It’s not because ginger cats believe themselves to be the rulers of our homes, but rather, due to a unique quirk of inheritance.
In all organisms, genes control which characteristics are inherited from their parents. Different genes control different characteristics. All genes exist in different versions (alleles) and each organism will have two alleles of every gene in their body (one on each chromosome in a pair).
If an organism has two of the same allele for a particular gene, then it is said to be homozygous for the corresponding trait. If its two alleles are different, then it is heterozygous for the trait.
If the two alleles are different, only one can determine what characteristic is present*. The allele responsible for the characteristic that is shown is known as dominant. The other allele is called recessive.
For an organism to display a recessive characteristic, both the alleles must be recessive. However, a dominant characteristic can be displayed if the organism carries both or only one of the dominant alleles.
(Credit: https://366.stem.org.uk)
22 of your pairs of chromosomes are ‘non-sex chromosomes’ (autosomes) of the organism. But the 23rd pair of chromosomes are sex chromosomes. There are two types of sex chromosomes, known as X and Y. All humans have a pair of these chromosomes which determines their gender. Males have an XY pair in which the X is inherited from their mother and the Y from their father. Females have an XX pair and inherit an X chromosome from each parent.
Most genes that control characteristics are found on the ‘non-sex chromosomes’ (autosomes) of the organism. However, a few characteristics will be coded for on the sex chromosomes.
The gene for blood clotting is found on the X chromosome, which means that haemophilia (a recessive disease that slows the blood clotting process) is described as an X-linked disorder.
Since girls have two X chromosomes, even if one of the X chromosomes contains a faulty allele of the blood clotting factor, they usually receive a correctly-functioning allele on the second X chromosome. The correctly-functioning allele will be dominant, meaning that girls who inherit only one faulty allele remain carriers, but do not actually have the disease. Girls rarely have haemophilia as it is very unlikely that both of her X chromosomes have a faulty allele of the blood clotting factor.
Boys, on the other hand, only inherit one X chromosome. This means that they have no “back up” allele. If a boy inherits one faulty allele of the blood clotting factor, they will definitely have haemophilia.
Haemophilia used to be considered “the royal disease” as, during the 19th and 20th centuries, Queen Victoria, through two of her five daughters, passed the faulty copy of the blood clotting factor to various royal houses across the continent, including the royal families of Spain, Germany and Russia.
The descendants with haemophilia are labelled in red (Credit: Wikipedia)
I had learnt about haemophilia and the way it is inherited in school, but had not come across any other examples of X-linked characteristics until I read Cat Sense by John Bradshaw. The book explores the science and history behind the appearance and behaviour of domestic cats; one of the most interesting sections of the book explains why ginger cats are almost always boys.
The allele for a ginger coat colour is recessive and carried on the X chromosome. Female cats have two X chromosomes, so, in order for a female to be ginger, she must have the ginger allele on both of her X chromosomes- which is very unlikely. If she has only one ginger allele, she will usually be a tortoiseshell cat.
If a male cat inherits one ginger allele, he will certainly have a ginger coloured coat. In fact, since they only have on X chromosome, male cats are almost never tortoiseshell. Very occasionally, male tortoiseshell cats crop up; they have two X chromosomes (with only one containing the ginger allele) and a Y chromosome as a result of abnormal cell division.
*(Generally, only one allele controls a characteristic, but there are exceptions to this in which two dominant alleles result in co-dominance and both control the characteristic. The classic example of this is blood type. The alleles controlling type A and type B blood can be co-dominant- resulting in the blood type AB. To extend the link to cats, co-dominance also arises in coat colour for cats. If a cat carries a dominant orange allele and a dominant brown allele of the gene controlling coat colour, then both colours appear on the coat in random patches (in some areas, the chromosome with the orange allele is switched on and, in other areas, the brown allele ‘wins’) producing a tortoiseshell tabby cat.)
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