Download Genetic Relationship

Genetic Relationship of Family Members
This model is based on the number of identical inherited chromosomes two related persons
would have, on average. Each person has exactly half of his father's chromosomes, and exactly
half of his mother's chromosomes. But all other relationships are statistical. For example, on
average two siblings share half of their chromosomes. But it is technically possible for two
siblings to share 100% of their chromosomes (identical twins) or 0% of their chromosomes (if
one sibling receives one set of his mothers chromosomes, and the other sibling happens to
receive exactly the other half of the mother's chromosomes; and the same on the father's side). In
particular, a person can, depending on the luck of the draw, be totally unrelated -- in a
chromosomal sense -- to a particular grandparent, or, on the other extreme, receive half of his
chromosomes from one grandparent, and thus be as related to that grandparent as to a parent or
sibling.
Also, note that all human share over 99% of their genetic material. (And almost that much with
apes!) So what we are considering here are the other, variable genetic traits such as skin color,
hair color, height, tendency to be thin or overweight, tendency to have certain heritable diseases,
differences in sense of smell or taste, presence of talents such as athletic coordination, musical
perception, etc.
To use the diagram, find the name of the relationship between yourself and your relative, then
look up to the fraction on the top line.
Notes:
- For simplicity, masculine relationship names are used when a neutral name is not available.
- Some relationship names are abbreviated due to space. For example, 1 Cous 1 Rem means 1st
-
-
-
Cousin Once Removed. Also, Grand is Abbreviated Grd or G. GG mean Great Grand; etc.
By Removed we mean that one of the two sides of the relationship is one generation further away
from the other than would otherwise be. Sometimes people misstate a relationship because they are
not familiar with this term. For example, Mr. A may refer to his first cousin’s child as his second
cousin. But that is not legally correct. Legally, that child is Mr. A’s first cousin once removed. Note
that neither of the persons is said to be “removed”. It is the relationship which is “removed”.
Actual second cousins are persons who share a common set of Great-Grandparents (just as first
cousins share a common pair of Grandparents).
Half relationships, such as half-brothers, occur when those brothers share only half their parentage,
such as when the father marries a second wife, with one son from each wife.
Combinations: For multiple marriages with no other complications you can usually add the
father-side and mother-side relationships. For example, if a man marries a woman and has child A,
then marries that woman's sister and has child B, the relationship of A to B is:
> half-sibling relationship on father's side, which contributes 1/4
> 1st cousin relationship on mother’s side (B is the child of A's Mother's Sister) which contributes
1/8. Added, the relationship is 3/8.
Legal note: This table is from my own analysis of the situation from a mathematician’s perspective.
This analysis may not apply in a legal setting, or when getting genetic counselling. Get further
advice if the matter is very important.
Typical genetic content shared:
1/1
1/2
1/4
1/8
1/16
1/32
GGGG Parent
(older generations than self
Parent
Self
Sibling
Son
GGG Parent
GGG Uncle
GG Parent
GG Uncle
GG ½ Uncle
Grd Parent
Grd Uncle
G ½ Uncle
1 Cous 2 Rem
Uncle
½ Uncle
1 Cous 1Rem ½ Cous 1 Rem
Half Siblng
Or: Dbl 1 Cous
1st Cousin ½ Cousin
Dbl ½ Cous
2nd Cousin
Nephew
½ Nephew
½ Cous 1 Rem
Grand Son
Grd Nephew G ½ Nephw
1 Cous 2 Rem
GG Son
GG Nephew
GG ½ Nephew
GGG Son
GGG Nephew
(younger generations than self)
1Cous 1Rem
GGGG Son
By Ron Jones.