Colour Genetics 1: The Basics

  Horses come in a wide variety of colours!  This colour series will go over the wide variety of coat colours and their genes!

  Let's start off with the basics shall we?

  Every individual receives one copy of genetic material from each parent, which ensures genetic diversity and decreases the chance of an individual having identical variations of the same gene (alleles) to increase the likely hood that the organism will survive long enough to reproduce itself.  Variation in a populations genome increases the likelihood that that population will survive, for if there is more variation, there is a higher likelihood that some individuals may live to reproductive age and reproduce, thus perpetuating the population.  Each gene may have several alleles that all code for the same basic function (ex. eye colour, height, form and function of organs etc.) and this is what accounts for the variations we see between individuals of the same species.  Genes are tightly packaged into chromosomes to keep cell division neat and tidy, and to prevent breakages in the DNA.  Which alleles get passed on to offspring is a 50/50 chance, as there are 2 sets of chromosomes that contain the genetic code for animals, and either set can be passed on through meiosis (a form of cell division which produces gametes).  There can be several alleles for a single gene, however, only the ones your parents have to pass on can be present in your genotype.

  Which gene is expressed (phenotype) in an individual is determined by several factors, but probably the most important one is dominance.  Not all genes are phenotypic, i.e. they are not visibly expressed or obviously affecting an individual due to either being silent or silenced by another.  Some alleles are dominant over others, which means that the recessive allele may not be fully, or at all, expressed.  Some genes (even from other chromosomes) may also affect others and either increase or decrease the expression of that gene.  It is hard to pinpoint all of the exact interactions between each and every single gene in an organism due to the sheer volume of genetic material, but some have been identified and continued research discovers more as time goes on.

  In genetics, genotypes (the specific alleles in an organism) are represented by letters!  Which letters are used depends on the organism and the letters that researchers have chosen that are easy to distinguish between Uppercase and lowercase.  When written, a slash between each letter is used to represent the given code on each chromosome, and as some codes may be 2 or more letters long, this helps to provide clarity. This (hopefully) eliminates confusion when looking at genotypes.  Since animals (most) have two sets of chromosomes, an individual can be either Dominant/Dominant, Dominant/recessive, or recessive/recessive for a particular gene (D/D, D/r, or r/r.  Typically the same letter is used but in upper and lower case, but I wrote it in this way to hopefully retain some clarity).  Which genotype the animal has will determine its phenotype.

  A gene that has the same allele (wether it be dominant or recessive) are known as homozygous for that allele.  Heterozygotes have one of each (dominant and recessive).

Homozygous dominant: D/D

Heterozygous: D/r

Homozygous recessive: r/r

  Typically, homozygous dominant and heterozygotes will both express the dominant phenotype if there aren't other factors at play, and recessive homozygotes will display the recessive phenotype as there aren't any dominant alleles to "drown out" the recessive.  Because heterozygotes have the same phenotype as dominant homozygotes, it can be hard to tell what alleles the animal carries, and are usually not visually distinguishable.  Genetic testing, looking at parental and offspring phenotypes, can give you an answer as to which of those two genotypes an organism has.

  Some alleles may be incompletely dominant or codominant.  In incomplete dominance, the organism will look like a blending of the two.  In codominance, both phenotypes may be expressed, and the organism will appear patchy as both phenotypes are expressing themselves.

  If we think of it in terms of flower colours, imagine we breed together a white and a red flower, and if, for our first example, we imagine that they are incompletely dominant, the offspring will be a mix, and in this case will appear pink!  In our second example, if they are codominant, the offspring will have white and red blotches on the petals.

  Incomplete and codominance shouldn't be confused with other reasons for variation.  For example, in some flower species, pink flowers may be a result of incomplete dominance and in others pink is the recessive genotype to orange.  Blotching or lightening may be another gene that partially removes pigment from certain areas based on the expression of yet more genes!  Unless the genotype has been sequenced, and researched, never assume... it's a messy thing trying to sort out all of the possible interactions!

  So this is a rather long post and potentially full of jargon, so please feel free to post any questions down below and I will try my best to answer them, or to even clarify them in future posts!  The next post in this series will actually relate to horses don't worry ;) and we will start to discuss the genes that affect horse coat colours!  Stay tuned!

Comments

Popular Posts