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.