Inheritance cards

A gene is the physical unit of heredity. It is a sequence of DNA that occupies a specific site or locus. on a chromosome and codes for a polypeptide. 

Polypeptides and proteins determine structures and functions in living organisms and so the gene is thought of as the sequence of DNA that codes for a characteristic. 

Small changes in nucleotide sequences of the same gene makes small changes in the polypeptides they encode and produce different characteristics. The different sequences are called alleles. 

A diploid individual has one of each chromosome from each parent, therefore one copy of each gene from each parent. 

If the alleles of a particular gene are the same from both parents, the individual is homozygous for that gene. 

If they are different, the individual is heterozygous for that gene. 

The genotype of an individual is all the alleles that they contain. 

Their phenotype can be described as their appearance. It includes much more than that e.g. characterisitcs that can't be seen; blood group. 

Both the genotype and the environment determine the characteristics of an individual so the phenotype is better thought of as an expression of the genotype in a specific environment.