Gerbil Genetics ~ Section 3

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Mendel's Laws

To understand the concept of how certain coat colours are passed on from generation to generation, and how some colours may skip a generation, it is worth understanding how traits are inherited, and by what laws these traits adhere to.

Mendelian traits

These are traits that are controlled by a single locus and will show a simple and predictable pattern of inheritance. A locus (Plural: loci) is a specific site or position on a chromosome that is occupied by a specific gene.  In our case with gerbils, a mutation in a single coat colour gene can cause the colour to be inherited according to Mendel's laws.

Mendelian Inheritance

These laws were derived by Gregor Mendel, a 19th century Austrian monk, when conducting his experiments with pea plants and their hybrids. In the years between 1856 and 1863 he grew and tested somewhere in the region of 28,000 plants. These experiments resulted in two basic generalisations which were later to become known as Mendel's laws of heredity, which were described in his essay on plant hybridization. Mendel's results were largely left neglected until much later when they were rediscovered around 1900, and were initially very controversial. In 1915 they were integrated with the chromosome theory of inheritance by Thomas Hunt Morgan, an American geneticist and embryologist and they went on to become the core of classical genetics.

Mendel's first law

This is essentially in four parts and they are known as the laws of segregation.

1) Alternative versions of genes account for variations in inherited characteristics.

This is the idea of alleles. Alleles are viable pieces of DNA coding that occupy a point on a given locus, and are usually the sequences that code for a gene. In diploid organisms such as mammals, they have two copies of each chromosome, and two alleles make up each individuals genotype. In our case with gerbils, it is the genes that control fur colour that we are interested in, and there may be several different versions, or alleles of these genes. The resulting colour of the fur of an individual gerbil will all depend on which two alleles it possesses for the gene and how the two alleles interact.

2) For each characteristic, an organism will inherit two alleles, one from each parent.

This means that when somatic cells (the cells of the body) are produced from these two alleles, that one allele will come from the mother, and the other from the father. These alleles can be the same, true breeding or homozygous, for example in gerbils, 'AA' will be homozygous, or these alleles can be mixed such as 'Aa' for hybrids or heterozygous individuals.

Zygosity

In genetics zygosity refers to the genetic condition of the zygote, and is referring to the similarity or dissimilarity of DNA between similar chromosomes at a specific allelic position or gene.

Homozygous

This means that the alleles at the same locus on a pair of similar chromosomes that are paired during meiosis are identical. It also refers to a genotype that consists of two identical alleles of a gene for a certain trait. So for example, in gerbils an individual could be either homozygous dominant 'AA' or homozygous recessive 'aa'.

Both 'AA' and 'aa' gerbils are true breeding and are known as homozygous.  This means they only have one type of gene at that locus, so this gene is the only one that can be inherited by their offspring. To demonstrate this we can cross two homozygous 'AA' Golden agouti gerbils together. This simple breeding demo below will show us the outcome of such a cross.

In gerbil offspring we also know that each pup receives one copy of each gene from both its mother and father at random. Now if two black parents have offspring they only have the non-agouti or black mutation at the A locus to pass on, the offspring can only receive 'a' from both parents, so they can only be black. We can see this working in this simple breeding demonstration with two 'aa' agouti gerbils.

Heterozygous

This means that the genotype consists of two different versions of the alleles of a gene for a particular trait. For example in gerbils, one of the copies of the gene may be the normal version, the other the mutation (Aa) Individuals who are heterozygous for a trait are referred to as heterozygotes.

'Aa' type gerbils are referred to as being heterozygous, which means they have two different types of genes at that particular locus, one being dominant or the normal version of the gene, the other is recessive and is the mutation. This type of genetic combination would occur if we mated two gerbils that had differing genes at the same locus.

To demonstrate this simply, if we use the two gerbil colours Golden Agouti and Black and breed them together, the offspring will receive one of their genes from each parent, so although the offspring may all look the same and be Golden Agouti, they will have different genes to their homozygous agouti parents. We can see this in the demo below.

Hemizygous

This means that there is only a single copy of the gene in an otherwise diploid organism. For example, in gerbils we can see this in the sex chromosomes in a male. Since there is only one copy of either the X or Y chromosome in the germ cell, the term homozygous or heterozygous are inappropriate.

3) If the two alleles differ, then A) the allele that encodes the dominant trait is fully expressed, and B) the allele encoding for the recessive trait will be masked by the dominant trait and will have no noticeable effect on the organism's appearance.

This essentially means that only the dominant trait is seen in the phenotype of the animal. This effect then allows the recessive traits, or the recessive alleles, to be passed down to the offspring even if they are not expressed in the first generation (F1 or filial generation)

Dominant and recessive genes

Each gerbil inherits a copy of each chromosome from its mother and a copy from its father. The parents therefore pass on one of each pair of these genes to their offspring (in our case the genes that create the characteristics of the coat colour).  As we now know, at each locus there may also exist alternative genes, these alternative genes we also know are named alleles. The individual loci and genes that give gerbils their coat colours are denoted letters to indicate a certain characteristic. Dominant characteristics are given a capital letter and recessive characteristics are allocated a lower case letter.

We also now know that each gerbil has two copies of each gene. In the above cases the gene that controls agouti patterning is given the symbol 'A', and the recessive mutation that gives us black, or non-agouti is given 'a'.

