Chapter 12: Mendel & Inheritance

Gregor Mendel

The founder of modern genetics known for his work with pea plants.

Dominant Trait

A trait that is expressed in the phenotype even if only one copy of the allele is present.

Recessive Trait

A trait that is only expressed in the phenotype when two copies of the allele are present.

True-Breeding Strains

Plants that, when self-fertilized, produce offspring identical to themselves.

F1 Generation

The first filial generation, consisting of offspring from the cross of the P generation.

F2 Generation

The second filial generation, consisting of offspring from self-fertilization of the F1 generation.

Phenotype

The observable traits expressed by an organism.

Genotype

The genetic makeup of an organism.

Law of Segregation

The principle that alleles separate during gamete formation.

Law of Independent Assortment

The principle that alleles for different traits assort independently during gamete formation.

Dihybrid Cross

A cross between individuals that involve two traits.

Incomplete Dominance

An inheritance pattern where the phenotype of heterozygotes is intermediate between those of the two homozygotes.

Codominance

An inheritance pattern where both alleles are fully expressed in the heterozygote.

Multiple Alleles

More than two alleles exist for a gene in a population.

Polygenic Inheritance

A trait controlled by multiple genes.

Pleiotropy

A single gene influences multiple traits.

Epistasis

One gene masks the expression of another gene.

Test Cross

A cross between an individual with an unknown genotype and a homozygous recessive individual.

Pedigree

A diagram that shows the occurrence of a genetic trait in several generations.

X-Linked Traits

Traits associated with genes located on the X chromosome.

Hemizygous

Having only one allele for a gene; males are considered hemizygous for X-linked traits.

Recessive Lethal Allele

An allele that is only lethal in the homozygous condition.

Dominant Lethal Allele

An allele that is lethal even in the heterozygous condition.

Autosomal Dominant

A pattern of inheritance in which a trait appears in every generation and affects both genders equally.

Autosomal Recessive

A pattern of inheritance where a trait can skip generations.

Incomplete Dominance Example

An example is when red and white flowers produce pink flowers.

Codominance Example

An example is AB blood type where both A and B alleles are expressed.

Homozygous

Having two identical alleles for a trait.

Heterozygous

Having two different alleles for a trait.

Allele

An alternative version of a gene.

Gamete

A mature haploid male or female germ cell that is able to unite with another of the opposite sex.

Self-Fertilization

The fertilization of a flower by its own pollen.

Cross-Fertilization

The fertilization of one plant by pollen from another plant.

3:1 Phenotypic Ratio

The expected ratio of dominant to recessive phenotypes in the F2 generation of a monohybrid cross.

9:3:3:1 Phenotypic Ratio

The expected ratio of phenotypes in the F2 generation of a dihybrid cross.

Punnett Square

A diagram used to predict the outcome of a genetic cross.

Gamete Ratio for Dihybrid Cross

1:1:1:1 ratio of gametes in AaBb x AaBb.

Probability Rule

A mathematical rule used to calculate the likelihood of a particular genetic outcome.

Environmental Influence

The effect of external factors on trait expression.

Genotypic Ratio from Monohybrid Cross

Expected ratio is 1:2:1 for AA, Aa, and aa.

Trait Expression

The appearance or observable character of an organism.

Dominant Disorders

Genetic disorders that appear in every generation.

Recessive Disorders

Genetic disorders that may skip generations.