MadSci Network: Genetics |
Hello AJ, The answer to your question depends in part on the sex of the child, but in all cases the difference is very small. In any case, both parents contribute a single copy of chromosomes 1 through 22. This adds up to approximately 2.87 billion bases of genetic information from each parent. In addition, the mother contributes one X chromosome, which is another 152 million bases. The father can contribute either 1 X chromosome (152 million bases), in which case the child will be a girl, or 1 Y chromosome (51 million bases) in which case the child will be a boy. The X and Y chromosomes are known as the sex chromosomes. In total, then, a person’s genome is about 6 billion bases. The other thing to keep in mind is the structure inside our cells that is called the mitochondria. This organelle is responsible for energy production, and is inherited only from the mother. Interestingly, the mitochondria has its own DNA, which is comprised of 16, 569 bases of genetic information. This is less than .0006% of the total DNA that the mother contributes. So, if the father contributes an X chromosome, and the couple has a baby girl, the mother has contributed about .0006% more DNA than the father, owing to the mitochondrial DNA. If the father contributes a Y chromosome, and the couple has a boy, the mother has contributed about 101 million bases, or about 3.5% more genetic information than the father, due mostly to the difference in size between the X and Y chromosomes, and just a tiny bit due to the mitochondrial DNA. For chromosomes 1-22, which account for ~95% of our genetic information, there is no difference whatsoever between the two scenarios. Siblings that have the same mother but different fathers will have the same mitochondrial DNA. This is not the case for siblings that have the same father but different mothers. As mentioned above, this accounts for only a tiny fraction of the total DNA in an individual. Now let’s consider the X and Y chromosomes. When the body produces gametes (eggs or sperm) through a process called meiosis, the chromosomes go through a process called recombination, or crossing over. Simply put, this is where the two copies of a chromosome line up and swap genetic code. We have two copies each of chromosomes 1-22. During meiosis, the two copies of chromosome 1 line up and swap code, as do chromosomes 2-22. In females, the two X chromosomes can also do this. As a result, the X chromosome that is contributed to the offspring is essentially a mix of the mother’s two X chromosomes. Therefore, each egg has a slightly different X chromosome. In men, there is only one X chromosome, and only 1 Y chromosome, so neither has a partner for recombination. Each sperm produced by the man should have either an X or a Y chromosome. Since there is very little recombination involving the X or Y chromosomes, there is essentially no variation in the X or Y chromosomes carried by the sperm. So, same sex siblings that have the same father have exactly the same Y chromosome (if they are male) or almost exactly the same X chromosome from their father. If they have the same mother, the X chromosome that each has from the mother will be slightly different. Let’s look at the different possible sibling combinations: 1) 2 boys If they have a different mother. but the same father, their Y chromosomes are identical. That’s 51 million bases. Their X chromosomes are different. If they have the same mother, their X chromosomes are 50% the same. 50% of 152 million bases is 76 million bases. If they have different fathers, then their Y chromosomes are different. Therefore, two boys with the same mother but different father will be slightly more similar than two boys with the same father but different mother. An emphasis on “slightly”…the difference is about 25 million bases, which represents about 0.4% of a person’s total genome. 2) 2 girls If they have a different mother, but the same father, then the X chromosome they each got from their father is 100% identical. That’s 152 million bases. Their other X chromosomes, which came from different mothers, are different. If they have the same mother, but different fathers, then the paternally inherited X chromosomes will be different. The maternally inherited X chromosomes will be 50% identical, so that’s 76 million bases in common. Therefore, two girls with the same father, but different mother, will be slightly more similar than two girls with the same mother, but different father. The difference in this case is about 76 million bases, or approximately 1.2% of a person’s total genome. 3) 1 boy and 1 girl If they have a different mother, but the same father, then both sex chromosomes will be different. The boy inherited a Y chromosome from the father, whereas the girl inherited an X chromosome from the father. The mothers are different, so the maternally inherited X chromosomes will be different. In this case, both sex chromosomes are different. If they have different fathers, but the same mother, the paternally inherited sex chromosomes are different. The maternally inherited X chromosomes will be 50% identical (76 million bases in common) Therefore, a boy and a girl with the same mother, but different fathers will be slightly more similar than a boy and a girl with the same father, but different mothers. The difference in this case is about 76 million bases, which accounts for approximately 1.2% of a person’s total genome. Alex Brands Lehigh University
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