Genes are the basic units of inheritance. All living things inherit genes from their parents and these genes influence how an individual turns out.
Genes are made of DNA, or deoxyribonucleic acid, a very large molecule that forms each living thing’s unique genetic barcode. Not only is it a barcode, but it is also a blueprint, and a repair, maintenance and operating manual for that animal.
DNA contains a very large number of thread-like molecules bundled up tightly into groups called chromosomes. These chromosomes are packed into the nucleus of each cell.
An animal has two copies of every chromosome – one from each parent – and therefore two copies of each gene. The specific location of a gene on a chromosome is called a locus.
The unbundled molecules of DNA resemble a huge twisted ladder (double helix). The “rungs” in the ladder consist of four chemical bases (or nucleotides) known as adenine (A), cytosine (C), guanine (G) and thymine (T). The bases are always in pairs, A with T and G with C. Genes are “instructions” and are made up of this base code or DNA sequence. The number of possible combinations of these bases within the DNA is in the thousands of millions – this explains the unlimited variety of life.
DNA Markers
Microsatellite
Microsatellites consist of specific sequences of bases repeated in a tandem fashion, for example, the two bases, C and A, repeated ten times (….TAGCCTCACACACACACACACACACAGGTCTA…). The number of these repeats found at any given microsatellite marker can differ between individual animals (or between pairs of genes in an individual).
SNP
A single nucleotide polymorphism, or SNP, is a DNA sequence variation occurring when a single base — A, C, G, or T — is changed, removed or added, so the sequence differs between individual animals (or between pairs of genes in an individual). For example, if the DNA sequence for one individual is …AAGCCTA… and for another is …AAGCTTA…, then they differ by a single nucleotide – there has been a change (or mutation) from C to T.
Allele, Genotype and Phenotype
The different forms of a microsatellite or SNP marker at a particular locus are known as alleles. An individual animal has two copies of each chromosome (and gene) so the pair of alleles makes up the genotype for that marker. If an allele is the same on each chromosome, the animal is homozygous for the gene at that locus, if the alleles are different, the animal is heterozygous.
The genotype of an animal affects its molecular, physical, behavioural, and other characteristics, which are called the phenotype.
Dominant and Recessive
At a heterozygous gene locus the two alleles interact to produce the phenotype – the interaction can often be described as dominant or recessive.
In the simplest case, if the phenotypic effect of one allele completely dominates the other i.e. the phenotype produced by the two alleles in heterozygous combination is identical with that produced by the homozygous genotype, then that allele is said to be dominant, and the other allele is recessive.
For a recessive disease, an animal has to have two copies of the mutated gene before they will exhibit the disease. If the animal is heterozygous, the normal copy of the gene will “mask” the mutated one so the animal will not show the disease but will be a carrier and will pass the mutation on to about half of their offspring.