A genetics lesson by Blakeman and a PS [with a bold new theory] from Marie

photo by Cal Vornberger
May 29, 2004

Marie,

About Punnett Squares:

If you remember back in either high school or college freshman biology, when you learned that genetic traits are stored on matching pairs of chromosomes, you did some genetic probability problems, just like
the ones that will have to be done when we
see a white-breasted Central Park eyass, perhaps next spring.

It goes like this. Pale Male, Sr. appears to be homozygous dominant for the erythrism trait. He's got a gene for this trait on each of the pair of chromosomes with the trait.

(And red-tail chromosomes are an extremely interesting story itself, as they have, as I recall, a hundred or more chromosome pairs or fragments, far more than most vertebrates. And I believe this can vary from bird to bird, as the chromosome fragments don't always uniformly reconstruct for mitosis, cell division. In the non-mitotic state, the chromosomes are in a somewhat dissolved state as chromatin, not as physically condensed chromosomes that could be stained and seen microscopically. Someone with more genetic experience with red-tails will know more about this cellular curiosity.)

Back to the Punnett Square stuff. During meiosis, when Pale Male is making sperm (quite continuously in February and March, as Central Park hawk watchers have noted) each sperm cell will get only one of the two matched chromosomes. This keeps the chromosome number from doubling in each generation. Same happened for us, as we got about half our genes from our mothers and half from our fathers. Standard stuff for sexually reproducing species on the planet.

Therefore, Pale Male could only donate a dominant gene for breast color. For convenience (the Punnett Square stuff from way back in school will come back), we'll let a big E represent the gene for erythrism. The white-breasted gene is apparently recessive, which means for it to get expressed (to show up), both of the chromosomes have to have the recessive trait. In Punnett Square problems, we let the lower case letter of the dominant trait represent the recessive trait. So for our problem, little e will represent normal (but recessive) white-breastedness.

Pale Male's genotype is almost surely EE, homozygous dominant, with two copies of the trait. Each of his sperm cells are going to have a single E. As an adult, Lola is white-breasted. Let's presume she was white-breasted as an eyass. If so, her genotype is ee. Her chromosome pair with the breast color trait has an "e" on each one. In meiosis, when Lola makes an egg, she donates one of her e genes. When the egg is fertilized by Pale Male, the new egg will have a dominant E and a recessive e. The genotype will be Ee, said to be heterozygous. The eyass that grows from this egg must necessarily have a colored breast, as the E gene is dominant. If it's there, it gets expressed, no matter what the other gene is. In order for a red-tail to have a white breast, both of its breast color genes must be e.

Let's presume that Pale Male is homozygous dominant, with a genotype of EE. Let's likewise presume that Lola is homozygous recessive, ee. If so, all of their progeny must be Ee, heterozygous for the trait. Genetically, the kids will all be half-and-half, while Dad is "pure." But all the kids will have erythric breasts, indistinguishable from Dad's. What is seen is known as the phenotype. The phenotypes of Pale Male and his progeny are identical. Their genotypes, the actual genes, aren't.

To keep track of all of that, we make (finally) Punnett Squares. Let's do one of those problems, just the one I mentioned above. First, draw a 4-cell grid (Remember?). Above the top, let's put Pale Male's possible genes. Above the upper left cell write "E." Because he has nothing else, put the other "E" above the cell on the right.

Lola's got only a pair of little e's, so put an e to the left of each of the two cells on the left edge of the grid. Now, just drop the letters into the cells down and across, which represent the probabilities and possibilities of all of the offspring. In this Punnett Square problem, 100% of the offspring will have Ee genotypes, with erythritic breasts. Real simple.

But what happens if two of Pale Male's eyasses, each with an Ee genotype, mate (not at all improbable)? In this case, a big E and a little e go along the top and the side of the square. The offspring predicted by the upper left cell is EE. The lower left and upper right cells are both Ee. The lower right cell got an e from both parents, and will be the only hawk with a white breast from this pair.

But of course, our red-tails don't produce four eyasses each year. And even if they did, they wouldn't always produce three red-breasted and one white-breasted eyasses. It's all chance. All of the offspring could be white-breasted. All could have colored breasts. The Punnett Square allows us to easily parse the probabilities, not the number of offspring.

So far, no little e's have been observed in the phenotypes of Junior's offspring. The prevalence of erythrism in all the Central Park eyasses now strongly suggests that the birds are descendants from Pale Male, as I originally surmised.

Len Soucy's observations are definitive. There appears to be a narrow genetic base in the Manhattan red-tails. But it's not yet an inbreeding problem, as the several mothers are from external genetic lines. All of them have had white breasts. Genetically, it's been extremely helpful that Pale Male has had multiple mates. Romantically, it would have been nice for the original female to still be mated and producing Pale Male offspring. But inbreeding problems down the line might have presented themselves. Not so likely now.

Next March and April, we have the good chance to make out two Punnett Squares, one for the offspring at 927, and the other for new eyasses at Trump Parc. Pale Male can only sire more red-breasted birds. But Junior probably possesses a little e from his mother. On Friday's quiz, everyone reading will have to tell the various probabilities of Junior producing both white-breasted and red-breasted eyasses next spring. Show your Punnett Squares to substantiate your probabilities.

(It's been hard leaving the biology classroom. Nice to have such good students here.)

Sincerely,

John A. Blakeman

PS from Marie

Somehow I'm having trouble understanding this. What doesn't hang together for me is this: We have seen 23 chicks emerge from eggs in the Fifth Avenue nest since the first successful brood in 1995. Every single one of them has had an orange-red breast. According to John Blakeman's explanation, this seems highly improbable! [Oh yes, now add the two chicks in the Trump Parc nest. That's 25 erythristic chicks.] So maybe it's not a matter of genetics. Here's an audacious idea: How about diet? When the Fifth Ave. and the Trump-Parc babies hatch they are pure white. The orange chests seem to develop a few weeks later. So maybe there's some pigment in CP rats or pigeons? Don't flamingos get their red-orange color entirely from something they eat? Another clue in this direction is Len Soucy's report that the young red-tails he keeps at the Raptor Trust, having started white-chested, turn orange-chested after a year or two. Doesn't that sound more like the effects of diet than genetics?

PPS. I have no idea when that reddish-orange chest coloration of the nestlings and fledglings changes to white. In the last photographs of the Trump-Parc chicks, taken in the first week of September, the breast coloring is still conspicuously reddish. And all the photos of fledglings of past years seem to show that orangy color. [I'll have to check this with some of the park's reliable hawk observers like Ann Shanahan.] But Pale Male Junior has a white breast. If indeed he is a descendent of Pale Male, when did his breast color change?

PPPS. In some of Lincoln's photos of Pale Male taken in previous years I've seen some reddish glints in his chest feathers. I always thought that was a matter of how the sun was hitting his feathers as the picture was taken. Now I'm wondering... But certainly in most photos of PM and Junior they look white-chested.