The nucleus of each human somatic cell contains 23 pairs of chromosomes, or 46 in all. One member of
each pair comes from the mother, and the other member comes from the father. Pair 1
autosomes and they contain the genes that contribute most to our physical appearance and behaviour.
pair are the sex chromosomes, which contribute to our physical and behavioural sexual
characteristics. (x and y).
Female = xx
Male = xy
The y chromosome contains the SRY gene that makes the SRY protein, which triggers testes
development and hence the male phenotype. Each cell contains two copies of every gene called alleles
(one gene pair contains two alleles), if they are identical they are homozygous, if they are different they
are heterozygous. The nucleotide sequence most common in a population is called the wild-type allele,
whereas a less frequently occurring sequence is called a mutation. In heterozygous pairs:
Only the allele from the mother may be expressed
Only the allele form the father may be expressed
Both alleles may be expressed simultaneously
Dominant vs. recessive
. Complete dominance / incomplete dominance / codominance.
Errors can arise during the reproduction of genes and passing them on to offspring is fallible. The altered
alleles are mutations. A mutation may be as small as a change in a single nucleotide base, called a single
nucleotide polymorphism (SNP). It results in change in one amino acid in a protein. A mutation can be