1. Chromosomes and Genes

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

3. 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

4. Each chromosome contains many genes

5. 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

6. 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

7. 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)

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

9. However, eye colour inheritance is not actually this simple…