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Genetics and Inheritance

Introduction
Genes in the Syrian Hamster
Simple Inheritance
Complex Inheritance
Sex Linkage
Introduction

Like all animals, the genes that a hamster inherits from its parents, (the genotype), affect the way it looks.  The animal receives genetic information from its parents in their eggs or sperm and, in the same way, passes the information on to its own offspring.  Inside the hamster's  cells are the chromosomes, which are tightly  coiled lengths of  DNA.  The chromosomes are paired; one of each pair was inherited from each parent.  Chromosomes within a pair have the same "slots"  for genes coding for particular characteristics in the same order.  The "options" for a gene for one particular slot, (locus), are called alleles.  For example, the allele rx codes for rex fur, but the allele Rx, (normal fur), may be found at the rex locus instead.   Each locus can contain only one of the possible alleles for a given gene, but the alleles for the same gene on the two chromosomes in a  pair can be the same as each other or different.

As the hamster grows its cells make copies of 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 new cell has one copy of each chromosome, pair.   When a hamster makes  gametes, (eggs or sperm), however, each one must receive one chromosome from each pair of chromosomes. That way, when it joins with a gamete from the hamster's mate, the resulting youngster will have two of each chromosome, i.e., a pair. This is acheived by a normal cell division, followed immediately by each cell dividing in two again. The chromosome pairs have not copied themselves again, so one chromosome from each pair goes to each new cell.

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

Mutations are changes in the genetic code. Mutations at a gene locus may cause something to be altered in the development of the characteristics controlled by that gene. Most mutations that are studied in hamsters affect coat colour, type and pattern.

Wild Syrian hamsters are golden coloured and short haired.  Changing this would make the animal less well adapted to its environment. For example, a white hamster would be more conspicuous to predators; it would therefore be more likely than normal coloured hamsters to be eaten and less likely 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,  hamsters with unusual characteristics can be protected and their mutations preserved by breeding.

Some alleles only alter the animal's appearence when they are present on both chromosomes .  These are called recessive genes/ alleles.  To show the effect of a recessive gene, the hamster has to receive a copy from each parent. The abbreviated code for recessive 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 dominant genes/ alleles and are given 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).

Common Mutations in the Syrian Hamster

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

Code

Name of mutation

a

Melanistic black

b Rust, (Guinea Gold)
Ba Banded, (White Banded)
cd Acromelanic gene - "Dark Eared White"
dg Dark Grey
Ds Dominant Spotting
e Cream
Lg Light Grey, (Lethal Grey)
l Longhaired/ Angora/ Teddy Bear
p Cinnamon
ru Ruby Eyed Fawn
rx Rex coated
s Spotting, (Piebald)
Sg Silver Grey
To Yellow, (sex linked, see later)
U Umbrous
Wh Anopthalmic, (Eyeless), White, (White bellied)
Examples of Simple Inheritance
First Cross

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

The Cinnamon allele, (p),  is recessive to its wild type counterpart, (P).  Since both animals are pure breeding, (homozygous), each has two copies of 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 E 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

  p from sperm p from sperm
P from egg Pp Pp
P from egg Pp Pp

The wild type allele, (P),is dominant, so this is what is shown in the youngsters' appearance.  This generation from the first crossing are called the First Filial generation, (F1).  It is important to note that the young F1 hamsters, although they  resemble their mother, are not homozygous, (pure breeding), like her.  They have two different alleles at their cinnamon locus.  They are described as heterozygous, although breeders would tend to describe them more simply 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.

Further Crosses

(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 the expected ratio of coat colours in the Second Filial, (F2), generation would be three golden pups to one cinnamon one.  How does this come about?

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.

  P from sperm p from sperm
P from egg PP Pp
p from egg Pp pp

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

Father to daughter, (Recessive backcross)

If the Cinnamon father and his golden daughter are mated then equal numbers of Cinnamon and Golden young would be expected.

As has been seen, all the male's sperm can only contain the allele p, while half of the young female's eggs will contain P and the other half p..

Crossing these together would give the following.

  p from sperm p from sperm
P from egg Pp Pp
p from egg pp pp

The possible genotypes are therefore;

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 it is the wild type gene, the gene for not having the mutation, that is recessive and is "carried".

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 has two p alleles at his Cinnamon locus.  The Dark Grey female, likewise, has two dg alleles at her Dark Grey locus.  The male, however, has wild type alleles, (Dg), at his Dark Grey locus and his mate has wild type alleles at her Cinnamon locus.  The male will produce sperm with the genetic makeup "p Dg", (considering the alleles at both gene loci). In the same way, the female's eggs will have the makeup "P dg".  All the youngsters will therefore have the genetic makeup "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 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 Cinnamon  and any that has the genotype dgdg will show Dark Grey).

 

Male's Sperm

Female's Eggs

  P Dg P dg p Dg p dg
P Dg PP DgDg Golden PP Dgdg Golden Pp Dg Dg Golden Pp Dgdg Golden
P dg PP Dgdg Golden PP dgdg Dark Grey Pp Dgdg Golden Pp dgdg Dark Grey
p Dg Pp DgDg Golden Pp Dgdg Golden pp Dgdg Cinnamon pp Dgdg Cinnamon
p dg Pp Dgdg Golden Pp dgdg Dark Grey pp Dgdg Cinnamon pp dgdg LILAC

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;

Colour

Genes, (names)

Genotype
Dove Black, Cinnamon aa pp
Flesh Eared White Dark Eared White, Cinnamon cdcd pp
Ivory Dark Grey, Cream dgdg ee
  Cream, Light Grey ee Lglg
Lilac Dark Grey, Cinnamon dgdg pp
Mink Cream, Cinnamon, Umbrous ee pp UU/Uu
Red Eyed Cream Cream, Cinnamon ee pp
Red Eyed Ivory Dark Grey, Cream, Cinnamon dgdg ee pp
  Cream, Light Grey, Cinnamon ee Lglg pp
Roan Cream, White Bellied (Eyeless White) ee Whwh
Sable Cream, Umbrous ee UU/Uu
Sex Linkage

So far we have only looked at alleles that can be inherited 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 hamsters have two identical chromosomes, called X chromosomes.  All hamster eggs, therefore, contain one X chromosome.  Males, 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, the 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 expresses 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, which is not simply either dominant or recessive. It is described as incompletely dominant.   This means that, if an animal has a yellow, (To), and a wild type, non Yellow, (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 "non Yellow") or Toto, (Tortoiseshell - pure breeding for neither).  Males, having only one X chromosome, are written as To_, (Yellow) or to_, (non Yellow), 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.

  Yellow male To_ Non Yellow Male to_
Yellow female ToTo All babies, male and female, will be yellow. All male babies will be yellow.  

All female babies will be tortoiseshells.
Tortoiseshell Female Toto Half of the male babies will be yellow, half will be non yellow.

Half of the female babies will be yellow, half will be tortoiseshell.
Half of the male babies will be yellow, half will be non yellow.

Half of the female babies will be non yellow, half will be tortoiseshell.
Non Yellow Female toto All of the male babies will be non yellow.

All of the female babies will be tortoiseshell.
All babies, male and female, will be non yellow.

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

Author Andrew Bryan - Duncton Hamstery
Copyright© Midland Hamster Club (UK)