Q: If a set of identical twin women married a set of identical twin
men and subsequently had children would their children genetically be siblings?
I think they will, my friend A.J. says I am crazy. Help! (Juliana,
Casselberry, Florida)
The fathers (blue) of families A and B are identical twins; likewise, the
mothers (red) are identical twins. Therefore, all of their children (yellow) are
genetic siblings. Drawing courtesy of Through-the-Maze, modified by the author.
A: You are right; A.J. is wrong. All the children in both
families are genetic siblings, in addition to being just plain siblings within
each family. See figure.
First, a quick
refresher
on definitions:
- Chromosomes are long strands of DNA molecules (with associated proteins)
that carry essentially all the hereditary information.
- DNA are big molecules that contain genetic information (genes) for making
proteins.
- Genes are snippets of DNA molecules that determine how our cells make
proteins and, therefore, how our bodies look and function.
How inheritance works: Each parent has a full set of 46
chromosomes, but each only passes 23 chromosomes to their child. Which 23 does
a parent pass on to a given child? It's a dice throw —
which results in a
unique set of 23. That's why siblings differ: each has a
different set of 23 chromosomes from each parent. (Only identical
twins get exactly the same set of 23 chromosomes from each parent.)
How does a parent's reproductive process create
new chromosomes from his or her chromosome pool to
pass to a child? "Before
fertilization, eggs in the mother and sperm in the father undergo
genetic
recombination where pieces of the homologous [similar in function]
chromosomes swap places," say reproductive biologist
J. Lannett Edwards and her student
Rebecca Payton in an email from the University of Tennessee. This ensures every egg and every
sperm has a unique combination of traits. "Therefore, when an egg having 23
chromosomes and a sperm also having 23 chromosomes unite to form a new
individual (46 chromosomes), that child has a combination of genes that is
different from both parents and different from any non-twin sibling."
Definition of genetic siblings: "Two
children are siblings if the chromosomes both children received were derived
from the same pool of chromosomes coming from their parents," Edwards and Payton say.
Consider the children, Andy and Annie, of Family A (parents: Al and Alice). Andy received
23 chromosomes from
his father, Al, and 23 from his mother, Alice. The same is true for
Annie.
However, because of genetic
recombination mentioned above, the set that Annie got from each parent is
somewhat different from Andy's set. Annie has in common with Andy some
chromosomes, but not
all. So the instructions for making Annie and Andy are similar, but not
identical.
Since Al and Alice are identical
twins to Ben and Blanch, the
chromosome
pool belonging to Al and Alice is
identical to the chromosome
pool belonging to Ben and
Blanch. "Because all the kids get their chromosomes from an identical pool, they
could be considered as 'genetic' siblings,"
Edwards and Payton say.
Further Reading:
Learn genetics, the basics and beyond by Louisa Stark, University of Utah
Cell
reproduction by Dennis O'Neil, Palomar College
Twin trivia
Q: Identical twin sisters, one has a child. If DNA analysis was done
on the child and both women, would both women match as the mother of the child?
(Danielle, Clinton, Washington)
Yes, they would both match, because they both have identical DNA.
Q: What is meant by the term 'alternative universe' and is it real,
or fictional? (Someone, World)
A: Usually 'alternative universe' refers to another look at an
established story, often one written by a fan. For example, 19-year old
Hannah Jones, a student at Barnard College, writes about a group of young
wizards who attend Hogwarts School. Her current story, for scads of fans
over the Internet, is an alternative look at Harry Potter
— his parents' story.
Alternative universes are fiction; parallel universes, on the other hand, may
not be. Parallel universes (known also as a meta-universe: a 'multiverse')
are hypothetical universes that comprise all physical reality, including our own
universe. Physicists have postulated various types of multiverses to
bypass seeming contradictions and problems in our current understanding of physics, for
instance, in quantum mechanics.
An example that physicist
Max Tegmark of MIT gives is throwing a 'quantum
die.'
A
quantum die: Which side does it land on? All?!
Me: I roll a die; it
comes up '4'.
Tegmark: The die lands on all of its faces simultaneously:
1, 2, 3, 4, 5 and 6.
Me: How can this be?
Tegmark: The die lands on each of its 6 different faces
simultaneously — but each in a different universe.
"In one sixth of the universes, it lands on 1, in one sixth, on 2, and so on.
Trapped within one universe, we can perceive only a fraction of the full quantum
reality.
"Although there is indirect supporting evidence,
the idea is controversial, and physicists argue about whether it is science or
science fiction."
Strange, but perhaps real.
Further Reading:
Multiple universes, Max Tegmark's library
(Answered Nov. 20, 2006)
