The Genetics of Horse Coat Color

The Genetics of Horse Coat Color
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For centuries, breeders have considered coat color an important physical attribute of a horse. Using basic genetic principles, breeders can predict the coat colors of offspring and mate horses that will produce foals with desired coat colors. This activity will demonstrate some of these genetic principles which can then be applied to other characteristics and species.
From top left: Chincoteague Pony, Friesian, Quarter Horse, Knabstrupper. Photos by Jim Shambhu. Come see these breeds and more at the Kentucky Horse Park!
*This educational packet is intended for high school biology students.
Name: ________________________________ Date: __________________________________
Class: _________________________________
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Table of Contents
Pg. 3 Why does coat color matter?
Pg. 4 The Genetic Blueprint: Genes
Pg. 5 How do we study genes?
Pg. 6 The Genetics of Horse Coat Color
Pg. 7-­‐8 Genetic Tool: Punnett Square
Pg. 9-­‐10 White Coats: Beautiful But Deadly
Pg. 11 Critical Thinking
Pg. 12 Vocabulary
Pg. 13 Genetics Vocabulary Crossword
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Why does coat color matter?
Horse breeders have selectively bred for certain coat colors for centuries. Though coat color has shown no correlation to improved work or performance by a horse, breeders still have a preference for coat colors called coat color bias. These preferences exist for many reasons. Horses compete in shows just as dogs do, and coat color is part of the breed standard against which each horse is judged. Breeders and owners that participate in horse shows will choose horses with coats that conform to the breed standard. Horses also pull wagons and carriages in parades throughout the country. Often, these horses have matching coats to accentuate their role as a team. Owners will then choose horses that have the desired coat to participate in the parade. Many people simply have a favorite color of horse that is aesthetically pleasing to them and want to own horses of that color.
An Arabian horse at a show. Photo by Jim Shambhu.
Shires Colin and TJ pulling a trolley. Photo by pixbysteve.com. Come see Colin and TJ at the Kentucky Horse Park!
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The Genetic Blueprint: Genes
Coat color is inherited, meaning that the condition of the trait is passed directly from parent to offspring. An understanding of this mechanism allows breeders to predict the coat colors of foals based upon their parents. Sweetie and her foal Surrie. Photo by Cindy Evans Photography. Come see Sweetie and Surrie at the Kentucky Horse Park!
The genetic “information” about all of an organism’s traits, such as a horse’s coat color, is contained on chromosomes inside their cells. A gene, a segment of the chromosome, contains the information for a specicic trait or suite of traits. Genes are further divided into individual units called loci. Each locus, either alone or in conjunction with other loci, incluences part of the trait controlled by its corresponding gene. For example, coat color is determined by a single gene, and several loci within that gene determine color. Locus for black coat
Chromosome
Copies of the same locus
Genes for coat color on homologous chromosomes. Loci with identical shades of color are molecularly identical. Loci with the same color but different shades still code for the same characteristic but are molecularly different.
Gene for coat color
Because each chromosome is paired with its homologous chromosome, another chromosome that contains genes for the same traits, there are two copies of each gene and locus in the genome that are not necessarily identical (their similarity is represented here by color). We will later see that this second “copy” of each gene is responsible for the vast variation seen among the coats of horses. In most cases, offspring receive copies of their parents loci and thus inherit traits from their parents.
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How do we study genes?
The molecular structure of a locus determines the physical appearance of its associated trait. The two copies of a locus may or may not be identical, and the variation in locus structure results in the physical variation of traits. Two single letters, either capital or lowercase, commonly represent the molecular structure of the locus. These letters are arbitrarily assigned by the scientist studying the locus. Each letter is called an allele and represents a specicic physical and molecular conciguration of the locus. Two different loci will have different letters (A and B, for example), but the two copies of a single locus will have the same letter and be written together (AA, aa, or Aa). Bb = Genotype: letters of the two loci that code for the same characteristic of the same trait.
A
A
D
d
b
b
K
K
w
W
Loci on homologous chromosomes that code for the same characteristics of a trait have the same letter, but sometimes have different cases. This concept was illustrated by the different shades of the same color in the diagram on the previous Loci on the same chromosome that code for different characteristics of the same trait have different letters.
Genes for coat color on homologous chromosomes.
Black coat
Phenotype: physical appearance coded for by the genotype.
The letters that represent loci are known together as a genotype. Every genotype has a corresponding phenotype, which is the physical appearance of the trait that results from the genotype’s molecular conciguration. Next, let’s use horse coat color as an example of genotype and phenotype.
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The Genetics of Horse Coat Color
One locus determines black and chestnut coat color. At the black and chestnut color locus, a B allele denotes black and a b allele denotes chestnut. The B allele is dominant, meaning that any genotype with this allele will produce the B phenotype. Both the BB and Bb genotypes produce a black coat. The b allele is recessive, meaning that it is masked by a dominant allele in a Bb genotype but is expressed in a bb genotype. The bb genotype, then, produces a chestnut coat. If you know the genotypes of two parents, you can predict the genotype and phenotype of their offspring.
