Dominant vs. Recessive Genes

Dominant Genes

is expressed only when two copies of the gene are present. is always expressed when present.

Recessive Genes

Opposed to dominant genes, is phenotypically expressed in the homozygous state but has its expression masked in the presence of a dominant gene.

is only expressed when no dominant genes are present.

How do we know of a trait is dominant or recessive?

Traits are determined by the genes on the chromosomes. A gene is a segment of DNA that determines a trait.

How do we know of a trait is dominant or recessive?

Chromosomes come in homologous pairs, thus genes come in pairs.

Homologous pairs – matching genes – one from female parent and one from male parent

How do we know if our traits are recessive or dominant?

Based on parent’s genotype/phenotype Could be: Sex – linked TraitsGenes for these traits are located only on the X chromosome (NOT on the Y chromosome). X linked alleles always show up in males whether dominant or recessive because males have only one X chromosome Incomplete dominance and Codominance When one allele is NOT completely dominant over another (they

blend) – incomplete dominance When both alleles are expressed – Codominance