The My Pet UK Genetics Guide

The Basics

Genetics is the science of heredity. Inheritance of colour, fur type and distinguishing characteristics are passed on from parent to young, generation after generation. They are passed onto the young animals from each parent equally, by means of genes.

To understand this, we need to look at DNA (deoxyribonucleic acid). Every cell of all living organisms contains DNA. It forms the blueprint used to build that organism and controls its distinguishing characteristics and features. DNA controls the appearance of the animal, and since DNA is passed from the parents to the offspring, they often look alike.

Sequence of bases

But what about the other half of the sequence? The bases present on this second half are determined by those on the first strand. A always pairs with T, and C always pairs with G. So, gap1 is T, forming an A-T bond. The next base is G. G always pairs with C, so gap2 is C, forming a G-C bond. Then we have another A on the first strand. Gap3 must also be T. C always bonds to G, so gap4 is a G. And so on, along the strand. The complementary sequence therefore runs: T C T G A A G C. We now have a complete section of DNA:

It is the bonds between the base pairs attached to each sugar/phosphate chain that hold the whole DNA molecule together.

How Is DNA Organised?

An organism's DNA is divided up into a number of separate sections called chromosomes. The number of chromosomes present depends on the animal: e.g. dogs have 78, cows have 60, humans have 46. The chromosomes are always arranged in pairs. For example, the 46 chromosomes in human cells are arranged in 23 matched pairs- one pair of sex chromosomes (X andY) and 22 other pairs known as autosomes. One chromosome from each of these matched pairs is inherited from the mother and the other from the father.

Each chromosome pair contains one chromosome from the father and one from the mother. So, in pair 1, the left-hand chromosome came from one parent and the right-hand from the other. We can't tell which one came from which parent by looking at this diagram. However, we can tell which chromosome came from which parent in the case of pair 23.

Determination Of Sex

Females are XX and males are XY. During gamete production, the chromosome content is halved in a process called meiosis. (When two gametes fuse, the chromosome number, n, returns to normal: 1/2n + 1/2 n = n). Every chromosome pair splits, with one chromosome going to one gamete, and the other going to a second gamete. All female gametes will contain one X chromosome, since females possess identical sex chromosomes in pair 23. But males have two different sex chromosomes, one X and one Y. Half of the male's gametes will contain one X chromosome, and half contain one Y chromosome.

For example, take the above image to represent one cell in the human male's body. This cell is about to undergo meiosis to produce male gametes. Each chromosome pair will split in half. For example, lets say all the left-hand chromosomes in each pair will go to one gamete, while all the right-hand chromosomes go to a second gamete. Each gamete will contain one chromosome from pair 1, one chromosome from pair 2, and so on all the way up to pair 23. So, one gamete will contain an X chromosome, and one will contain a Y chromosome, since pair 23 has also split. It is therefore the male that determines the sex of the offspring. If two X gametes fuse, its a girl, if an X and a Y fuse, its a boy. Therefore in the above image, the X chromosome of pair 23 came from the mother, and the Y came from the father.

Sex determination can be represented in a chart. The male and female chromosomes that are combining are highlighted in bold. The possible combinations are shown:----------------------------------------

The possible outcomes, as we know, are either XX or XY. The chart tells us the probability of each outcome. The probability of XX is 2/4, or 1/2. The probability of XY is also 2/4 or 1/2. This means there is a 50-50 chance for each sex, each sex has an equal chance of occurring.

Genes

Each chromosome contains several genes. A gene is a section of DNA which codes for a certain characteristic. For example, a chromosome may contain the gene for coat colour, or fur type, or eye colour. Which genes are passed on by the parents to the offspring, and which genes are displayed, depends on a number of factors.

Each gene is given a letter so that it can be identified. Lets use coat colour in gerbils as an example. There are many, many different genes for coat colour, but we will use Agouti and Albino. The Agouti gene is represented by "A", and the Albino gene by "c". If we mate a purebred Agouti male, with a pure-bred Albino female, what will the outcome be?

First, we have to determine each parent's genotype. This is simply the genes that are present in their genetic make-up. The Agouti male's genotype is AA. Why? He is an Agouti gerbil, and the gene for Agouti fur colour is represented by the letter A. And he is a pure-bred gerbil, so is homozygous for the Agouti gene, meaning that he possesses identical fur-colour genes, resulting in AA. So, his genotype is AA.

The Albino female's genotype is cc. She is an Albino gerbil, and the gene for Albino fur colour is represented by the letter c. She is also a pure-bred gerbil, so is homozygous for the Albino gene, resulting in a genotype of cc.

Here is a summary of the information we have gathered so far. Phenotype is just a word meaning the physical appearance of the parent, determined by its genotype.

We will determine the outcome of the mating using a chart identical to the one used before when explaining sex determination. The male and female gametes are highlighted in bold, with the possible outcomes below.

Here there is only one possible outcome, Ac. All the offspring will have the same genotype of Ac. They are heterozygous, meaning they have two different fur-colour genes.How can we find out what colour they will be? There are only two options, they will either be Agouti or they will be Albino. They can't be a mixture of both.

We need to look at gene dominance. One gene is "stronger" than the other, and this is the gene that will be expressed (determine the outcome of the particular characteristic). The "stronger" gene is called the dominant gene. When representing genes, the dominant gene always has a capital letter, and the "weaker" or recessive gene has a small letter. Here we have two genes, A and c. A is a capital letter, meaning that the Agouti gene A is dominant over the Albino gene, c. When both genes are present, as in the Ac offspring, it is the dominant A gene that will be expressed, meaning that all the Ac offspring are Agouti gerbils. They still carry the c gene, but it is not expressed, it is "hidden" and doesn't affect their fur colour. Their genotype is Ac, and their phenotype is Agouti.

