From Newsgroup: talk.origins

I understand that we get half our DNA from each of our parents. How
does that work backwards? I am into family research and have
identified all 8 of my great-grandparents; did each of them
contribute exactly 1/8 of my DNA or is that an approximation?

In my case, I have 8 distinct great-grandparents What happen if there
is some cross-breeding? Let's say that two of my grandparents were
cousins so I only have seven different great-grandparents, Would that
mean one of the 7 would have contributed 1/4 of my genes and the other
six contributing 1/8 ?

--- Synchronet 3.21a-Linux NewsLink 1.2

From Newsgroup: talk.origins

On Thu, 11 Sep 2025 22:09:39 +0100, the following appeared
in talk.origins, posted by Martin Harran
<martinharran@gmail.com>:

I understand that we get half our DNA from each of our parents. How
does that work backwards? I am into family research and have
identified all 8 of my great-grandparents; did each of them
contribute exactly 1/8 of my DNA or is that an approximation?

AFAIK it's exact; each parent contributes one-half of the
genes for the offspring (I don't know how or if the X-Y size
difference affects this).

If I'm mistaken, someone will correct me. ;-)

In my case, I have 8 distinct great-grandparents What happen if there
is some cross-breeding? Let's say that two of my grandparents were
cousins so I only have seven different great-grandparents, Would that
mean one of the 7 would have contributed 1/4 of my genes and the other
six contributing 1/8 ?

Even if they were cousins, their genetics are not identical;
probably not even close. Only identical twins or clones have
identical genetics, Again, AFAIK.

--

Bob C.

"The most exciting phrase to hear in science,
the one that heralds new discoveries, is not
'Eureka!' but 'That's funny...'"

- Isaac Asimov

--- Synchronet 3.21a-Linux NewsLink 1.2

From Newsgroup: talk.origins

On 9/11/2025 4:09 PM, Martin Harran wrote:

I understand that we get half our DNA from each of our parents. How
does that work backwards? I am into family research and have
identified all 8 of my great-grandparents; did each of them
contribute exactly 1/8 of my DNA or is that an approximation?

In my case, I have 8 distinct great-grandparents What happen if there
is some cross-breeding? Let's say that two of my grandparents were
cousins so I only have seven different great-grandparents, Would that
mean one of the 7 would have contributed 1/4 of my genes and the other
six contributing 1/8 ?

I was involved in genomic selection. How it works is you figure out the genomic relationship between all individuals back to around 3 or more generations. You use the genetic variation segregating in the
population to determine how much of their genomes they have in common.
Since the lines had been closed for decades all the animals are related
to each other in some way so you get a genomic relationship with animals
not derived from the same families just 3 generations back. It is a
better and more accurate means of determining how related each
individual is instead of relying on pedigree that relies on probalistic outcomes of vagaries of segregation. On average you would inherit about
25% of the genetics of one grandparent from one of your parents, but
because of random segregation from that parent one of the parent's
parents can get short changed and you inherit more than 25% from one and
less than 25% from the other.

In the case of cousin matings the progeny can end up looking like they inherited more than 50% of one of their parents genetics. This is
because some of the same genetics are coming from both cousins involved
in the mating. The progeny end up with an average of 6.25%
homozygousity from their parent's shared ancestor for first cousin
matings. Since the cousins share 25% of their genetics their progeny
only inherit on average 12.5% from one parent, but some of the 25% that
is not transmitted from one parent can be transmitted from the other
parent. This makes the progeny of cousin matings look like they
inherited more than 50% of their genetics from one parent because both
cousins have genetics in common.

That is what you are going to be seeing in your backtrack analysis if
you could obtain DNA from those individuals. For your scenario of
cousin matings among one set of grandparents you still have 8 great grandparents, but two of the great grandparents are sibs so you would
have 14 in stead of 16 great great grandparents. One of your parents
would be inbred. By the time the genetics segregated down to you it
would look like the inbred grand parent mating contributed more in your
genome than 25% each. You would have inherited some of the inbred
genetics from both of them making it look like your inbred parent had inherited more than 50% of their genome from their cousin parents, and
passed half of that more than 50% down to you.

