בבא בתרא קי, א
אמר רבא הנושא אשה צריך שיבדוק באחיה ... תנא רוב בנים דומין לאחי האם
Ravah said: If a man wants to marry a women he should examine the character of her brothers...It was taught in a Baraita: Most sons resemble their maternal uncles.
In today's page of Talmud we continue a discussion of the rights of inheritance, which evolves into a discussion of the influence that ancestors have over their descendents. The Baraita (a collection of teachings from the era of the Mishnah) cited in support of Ravah's advice notes that not all sons resemble their maternal uncles, but most do.
Shared Genetic Material
The amount of material we share with our relatives varies by the relationship. Only identical twins share all of their genetic material. We share 50% of our genes with each of our parents, and 50% with each of our siblings. But as you can see below, we share only 25% of our genetic material with our uncles or aunts - the same amount we share with our grandparents.
The amount of genetic material we share with our uncles or aunts does not vary with whether they are on our maternal or paternal side. A boy will no more genetically resemble his maternal uncle than they will his paternal uncle. And for girls, the rule is the same regarding their aunts.
We find this statement repeated in the Jerusalem Talmud, where it appears as part of a lengthy list originally stated by Abba Shaul:
תלמוד ירושלמי קידושין פרק ד
אבא אוריין איש צדיין אומר משום אבא שאול כו'. רוב חמרים רשעים. רוב גמלין כשירין. רוב ספנים חסידים. רוב ממזירין פקחין. רוב עבדים נאים. רוב בני אבות בויישנים. רוב בנים דומין לאחי האם כשר שברופאים לגיהנם. הכשר שבטבחין שותף עמלק
Most donkey drivers are evildoers, most camel drivers are upright, most sailors are pious, most Mamzerim are cunning, most slaves are haughty, most people of fine genealogy are embarrassed [to take from the public charity fund], most sons are like their uncles [i.e. their mother's brothers], the best doctors go to hell; the best butcher is a partner of Amalek...
When understood in this context, the connection between uncles and nephews is nothing more than a pithy generalization, and about as likely to be correct as the others on the list. Like all talmudic aphorisms, it was uttered in a different time and place from our own. If you really want to know who your child will resemble, you should look no further than you and your spouse. That will be the most likely predictor of your child's character, whether genetically encoded or socially constructed.