Chromosomes and Genes

Within each human somatic cell there are 46 chromosomes (23 pairs) Karyotype of metaphase chromosomes homologous pair sex chromosomes

Each chromosome is made of DNA Long strands of chromatin are packed tightly together to make chromosomes A segment of DNA is called a gene

Each chromosome contains many genes

A pair of homologous chromosomes have the same genes but the specific information may differ slightly For example: this homologous pair contains the gene for eye colour; but one chromosome has the gene for brown eyes, while the other has the gene for blue eyes

A = brown eyes; a = blue eyes These are called alleles (the different forms of expression of a gene) Some alleles are dominant, while others are recessive

If a person has the dominant and recessive alleles (heterozygous), only the dominant will be observed in their phenotype If a person has 2 dominant or 2 recessive alleles, their genotype is homozygous Genotype = the specific alleles a person has (e.g. Aa) Phenotype = the observable traits a person has (e.g. brown eyes)

E.g. Aa = brown = heterozygous (hybrid) AA = brown = homozygous (pure dominant) aa = blue = homozygous (pure recessive)

However, eye colour inheritance is not actually this simple…