Single genes and disorders

Allele: one of two or more alternative forms of a gene Genotype: genetic constitution of an organism Phenotype: Observable properties of an organism produced by a combination of genotype and environment

Alteration in DNA or chromosome sequence or structure. The sources of alleles and genetic variation. Nonsense - change in sequence coding for a stop codon Missense - change to amino acid sequence of protein Synonymous - base substitution but no change to amino acid Point - single base substitution

Can lead to a frameshift mutation which throws off all the codons for the amino acids.

A gene being transcribed has regulatory points where proteins bind and if these change, the proteins will not bind and this affects transcription.

Homozygote = a diploid organism having the same alleles at one or more genes Heterozygote = a diploid organism having different alleles at one or more genes

The heterozygote makes 50% of normal level gene product but this is not enough for normal function.

Dominant and recessive

• Males and females affected equally - Probability of an affected child is 50% is one parent is homozygous recessive and the other is heterozygous dominant

• More frequent in males as they have only one X chromosome • Cannot be passed from father to son as Y chromosome comes from father • All daughters of affected fathers are carriers • By probability, half the sons of a carrier will be affected and half their daughters will be carriers.

Most common traits are multifactorial: arising from the action of two or more genes and interaction between genes and the environment.