Dominance and codominance

Dominant alleles

• Determine the phenotype in individuals that are homozygous for the dominant allele and heterozygous

• Recessive alleles only determine the phenotype if an individual is homozygous with 2 recessive

Genetic diseases

• Most genetic diseases are caused by a recessive allele

• A person can be a carrier of the disease if they are heterozygous dominant.

• If two parents are carriers of the same recessive allele, the chance of their offspring inheriting the disease is 25%.

Phenylketonuria

• A recessive genetic condition caused by mutation in a gene on chromosome 12 that codes for the enzyme phenylalanine hydroxylase (PAH)

• The enzyme converts the amino acid phenylalanine into tyrosine and in cases of phenylketonuria this metabolism is impaired

• The excess phenylalanine is instead converted into phenylketone, resulting in a toxic build in the blood and urine (hence phenylketonuria)

What happens if PKU is left untreated

• Can impair brain development leading to intellectual disability

• Babies with PKU present as normal at birth because the mother can break down the phenylalanine during pregnancy

Treating PKU

• Treated by enforcing a strict diet that restricts the intake of phenylalanine to prevent its build up within the body

• This low-protein diet should include certain types of fruits, grains, vegetables and special formula milk

• Patients who are diagnosed early and maintain this strict diet can have a normal life span without damaging symptoms

Gene pools

• A gene pool is all genes of all individuals in a sexually reproducing population

• Every new individual inherits a selection of genes from the gene pool