Heredity and Basic Genetic Terms Used in Coat Color Genetics of Cats
Goals of the Series of Blogs on Coat Colors Genetics of RagaMuffins
To enable breeders to gain a basic understanding of:
- Genetic Terms
- Sex-Linked Traits
To assist in the prediction of potential color/pattern outcomes of litter.
Basic Genetic Terms
Homozygous vs. Heterozygous
Dominant and Recessive Alleles in Cat Coat Color Genetics
Every cell contains a nucleus. Within each nucleus of every cell
Each cell of a cat contains 19 pairs of chromosomes.
The sire and the dam each contributes one chromosome to each of the 19 pairs of chromosomes so that the offspring get half of their chromosomes from their dam and a half from their sire.
On each chromosome are genes, that are arranged like beads on a string. The position of a gene on the chromosome is called its locus.
Coat Color Genetics of Cats and Basics and Heredity
Genes mutate and can take two or more alternative forms. An allele is one of these forms of a gene and usually results in a new appearance (called a phenotype). Since chromosomes appear in pairs, cats have two alleles for each gene.
We already discussed that one chromosome in the pair comes from the dam and the second chromosome in the second chromosome in the pair comes from the sire. As a result, every kitten inherits one allele from each parent for each gene.
Alleles and Genes Further Explained for Cat Coat Color Genetics
Refers to a specific variation of a gene A section of DNA that controls a specific trait
Ex: Pointed cats, black cats, blue cats Ex: Color restriction, coat color, the tabby pattern
pattern of tabby
Since a kitten inherits one allele from each parent, the two alleles may be the same (called homozygotes) or the two alleles may be different (called heterozygotes).
Use of Letter Symbols in Cat Coat Color Genetics
Italicized letters are used to symbolize genes for certain characteristics. At first, it may seem confusing, but if you continue to use the italicized letters, this will assist you in getting a better grasp of the genetic concepts.
A cat has two alleles for each trait (one from the sire and one from the dam), so you will use two letters to describe the genotype for a particular trait. One letter will indicate the allele the kitten inherited from the sire and the other letter will indicate the allele the kitten inherited from the dam.
If an allele is dominant, the letter symbol for such allele is CAPITALIZED. Recessive and co-dominant alleles are lower-case.
Polygenes are genes that each have a small quantitative effect on major genes and the way the cat looks. The effect of polygenes is cumulative on coat color genetics.
An example of polygenes influencing a major gene is the color of a red cat.
Both of the cats above have the same major allele – O – which is responsible for their red coats. The influence of polygenes is what is responsible for the variation in the color of the red coat.
Polygenes can cause variations in how major genes are expressed in the color of the cat’s coat. Such an effect can be cumulative which will result in wide variations. Examples of the effect of polygenes can be found on the cats and kittens of the breeders of The RagaMuffin Kitten Breeder’s Society.
Of the 19 pairs of chromosomes, the cat has, one pair is related to the gender of the cat. As with all mammals, the female has two Y chromosomes and the male has one X chromosome and one Y chromosome.
Each female egg contains one X chromosome, while each male sperm may carry either one X chromosome or one Y chromosome.
Phenotype and Genotype
The outward appearance of a cat is called its phenotype. For example, both cats below have the phenotype of short-haired black cats. In other words, they both look like short-haired black cats.
The phenotype of a cat (what it looks like) only tells us part of the story of what cat coat color kittens that cat can produce.
The genotype of a cat is the set of alleles a cat has inherited from each of its parents.
Even though two cats have identical phenotypes, they could have different genotypes. The genotype of the sire and the dam will determine the possible coat color of the kittens.
If a cat is homozygous for a particular trait, this simply means that the two alleles the cat has inherited from its sire and dam for such trait are the same.
If a cat is heterozygous for a particular trait, this simply means that the two alleles the cat has inherited from its sire and dam for such trait are different.
