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Introduction to Angelfish Genetics


Genetics is a branch of biology that studies heredity and how certain characteristics or traits are passed from parents to offspring.

Genetics means the study of inherited variation also known as polymorphism:

(1)The study of genes through their variation.
(2) The study of inheritance. Polymorphism is the occurrence, together in the same population, of more than one allele or genetic marker at the same locus with the least frequent allele or marker occurring more frequently than can be accounted for by mutation alone. Genes are the fundamental physical and functional unit of heredity, which carries information from one generation to the next.


We begin with explaining the chromosome. The structure of the chromosome consists of two long strands of Deoxyribonucleic Acid (DNA) that are bound together, and twisted into the shape of a helix. The DNA helix is organized additionally by becoming wrapped around proteins called histones forming a DNA-protein complex known as a nucleosome. During cell division, the nucleosomes coil further into a dense, visible (stainable) mass of DNA most people think of as a chromosome.

Homologous chromosome

Gene Locus on Chromosomes

Homologous chromosomes are a matching pair of chromosomes containing the same genetic loci in the same order. One of the homologous chromosomes is received from the father, and the other is received from the mother.

Because there are two chromosomes in the pair, the pair contains two different alleles at every loci, one on each chromosome. This allows for a great deal of variation in individuals, by means of dominant traits, recessive traits, polygenic traits, and masking genes. In the figure to the right, we show a set of chromosomes showing only one locus. However, a chromosome pair can have more than one locus as you will learn with angelfish genetics (see next figure).

Loci on the Chromosomes

Locus is a single set of alleles on the chromosome.

Ex: D/+ (Black Lace) shows only one position (location) on the chromosome.

Loci is more than one set of alleles on the chromosome.

Ex: D/+ - S/+ (Black Ghost ), this shows two positions (locations) on chromosome.

Understanding the basics of the chromosome will help us when learning the genetics of angelfish, the alleles, and their interactions.

Below are a few common terms to help you understand other sections in the genetics category of this site. For a full glossary of terms used in genetics, please see our "Genetic Terminology" page.

Allele: One alternative of a pair or group of genes that could occupy a specific position on a chromosome.
Chromosome: A linear strand of DNA harboring many genes.
Dominant: An allele producing the same phenotypic effect whether inherited heterozygously or homozygously; an allele that "masks" a recessive allele.
Gene: A unit of genetic information that occupies a specific position on a chromosome and comes in multiple versions called alleles.
Genotype: The genetic constitution of an organism.
Heterozygous: Having a genotype with two different and distinct alleles for the same trait.
Homozygous: Having a genotype with two of the same alleles for a trait.
Phenotype: The physical or observable characteristics of an organism.
Recessive: An allele producing no phenotypic effect when inherited heterozygously and only affecting the phenotype when inherited homozygously; an allele "masked" by a dominant allele.

It is important to remember that an Angelfish's color, growth rate and adult size are a product of not only it's genetic material but also environmental factors. These factors include, but are not limited to, tank size, temperature, frequency of feeding, nutrition, water quality, and the length of day called the "photo period." Dr. Joanne Norton has spent over 20 years studying the genetics of Angelfish. Her articles will help you to understand how the genetics and environmental influences work together to produce many of our angelfish varieties. The "genetics terminology" will allow you to have a better understanding of the term itself and how it works.


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