BB =
Black coat
Bb =
Black coat
The B allele is dominant, so its phenotype is expressed, even when the b allele is present in the genotype.
bb =
Chestnut coat
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Genetic Tool: Punnett Square
A Punnett square is a simple genetic tool that can be used to demonstrate the possible offspring between two parents using their genotype. Let’s set up a square for a chestnut horse and a Bb black horse. One cross is done for you as an example.
Write the cirst allele for the Bb horse here.
Write the second allele for the Bb horse here.
B
Write the cirst allele for the bb horse here.
b Bb
Write the second allele for the bb horse here.
The crosses in the boxes represent the possible genotypes of the offspring from this mating. In this case, we see that statistically, half of the offspring of these parents over their breeding time will be black and half will be chestnut.
Genetic Tool: Punnett Square (cont.)
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Let’s see what happens when two Bb black horses breed. Fill in the following Punnett square:
Fill in the blanks:
______(fraction) of the offspring will be black, but two different genotypes are represented among the black offspring. Of the black offspring, 1/3 will be ______(genotype) and 2/3 will be _______(genotype). The remaining _______(fraction) will be chestnut.
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White Coats: Beautiful But Deadly
White Prince, a rare white Thoroughbred. Photo by Jim Shambhu. Come see White Prince at the Kentucky Horse Park!
Question: A white mare and white stallion mate and have a white foal that dies shortly after birth. Assuming that their next offspring lives, what are their chances of having a white foal? A colored foal? Create a Punnett square to check your answer. White coats have long been a desirable color for their beauty, but are also more rare among horses. Why? White coats present another different but interesting mechanism of genetic inheritance. More often than not, white coats are a result of a genetic mutation at a particular locus as the foal is developing. This mutation can then be passed to the offspring of white horses. They are controlled by a locus where the W allele is dominant and epistatic, meaning that when present, it will mask all other genotypes that incluence the same phenotypic trait. For example, if a horse has the B allele for being black but also possesses the W allele for being white, the horse will be white. The white homozygous genotype (both alleles are dominant, WW), however, is lethal, and all foals with this genotype die shortly after birth. Horses with the heterozygous genotype (one of each allele, Ww) have white coats and live normal, healthy lives. The homozygous recessive genotype(both alleles are recessive, ww) actually produces a colored coat, the color of which is controlled by alleles at other loci. White Coats: Beautiful But Deadly (cont.)
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Let’s observe the epistatic effects of the white locus by setting up a Punnett square between two heterozygous mates, WwBb. First, list the possible genotypes passed on by the parents by listing every possible combination of a single allele from each locus. For example, for a WwBB individual, the possible combinations are WB and wB. The number of squares within your Punnett square will depend upon the number of genotypic combinations of the two parents. Possible genotypes from parent 1:
Possible genotypes from parent 2:
Now set up the Punnett square for crosses between these two parents:
List the ratios and phenotypes of all the possible offspring between these two mates.
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Critical Thinking
In the absence of genetic mutation, two non-­‐white, non-­‐albino horses can never have a white foal. Why? Demonstrate your answer with a two-­‐loci Punnett square and a brief explanation.
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Vocabulary
Allele: Molecular conciguration of a locus; combination of at least two of these comprise a genotype
Chromosome: Present in pairs in animal cells; contain genes and genetic information
Coat color bias: Showing preference for a certain coat color
Dominant: Masks recessive alleles in a heterozygous genotype
Epistatic: Masks all other genotypes that incluence the same phenotype
Gene: Segment of a chromosome; contains information for a specicic trait or suite of traits
Genotype: Letters that represent the molecular conciguration of loci
Heterozygous: Have one dominant and one recessive allele at the same locus
Homologous: Two copies of a chromosome that contain genetic information about the same traits
Homozygous: Have either two dominant or two recessive alleles at a single locus
Inheritance: Mechanism through which a trait is passed directly from parents to offspring
Loci: Individual units of genes
Phenotype: Physical appearance of a trait
Punnett square: Genetic tool used to predict genotypes and phenotypes of offspring
Recessive: Masked by a dominant allele in a heterozygous genotype
Genetics Vocabulary
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ACROSS
DOWN
3 Letters that represent the molecular conciguration of loci
1 Molecular conciguration of a locus; combination of at least two of these comprise a genotype
8 Masks all other genotypes that incluence the same phenotype
2 Masks recessive alleles in a heterozygous genotype
11 Have one dominant and one recessive allele at the same locus
12 Showing preference for a certain coat color
13 Segment of a chromosome; contains information for a specicic trait or suite of traits
15 Mechanism through which a trait is passed directly from parents to offspring
4 Genetic tool used to predict genotypes and phenotypes of offspring
5 Have either two dominant or two recessive alleles at a single locus
6 Physical appearance of a trait
7 Masked by a dominant allele in a heterozygous genotype
9 Two copies of a chromosome that contain genetic information about the same traits
10 Present in pairs in animal cells; contain genes and genetic information
14 Individual units of genes