Forms of inheritance

What are inheritance forms?

Actually it is very simple: inheritance forms are the ways in which genes are passed from parent to child. But all these different ways can be quite complicated. So far, not all are addressed in African Pygmy Hedgehogs but it is always useful to know the differences.


Dominant & recessive

A dominant gene is a gene that is only needed from one parent to show up on a hedgehog, it’s predominant. A dominant gene is denoted by capital letters (A). A gene is dominant if one half of an allele shows and the other half is not visible. The half that doesn’t show is referred to as recessive and is denoted by small letters (a). If an animal has two recessive or two dominant genes, it is called homozygous (aa or AA). If an animal has one dominant gene and one recessive, it’s called heterozygous (Aa). In a heterozygous combination, the dominant form is always written first, whether it is  passed on from the father or mother.


Codominance

Codominance is the phenomenon whereby an animal has a dominant allele and a recessive allele but both halves show. A good example for this is Reversed Pinto. The gene for Pinto is dominant, but the allele that allows for Reversed Pinto is not. Yet Reversed Pinto does show, even though you would expect the Pinto gene to hide it. A codominant gene is denoted by first the dominant allele, then a roof and then the recessive gene. Where possible, it can also be denoted by putting the recessive allele in superscript (Reversed Pinto is denoted as Pi^rv or Pirv). Although in these cases, the recessive allele always requires the specific dominant gene in order to be expressed in the fenotype. The recessive gene can be passed through amongst the children, many generations, without being noticed! Conversely, the dominant gene does not require the recessive gene in order to show.


Incomplete dominance

Incomplete dominance seems much like codominance. Only with incomplete dominance, the recessive allele has less influence. However, it does show that the gene is carried, so the dominant allele is ‘incomplete’. The second difference is that the recessive gene, doesn’t need the dominant allele in order to show up. If an animal has two of these recessive alleles, it will be shown as the true color. Here, too, it applies that the dominant gene, does not need the recessive gene to be expressed. Incomplete dominant genes, however, have no separate denotation to distinguish themselves from normal recessive and dominant alleles. With these genes you must learn the effect of imcomplete dominance by yourself.


Sex-linked inheritance

This inheritance is perhaps the most difficult. A gene that’s sex-linked inherited, is linked to the X-chromosome. It thus inherits different in a male and a female, because a male has an X- and a Y-chromosome and a female has two X-chromosomes. And the most difficult is perhaps that a sex-linked gene can occur in both recessive and dominant form.

In the case of a dominant gene, a male needs the dominant allele in order to let the color show (denotated as Ay, the y stands for the missing allele on the Y-chromosome). The man can only pass on the allele that he has on his X-chromosome. All his offspring will get that allele too. This also applies to a recessive sex-linked gene. A female does have a complete gene and the dominant gene can thus pass in homozygous form and in heterozygous form in both dominant and recessive forms.

Because with the male, in a sex-linked gene only one allele is available, the phenotype can vary considerably. A good example of this is Tortoiseshell/Yellow in Syrian hamsters. A male with this dominant gene is called Yellow, because his fur is yellowish (TOy). A female can get two different colors in this gene. In a heterozygous form it is Tortoiseshell (Toto), but with a homozygous dominant form (TOTO) it is Yellow.

Photos by Martin Braak.


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