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The genetics of Maine Coon colors

Chromosomes: the vectors of heredity

Heredity refers to the transfer of biological characteristics from a parent organism to offspring. DNA (short for deoxyribonucleic acid)--in the form of large molecules known as chromosomes--is the carrier of genetic instructions for the development and functioning of living organisms. The word chromosome comes from the Greek χρῶμα (chroma, meaning color) and σῶμα (soma, meaning body) due to its property of being stained very strongly by some dyes. Each chromosome contains many genes, or locatable regions along the chromosome that can be thought of as a "unit" of inheritance. The physical appearance of an organiss can be thought of as the product of genes interacting with each other and with the environment.

In living organisms, DNA usually exists as two long strands that are twisted together in the shape of a double helix, right. The backbone, which is made of sugars and phosphate groups, holds each strand together. Attached to the backbone of each strand are four different bases, known as adenine, cytosine, guanine and thymine. The DNA double helix is held together by hydrogen bonds between the bases on each strand, whereby adenine on one strand only bonds with thymine on the other strand, and cytosine on one only bonds with guanine on the other. It is the sequence of these four bases that encodes genetic information.

A domestic cat has 19 different chromosomes (humans have 23). Each occurs twice, for a total of 38. 19 are inherited from the father and 19 from the mother. Karyologists, or people who study chromosomes, have shown that the majority of felids possesss a similar number, which probably explains why it is possible to obtain hybrids between many of the species.

The gene pool of a population is the complete set of genetic material found among the living members of that population. Differences in the traits among individual organisms are due to genetic differences, or genetic variation. Generally, the larger the gene pool, the more genetic diversity (or variety in physical traits) within that population.

Ordinary cell division is essential for organisms to grow, and prior to a cell dividing into two, it must replicate (or make an exact copy of) its DNA, theoretically resulting in two identical cells where there was previously only one. The genetic code of the original cell must be precisely duplicated, so that both of the resulting cells will be identical, able to perform identical functions within the organism. Certain proteins are responsible for "reading" and "copying" the sequences of bases (adenine, cytosine, guanine and thymine) in the long strings of genetic instructions. Chromosomes must exist in duplicated form just prior to cell division, and the two copies are said to be joined by a centromere (2, right). Compaction of the duplicated chromosomes during cell division results in the classic four-arm structure. Each of the two DNA strands is referred to as a chromatid (1). Each chromatid has a long arm (4) and a short arm (3).

Over time, errors can occur during replication, causing a change in the sequence of the gene's bases known as a mutation. These mutations can then be inherited by future generations. Deletions or additions in the genetic code that result in changes in the proteins made by cells can alter how the cells function, and those functions can alter the physical appearance of the animal.

Mutations create variation in the gene pool by causing new traits to appear over time. Individuals possessing the most advantageous traits for survival are more likely to survive longer to reproduce than those with unfavorable traits. Favorable mutations thereby accumulate in the gene pool and become more common in later generations, resulting in evolutionary change (Darwin's theory of evolution). Conversely, mutations deleterious to survival are removed from the gene pool. This process by which favorable inherited traits propagate throughout a reproductive population is known as natural selection.

Isolating defective genes has led to DNA tests that can detect the presence of cetain of these undesirable genes. For diseases for which DNA testing is available, testing allows breeders to screen for defective genes, with the potential to eradicate disease as cats with unhealthy traits are excluded from the breeding population.


Basic principles of heredity

The modern science of genetics, which seeks to understand the process of inheritance, began with the work of Gregor Mendel in the mid-nineteenth century. The first to apply mathematics to a biological problem, Mendel observed that organisms inherit traits in a discrete manner—through basic units of inheritance that we now call genes.

Remember, cats have 19 pairs of chromosomes. The matching chromosomes in a pair are called homologous chromosomes. They contain the genetic information for the same biological features. Another way of saying that is to say, cats have two copies of each gene. Each of those 19 different chromosomes in a cat are shown right. Then why, you ask, are there 20 chromosomes pictured? You may remember from biology class that females have two X chromosomes and males have an X and a Y chromosome, which determine gender. Those chromosomes constitute one pair of sex chromosomes. The other 18 chromosomes in a cat are the same in both sexes and are referred to as autosomal (non-sex) chromosomes.

