Genes: the units of heredity
A gene is a segment of DNA on a chromosome that carries the coded instructions for a particular characteristic, such as eye colour or seed shape. The fixed position of a gene on a chromosome is called its locus. Because chromosomes occur in homologous pairs, each gene is present in two copies, one inherited from each parent.
Alleles and their forms
An allele is one of the alternative forms of a gene. For a single gene there may be two contrasting alleles — for example an allele for tall and an allele for short. An allele that is expressed even when only one copy is present is dominant; an allele expressed only when two copies are present is recessive. A dominant allele masks a recessive allele in a heterozygote.
Example
For flower colour, R (red) is dominant and r (white) is recessive. RR and Rr plants have red flowers; only rr plants have white flowers. R and r are two alleles of the same gene occupying the same locus on homologous chromosomes.
Genotype and phenotype
The genotype is the genetic make-up of an organism, written as allele symbols (e.g. RR, Rr or rr). The phenotype is the observable characteristic that results (e.g. red or white flowers). An organism with two identical alleles is homozygous (RR or rr); one with two different alleles is heterozygous (Rr). Understanding the difference between genotype and phenotype is essential, because two organisms that look exactly the same can carry different alleles, and this hidden difference only shows up in their offspring when they are crossed with other individuals.
Key idea
Gene = unit of heredity at a fixed locus. Allele = a version of a gene. Genotype = allele combination; phenotype = the trait you can observe.
Remember
Homozygous dominant (RR) and heterozygous (Rr) can share the same phenotype but have different genotypes.