Genetics and Inheritance

Introduction

Genes in the Syrian Hamster

Simple Inheritance

Complex Inheritance

Sex Linkage

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Introduction

Like all other mammals, including human beings, a hamster's  genotype, (genetic makeup), controls much of its phenotype, (what it looks like).  The genes that make up each animal's genotype are inherited from its parents in their eggs or sperm.  In the same way, a hamster will pass these genes on in its own sex cells.  Within the nucleus of the hamster's  cells the genes are located on chromosomes.  These are tightly  coiled lengths of  DNA.  The chromosomes are  in pairs; one of each pair was inherited from each parent.  Apart from a special pair of chromosomes, (the sex chromosomes), the  chromosomes in each pair look the same.  They have the same "slots", (loci, singular, locus)   for the genes coding for   particular characteristics.  The "options" for a gene for one particular characteristic are called alleles.  For example, the allele rx codes for rex fur.  The other allele that can be found at the rex locus is Rx, which codes for non-rex, (normal), fur.   Each locus can contain only one of the possible alleles for a given gene, but the alleles for the same gene on the chromosomes in a  pair can be the same as each other or different.

As the hamster grows its cells replicate themselves by cell division.  Each pair of chromosomes is copied, (so that the cell contains two copies of each pair), and then the cell divides in two.  Each half has one copy of each chromosome, still in their pairs.   When a hamster makes  sex cells, however, each one must receive only one of each pair of chromosomes.  That way, when it joins with a gamete from the hamster's mate, the resulting fertilized egg will have two of each chromosome, i.e., a pair.  Without this "reduction division", (meosis), each generation of hamsters would have twice as many chromosomes as the generation before.

To recap. in each of the hamster's body cells are chromosomes,  in pairs.  At intervals along the length of each chromosome are loci   for the various genes on that chromosome.  Each gene codes for a different characteristic, e.g., eye colour.  The hamster's gametes contain only one chromosome from each pair and so only one copy of each gene on that chromosome.

Mutations occur when the genetic code is copied wrongly at a gene locus.  Instead of giving the correct instructions, the code may now cause something to be altered in the development of that feature.   Mutations may be hard to detect, especially if the changes that they cause are to do with the way the animal's body chemistry works.   Most mutations that are studied in the Syrian and other hamsters are to do with coat colour, type and pattern.

Wild Syrian hamsters are always golden coloured and short haired.  Any mutations that changes this tend to make the animal more conspicuous and thus less likely to survive to breed.  Mutant alleles for coat colour/ type tend not to survive in the wild; the natural allele is more successful.  For this reason, the allele at each locus that is not a mutation is known as "wild type".  In captivity, however,  mutations can be propagated and selected for, instead of the wild type allele.

Some alleles for a given gene only affect the animal's appearence when they are present on both chromosomes, i.e. the hamster has a copy from each parent.  These are called recessive genes/ alleles.   The abbreviated code for such alleles is in lowercase.  Others show their effect even if the hamster has only one copy, (i.e. it doesn't matter what the allele on the other chromosome is).  These are called dominant genes/ alleles and are designated with a code letter in uppercase.  For a recessive mutation like Dark Grey the mutant allele, (dg), is recessive and the wild type dominant, (Dg).  For a dominant mutation like Silver Grey the situation is reversed; the mutant allele, (Sg), is dominant and the wild type recessive, (sg).

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Common Mutations in the Syrian Hamster

Below is a list of common coat mutations in Syrian Hamsters and the code for the mutant alleles.

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Examples of Simple Inheritance

First cross

Further Crosses

First Cross

If a pure breeding Cinnamon male is mated to pure breeding Golden female, the resulting pups are all golden. How does this come about?

The Cinnamon allele, (p),  is recessive to its wild type counterpart, (P).  Since both animals are pure breeding, (homozygous), each has two copies of only one allele at the Cinnamon locus.  The Cinnamon hamster will have two cinnamon alleles, listed as pp.  The Golden hamster will have two wild type alleles, (PP).

When the Golden female makes eggs she will "send" one chromosome of each pair to each egg.  Since she has the allele P at both of her cinnamon loci, all of her eggs will receive a chromosome with the allele P  on it.  By a similar deduction, all the Cinnamon male's sperm cells will contain only  the allele p.   When these gametes fuse to form an embryo hamster the genetic makeup that it will inherit is Pp. 

The wild type allele is dominant, so this is what is expressed in the hamster's phenotype.  This generation from the first crossing are called the First Filial generation, (F1).  It is important to note that the young F1 hamsters, for all they  resemble their mother, are not homozygous, like her.  They have two different alleles at their cinnamon locus.  Breeders describe these hamsters as "carrying Cinnamon", or "being split for Cinnamon".  Although it cannot be seen the Cinnamon allele, (p), is in the makeup of these hamsters, inherited from their father.

Let us imagine that from the litter of young hamsters, a  male and a female are kept and bred from.

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Further Crosses

F1 Brother - Sister

Recessive parent, (Cinnamon) - F1 Offspring

(Please note; these matings do not necessarily have to be incestuous;  all that is necessary would be that the hamsters in the pairings had the appropriate genotype.  Also, the ratios of each colour expected are just that; the ratios expected.  Individual results will vary, but over many such matings the ratios will hold true).

Brother to Sister, (F1 self cross, carrier to carrier)

If a brother and sister are mated together then there are both Cinnamon and Golden coat colours in the Second Filial, (F2), litter. How does this occur?

When the heterozygous young male makes his sperm cells, half will contain the chromosome with the p allele from his father, the  other half the P bearing one from his mother.   The same will be true of the eggs that his sister makes.

