Genetics

Genes and alleles of genes

A gene determines a trait, and different alleles or forms of gene exist. The color gene in peas has two alleles: yellow and green.

Genotype and phenotype

Genotype is the genetic makeup of an organism (written as alleles of specific genes), while phenotype is how the organism looks.

Homozygous and heterozygous

When both alleles of a gene are the same, the individual is homozygous for that gene (or pure breeding). If the two alleles are different, the organism is heterozygous (also called a hybrid).

Dominant and recessive

The dominant allele is the one that controls phenotype in heterozygous and genotype; the recessive allele controls phenotype only in a homozygote.

Monohybrid or dihybrid cross

A cross between individuals who are both heterozygotes for one gene (monohybrid) or for two genes (dihybrid).

Phenotype

Observable characterstic

Alleles

Alternate forms of a gene

Independent assortment

Alleles of one gene separate into gametes randomly with respect to alleles of other genes

Gametes

Reproductive cells containing only one copy of each gene

Gene

The heritable entity that determines a characteristic

Segregation

The separation of the two alleles of a gene into different gametes

Heterozygote

An individual with two different alleles of a gene

Dominant

The allele expressed in the phenotype of the heterozygote

F_¹

Offspring of the P generation

Testcross

The cross of an individual of ambiguous genotype with a homozygous recessive individual

Genotype

The alleles an individual has

Recessive

The allele that does not contribute to the phenotype of the heterozygote

Dihybrid cross

A cross between individuals both heterozygous for two genes

Homozygote

Having two identical alleles of a given gene