Glossary of Genetic Terms and Information
The following are scientific genetic terms used to discuss the inheritance and expression of genes.
DNA - Stands for [D]eoxyribo[N]ucleic [A]cid. DNA carries genetic information.
Chromosome - The structure within cells that houses DNA (genes).
Sex Chromosome - One of two chromosomes which determine the sex of an organism.
Locus - The location of a specific gene on a chromosome; plural loci. Each chromosome has sets of loci.
Gene - A segment of DNA. A specific set of nucleotides which determine an organism's traits.
Allele - A mutation which replaces another gene on the same locus. As an example, you have a locus for eye color and a brown gene or a blue gene can go in that locus, but not both in the same locus, as they replace one another.
Genome - The entire set of chromosomes for an organism.
Genotype - The set of actual genes an organism has. These may or may not show in the phenotype.
Phenotype - The visual or behavioral traits an animal displays, which may or may not reflect all genes present.
Express - To show. Example: "The gene expresses in the phenotype" means the gene is visible or affects the phenotype.
Co-expression - When two genes for the same thing express in the phenotype at the same time.
Polygenic trait/mutation - A trait or phenotype which takes multiple (poly) genes (genic) to express.
Wild Type - The phenotype found in the wild. For peafowl, this would be blue and barred wing, with no white.
Morph - The word for a phenotype that results from a mutation or group of mutations. Morph names do not always match the genotype. For example, Platinum is the morph name for a bird with two bronze AND two opal mutations where the mutations co-express in the phenotype.
COLOR mutation - a genetic mutation which affects the color of a bird's feathers. Autosomal colors are carried on the non-sex chromosomes. Sex-linked colors are carried on the sex chromosomes. Recessive colors require the bird to have 2 matching copies of the same color mutation, dominant colors require the bird to have 1 or 2 copies of the mutation to display in the phenotype.
PATTERN mutation - a genetic mutation which affects the pattern in which color is displayed. All peafowl pattern mutations are autosomal.
Leucistic mutation - A mutation resulting from the genetic condition leucism.
Heterozygous (het) - Literally "two different genes," but largely used in reference to only having one copy of a mutated gene (and one wild type gene). However, a bird with two different alleles that are both mutations would still be considered a heterozygote, it would just be a "compound heterozygote."
Hemizygous - When an individual has only one chromosome of a pair/the other part of the chromosome pair is missing or doesn't exist. This most commonly applies to the sex chromosomes and sex-linked genes; a hen with Z and w is hemizygous for Z and w (because there is no second Z or second w). Sex linked genes display on hens in single copy because they are hemizygous. The other Z chromosome is missing, so there's no way they can be heterozygous or homozygous.
Compound Heterozygous - When an individual has two different alleles, one on each chromosome, but in the same locus, and no wild type gene. An example of this would be "pied" in peafowl (W/P).
Sex-Linked gene - A trait carried only on the sex chromosome.
Autosomal gene - A trait carried only on the non-sex (autosomal) chromosomes.
Recessive gene - A gene which must be present on all copies of an autosomal chromosome (homozygous) in order to express.
Sex-linked Recessive gene - A gene which must be present on all copies of a sex chromosome (sex-linked) in order to express.
Dominant gene - A gene which must be present on only one copy of an autosomal chromosome (heterozygous) in order to express.
Incomplete Dominant gene - A gene which expresses in the phenotype in heterozygous and homozygous form, but expresses different in heterozygous form than in homozygous form.
Partial/Incomplete Dominance - A gene which expresses partially in the phenotype in homozygous form, typically presenting as a blend between two genes.
Co-dominance - When two genes express fully but separately in the phenotype.
Haplosufficient - Used to describe the wild type genes. Sufficient to express in single-copy form, but importantly, without being dominant.
Haploinsufficient - Used to describe the wild type genes. Insufficient to express in single-copy form, without being recessive.
Genetic Inheritance
Genetic inheritance of color and pattern mutations in peafowl is mainly Mendelian inheritance (governed by recessive/dominant genes), but not all traits are single-gene inheritance. One of the most prevalent examples is hybrid trait inheritance; black leakage on wings that resembles the blackshoulder mutation (but is actually the result of having mixed blue-green parentage) is an example of non-Mendelian genetic inheritance. Another common example is the specific pattern of white on a pied bird- offspring do not inherit the exact same pattern as their parents. This guide will discuss the mutations that are inherited 1:1.
Chromosomal crossover occurs during meiosis. Meiosis is the process by which sex cells divide to combine DNA from the parent cells. One of the stages of this process is called prophase 1, which itself has several stages. The third stage of prophase 1 is called the pachytene stage. During the pachytene stage, chromosomes pair up and exchange DNA code through genetic recombination, which is exactly what it sounds like: the genetic code (simply speaking) gets split and recombined into a different pattern to make offspring that are not quite like mom and not quite like dad, a process which often serves as a point during which damaged DNA can be repaired.
DNA - Stands for [D]eoxyribo[N]ucleic [A]cid. DNA carries genetic information.
Chromosome - The structure within cells that houses DNA (genes).
Sex Chromosome - One of two chromosomes which determine the sex of an organism.
Locus - The location of a specific gene on a chromosome; plural loci. Each chromosome has sets of loci.
Gene - A segment of DNA. A specific set of nucleotides which determine an organism's traits.
