note that throughout this post, “son” and “daughter” refers only to non-twin, full siblings.
fathers are slightly more similar genetically to their daughters than they are to their sons … if my calculations are (*ahem*) correct.
ken weiss, professor of anthropology and genetics at penn state, has pointed out that:
“[S]ame-sex sibling sets are closer [in terms of genetic proximity] than brother and sister sets….
“Weiss offered a lesson in basic biology: Get a piece of paper and a pen. Draw a circle for the mother and a square for the father. Populate the circle with ‘XX’ for the female sex chromosome and the square with ‘XY’ for the male chromosome. Draw two more circles under the symbols for the parents to represent two sisters.
“‘The two daughters below them are going to inherit an X from their mother, one or the other. Each one gets an independent choice,’ said Weiss. ‘But if they’re daughters, they have to be XX, which means they must both inherit the same X from their father, since he only has one to give them. Everything else is scrambled equally between the two sisters from the two parents. They have one X that they share, and one X that they may or may not share, depending on the luck of the draw from the mother.’
“Now, if you were to draw two squares for brothers under the parents, you would pull one or the other X from the mother and the Y from the father, because to be a son you must be XY and the Y can only come from Dad.
“Humans have 46 chromosomes, inheriting 22 non-sex chromosomes, and one sex chromosome, from each parent. What they inherit from the other chromosomes is similar regardless of sex, which overall makes the child 50 percent related to each other and to each parent. In this sense, Weiss said, ‘Two brothers are as equally close to each other as two sisters. A brother and a sister are not as closely related as two brothers or two sisters. They’re a bit more distantly related.’ This is because one will have an X and the other a Y from their father, whereas two brothers must share the same Y, and two sisters the same X, from him.'”
this is more of the differential x-chromosome inheritance (plus differential y-chromosome inheritance) issue that fox, et. al., discussed in their study of the grandmother effect: different grandmothers (paternal vs. maternal) have different levels of relatedness to their various grandchildren dependent upon how their x-chromosomes are inherited by each type of grandchild.
only weiss is talking about members of the nuclear family: ma, pa, and the kids. he’s saying that brothers are slightly more related to each other, and sisters are slightly more related to each other, than either type of sibling is to the other type — because brothers inherit (virtually) the same y-chromosome from their father (which the sisters do not) AND sisters inherit (virtually) the same unrecombined x-chromosome from their father (which the brothers do not).
fox, et. al., calculated how related each type of grandmother is to each type of grandchild.
here are my calculations for how related the various members of a nuclear family are (assuming there’s no inbreeding). fox, et. al., based their calculations on the number of genes on the x- and y-chromosomes. i’ve decided to work the numbers according to total base-pairs ’cause, hey, junk dna ain’t all junk.
so, here we go…
for females, the autosomes represent 95% of their dna material; the two x-chromosomes, 5% of their dna material.
now, sons inherit half their autosomal dna from their father and half from their mother, plus a (virtually) unchanged y-chromosome from their father and a recombined x-chromosome from their mother (triangles=males; circles=females):
so, the genetic relatedness between a father and son (from both the father and son’s points-of-view) is:
(95.5% x 0.5) + 2% = 49.75%
and the genetic relatedness between a mother and son (from the mother’s pov) is:
(95% x 0.5) + (5% x 0.5) = 50%
the genetic relatedness between a mother and son from the son’s pov is:
(95.5% x 0.5) + (5% x 0.5) = 50.25%
daughters inherit half their autosomal dna from their father and half from their mother, plus a (virtually) unchanged x-chromosome from their father and a recombined x-chromosome from their mother:
so, the genetic relatedness between a father and a daughter (from the father’s pov) is:
(95.5% x 0.5) + 2.5% = 50.25%
from the daughter’s pov it is:
(95% x 0.5) + 2.5% = 50%
and the genetic relatedness between a mother and a daughter (from both the mother and daughter’s pov) is:
(95% x 0.5) + (5% x 0.5) = 50%
as weiss pointed out, brothers share (virtually) the same y-chromosome that they inherited from their father, while sisters share (virtually) the same x-chromosome that they inherited from their father. brothers and sisters do not share y-chromosomes or paternal x-chromosomes.
so, the genetic relatedness between full, non-twin brothers
can be up to is**:
(95.5% x 0.5) + (2.5% x 0.5) + 2% = 51%
and the genetic relatedness between full, non-twin sisters
can be up to is:
(95% x 0.5) + (2.5% x 0.5) + 2.5% = 51.25%
and between brother and sister (from the brother’s pov)
can be up to it is:
(95.5% x 0.5) + (2.5% x 0.5) = 49%
(95.5% x 0.5) x 0.5 + (95.5% x 0.5) x 0.5 + (2.5% x 0.5) = 49%
and between brother and sister (from the sister’s pov) it is:
(95% x 0.5) x 0.5 + (95% x 0.5) x 0.5 + (2.5% x 0.5) = 48.75%
so, fathers are genetically closer to their daughters than to their sons, and genetically closer (from the father’s pov) to daughters than mothers are to their daughters. mothers are equally close genetically (from the mother’s pov) to all their children, but are genetically closer to sons than fathers are to their sons.
full (not including twin) sisters can be the most closely related genetically of all nuclear family members (@51.25%), while full (not including twin) sisters and brothers can be the least related (@
49% 48.75% [sister’s pov]).
do, please, check my math! (^_^)
**the relationships between siblings are probabilities, not certainties.
(note: comments do not require an email.)