![]() ![]() After X-inactivation, this means that some of her cells will produce red pigment, and some will produce black. ![]() (If this idea sounds strange to you then make sure you also read about chimerism !) Most of the time this isn’t noticeable, but in tortoiseshell cats it is right there for everyone to see! A tortoiseshell cat has inherited different colour alleles from their parents: she may have a red allele from her father and a non-red allele from her mother. Some of your body will be using your father’s X chromosome, and some of your body will be using your mother’s X chromosome. This means that if you are a female you are actually a ‘mosaic’ of different cells. Crucially this X-inactivation stays the same when these cells divide. So if you have 100 cells when X-inactivation happens, afterwards you will have ~50 cells with only the maternal X chromosome active, and ~50 cells with only the paternal X chromosome active. The zygote will start growing by making copies of its genetic information and then dividing so that each new cell will also contain a copy of the complete genome for that organism.įairly early in the development of a female the cells ‘switch off’ one of their X chromosomes in a fairly random way. The zygote contains the complete library of genetic information for that organism – half of it from the male parent and half from the female parent. Mammals start off as a single cell called a zygote – this is the cell that results when a female’s egg cell is fertilised by a male’s sperm cell. We haven’t covered the developmental process in this series (because it is an immensely complex subject area in its own right!) but we will cover a little now. The key part is that (in humans and cats) this inactivation is done at random, and at an early point in the developmental process. This involves ‘switching off’ one of the female’s X chromosomes. ![]() To avoid this situation mammals evolved something called ‘X-inactivation’. Because females have two copies of the X chromosome, whereas males only have one, you would expect females to produce twice as many X-chromosome gene products as males. We discussed in a previous article that mammals have two copies of each chromosome aside from the sex chromosome. ![]()
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