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."

Identical twins get exactly the same chromosome set from each parent through a fluke in the reproductive process.  After the sperm fertilizes the egg (which forms an individual with 46 chromosomes) the resulting embryo will "sometimes split into two identical embryos." Identical twins result.

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)