4.3+Theoretical+Genetics

4.3.1: Define //genotype, phenotype, dominant allele, recessive allele, codominant alleles, locus, homozygous, heterozygous, carrier// and //test cross//.

**Genotype:** the alleles of an organism.

**Phenotype:** the characteristics of an organism.

**Dominant allele:** an allele that has the same effect on the phenotype whether it is present in homozygous or heterozygous state.

**Recessive allele:** an allele that only has an effect on the phenotype when present on the homozygous state.

**Codominant alleles:** pairs of alleles that both affect the phenotype when present in a heterozygote.

**Locus:** the particular position on homologous chromosomes of a gene.

**Homozygous:** having two identical alleles of a gene.

**Heterozygous:** having two different alleles of a gene.

**Carrier:** an individual that has one copy of a recessive allele that causes a genetic disease in individuals that are homozygous for this allele.

**Test cross:** testing a suspected heterozygote by crossing it with a known homozygous recessive.