Ron Okimoto



--- Synchronet 3.21a-Linux NewsLink 1.2

From Newsgroup: talk.origins

On 11/09/2025 22:09, Martin Harran wrote:

I understand that we get half our DNA from each of our parents. How
does that work backwards? I am into family research and have
identified all 8 of my great-grandparents; did each of them
contribute exactly 1/8 of my DNA or is that an approximation?

It's an approximation. There's a number of factors that mean that the
50% is an approximation. What is exact, in almost all humans, is that
the same number of nuclear chromosomes is inherited from both parents.

Firstly, if you're male you inherit less nuclear DNA from your father
than from your mother (a Y chromosome rather than X chromosome). This
amounts to about 1.5% less.

Secondly, mitochondrial DNA is uniparentally inherited (though there is
some paternal leakage); you inherit that from your mother only. But this
is only 0.005% of genome. There is also the potential complication of heteroplasmy.

Thirdly, chromosomes are not the exact same length in all individuals,
with insertion, deletion, duplication, ... variants segregating in the population. I suspect that we don't have good data on the degree of
variation in genome size; the difficulty of sequencing repetitive DNA
means that the exact size of a genome is not precisely known, and I
suspect that measurement noise blurs the data from flow cytometry. (Flow cytometry estimates the mass of DNA per cell.)

Fourthly, microchimerism means that some children contain maternal
derived cell lines, though this is less common than the occurrence of
fetal derived cell lines in mothers.

The number of genes inherited from each parent is a closer match, but
there's still the matter of the mitochondrial genes, the smaller number
of genes on the Y-chromosome than on the X-chromosome, and copy number variation in nuclear genes.

When it comes to multigeneration inheritance, ignoring crossing over
each of the 23 chromosomes inherited by a parent from one grandparent
has a 50% chance of being passed onto a grandchild. So the amount of the genome inherited by the grandchild could be between none and a 1/2, with
1/4 being the most probably result. The chance of none is 1 in 8
million. When crossover is taken into account the result is that there
are more genetic segments involved, inheritance is more strongly peaked
around 25% of the genome, and the chances of 0 or 50% are so low that it
may not have happened prior to the last couple of centuries.

Excluding identical twins and chimaeras from consideration

Most everybody of European descent is descended from Charlesmagne, but
that doesn't mean that they've inherited any of his genome.

In my case, I have 8 distinct great-grandparents What happen if there
is some cross-breeding? Let's say that two of my grandparents were
cousins so I only have seven different great-grandparents, Would that
mean one of the 7 would have contributed 1/4 of my genes and the other
six contributing 1/8 ?

On average 1/4, but varying between 0 and 1/2. I want to say more
strongly peaked around 1/4 than grandparent contributions, but I haven't
done an analysis to confirm that.
--
alias Ernest Major

--- Synchronet 3.21a-Linux NewsLink 1.2

From Newsgroup: talk.origins

On 9/11/25 2:09 PM, Martin Harran wrote:

I understand that we get half our DNA from each of our parents. How
does that work backwards? I am into family research and have
identified all 8 of my great-grandparents; did each of them
contribute exactly 1/8 of my DNA or is that an approximation?

Because of recombination, it's all approximate.

In my case, I have 8 distinct great-grandparents What happen if there
is some cross-breeding? Let's say that two of my grandparents were
cousins so I only have seven different great-grandparents, Would that
mean one of the 7 would have contributed 1/4 of my genes and the other
six contributing 1/8 ?

Yes, approximately.

--- Synchronet 3.21a-Linux NewsLink 1.2

From Newsgroup: talk.origins

On Thu, 11 Sep 2025 22:09:39 +0100, Martin Harran
<martinharran@gmail.com> wrote:

I understand that we get half our DNA from each of our parents. How
does that work backwards? I am into family research and have
identified all 8 of my great-grandparents; did each of them
contribute exactly 1/8 of my DNA or is that an approximation?

In my case, I have 8 distinct great-grandparents What happen if there
is some cross-breeding? Let's say that two of my grandparents were
cousins so I only have seven different great-grandparents, Would that
mean one of the 7 would have contributed 1/4 of my genes and the other
six contributing 1/8 ?

Thanks all for the explanations. I understand it a bit better now :)

--- Synchronet 3.21a-Linux NewsLink 1.2