The family that spits together…

You’ve always known that you have your dad’s curly hair, your mother’s eyes, and your grandmother’s coloring. But now that you’ve got your data back from 23andMe, you find yourself wondering whose side of the family the wet ear wax comes from (everyone denies having it), as well as whom to thank for the malarial resistance that came in handy during your last backpacking trek through Asia. Convince your family members to spit for science, and the answers to these and other burning questions may be at hand.

Consider Erin Mendel, a member of the family whose data is visible both to customers and holders of free demo accounts. Using the Compare Genes feature (reproduced below), you can see that among the genes associated with pigmentation Erin is closest to her brothers and mother, and less like her father Greg.

Erin\'s One-to-Many comparison chart

The Cards You’re Dealt

Assign each of your grandparents a suit — spades, hearts and so on – in a deck of cards. In the game of inheritance, their chromosomes (or chunks of genes) are shuffled (or recombined) and then dealt to their children so that each grandparent contributes one chromosome out of each pair. Your dad then ends up with 23 chromosomes of one suit from his father, and 23 of another from his mother. Your mom has a similar set of paired chromosomes from her two parents. So when your parents’ chromosomes

Erin Mendel\'s GrandTree

are shuffled, mixed together and dealt in turn to you and your siblings, you end up with a mix of chromosomes bearing all four of your grandparents’ suits. The illustration on the right shows the proportion of her genes that Erin inherited from each grandparent.

Since each child’s genetic information is produced by shuffling and dealing a new hand from the same genetic deck, there are going to be segments on their chromosomes where siblings are completely identical (having gotten the same suits from mom and dad). If a pair of siblings got the same chromosomal segment from one parent, but not from the other, they will be what geneticists call “half-identical.” In general, parents are half-identical to their children everywhere, because they passed their offspring one out of each pair of chromosomes. The exception is when two parents are related to each other.

Family Comparisons
Using the Family Inheritance option, Erin can see which segments she shares in common with various members of her family.

On the Genome-Wide Comparison chart, bars representing the 22 chromosomes and both X and Y chromosomes can be dark blue (representing a full match), light blue (“half-identity”), white (no match) and gray (not enough information). Below is a comparison of Erin and her father, showing only the first six chromosomes. Erin is only half-identical to her dad throughout her genome because one of her chromosomes out of each pair came from her mom Lilly.

Erin and Greg genome-wide comparison

So what does this all mean? Let’s consider the following example. When Erin compares herself to other people at the immune system compatibility trait, she sees red triangles above a region of chromosome 6 known as the MHC, or major histocompatibility complex. The human MHC is known as the HLA (human lymphocyte antigen) system. These genes determine how the immune system recognizes and distinguishes bacteria and other foreign invaders from the body’s own tissues. Aside from tests to verify that the blood types of donor and recipient match, the HLA system is what gets checked to minimize the chances that a transplant recipient’s immune system will reject the organ or tissue donated.

Erin and Alan\'s ISC comparison

Because they are unrelated, Erin’s parents Greg and Lilly have no identical segments in the HLA system so they probably wouldn’t be a good match for each other. One’s histocompatibility, as this gene harmony matching process is known, is inherited.

Greg and Lilly\'s ISC comparison

On the other hand, Erin might decide to make sure her brother Ian gets great birthday and Christmas presents from now on. When Erin does the one-on-one comparison with Ian using the Family Inheritance option, the results on the right suggest that he might be the best match for her if she ever needs a new kidney or some bone marrow. With “completely identical” HLA systems, the chances are good that they could donate marrow or a kidney to each other without immune rejection.

Erin and Ian\'s ISC comparison