1. Gene: basic unit of heredity that corresponds to a sequence of nucleotides along a molecule of DNA.
 

2. Allele: alternate forms of a gene. For instance, the alternate form of the gene for black (B) fleece in sheep is brown (b). In some cases, there are more than two forms or alleles for a gene but any individual will only have two alleles (one inherited from its mother and one from its father).
 

3. Chromosomes: thread like structures of DNA, found in pairs in higher plants and animals. Humans have 23, sheep have 27.
 

4. Locus, plural loci: location of a gene on a chromosome. An individual has two alleles at any locus, one allele inherited from its mother, one from its father.
 

5. Homozygous: having two identical alleles at one locus. For instance BB (also written BBBB) in sheep. This individual carries two copies of the B (black) form of the gene.  It can only contribute a B allele to its progeny.
 

6. Heterozygous: having two different alleles at a particular locus. For instance, Bb (also written BBBb) in sheep. This individual carries one copy of the dominant B (black) form of the gene and one copy of the recessive b (brown) form of the gene.  There is a 50/50 chance of contributing one or the other allele to the progeny.
 

7. Dominant: an allele that will be expressed physically (in the phenotype) when only one copy is present in the genotype. For instance B is dominant to b; so a sheep that is Bb will be black. Alleles for any gene are usually ordered from most dominant to most recessive: B > b.
 

8. Recessive: an allele that is only expressed physically when there are two copies present (homozygous) or, where there are more than two forms of alleles, when no more dominant allele is present. For instance, only bb (also written BbBb) will result in a brown sheep as b is recessive to B (b < B).
 

9. Codominance: where allele forms are not dominant to each other. The homozygous forms express normally but the heterozygous form shows characteristics of both alleles in the phenotype. For example in some cattle the R gene has two forms R and r, which are codominant: R=r. This means that RR produces red cattle, Rr produces roan cattle with both red fibres and white fibres and rr produces white cattle.  In Shetlands, the gulmoget pattern and the katmoget pattern may be codominant.
 

10. Incomplete dominance: the heterozygous form is a blend of both homozygous forms. For instance a red flower (RR) crossed with a white flower (rr) may yield all pink flowers (Rr) in the progeny if there is incomplete dominance of the alleles.
 

11. Epistasis: is the interaction of a number of loci or genes. For instance, A is the Agouti pattern gene in sheep. Awt (white) is the dominant allele for that gene. If Awt is present at the A locus, then any expression of the B gene for black or brown is suppressed or covered up.  If the genes are expressed in the phenotype independently of each other, they are not epistatic.
 

12. Genotype: the particular alleles for each gene found in an individual. For instance, a sheep will have one of three possible genotypes for the B gene: BB (also written BBBB), Bb (also written BBBb), and bb (also written BbBb). If there are more than two allele forms for a gene, the number of possible genotypes increases.
 

13. Phenotype: the physical expression of a trait. Phenotype is the sum of the genotype and the environmental influences operating on an individual. Distinct genotypes are not always expressed as distinct phenotypes. For instance, both the BB (also written BBBB) and the Bb (also written BBBb) genotypes will produce black sheep so long as they are not suppressed by a pattern allele at the Agouti locus.
 

14. Heritability: the degree to which a trait is likely to be passed from parent to offspring. It is the proportion of phenotypic variation that is due to genetics, not environment.