How can we prove through DNA that whether a man or a woman are brother or sister or not?
It depends how complicated you want this really, (A level or degree.) This is a fairly simple explaination which gives you a good idea. Nowadays, DNA technology is used to figure out who is the father of a child. DNA paternity testing makes it possible to determine a child’s biological father to a very high degree of certainty. Everyone, except identical twins, has a unique set of DNA. DNA is made up of 4 bases or letters, A, C, G, and T. These 4 letters form the written code that makes up the DNA sequence. Now when someone says that everyone’s DNA is unique, what they mean is that occasionally one of these letters is different for different people. On average, two people at random have a different base every thousand bases or so. This is where the statistic that says that everyone’s DNA is 99.9% the same comes from. Since you get half your DNA from your father and half from your mother, your DNA is more than 99.9% the same as your parents. Your DNA is also more similar to that of your
There are ratios seen in DNA which are very reliable. Identical twins have 100% similar DNA, parents each share 50% with each child, other siblings have 25% similarity between each other. This relates to how the DNA splits when the sperm and eggs form- in each egg there is 50% from the mum and 50% from the dad. Because they only give 50% each, siblings who have the same mother and father only share 25% on average. If your father produced a child with a woman other than your mother, you would only have 12.5% similarity with this person (although still a brother/sister). So if a test is performed and a man and woman have 12.5-25% similar DNA, there is only an incredibly slight probability that they are not brother and sister. Considering there are millions and millions of nucleotides which have to be compared, the chances that two people would randomly have this much similar DNA is effectively zero.
This can only be expressed as a very high degree of probability. Ideally you need the DNA profiles of their Mum and Dad as well. DNA profiling looks at 15 -17 different loci in the DNA. There is a great deal of variation at each locus, that is each locus is highly polymorphic. Any one person has two out of many different alleles at any given locus. Mum passes one allele from each locus to a child and Dad does the same. Although the exact combination of alleles for all the loci from Mum and Dad varies from child to child they will show a high degree of similarity because they are only being provided by those that the parents have. Thus a brother and sister will have DNA profiles that are similar. If the similarity is strong this is strong evidence in favour of them being brother and sister (other relationships are possible of course). The similarity can be expressed as a probability. What level of probability you accept as proof depends on the context. In mathematical terms it never get