Allele - A mutation which replaces another gene on the same locus. As an example, you have a locus for eye color and a brown gene or a blue gene can go in that locus, but not both in the same locus, as they replace one another.
Genome - The entire set of chromosomes for an organism.
Genotype - The set of actual genes an organism has. These may or may not show in the phenotype.
Phenotype - The visual or behavioral traits an animal displays, which may or may not reflect all genes present.
Express - To show. Example: "The gene expresses in the phenotype" means the gene is visible or affects the phenotype.
Co-expression - When two genes for the same thing express in the phenotype at the same time.
Polygenic trait/mutation - A trait or phenotype which takes multiple (poly) genes (genic) to express.
Wild Type - The phenotype found in the wild. For peafowl, this would be blue and barred wing, with no white.
Morph - The word for a phenotype that results from a mutation or group of mutations. Morph names do not always match the genotype. For example, Platinum is the morph name for a bird with two bronze AND two opal mutations where the mutations co-express in the phenotype.
COLOR mutation - a genetic mutation which affects the color of a bird's feathers. Autosomal colors are carried on the non-sex chromosomes. Sex-linked colors are carried on the sex chromosomes. Recessive colors require the bird to have 2 matching copies of the same color mutation, dominant colors require the bird to have 1 or 2 copies of the mutation to display in the phenotype.
PATTERN mutation - a genetic mutation which affects the pattern in which color is displayed. All peafowl pattern mutations are autosomal.
Leucistic mutation - A mutation resulting from the genetic condition leucism.
- Technically pronounced 'lew-kiz-em' but often pronounced 'lew-si-sem' to the point where either pronunciation is acceptable.
- Leucism is a genetic condition (not a color or a pattern) which causes pigment cells to fail to move to the correct locations during development, resulting in the bird's inability to deposit pigment onto their feathers, leaving them white.
- There are three forms of leucism: total leucism (fully white animal), partial leucism (animal with white patches), and pale leucism (animal with reduced pigment).
Heterozygous (het) - Literally "two different genes," but largely used in reference to only having one copy of a mutated gene (and one wild type gene). However, a bird with two different alleles that are both mutations would still be considered a heterozygote, it would just be a "compound heterozygote."
- Heterozygous is the scientific term for the word "split" in peafowl- people use the word "split" because the first folks to talk about peafowl color mutations didn't know enough about genetics. I will use the scientific term instead of split, as heterozygous is more accurate, and makes it easier to understand some genes and their expression (like white and pied), but "split" may appear in some places.
Hemizygous - When an individual has only one chromosome of a pair/the other part of the chromosome pair is missing or doesn't exist. This most commonly applies to the sex chromosomes and sex-linked genes; a hen with Z and w is hemizygous for Z and w (because there is no second Z or second w). Sex linked genes display on hens in single copy because they are hemizygous. The other Z chromosome is missing, so there's no way they can be heterozygous or homozygous.
Compound Heterozygous - When an individual has two different alleles, one on each chromosome, but in the same locus, and no wild type gene. An example of this would be "pied" in peafowl (W/P).
Sex-Linked gene - A trait carried only on the sex chromosome.
Autosomal gene - A trait carried only on the non-sex (autosomal) chromosomes.
Recessive gene - A gene which must be present on all copies of an autosomal chromosome (homozygous) in order to express.
Sex-linked Recessive gene - A gene which must be present on all copies of a sex chromosome (sex-linked) in order to express.
Dominant gene - A gene which must be present on only one copy of an autosomal chromosome (heterozygous) in order to express.
Incomplete Dominant gene - A gene which expresses in the phenotype in heterozygous and homozygous form, but expresses different in heterozygous form than in homozygous form.
Partial/Incomplete Dominance - A gene which expresses partially in the phenotype in homozygous form, typically presenting as a blend between two genes.
Co-dominance - When two genes express fully but separately in the phenotype.
Haplosufficient - Used to describe the wild type genes. Sufficient to express in single-copy form, but importantly, without being dominant.
Haploinsufficient - Used to describe the wild type genes. Insufficient to express in single-copy form, without being recessive.
- A note about wild type genes: wild type genes are neither dominant nor recessive. It is what expresses when there is no other gene capable of expressing. ALL color/pattern mutations, in every animal species in the world, are alleles of the wild type genes, which is to say they replace the wild type genes.
Genetic Inheritance
Genetic inheritance of color and pattern mutations in peafowl is mainly Mendelian inheritance (governed by recessive/dominant genes), but not all traits are single-gene inheritance. One of the most prevalent examples is hybrid trait inheritance; black leakage on wings that resembles the blackshoulder mutation (but is actually the result of having mixed blue-green parentage) is an example of non-Mendelian genetic inheritance. Another common example is the specific pattern of white on a pied bird- offspring do not inherit the exact same pattern as their parents. This guide will discuss the mutations that are inherited 1:1.
Chromosomal crossover occurs during meiosis. Meiosis is the process by which sex cells divide to combine DNA from the parent cells. One of the stages of this process is called prophase 1, which itself has several stages. The third stage of prophase 1 is called the pachytene stage. During the pachytene stage, chromosomes pair up and exchange DNA code through genetic recombination, which is exactly what it sounds like: the genetic code (simply speaking) gets split and recombined into a different pattern to make offspring that are not quite like mom and not quite like dad, a process which often serves as a point during which damaged DNA can be repaired.