In mammals, sex is determined by the X and Y chromosome. Two X chromosomes represents a female and one X and one Y chromosome represent a male. Genes that are located one these chromosomes are called "sex-linked" genes as they are located on either the X and Y chromosome.
An interesting way of examining sex linkage is through cats as they are both cute and have interesting genetics, especially when it comes to their tortoiseshell fur. Tortoiseshell is when a cat, usually female will inherit both genes however instead of its fur being either the dominant gene or the homozygous recessive gene there will be a patchwork of both colours! More precisely there will always be a match of orange and black!
Beautiful!
However it is worthwhile asking how does happen?
Simply, these genes a sex linked. The orange gene is carried on the X chromosome. Therefore males can only be black or orange. Females can be black, orange or tortoiseshell. If you simply look back to the intro you can see that males only have one X chromosome and one Y chromosome (showing that females are superior to males biologically but anyways) whereas females carry both X chromosomes. And for females one X chromosome is unactivated in the embryo at a certain point and this is completely random. It inactivates by supercoiling into a structure known as Barr Body. Named after Murray Barr, this is an inactive X chromosome in a female somatic (any cell in the body expect egg and sperm cells, they are diploid containing two sets of chromosomes) cell. They are rendered inactive by a process called lyonization. This is the process by which one of the two X chromosomes in female mammal is rendered inactive. This X chromosome is rendered inactive by being packaged in a way that will make it transcriptionally inactive also known as a heterochromatin.
To continue on our cats, at a certain point lyonization will occur in the female embryo, making one X chromosome inactive therefore only the alleles in the active X chromosome are expressed. Therefore in a patch of skin where the X chromosome carrying the B (orange fur) allele is unactivated, only the b (black fur) allele will be expressed. In a patch of skin where the X chromosome carrying the b (black fur) allele is unactivated, that patch of skin will only be expressed with the B allele thus orange fur.
This usually only happens in females however it is possible very rarely that a male cat will inherit both the XX chromosomes and the Y chromosome, which is more commonly known as the Klinefelter Syndrome. The same process would then happen however the cat would most likely be infertile, making this even more rare.
However it is noticed that some cats have larger patches than others and this is suspected to be the cause of earlier lyonization causing larger patches.
Genetics is so beautiful!
An interesting way of examining sex linkage is through cats as they are both cute and have interesting genetics, especially when it comes to their tortoiseshell fur. Tortoiseshell is when a cat, usually female will inherit both genes however instead of its fur being either the dominant gene or the homozygous recessive gene there will be a patchwork of both colours! More precisely there will always be a match of orange and black!
Beautiful!
However it is worthwhile asking how does happen?
Simply, these genes a sex linked. The orange gene is carried on the X chromosome. Therefore males can only be black or orange. Females can be black, orange or tortoiseshell. If you simply look back to the intro you can see that males only have one X chromosome and one Y chromosome (showing that females are superior to males biologically but anyways) whereas females carry both X chromosomes. And for females one X chromosome is unactivated in the embryo at a certain point and this is completely random. It inactivates by supercoiling into a structure known as Barr Body. Named after Murray Barr, this is an inactive X chromosome in a female somatic (any cell in the body expect egg and sperm cells, they are diploid containing two sets of chromosomes) cell. They are rendered inactive by a process called lyonization. This is the process by which one of the two X chromosomes in female mammal is rendered inactive. This X chromosome is rendered inactive by being packaged in a way that will make it transcriptionally inactive also known as a heterochromatin.
To continue on our cats, at a certain point lyonization will occur in the female embryo, making one X chromosome inactive therefore only the alleles in the active X chromosome are expressed. Therefore in a patch of skin where the X chromosome carrying the B (orange fur) allele is unactivated, only the b (black fur) allele will be expressed. In a patch of skin where the X chromosome carrying the b (black fur) allele is unactivated, that patch of skin will only be expressed with the B allele thus orange fur.
This usually only happens in females however it is possible very rarely that a male cat will inherit both the XX chromosomes and the Y chromosome, which is more commonly known as the Klinefelter Syndrome. The same process would then happen however the cat would most likely be infertile, making this even more rare.
However it is noticed that some cats have larger patches than others and this is suspected to be the cause of earlier lyonization causing larger patches.
Genetics is so beautiful!