When these gametes join together the following crosses can occur.

From this "Punnett Square" it is easy to see that the possible genotypes and ratios are;

• One PP youngster.  This will be a pure breeding Golden animal, like its grandmother.  It will, however, be impossible to tell it apart from its "Cinnamon carrying" littermates, except by breeding from it.
• Two Pp youngsters.  These have at one P allele and one p one.   The P allele is dominant, so they are Golden in appearance, not Cinnamon.  Like their parents they are "carrying Cinnamon"; with the correct mate they can produce Cinnamon young. 
• One pp youngster.  This has two p alleles and will thus be Cinnamon in colour.   Since it has only the p allele, it will be pure breeding for Cinnamon.

It can thus be seen that the expected ratio is three Golden young, (dominant phenotypes), to one Cinnamon, (recessive phenotype).

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Recessive parent to F1 offspring, (Recessive backcross)

If the Cinnamon father and his golden daughter are mated together then both coat colours appear in the litter about the same proportion. How does this happen?

As has been seen, all the male's sperm contain only allele p, while the young female's eggs can contain either P or p..

Crossing these together would give the following.

The possible genotypes are therefore;

• Two Pp youngsters.  These have a P allele and a p one.   They will thus be Goldens carrying Cinnamon, like their mother.
• Two pp youngsters.  These have two p alleles and will thus be Cinnamon in colour.

Therefore the expectation is that the dominant and recessive phenotypes will appear in roughly equal numbers, although in this case all the dominant coloured animals are carrying the recessive gene.

The same ratios would apply if the young male were to be mated back to his mother.  In this case it would be expected that half the litter would be of Pp genotype and the other half PP.  In other words, half would be goldens carrying Cinnamon and the others pure breeding goldens.  It would, however, be impossible to tell which were which by looking at them.

Recessive genes, inherited in the same way as has been described for Cinnamon, include; dark grey, cream, dark eared white, black, rust, ruby eyed fawn, piebald, rex and longhair.  Dominant genes, such as Umbrous, Banded or Dominant Spotting, are also inherited in this way, except that of course it is the wild type gene, the gene for not having banding or whatever, that is recessive and is "carried".

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Crosses involving two gene loci

If a pure breeding Cinnamon male is mated to a pure breeding Dark Grey female, the resulting litter are all goldens.  What is happening here?

The Cinnamon male is Cinnamon because he has two p alleles at his Cinnamon locus.  The Dark Grey female is likewise pure breeding for dg at her Dark Grey locus.  The male, however, has only wild type alleles, (Dg), at his Dark Grey locus and his mate has only wild type alleles at her Cinnamon locus.  The male will thus produce sperm with the genetic makeup "p Dg" and the female will make eggs with the makeup "P dg".  When these join to make the young hamsters all the youngsters will have the genotype "Pp Dgdg".  They have a wild type gene at the Cinnamon locus and so show do not show the effect of the Cinnamon allele, p.  In the same way, they do not show the effect of the Dark Grey allele, dg.   They therefore show no mutations in their coat colour - and so are the wild type Golden colour.  The really interesting situation comes when the youngsters are bred together.  They carry BOTH Cinnamon AND Dark Grey.  The sex cell that they can produce are therefore as follows; P Dg, P dg, p Dg and p dg.   When the young from this cross are mated together, the following genotypes occur.   (Remember, any animal with the genotype pp will show be Cinnamon  and any that has the genotype dgdg will be Dark Grey).

The results of this mating are Goldens, Cinnamons and Dark Greys, but a new colour, Lilac, is created by the "mixing" of the Cinnamon and Dark Grey genes.  The Lilac animal is pure breeding for both Cinnamon and Dark Grey.

Other coat colours created by combining two or more genes include;

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Sex Linkage

So far we have only looked at alleles that can be inherited equally from either parent.  This is because the pairs of chromosomes in the hamster's cells look the same.  This is not the case with the sex chromosomes.

Female mammals, including hamsters and humans, have two identical chromosomes, called X chromosomes.  All hamster eggs, therefore, contain one X chromosome.  Male mammals, however, have sex chromosomes that look different.  One of the pair is a normal X chromosome, the other is a shortened version of an X chromosome, called a Y chromosome.  Half of the sperm produced by a male hamster contain an X chromosome and half contain a Y.  If an egg is fertilised by an X bearing sperm an XX female results.  If the sperm was Y bearing, an XY male will be formed.  Since the sperrm are produced and released in equal numbers that ratio of males: females is 1:1.

Any gene that has its locus on the part of the X   chromosome that is missing on the Y chromosome express itself differently in males and females.  The main gene that fits this description in the Syrian hamster is Yellow, (To).  It is an unusual gene; instead of being simply either dominant or recessive it is incompletely dominant.   This means that, if an animal has a yellow, (To), and a wild type, (to), allele at the Yellow locus, it will show both.   The coat will be a mosaic of yellow and non yellow patches.  This can only happen in XX animals, (i.e., females), and is called "Tortoiseshell".   Females can thus be ToTo, (pure breeding Yellow), toto, (pure breeding wild type) or Toto, (Tortoiseshell - pure breeding for neither).  Males, having only one X chromosome, are written as To_, (Yellow) or to_, (wild type), and both are pure breeding for what they show..

A trick to avoiding getting overwhelmed when working out what to expect when mating animals with the Yellow gene is to follow this grid.

The Yellow gene combines with Cinnamon to produce Honey and with Dark Grey to produce Smoke Pearl.

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