No. Even two siblings to the same parents are astronomically unlikely to end up with the same DNA unless they started out from the same fertilized egg. With no crossovers, the probability of getting the same chromosomes would be one in 2^46 (The same chromosome for 23 chromosomes from each parent i.e. about 70 trillion), but zero crossovers is astronomically unlikely, and with crossovers the probability increases by many, many orders of magnitude.
But of course, it is quite possible for two relatives to look enough alike that it’s difficult for a casual observer to tell them apart, especially if they’re as close as half-siblings.
The 99% I assume is the third sigma of the distribution.
Meaning 2/3 of people (actually about 68%) fall much closer to the center, one sigma (Standard deviation)
I once knew a family with five children. Four out of the five looked very much like their mother and were virtually identical (to the extent that I, a casual acquaintance, mistook them for one another). The fifth looked nothing like his brothers and sisters.
“Looks” are not an indication of chromosomal or genetic identity.
No, I meant humans share ~99% of their genome with every other human and DNA-test-comparisons compare a small fraction of the ~1% that differs and even then doesn’t look at how many of those bases are the same, but whether they are the same over consecutive tested bases along a segment.
Something that hasn’t been mentioned is that everybody is either homozygous or heterozygous for any given gene. As in, either you have the exact same version of that gene on both the pair of chromosomes (having by chance gotten the same gene from each parent) or you got a different variant from each parent.
For a well-known example, blood type. Your blood type can be AA, OO, BB, AO, BO, or AB. For normal uses, AA and AO are the ‘same’, and the same for BB and BO.
Where it touches on this topic, when your blood type is one of the first three, all of you children will inherit the same gene regardless of which chromosome they got it from. While with the latter three, the children may or may not happen to get the same gene.
The thing is, I have no idea what percentage of genes the average person is homozygous for. You could theorize a woman who is 100% homozygous for everything – and then her children would inherit identical gene sets from her, and thus their total genes (assuming a normal father) would be around 75% identical.
Since apparently the true number is just around 50%, humans must not be homozygous very much.
Humans are homozygous for most. But when we talk about genetics we’re mostly interested in what makes us different, and for those variants we range from mostly homozygous to half and half, depending on the prevalence of the variants, and the prevalence varies between populations.
Say there is a prevalence of 10% of a variant B, with the “original” being A, you have a 10% chance of getting B from either parent, so a 1% chance of being homozygous that way, and a 90% chance of getting A from either parents, so a 81% chance of being homozygous the other way. That leaves an 8% chance of being heterozygous. Three variants existing makes things more complicated, but if we stick with two, maximum heterozygousity occurs when both versions are equally common. Then you’ll have 25% chance of either homozygousity and 50% chance of being heterozygous.
At least, for genes for which there are only two variants.
Sorry, I was referring to the chart in the link. But I see it’s the wrong link. What I meant was this one:
It states for example, full siblings have 50% genes in common, but 99% of us range from 38% to 61%. Presumably that’s therefore 3 sigma, and 68% of siblings (one sigma) are between 46% and 54% in common assuming a standard Bell curve distribution. (If I remember my statistics class) Zero in common would be one in a million or more.
For example, Harry (formerly known as Prince) is as melanin-free as they come, and we can see that Meghan has very few melanin genes. Skin colour is a product of multiple genes that say “produce melanin”. Any child of theirs can have no more melanin genes than Meghan, and statistically will probably have about half as many. (Photo evidence bears this out) If Princess Whatsername suggested the child would ever be darker, she was way off in left field. It’s impossible.
In some places like South Africa, I remember reading about where they used to classify some citizens by the darkness of their skin, the issue was more obvious. Two parents with say, half the melanin genes of a full native African would have some children with more than either parent and some with less, thus resulting in the bizarre result that siblings in the same immediate family could have different classifications (“black” or “coloured”)
You can theoretically count it by just examing one gene.
Farther has genes a and b and mother in same loci has genes c and d.
Then the offsprings are either ac, ad, bc or bd. Each probability of 25%. Then we take all pairs:
ac and ac have full common
ac and ad have half common.
ac and bc have half common.
ac and bd have no common.
ad and ac have half common.
ad and ad have full common.
etc.
And you find out that there are as many full commons as no commons so the total average is half common.
Now exponentiate that with all the non-codependent genes and forget that sex-chromosomes muddles the figures and you find out that though the deviation increases the mean stays the same.
Think of DNA as a coin flip - do you get this gene from your mother or father? The odds you’d flip a (fair) coin and get 200 heads in a row is pretty much zero. (1 in 2^200 or about 1 in 10^60)
The difference is that genes come in packages called chromosomes, and you get 23 from Dad and 23 from Mom - one of each of their 23 pairs. During replication, these chromosomes can mix-and-match between pairs so the chromosome from, say, mom may not be intact the same as she got from one of her parents, it may be a mix of grandma and grandpa’s versions of that chromosome.
The one difference is X and Y pair. Males have an X and Y, females 2 X’s. So a child gets one of these from each parent - one of the 2 X’s from the mother, and either the X or Y from the father. A child who gets an X now has 2, and is a girl (biologically). A child who gets a Y from dad has XY and is a boy.
To dig up another royal example… Queen Victoria appears to have had a mutation on one X chromosome that failed to produce the blood-clotting factor. There was a 50-50 chance any of her children inherited that X - but like Victoria, the women of the family inherited a second X from Albert and so with at least one X telling the body how to make the factor; men with her defective X only had a Y which does not include this gene.
At least two of Victoria’s daughters (Alice and Beatrice) apparently inherited this gene but it was not obvious until they had male children, some of whom inherited the disease. He sons were lucky and did not get the defective X (i.e. Edward VII showed no sign of it, nor the royal family descended from him.) However, one son did - Prince Leopold, who died at 31. Wikipedia shows a family tree of those believed to be carriers.
This is a good example of of a defective gene - it can be recessive, like hemophilia, meaning unless the person has no good versions of the gene, they may never know they have it. But - they can pass this on to their offspring. Genes which are on the X chromosome typically have no matching version on the Y and so affect mainly males. If a female were to inherit the defect on both X from both parents, then they too would exhibit symptoms. This is more likely in more closed populations, where they habitually intermarry with a smaller common pool of population.
Another example of this is Tay-Sachs: it’s common in closed communities, where because of its recessive nature, most carriers are unaware (until nowadays) that they carry it. But if they marry someone also carrying the defect, then it’s 50-50 for each to pass it on (AB and AB gives AA, AB, BA, BB) - a 25% chance the child has AA both defective genes, 25% that they are not a carrier (BB), and 50% chance that they have one defective gene (AB or BA) but no symptoms and so if they have children with another carrier, those children run the same risk. Normally, the risk is close to zero - but by typically marrying someone who is from the same small population, the odds of matching with another carrier are good.
The disease is rare in the general population.In Ashkenazi Jews, French Canadians of southeastern Quebec, the Old Order Amish of Pennsylvania, and the Cajuns of southern Louisiana, the condition is more common. Approximately 1 in 3,600 Ashkenazi Jews at birth are affected.
Note that Cajuns (Acadians) were expelled to Louisiana from New Brunswick so essentially the same root French population.