Slightly less basic genetics
Want to understand a little bit more about Color Genetics in horses? Here's a slightly less basic primer on genetics in general and some suggestions for further reading.
Understanding How Genes Work
Genes are the blueprints our body follows to build us. They tell our cells how to build our bodies - things like what color eyes to give us, if we have curly hair or straight hair, or if our noses are small or large, snubbed or crooked.
Every human being has a unique genetic blueprint given to them in completed form at conception. This basic blueprint never changes and is with us through our whole lives. A copy of it is printed and stored in every cell of our bodies. In humans this blueprint consists of about 20,500 different genes which work together to create every aspect of our bodies. Taken as a whole group, this full set of genes is called a "genome."
Our complete genetic blueprint (genome) is like a large book with 20,500 chapters, each filled with instructions for building a different aspect of our our bodies. For instance, every human being has a chapter for eye color, and each eye color chapter is in the same place in each human's book (Lets say, its chapter five). Thus, a geneticist (scientist) knows that she can look in the fifth chapter of the genome of any human being on the planet and she will see instructions for what color eyes this person is going to have. What is different in each human being is what version of each of those genes are present. Blue, brown, green or hazel are all versions of the eye color gene which exist in the human population.
Contributions from the Mother and the Father - two copies of each gene
At conception, every baby is given not one but two copies of every gene in the genome. In effect, each human receives two large books of instructions with 20,500 gene-chapters each. One of these books comes from the baby's father and one comes from the baby's mother.
When a baby is conceived, these two books are meshed together so that they make up one large book in which every chapter is doubled: There are now two chapters on eye color, two chapters on height, two chapters on whether the person will have curly hair or straight hair, blond hair or brown.
This could be very confusing. Each chapter contains two complete sets of instructions for how to make each characteristic. But those instructions are often different from each other. (For instance, one says make the eyes brown, the other says make the eyes blue). So which chapter does the body follow?
There are two different ways that the body has worked out to determine which of the two sets of instructions to follow for each gene:
Each different chapter in the human genome works one of these two ways and always works that way in all humans, no matter which human being the genome belongs to. (For instance, hair color in humans always creates a mix of the gene from the mother and the gene from the father. Eye color always creates only one color or the other.)
Genes Determined by Dominance
If a particular trait is determined by the second kind of gene - the kind, like eye color, in which one copy of the gene is used and the other is never used at all - how does the body decide which copy of each gene to use?
Each possible version of a gene (like brown, blue, green or hazel in eye color) is ranked against all the others in order of dominance. In the case of eye color, brown eyes are the most dominant, then green, then blue (hazel is determined by something else). When a baby is created with its double copy of each gene, the most dominant version of each gene is the one that gets to be used - the other has no effect on the person at all.
Genes are the blueprints our body follows to build us. They tell our cells how to build our bodies - things like what color eyes to give us, if we have curly hair or straight hair, or if our noses are small or large, snubbed or crooked.
Every human being has a unique genetic blueprint given to them in completed form at conception. This basic blueprint never changes and is with us through our whole lives. A copy of it is printed and stored in every cell of our bodies. In humans this blueprint consists of about 20,500 different genes which work together to create every aspect of our bodies. Taken as a whole group, this full set of genes is called a "genome."
Our complete genetic blueprint (genome) is like a large book with 20,500 chapters, each filled with instructions for building a different aspect of our our bodies. For instance, every human being has a chapter for eye color, and each eye color chapter is in the same place in each human's book (Lets say, its chapter five). Thus, a geneticist (scientist) knows that she can look in the fifth chapter of the genome of any human being on the planet and she will see instructions for what color eyes this person is going to have. What is different in each human being is what version of each of those genes are present. Blue, brown, green or hazel are all versions of the eye color gene which exist in the human population.
Contributions from the Mother and the Father - two copies of each gene
At conception, every baby is given not one but two copies of every gene in the genome. In effect, each human receives two large books of instructions with 20,500 gene-chapters each. One of these books comes from the baby's father and one comes from the baby's mother.
When a baby is conceived, these two books are meshed together so that they make up one large book in which every chapter is doubled: There are now two chapters on eye color, two chapters on height, two chapters on whether the person will have curly hair or straight hair, blond hair or brown.
