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Chromosomes and Inheritance

In an earlier post, we saw about genes and compared it to a sentence, where the codons are its words and the nucleotides are its letters. Let us move forward from there to see where this DNA sits in our body.

It is common knowledge that our body has billions of cells. Cells make up everything in our body. From our fingers, to bones to heart. At the centre of each cell is a nucleus. It is this nucleus that contains what are called chromosomes. The chromosome is nothing but a long piece of DNA that is coiled up and it is this DNA that we talked about in the last post. Each chromosome will contain anywhere between a few hundred to few thousand genes.

Every species contains a specific number of chromosomes. We humans have 46 chromosomes in each of our cells. But these chromomes exist in pairs. That is, we have 23 pairs of chromosomes. To get our idea clearly here is the hierarchy of organisation: Body contains cells, cells contain nucleus, nucleus has chromosomes, each chromosome has genes.

Continuing the analogy from the previous post, if we can look at a gene as a meaningful sentence of words, the chromosome can be looked at as a chapter. All the chromosomes together, which forms the genome of a human being can be considered the book of life.

In the rest of this post, I will talk about chromosomes with reference to our human body. As mentioned before, we humans have 23 pairs of chromosomes in each of our cells. Out of these 46 chromosomes, two of them are the sex chromosomes. They are the famous X and Y chromosomes. Males have 1 X and 1 Y chromosome, while females have 2 X chromosomes. The remaining 44 are called autosomes.

Let us number the 22 autosomal pairs from 1 to 22. Since chromosomes occur in pairs, let us name them 1A-1B, 2A-2B and so on till 22A-22B, and then the sex chromosomes. If you are wondering what the pairing is all about, let me clarify that. To build a human body, 22 chromosomes and 2 sex chromosomes are sufficient. You can just take all the A chromosomes (1A, 2A, 3A and so on) with two sex chromosomes and build a body. Similarly, all the B chromosomes will be sufficient to build a body. You can also take 1A, 2B, 3A, 4B and any such combination. This process is called recombination.

But recombination is not just choosing 1 chromosome over another. Recombination usually works by choosing parts of different chromosomes. Thus, you can even choose the top half of 1A and the bottom half of 1B, and make a new chromosome. The chromosomes can be taken apart to be brought together in different combinations.

Back to our analogy. We can consider each cell in our body to have not one but two copies of the book of life. Either of the books is sufficient to give you the instructions to build a body. So you can pick one chapter (chromosome) from one book and another chapter from the second book. You can also split up chapters, such that you take some sentences from the chapter 1 of the first book, and the remaining sentences from chapter 1 of the second book, thus creating a new chapter. You can go on creating chapters like this and form a different book.

Suppose for example, there is a gene to determine what colour of eye, one has. Let us assume that it resides on the 13th chromosome. Let us also assume that 13A has the gene for the black eye colour, and 13B has the gene for the brown eye colour. During recombination, any of the two colours could be chosen, to be part of the daughter cell.

When does recombination occur?
Normally when cells reproduce in our body (all the growth and repair of our body is because of cell reproduction), it produces an exact copy of the 23 pairs (with some rare mistakes, which are called mutations). Such a reproduction of cells (not to be confused with reproduction of individuals) is called mitosis. Thus if a heart has to grow in size, the heart cells will keep reproducing till the desired size is achieved.

But there is a special type of reproduction of cells where in a cell with 46 chromosomes, gives birth to cells with 23 chromosomes. Such (half)cells, called gametes, is what we call as a sperm in a male and an egg in a female. The sperms and eggs are special kinds of cells which have only half the number of chromosomes as its parent cell. You must be able to guess now that when an egg and a sperm come together, they form a fertilised egg having 46 chromosomes, leading to the formation of a new life (now we are talking of the reproduction of the individual).

There is something else that is more important about this kind of cell reproduction. When this special cell reproduction happens, it undergoes recombination and all that we discussed previously (about chapters and books) comes into play. Thus it is really not predictable to say which part of the 1st chromosome, which part of the second chromosome and so on are part of the newly formed daughter cell. So it might either have a brown eye gene or a black eye gene, brown hair or black hair and so on for different features.

Sex determination
The egg from the female will always have only X chromosome, since it is the daughter of a cell from the female which has X chromosomes. But the sperm (the half cell coming from the male which has both X and Y chromosomes) could have either X or Y chromosome. A sperm with a X chromosome combining with an egg will lead to the child being a female, while a sperm with a Y chromosome combining with an egg will mean that the child is a male.

An aside: I remember this fact that the gender of the child depends on the male half cell (sperm) and not the female, was once used in AID India’s health team to make a poster, to counter the ridicule that women face when they fail to deliver boys. The poster tries to tell the viewer that if anybody is to blame it is the father and not the mother. In reality of, course, it is nobody’s conscious decision.

Now we have come to the final part of the post. It is because of this recombination, that we inherit some traits from the father and some from the mother. This recombination, also ensures that each one of us is a unique individual and not a clone of either of our parents. So a specific gene from your father, has a 50% chance of being in your genome too. Similarly a gene from your mother could have a 50% chance of being in your genome too. But any gene in your body should have come from either of your parents. And since your siblings share the same parents, a gene in your genome has a 50% chance of being in your sibling’s genome too.

There are some interesting details that I have not talked about here. We will discuss them in the coming posts.

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