The young Ac gerbils from this mating are not pure-bred. They are not homozygous for a fur-colour gene, they are heterozygous. They are cross-bred, a result of cross-breeding gerbils with different fur-colours. Mating two cross-bred gerbils will give a different outcome to that obtained by mating pure-bred gerbils:

This time there are three possible outcomes: AA, Ac and cc. There will be a mixture of Agouti and Albino offspring. 1/4 of the offspring will have the genotype AA, they will be homozygous for the Agouti gene and therefore will have Agouti fur. 2/4 or 1/2 of the offspring will have the genotype Ac, they will be heterozygous like their parents. Since A is dominant over c, they will also have Agouti fur. The final 1/4 of the offspring will have the genotype cc. Since the dominant A gene is not present, the c gene can be expressed and these gerbils will be Albino.

This may seem a surprising result - how could two Agouti gerbils have an Albino baby? It is only because both the parents were heterozygous and carried the "hidden" Albino gene, c, that this result could be obtained.

Test Crosses - Determining The Genotype

We have found that if an organism shows a recessive characteristic in its phenotype (e.g. Albino coat colour), we know its genotype must be homozygous for the recessive gene (e.g. cc). But if an organism shows the dominant characteristic (e.g. Agouti coat colour) we can't tell if it is homozygous (e.g. AA) or heterozygous (e.g. Ac) for that characteristic. Both genotypes would produce the same phenotype.

The simplest way to determine the animal's genotype is to perform a test cross.

Lets use an example. A cat breeder has a short-haired female cat. She knows that the allele for short hair, H, is dominant to the allele for long hair, h. (An allele is just a particular form of a gene. In this case, the gene for hair length has two alleles, one which codes for short hair and one which codes for long hair).

The short-haired cat could therefore be either homozygous, HH, or heterozygous, Hh, for the allele for short hair. The breeder can determine the genotype of her cat by mating it with a long-haired cat (which has the genotype hh). This is called a test cross.

If the short haired female cat was homozygous (HH):

There is only one possible outcome: all the kittens are heterozygous (Hh), and since H is dominant, all the kittens would have short hair.

If, however, the short haired cat was heterozygous (Hh), there would be a mixture of short and long-haired kittens:

This time there are two possible outcomes, Hh and hh. 1/2 the kittens will be Hh and have short hair, and the other 1/2 will be hh and have long hair.

To interpret the results: if the kittens are all short-haired, then the mother is homozygous and has the genotype HH. If there is at least one long-haired kitten in the litter, the mother is heterozygous, Hh.

Codominance

So far we have said that a heterozygous animal's phenotype is determined by the dominant gene (or allele), and the recessive gene (or allele) is "hidden" and has no effect. But sometimes, both alleles of a gene have an effect on the animal's phenotype when present together. In this case, the animal has characteristics half way between the characteristics of the homozygous ones. Alleles which behave in this manner are said to be codominant.

For example, in some breeds of cattle two of the alleles for coat colour are codominant. When representing a codominant allele, a capital letter is used for the gene, and then two different superscripts represent the different alleles. In this case, "C" represents the coat colour gene. "CR" and "CW" are the symbols we will use for the alleles for red and white fur colour. The possible combinations are listed below:

Roan is a pink colour produced by a mixture of red and white hairs. To put this idea into practice, what would happen if we mated a roan cow with a roan bull?

There are three possible outcomes. 1/4 of the calves will be CRCR, and have a red coat. 2/4 or 1/2 of the calves will be CRCW, and have a roan coat. The final 1/4 of the calves will be CWCW, and have a white coat.

Multiple Alleles

Some genes have more than two possible alleles. A good example is the gene which determines your blood group. There are four different blood groups : A, B, AB and O. The symbol for the blood-group gene is "I". This gene has three alleles: IA (blood group A) IB (blood group B) and Io (blood group O). IA and IB are codominant, while Io is recessive to the other two. The possible genotypes are listed below:

A mother may be surprised that her baby has blood group O, when hers is A and the father's is B. The genotype for group O is IoIo, so the baby must have received an Io from each parent. The mother must have the genotype IAIo and the father must have the genotype IBIo.

1/4 of the children will be AB, 1/4 will be A, 1/4 will be B and 1/4 will be O. All blood groups are possible.

The end!

This guide was written by me, Charlotte Owen. It's my work, please don't copy it. Thanks.

A Closer Look At DNA

DNA is arranged in a double-helix, like a spiral staircase. Each piece consists of two long sugar/phosphate strands, which make up the backbone of the DNA molecule (these would be the rails of the staircase) connected to each other by pairs of organic bases (the rungs).

There are four different organic bases present in DNA : Adenine (A), Thymine (T), Cytosine (C) and Guanine (G). These bases make up sequences along the DNA strand. The diagram below shows a DNA strand with one side of its sequence of bases:

Here you can see all 46 chromosomes of a human male. But wait. How can you tell it is a human male?

Firstly, the number of chromosomes tells you the species. Here there are 23 pairs, making a total of 46 chromosomes. This means its a human. To tell the sex of the human in question, you need to look at the last pair of chromosomes, pair 23. These are the sex chromosomes. Females have two X chromosomes, while males have an X and a Y. Here you can see that pair 23 is XY. It's a boy!