B1- Variation and Inheritance

Variation

Differences between individuals of the same species are variations. Genetic variations occur because individuals inherit different combinations of genes. They can be caused by mutations, differences between individual gametes or the random nature fertilisation. 

Individuals develop in different conditions, so some variations are due to environmental causes. Some characteristics are determined by a combination of genetic and environmental factors.

E.g.

Genetic- Nose shape and eye colour

Environmental- Language and scars

Combination- Body mass, Intelligence and Height.

Scientists are currently debating whether genetics of environment has the greatest influence in the development of characteristics like intelligence, health and sporting ability. It's unlikely that any characteristics are the sole result of one factor.

Alleles

All the instructions to make an individual are held on chromosomes kept in the nucleas of all body cells. A section of chromosome which codes for an inherited characteristic or protein is called a gene. A person has 23 pairs of chromosomes; different species have different numbers of chromosome pairs.Gametes have half the number of chromosomes of normal body cells. The different versions of genes are called alleles. Alleles that control characteristics are described as being Dominant or Recessive.

Dominant Alleles control the development of a characteristic, even if present on only one chromosome in a pair. Recessive alleles control the development of a characteristic…