Chapter 12: Heredity

Gene

A section of DNA that contains instructions for a functional for a product, such as a protein or RNA, which can influence a trait.

Alleles

Different versions of the same gene found at the same locus on homologous chromosomes. For example, A and a.

Genotype

The combination of alleles an organism has for a gene, such as AA, Aa, or aa.

Phenotype

The observable trait produced by an organism’s genotype and environment, such as purple flowers.

Homozygous

Having two identical alleles for a gene, such as AA or aa.

Heterozygous

Having two different alleles for a gene, such as Aa.

Dominant allele

An allele that affects the phenotype when at least one copy is present. It is usually represented by a capital letter.

Recessive allele

An allele whose effect is usually hidden by a dominant allele. In complete dominance, it appears in the phenotype only when two recessive copies are present.

Mendel’s principles

The principles that alleles occur in pairs, separate during gamete formation, and alleles of different unlinked genes assort independently.

Independent assortment

The random and independent separation of alleles from different unlinked genes into gametes during meiosis.

Dependent assortment

The tendency of genes located close together on the same chromosome to be inherited together. These linked genes do not assort completely independently, although crossing over may separate them.

Monohybrid

An organism that is heterozygous for one gene, such as Aa.

Dihybrid

An organism that is heterozygous for two genes, such as AaBb.

Test cross

A cross between an organism with a dominant phenotype but unknown genotype and a homozygous recessive organism to determine the unknown genotype.

Reciprocal cross

Two crosses in which the sexes of the parents are switched. It can help determine whether a trait is sex-linked or affected by the parent it came from.

Monohybrid cross

A genetic cross that follows the inheritance of one gene, often Aa × Aa.

Dihybrid cross

A genetic cross that follows the inheritance of two genes at the same time, often AaBb × AaBb.

True-breeding

An organism that is homozygous for a trait and produces offspring with the same trait when self-fertilized or crossed with the same type.

Dominant

A trait that appears in the phenotype when at least one dominant allele is present.

Recessive

A trait that, under complete dominance, appears only when an organism has two recessive alleles.

Segregation

The separation of the two alleles for a gene during meiosis so that each gamete receives only one allele.

Punnett square

A diagram used to predict the possible genotypes and phenotypes of offspring from a genetic cross.

Probability

The chance that a particular genetic outcome will occur. It may be written as a fraction, decimal, or percentage.

Genotypic ratio

The relative numbers of each genotype expected among the offspring. For Aa × Aa, the ratio is 1 AA : 2 Aa : 1 aa.

Phenotypic ratio

The relative numbers of each observable phenotype expected among the offspring. For Aa × Aa with complete dominance, the ratio is 3 dominant : 1 recessive.

Non-Mendelian inheritance

Any inheritance pattern that does not follow simple complete dominance, such as incomplete dominance, codominance, epistasis, pleiotropy, or polygenic inheritance.

Incomplete dominance

An inheritance pattern in which neither allele completely dominates, causing the heterozygote to have an intermediate phenotype.

Codominance

An inheritance pattern in which both alleles are fully and separately expressed in a heterozygote, such as the IA and IB alleles in AB blood.

Pleiotropy

When one gene affects multiple different traits.

Epistasis

When one gene masks or changes the expression of another gene.

Polygenic inheritance

When multiple genes work together to influence one trait, often producing continuous variation such as height or skin color.

P generation

The original parental generation used in a genetic cross.

F1 generation

The first filial generation produced by crossing the P generation.

F2 generation

The offspring produced by crossing or self-fertilizing members of the F1 generation.

Blending theory of inheritance

The incorrect historical idea that parental traits permanently mix in offspring. Mendel showed that alleles remain separate and can reappear in later generations.