The sire is homozygous for cs and the dam is
homozygous for C; all offspring are heterozygous with genotypes C, cs.
Dominant and Recessive Alleles and Effect on Coat Color Genetics
If an allele is dominant, that means that all the cat needs is one copy of the allele for the trait to be expressed (the phenotype will reflect the dominant allele).
For example, the Agouti gene (A) is the gene that “turns on” the tabby genes and makes the phenotype of the cat (what it looks like), a tabby. Another possible allele at the Agouti locus is a (non-Agouti). The Punnett square below illustrates this concept.
Sire’s genotype (A, a)
Dam’s genotype (A, a)
If a RagaMuffin cat carries one dominant allele and one recessive allele, the dominant allele is in control and the recessive allele is not operative. Therefore the dominant allele will determine the phenotype of the kittens. For example, a cat carrying one A allele and one a allele (non-agouti) is heterozygous for the Agouti trait since the two alleles it carries for that trait are different.
Genotype A, a: a, the recessive allele, is not operative in this cat’s phenotype. The dominant A allele is in control. The recessive allele is carried and only becomes operative and effects the phenotype when the cat is homozygous for the particular recessive allele. For example, as seen in the chart above, if a cat is a, a for the Agouti trait (homozygous for the non-Agouti alleles), this would mean the cat’s phenotype (what is looks like) is not a tabby since the Agouti allele is required to “turn on” the tabby genes that cat has but is not expressing.
Genotype a, a: Since the cat has two copies of the recessive allele for the Agouti trait, the recessive non-Agouti (non-tabby) alleles are expressed in the phenotype.
Importance of the Concept of Dominant and Recessive Alleles
The concept of dominant and recessive alleles is critical to understanding probable outcomes in RagaMuffin coat colors for a litter.
If a cat is expressing a phenotype that results from recessive alleles you know that the cat MUST have two copies of the recessive allele for the trait. The only way for a cat to express a recessive phenotype is for it to have two copies of the recessive allele. If it has a copy of the dominant allele, the recessive allele will not be expressed. Therefore a non-tabby cat MUST have the genotype a, a. It cannot have an A allele.
Each of the tabby cats below has the same phenotype. In other words, they both look like brown mackerel tabby cats.
However, since the Agouti allele, which allows the tabby genes to be expressed, is a dominant allele, a cat only needs one copy of the A allele to have a tabby phenotype.
That means that the brown tabby cats above could have either of the following genotypes for the Agouti gene:
A, A or A, a. Both of these pairings of alleles will produce a tabby cat because A is the dominant allele, so only one copy is needed and the cat will be a tabby.
(A, A) (A, a)
Cats That Have Dominant Phenotypes
Whenever you look at a cat that is “expressing a dominant allele” (has a different phenotype), you must remember the possibility that the cat may carry the recessive allele for that particular trait. Above, one of the silver tabby cats had an A, a genotype (carried the recessive (non-Agouti) allele).
Genotype: A, _ (we do not know what the second allele is, A or a. From the cat’s phenotype, all we know is that the cat must have at least one A allele because it is a tabby cat.
The two silver tabbies in the photos above could both be carrying the a (non-agouti) allele. This is important to keep in mind because if bred together, both of them could pass the a allele to the offspring, as seen in the Punnett square above. So two tabby cats can have a non-tabby cat. Any offspring with the genotype a, a will not be tabbies.
Using this logic, if you were to breed two tabbies together and you got any non-tabby offspring, the genotype with respect to the Agouti trait for both the sire and the dam must be A, a. The reason is in order for two tabby cats (who must have at least one A allele) to have a non-tabby offspring, the offspring’s genotype MUST be a, a. This means the offspring got an a allele from each of its parents. Since we know each of the parents MUST have at least one A allele and one a allele, we know that the genotype for each of them is A, a.
This is some basic information about cat coat color genetics. We will do blogs in the future covering additional aspects of this complicated subject, so stay tuned!
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