Each gene is positioned along a chromosome in a fixed position known as its locus (the plural is loci). The process of determining the locus (or chromosomal location) for a particular biological trait is known as gene mapping.

Each homologous autosomal chromosome in a pair contains the same genes at the same loci. So to say it another way, if you look at two homologous chromosomes in a pair, they will both have a gene that affects the same physical traits at the same location along those two chromosomes. As an example, there is a gene affecting coat color called the brown gene at the same position along two of a cat's chromosomes.

The variation in appearance among individual cats is increased by the possible variations in the DNA sequence that can occur at a given gene locus, called alleles. For example, the brown gene on each homologous chromosome may carry different alleles or the same allele. To carry our example further, there are three possible alleles that may appear at this brown gene, which geneticists have named B, b, or b1. So, since a cat has two copies of every gene, an individual cat may have two of the same allele on both of its brown genes, as in BB cats, or it may have two different alleles, such as in Bb cats.

An individual is said to be homozygous for a given trait if, at a specific locus on both homologous chromosome, the alleles that control that trait are the same.
If the alleles are different at the two loci, the individual is said to be heterozygous for the affected trait.

These concepts are important to understanding heredity, because it is the relationship between the effects of two or more alleles that ultimately determines appearance. Sometimes cats with different genetic makeups appear different and sometimes they appear the same. A cat with two B alleles will be a different color than a cat with two b alleles or two b1 alleles. Yet, a BB cat and a Bb cat will look the same despite different genetic makeups, while a bb cat and a b1b1 cat will be different colors. How do we explain this?

Let's consider a specific trait and examine these concepts further... We know tabby Maine Coons exhibit two different patterns, mackeral and classic. The allele responsible for the mackerel pattern (right) is designated by the letters "Mc", and the allele for the classic pattern by the letters "mc". Since each cat has a pair of chromosomes (one inherited from its mother and one from its father), offspring will possess one of three possible variations in their genetic code affecting pattern: McMc, Mcmc or mcmc. The use of the capital letter "M" to designate the allele responsible for the mackerel pattern reflects the fact that the mackerel-pattern allele MC is dominent over the classic-pattern allele mc. If the Mc allele is present on one or both loci of homologous chromosomes, the cat will exhibit a mackerel pattern. The classic-pattern allele is considered recessive, since it must be present on both homologous chromosomes in order for the cat to have a classic pattern. In most cases a dominance relationship is seen when the gene in its dominant form encodes an enzyme, and its recessive counterpart does not. In many cases, a normal function can be maintained with only half the amount of the enzyme. In these cases a single copy of the dominant allele produces enough of the gene’s product to give the same effect as two normal copies.

The inheritance of tabby striping over three generations can be represented as in the chart, right, where "red" symbols represent a cat possessing the Mc allele. Not all offspring of a mackerel tabby will necessarily be a mackerel tabby, even though the mackerel allele is dominant. A classical-pattern tabby cat carries the alleles mcmc, whereas a mackerel-pattern tabby is either McMc (homozygous for the mackerel pattern) or Mcmc (heterozygous for the mackerel pattern). Intuitively you may believe that the mackerel pattern would be more common, and you would be correct, statistically speaking. Yet by understanding how traits are inherited, cat breeders prefering one look over another, say that of a classic tabby over the mackerel tabby, can choose which cats to breed based in part on their tabby striping, thereby increasing the chances of producing classic tabby offspring (a process known as selective breeding).

So getting back to the genetics, here is a very important genetic rule:

A dominant allele (like the Mc allele for the mackerel pattern) will be expressed even if the gene is heterozygous (Mcmc, when you consider both alleles on homologous chromosomes). Recessive alleles (mc) need to be homozygous (mcmc) to be expressed.

Now, it is try that most traits are affected by many genes (remember the discussion of polygenic rufousing when exploring tabby striping on the previous page, where multiple genes affect the nuance shades of color among tabbies with stripes?), most genes are involved in the development of multiple traits, and there are relationships referred to as incomplete or partial dominance, which produce an intermediate or blended look in a trait. However, an understanding of the basic concept of dominance helps us to predict possible genetic outcomes for many traits, such as that of tabby stripes.

Now the fun part. Let's examine how this knowledge applies to color inheritance in Maine Coon cats as we explore the genetics of color.


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