# 50% similarity between siblings

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#1
Why do we say that siblings an farental twins are 50% similar?
if each sperm and each egg contains any random combination of each parents full genome (due to crossing over) how can such a figure be used?
Thanks
0
6 years ago
#2
50% of our genes are from our mother and 50% are from our father - 23 chromosomes from each. Our parents have 2 copies of each chromosome, so it is 50/50 which copy of each chromosome we will inherit. It is like flipping a coin 46 times , and on average as it is random, and the parents have 2 copies of each chromosome , it is likely that around half of the chromosomes we have will be the ones that the sibling inherited. A good way to test it out is by flipping a coin and recording if it is heads or tails each of the 46 times. Then by repeating it again, you can compare and the chances are there should be around 50% the same
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#3
(Original post by maxwynne)
50% of our genes are from our mother and 50% are from our father - 23 chromosomes from each. Our parents have 2 copies of each chromosome, so it is 50/50 which copy of each chromosome we will inherit. It is like flipping a coin 46 times , and on average as it is random, and the parents have 2 copies of each chromosome , it is likely that around half of the chromosomes we have will be the ones that the sibling inherited. A good way to test it out is by flipping a coin and recording if it is heads or tails each of the 46 times. Then by repeating it again, you can compare and the chances are there should be around 50% the same
ok so its not 50% ALWAYS its 50% - most likely (on average)...right?
thanks
0
6 years ago
#4
yeah, exactly.. this might explain it better..
A good way to look is by using cards. If you deal 26 cards for a sibling ( half a deck of 52 - using them as a metaphor of one parents chromosomes) and record them. Then shuffle and do it again for each sibling. The chances are the sibling will share around 13 cards which is half of the 26 dealt. So the two siblings share 25 % of your mothers DNA . The same thing works for the father, showing you should share 25% of your fathers DNA, making 50% overall.
Obviously it's averages, and sometimes the siblings may share less than 25% of one parents DNA ( if only 10 of the cards match for example, metaphorically speaking).

1
6 years ago
#5
(Original post by jsmith6131)
ok so its not 50% ALWAYS its 50% - most likely (on average)...right?
thanks
Its an extremely simplified model used when looking at probabilities of single mutations e.g. those that confer diseases. It does not imply anything about actual DNA.
1
#6
thanks all (
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