says that genes are sorted into gametes independently.
• The law of segregation states that paired genes separate equally into gametes.
• The product rule calculates the probability of independent events occurring together.
• The sum rule calculates the probability of at least one of two mutually exclusive events.

Chromosome and Gene Relationships

• Genotype is the genetic makeup, including visible and non-expressed alleles.
• Phenotype is the observable physical and physiological traits of an organism.
• Homozygous means having identical alleles; heterozygous means having different alleles for a gene.
• Hemizygous describes having only one allele for a characteristic (e.g., single X chromosome in males).
• Linkage occurs when genes close together on a chromosome are likely inherited together.
• A model system is a species used to study biological phenomena for broader application.

Sex-linked Traits and Special Terms

• Sex-linked genes are found on sex chromosomes.
• X-linked genes are found on the X, but not Y, chromosome.
• A reciprocal cross switches the traits of male and female parents in paired crosses.
• A test cross mates a dominant-expressing individual of unknown genotype with a homozygous recessive to determine genotype.
• Recessive traits are only expressed if present in two copies; dominant traits are expressed if at least one copy is present.
• Recessive lethal alleles are fatal only in homozygous form.

Visual Tools and Key Concepts

• A Punnett square is a diagram showing possible genetic outcomes from a cross.
• Traits are variations in heritable characteristics.

Key Terms & Definitions

• Allele — gene variant at a specific chromosome location.
• Autosomes — non-sex chromosomes.
• Codominance — both alleles fully expressed in heterozygotes.
• Continuous variation — range of trait values with small gradations.
• Discontinuous variation — traits are distinct and separate.
• Dominant — allele expressed with one or two copies.
• Epistasis — one gene affects expression of another.
• Genotype — genetic makeup of an organism.
• Phenotype — observable traits of an organism.
• Heterozygous — two different alleles for a gene.
• Homozygous — two identical alleles for a gene.
• Law of independent assortment — genes sort independently into gametes.
• Law of segregation — genes separate equally into gametes.
• Linkage — genes inherited together due to chromosome proximity.
• Punnett square — diagram to predict genetic crosses.
• Recessive — trait expressed only with two copies of the allele.
• Sex-linked — gene located on a sex chromosome.
• Test cross — identifies genotype by crossing with homozygous recessive.
• Trait — heritable characteristic variant.

Action Items / Next Steps

• Review all definitions and examples for each key term.
• Practice drawing and interpreting Punnett squares.
• Study the laws of inheritance and their genetic implications.