Genetically Unlikely Punnett Squares

Some context — it might be! — is needed for this.

In illo tempore, two families there were, just as you know. Of an age, and neighbors, but not friends, nor even acquaintances. Townspeople, just as you know. One family, the one that lived beneath the belching smokestacks of the lumber mill, gave birth to twins, boy children, as like as two peas, as fair to behold as two moons. How sweet their eyes! How clear their skin! The other family, that lived, just as you know, above the grocery store by the Arena, also at this time (or, it might be, just a little later) gave birth to twins, girl children. Of these girls what is there to say? Two stars they were, as fair as He That Fell, and mild and sweet.

Time passed, just as you know, and children became adults. In the unconcerned way of townspeople, the twins met and fell in love, each with each. In the way of things they were married, man to woman, woman to man. And between the brothers and between the sisters was affection and companionship and charity. When the men were twenty-five, and eke the women, the women went in to the men — or the men went in to the women — and two children were born, one for each couple.

So. Listen, then, that this story might ring awhile longer between the hills…