A- Agouti

a- non-agouti (Black)

So our gerbil at the A locus can either be 'AA' (homozygous dominant), 'Aa' (heterozygous) or 'aa' (homozygous recessive). When it has either 'AA' or 'Aa' the gene still has the functioning protein to create the banding in the gerbils coat, but if the gerbil is aa the protein is non functional, so our gerbil will be black and will not have the agouti pattern. This demonstrates how simple dominance works.

Note that in the simple demonstrations shown we will only be dealing with four offspring. This is mainly to show that when dealing with simple inheritance of a single characteristic that the offspring will have a probability of inheriting the genes in the ratios that are given. However in true breeding terms, do not expect the colours to appear like this in every litter as this is also governed by the rules of probability, and like the flip of a coin, where you can never be sure of which side it that it lands on, this also holds true for each gerbil colour that is produced in a litter.

However it should be noted here that not every trait is simply dominant or recessive, and some gene traits exist in organisms which are either incompletely dominant or they can also be co-dominant.

4) The two alleles for each characteristic will segregate during gamete production

This means that each gamete will contain just one allele for each gene and this will allow the alleles from the mother and father to be combined in their offspring, and this ensures natural variation. It should be noted here that it is often thought that it is the gene itself that is dominant, recessive, and incompletely dominant or co-dominant, where in actual fact it is the trait or the gene product that the particular allele encodes that is either dominant or recessive, etc.

Before we venture any further it's probably time to introduce a tool that enables us to track and understand the gene interactions of these crosses that we are going to make. It is quite straightforward and easy to use, and is known as the Punnett square.

The Punnett square and the probability of inheritance

The Punnett square is a predictive tool that is still used today which helps determine the mathematical probability of offspring in crosses having a particular genotype. This concept was designed by Reginald Punnett, a British geneticist who helped introduce genetics to the public with his published work Mendelism in 1905. He also co-founded, along with William Bateson the Journal of genetics, the scientific journal of genetics and evolution in 1910.

The diagram helps determine the outcomes of crosses by graphically comparing all the possible combination of alleles from the mother with those of the father. To use this method, all you have to do is to create a table, where at the top you write the genotype of one parent, then down the side the genotype of the other parent. You then record for each of the parents, all the possible combinations of their genes that the animal or plant can pass on. So using our purebred cross for an example, a gerbil that is 'AA' can pass just 'A' to its offspring and a gerbil that is 'aa' can only pass on 'a' to its offspring, but both have two copies of the gene so it is noted down twice as it helps us define the ratios and probabilities.

As you can see by the example below, you have four boxes where you can fill in all the genetic combinations, and to complete the table you cross reference the relevant genes from both parents. So in our example in the top left hand box we will enter 'A' from the mother, and 'a' from the father which gives us the combination 'Aa'. We know that the dominant gene masks the effect of the recessive one, so this produces a golden agouti, and we can also include this information as well. We can carry on and fill in the next box in the top right hand corner and write 'Aa', which we again know is Golden Agouti, so can write this in too, etc. we then carry on cross referencing for the two lower boxes.

Monohybrid cross

A monohybrid cross typically compares just one trait and is a type of cross between two individuals who are identically heterozygous at one particular locus, for example if we again stick to the Agouti locus in gerbils, it would be a cross that is Aa X Aa, and monohybrid inheritance, is the inheritance of a single characteristic, and the different types of the characteristic are controlled by the differing alleles of the same gene.

Filials

When we are examining diagrams that show us the relationships between the families over the generations, you may see them being labeled as P1, then F1, F2, etc. Quite simply, P1 means the parents, or the parent generation, and then each successive generation is labeled with the F prefix. F is shorthand for filial which literally means "having or assuming the relation of a child or offspring" When Mendel conducted his pea experiment he noticed the temporary loss of a variation, in the first generation (F1) of offspring when breeding two purebred varieties together. However in the following generation (F2) the variety appeared again in the 3:1 ratio. Two of the offspring would be heterozygous, one would be homozygous dominant, and the other would be homozygous recessive, this being the purebred variety that disappeared in the F1 generation. We can see the same effect happening when we examine a cross at the Agouti locus with gerbils.

In the case of our Agouti locus in gerbils we know that there are two alleles, one for agouti or 'A' which is dominant, and the other for non-agouti or black, which is 'a'. So for example if we crossed two pure breeding strains of this gene, 'AA' for golden agouti with 'aa' which is black, we know that 'A' is dominant to 'a', so this will mask the black characteristic, so all the offspring in the F1 generation will be Golden agouti, but they will also be heterozygous, that being 'Aa'. For such monohybrid crosses, the parent genotypes are homozygous, and their offspring, or the F1 generation will all be heterozygous. We saw this earlier in our first example using the Punnett square.

So what would happen if we mated two of our heterozygous golden agoutis together?

Again we can plot this out using the Punnett Square below;

As you can see we have a ratio of three golden agoutis to one black gerbil, or a 3:1 phenotypic ratio, but we also have a 1:2:1 genotypic ratio in our example above. The golden agoutis are a combination of both heterozygous and homozygous individuals. While it holds true that this type of cross will average out in a 3:1 ratio over time, we cannot expect the colours to appear like this in every litter, because as we mentioned earlier, the colours appearing are also governed by the rules of probability.

In general, monohybrid crosses are used to determine the F2 generation from a pair of homozygous grandparents, usually one being dominant, the other recessive. The pairing of these F1 offspring gives us the F2 generation, with a 75% chance for the dominant phenotype to appear and a 25% chance for the recessive phenotype to appear.

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