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Genetics - The Basics

Definitions
Genetics : 
The study of laws governing similarities (hereditary) and differences (variations) between the offspring and their parents

Gene : 
A hereditary unit located in a particular locus of the chromosomes, which comprises of a segment of DNA which codes for a particular polypeptide. This determines the specific phenotypes of an individual, and can recombine (interact) with other genes and can undergo mutation and change the phenotype of your fish.

Homologous chromosomes : 
A pair of chromosomes, one which is of paternal origin and the other, maternal. They are morphologically similar in length, size, shape, location of the centromere and the have the arrangement of the same number of genes in the same linear order. Each chromosome is made up of a super-condensed (ultra-coiled) strand of DNA. 

Alleles :
Alleles are one of the alternative forms of a given gene. They occupy the same position (locus) on homologous chromosomes, and are responsible for determining contrasting characters of the same trait. In diploid animals like Bettas, only 1 or 2 forms exist at any one time, and the expression of these alleles are determined by whether they are dominant, recessive, co-dominant or even incompletely dominant. A gene is said to be homozygous if both alleles are identical, and heterozygous when the alleles are different.

For example, in bettas, dt (double tail gene) and DT (single tail gene ) are alleles of each other as they control the same trait (the type of tail produced). Hence if a fish contains the genes dtdt or DTDT, we call it homozygous (both alleles are of the same type). If it contains dtDT, then it is heterozygous. I will elaborate more on the nomenclature of alleles later on.

Dominant:
This is a condition when the allele is able to produce its effect in hoth homozygous and heterozygous condition. For example, the dominant single-tail gene (DT) will produce single-tailed specimens when the fish contains either the combinations DTDT (homozygous) or DTdt (heterozygous). In other words, a dominant allele will cancel out the effects of the the other allele, and will express itself, no matter what. (how selfish! =)

Recessive:
Recessive genes can only produce its effects in the homozygous condition. Again taking the example of the double-tail gene, dt, a fish with dtdt will have double-tail characteristics. A fish with dtDT will, however, be single-tailed, as the dominant DT (single-tail) gene will overwrite the effects of the dt gene.

Genotype:
Refers to all the genes that an individual carries, in other words, its genetic composition. Some of these genes may not be shown in the actual appearance of the fish.

Phenotype:
This refers to the actual appearance of the fish, i.e. a measurable characteristic or distinctive trait of the fish.

For example, a halfmoon genotype betta is NOT a real halfmoon, it's most likely a delta tail, but contains the genes that may result in a real halfmoon when spawned. A halfmoon phenotype, on the other hand, is a real halfmoon, and shows the 180^o tail (caudal) spread.