Struck me that it might be an idea to explain how the base colours work too!
These are a lot simpler than you'd think. To start with, all horses are genetically either black or chestnut as a starting point. It works like this....
Strictly speaking, a gene isn't dominant or recessive. A gene contains codes for development, nothing more. Each gene, however, contains what's called 'alleles' - these can be dominant or reccessive. There is no separate gene for red and black - they're both caused by the same gene. It depends on whether the horse carries 2 reccessive alleles of that gene (e) or a dominant allele of the gene (E). Put very simply - it depends whether the gene is 'switched on' for black or not.
If a horse carries the dominant allele - E - he'll be black. If not, and he has instead two reccessive alleles - e - he'll be red (or chestnut). Dominant means he only needs one E allele to show that colour - in order to show red he must have two copies of the reccessive form e.
So, there's the start - if a horse has ee, he'll have a red base. This horse can only ever contribute e (red) to a foal.
If a horse has Ee, he'll have a black base - but if he bred, he could contribute either E (black) or e (red) to the foal. This is why black can throw non-black foals.
If a horse is EE, he's homozygous for black. This mean that he can only contribute E to a foal - breed two EE blacks and you'll only ever get black foals.
And onto bay....
Bay is caused by a gene called Agouti. Again, whether a horse carries bay depends on whether the allele for this gene is dominant (A) or reccessive (a) - whether bay is 'switched on', in other words.
A horse with the switched on copy of bay (A) will carry bay. A horse without (a) won't.
Agouti works by restricting black pigment to the points - legs, mane and tail. Because it only works on black, a chestnut horse can carry bay without showing it. A black will always be bay if it carries bay.
This is how a bay foal can be born to a black parent and a chestnut parent - the black passes on E for black, the chestnut (carrying an 'invisible' A) passes on e, which doesn't show on the coat colour, and A, which causes the black pigment to be restricted to legs, mane and tail - so there's your bay foal.
A horse that carries two A alleles will only ever throw bay or chestnut foals, never black, even to a homozygous black - because they will always pass on A that causes black to become bay.
All other coat colours are caused by genes and their alleles that modify these base colours. For example, cream on a red base to give palomino; tobiano on a black base to give piebald; roan on a red base to give strawberry roan.
These are a lot simpler than you'd think. To start with, all horses are genetically either black or chestnut as a starting point. It works like this....
Strictly speaking, a gene isn't dominant or recessive. A gene contains codes for development, nothing more. Each gene, however, contains what's called 'alleles' - these can be dominant or reccessive. There is no separate gene for red and black - they're both caused by the same gene. It depends on whether the horse carries 2 reccessive alleles of that gene (e) or a dominant allele of the gene (E). Put very simply - it depends whether the gene is 'switched on' for black or not.
If a horse carries the dominant allele - E - he'll be black. If not, and he has instead two reccessive alleles - e - he'll be red (or chestnut). Dominant means he only needs one E allele to show that colour - in order to show red he must have two copies of the reccessive form e.
So, there's the start - if a horse has ee, he'll have a red base. This horse can only ever contribute e (red) to a foal.
If a horse has Ee, he'll have a black base - but if he bred, he could contribute either E (black) or e (red) to the foal. This is why black can throw non-black foals.
If a horse is EE, he's homozygous for black. This mean that he can only contribute E to a foal - breed two EE blacks and you'll only ever get black foals.
And onto bay....
Bay is caused by a gene called Agouti. Again, whether a horse carries bay depends on whether the allele for this gene is dominant (A) or reccessive (a) - whether bay is 'switched on', in other words.
A horse with the switched on copy of bay (A) will carry bay. A horse without (a) won't.
Agouti works by restricting black pigment to the points - legs, mane and tail. Because it only works on black, a chestnut horse can carry bay without showing it. A black will always be bay if it carries bay.
This is how a bay foal can be born to a black parent and a chestnut parent - the black passes on E for black, the chestnut (carrying an 'invisible' A) passes on e, which doesn't show on the coat colour, and A, which causes the black pigment to be restricted to legs, mane and tail - so there's your bay foal.
A horse that carries two A alleles will only ever throw bay or chestnut foals, never black, even to a homozygous black - because they will always pass on A that causes black to become bay.
All other coat colours are caused by genes and their alleles that modify these base colours. For example, cream on a red base to give palomino; tobiano on a black base to give piebald; roan on a red base to give strawberry roan.