Genetics

A phenotype is the characteristic (the gene) that you display.

A genotype is the gene(s) that you carry.

An allele is two different copies of the same gene. E.g. Brown eyes (B) , blue eyes (b).

An allele will either be dominant or recessive. Two dominant alleles and the gene will be described as 'homozygous dominant'. Two recessive alleles and the gene will be described as 'homozygous recessive'. A dominant allele and a recessive allele together, the gene is described as 'heterozygous'.

In a cell human body cell there is a nucleus. In this nucleus there is long lengths of Dna coiled up and a short section of this is a gene. The human sex cells (sperm and ovum) should have 23 chromosomes each so that the baby will have a total of 46. One less will not be enough to make a baby and one more will cause the baby to have Down's Syndrome.

X and Y chromosome determine whether a child will be a make or female. A combination of ** will produce a female child and a combination of XY will produce a male child.

Sickle Cell Anaemia and Cystic Fibrosis are both genetic disorders. They are both caused by a rece**ive allele. This means that only a homozygous dominant person can inherit this disease.

Example: Sickle Cell Anaemia

S= dominant allele s=rece**ive allele

Po**ible combinations: **- homozygous dominant. Not affected.

**- heterozygous. Not affected but a carrier. Rece**ive allele can be pa**ed on

**- homozygous rece**ive. A sufferer of sickle cell.