This could be very confusing. Each chapter contains two complete sets of instructions for how to make each characteristic. But those instructions are often different from each other. (For instance, one says make the eyes brown, the other says make the eyes blue). So which chapter does the body follow?
There are two different ways that the body has worked out to determine which of the two sets of instructions to follow for each gene:
- With some genes the body follows both, thus giving the baby a little of the mother's characteristic and a little of the father's. (In humans, hair texture is like this - notice how different the hair of a person of African decent is from that of a person of European decent. Any baby born to these two people will have hair that is partway in between the two types.)
- However, with most genes, only one of the doubled sets of instructions will be followed. The other will sit in the person's genome for the rest of their life and do nothing at all. (Eye color, works this way. A mother may pass on the gene for blue eyes and the father a gene for brown eyes. When this happens, the baby will have brown eyes - not blue eyes or bluish-brown eyes but brown. The blue-eye gene will have no effect on the person whatsoever.)
Each different chapter in the human genome works one of these two ways and always works that way in all humans, no matter which human being the genome belongs to. (For instance, hair color in humans always creates a mix of the gene from the mother and the gene from the father. Eye color always creates only one color or the other.)
Genes Determined by Dominance
If a particular trait is determined by the second kind of gene - the kind, like eye color, in which one copy of the gene is used and the other is never used at all - how does the body decide which copy of each gene to use?
Each possible version of a gene (like brown, blue, green or hazel in eye color) is ranked against all the others in order of dominance. In the case of eye color, brown eyes are the most dominant, then green, then blue (hazel is determined by something else). When a baby is created with its double copy of each gene, the most dominant version of each gene is the one that gets to be used - the other has no effect on the person at all.
The Importance of the Second Copy
Why are there two copies of each gene if only one is used? Does the second copy really have no importance at all?
The second copy has no importance to the person it belongs to at all. But it DOES have importance for their children.
None of your children are perfect clones of one parent or the other - they are a unique mix of their mother's traits, their father's traits and, sometimes, traits which came from grandparents or other ancestors too far back to be remembered. The ability of a species to create such variety makes it more likely to survive in a wide variety of situations, thus making the species stronger. Here's how it works...
Each egg in the mother's body and sperm in the father's body is given one copy of each of that parent's genes (rather than the double copies that exist in every other cell of their body). What specific mix of gene versions each egg or sperm gets is random and different from every other egg or sperm in the parent's body (which is why siblings can be so different from each other). When a specific egg and sperm meet, they mix their single copies to make one, whole, double copied book (genome) used to create the baby they are forming. That baby ends up with two versions of every gene - in the case of eye color, perhaps one for blue eyes and one for brown. In this case, the baby would have brown eyes and, short of a genetic test, there would be no way to know that this baby also contained a copy of the gene for blue eyes. But that blue eye version of the gene is just as likely as the brown version to be passed on to the next generation. Thus, this person's child could have blue eyes.
Why are there two copies of each gene if only one is used? Does the second copy really have no importance at all?
The second copy has no importance to the person it belongs to at all. But it DOES have importance for their children.
None of your children are perfect clones of one parent or the other - they are a unique mix of their mother's traits, their father's traits and, sometimes, traits which came from grandparents or other ancestors too far back to be remembered. The ability of a species to create such variety makes it more likely to survive in a wide variety of situations, thus making the species stronger. Here's how it works...
Each egg in the mother's body and sperm in the father's body is given one copy of each of that parent's genes (rather than the double copies that exist in every other cell of their body). What specific mix of gene versions each egg or sperm gets is random and different from every other egg or sperm in the parent's body (which is why siblings can be so different from each other). When a specific egg and sperm meet, they mix their single copies to make one, whole, double copied book (genome) used to create the baby they are forming. That baby ends up with two versions of every gene - in the case of eye color, perhaps one for blue eyes and one for brown. In this case, the baby would have brown eyes and, short of a genetic test, there would be no way to know that this baby also contained a copy of the gene for blue eyes. But that blue eye version of the gene is just as likely as the brown version to be passed on to the next generation. Thus, this person's child could have blue eyes.