I feel like I should know this, but I don’t. It’s been a while since I took biology.
Do my eggs and my sister’s eggs have basically the same genetic material in them, since we’re full sisters?
Also, I’m under the impression that the variation between siblings is the result of the way the genes between the two parents combine and are expressed after the fertilization of the egg. Does this mean that all my eggs have the same genetic makeup and all my husband’s sperm has the same genetic makeup, but they’ll combine randomly (or according to dominance/recessiveness) to produce different results, or is each egg/sperm unique?
Each sperm and egg is unique. Your eggs are more closely related to your sister’s eggs than compared to a random woman’s eggs pulled from the general population but the differences are still huge. Even if you were identical twins the eggs wouldn’t be the same due to meiosis.
The above link, from a month or so ago, may or may not answer the question that Renee asks, but it does provide some information on the whys and hows of two eggs from the same woman not being identical.
Each sperm and egg is unique. You have two copies of every chromosome (46 chromosomes are actually 23 pairs), one copy is from your mother and one copy is from your father. Your eggs have only one copy. But the copy that each egg gets is unique because when your eggs were being made they didn’t get a complete set of just your mother’s chromosomes or just your father’s chromosomes. They didn’t even get whole chromosomes from either your father or your mother. The chromosomes in your eggs are a mixture of your father’s and you mother’s chromosomes. The swapping of genes that occurs to produce these mixed chromosomes is called crossing over. The degree of crossing over is different for every egg that is made so each egg is different. Sperm go through the same steps when they are being made, so each sperm a man produces is unique too.
I think maybe I phrased my question wrong (at least the first part of it). Let me try again:
Given that my sister and I have the same parents, are the odds of any particular combination of chromosomes in any given egg equally likely to show up in either one of us? In a practical sense, are my children any more likely to be very similar to myself than to my sister?
Thinking as I type…I guess they would have to be dissimilar because otherwise it would not be possible through genetic testing to determine whether a child was mine or my sister’s.
If I’ve understood what Dr Lao told us right: There are 23 chromosomes in each egg. Lets number them 1 through 23. The egg has to pick for each number whether it wants your mothers version of that chromosome or father’s version.
So the odds of two eggs making the same ‘picks’ would be 1 in 2^23 which is 1 / 8,3886,08.
So a little number chrunching says that in order for you and your sister to have just two eggs with identical genetic material (Barring any random mutations, which do happen) you’d both need to come from genetically identical eggs from your mother, genetically identical sperm from your father (the odds of each is 1 / 8,3886,08, so square that)
Then you’d each need one of your eggs to make the same picks as one of your sisters, again the odds would be 1 / 8,3886,08, so square it again and mutiply them together.
I recon the odds are 1 in 4,951,760,157,141,521,099,596,496,896 of you having one egg that’s the same chromosomes as your sister.
I don’t think I was entirely clear. Your eggs don’t get your mother’s version or your father’s version of any of your chromosomes. The chromosomes in your eggs are unlike any of the chromosomes in any other cell in your body. Each chromosome of each egg contains a mixture of the father and mother version of that chromosome. The amount of mixing varies from egg to egg. So, each egg is unique.
And, likewise, the eggs that created your siblings and you contained unique mixtures of your mother’s parents’ genetic contribution to her. Ditto for your father.
Let’s look at this at the gene level, rather than the chromosone level, so we don’t have to worry about crossovers. You still won’t see all the same genes on your eggs as on your sister’s. Consider a simple gene like hair color, for instance: If your mom has blond hair, and your dad has brown hair, their kids can have either. Let’s say that your sister has blond hair, and you have brown hair. That means that you have one brown hair gene, and your sister doesn’t have any brown hair gene (because if she did, she wouldn’t be blond, since brown is dominant). About half of your eggs will then have a brown-hair gene, and half won’t. But none of your sister’s eggs will have a brown gene, since she herself doesn’t have a brown gene. Likewise for any other gene you might consider.
You and your sister will, of course, have a lot of genes in common, since you’re sisters. So you might well see some resemblence between your kids and her, or vice versa. But your kids definitely have more in common with you, genetically, than with your sister.
As Dr. Lao has pointed out, we should be talking about genes rather than chromosomes. That said, geneticists commonly talk in terms of the coefficient of relatedness (the link refers to horses, but it was the simplest discussion I could find). This is an expression of the probability of the number of gene alleles you share with another individual by virtue of descent from a common ancestor.
For a parent and offspring, the coefficient of relatedness is 50%; that is, the offspring derives half its alleles from each parent. The coefficient between full siblings is also 50%; for each allele, they have a 50:50 probability of having inherited the same one from a parent.
The relatedness between an aunt or uncle and niece or nephew is 25%. This should be evident because your sister has a relationship of 50% to you, and you have a relationship of 50% to your offspring; so your sister will be half as related to your offspring as they are to you.
Note that these are the probabilities that alleles are identical by descent, not that they are the same in general. For example, if you and your spouse both have blue eyes, then your children can only have alleles for blue eyes. However, only have of these alleles will have been acquired from you by descent.