New Zealand Science Teacher

A new way of looking at the dihybrid Punnett square

01/12/2015

The Punnett square is a firmly established tool used in explaining dihybrid crosses. So firmly established, that we don’t seem to ask whether it could be improved, writes MARTIN HANSON.

In the example below I have used Mendel’s cross between round, yellow seeded (RRYY) and wrinkled, green seeded varieties (rryy). The dihybrid has the genotype RrYy and makes four kinds of gamete, RY, Ry, rY and ry.

The 16 different ways four kinds of gamete can combine at fertilisation are usually shown in the checkerboard diagram named after the geneticist R. C. Punnett.

The four different kinds of gamete are conventionally written in the order RY, Ry, rY, ry.

The 16 different ways in which the gametes can combine yield nine different genotypes and four phenotypes in a pattern that depends on the order in which the gametes are written along the top and side of the square.

The conventional order in which the gametes are written is one of 24 possible sequences, so what’s so special about this one?

The answer is probably nothing, but the convention seems to have been set in stone ever since Punnett published his work early in the 20^th century.

There is, however, a different sequence that is arguably more informative than the traditional one, and is shown in Fig. 1, together with the traditional one for comparison.

The alternative Punnett is more useful than the conventional one, for several reasons:

• It is easier to count, and therefore check, the phenotypes and genotypes.
• Most importantly, it is easier to see that the 9:3:3:1 ratio is actually the product of two independent 3:1 ratios (12/16 to 4/16) for colour and shape, respectively (Fig. 2).

 

• The 3:1 ratios are independent of each other, as Fig. 3 shows:
• Of the yellow seeds, ¾ are round and ¼ are wrinkled.
• Of the green seeds, ¾ are round and ¼ are wrinkled.
• Of the round seeds, ¾ are yellow and ¼ are green.
• Of the wrinkled seeds, ¾ are yellow and ¼ are green. 

• The concept of independence in the segregation of chromosomes is, I believe, one of the most difficult to grasp in Level 2 genetics, so any visual tool that makes it clearer is surely worth adopting.
• As students are younger than their teachers, they should have more adaptable brains. What would constitute change for teachers would be a first-time experience for students, who would easily cope.
• That said, the only change teachers would have to cope with would be listing the genotypes of the four kinds of gamete in a different order.

The acid test is, or should be, the students. Just ask a class of Level 2 students who have not yet been taught dihybrid genetics, and show them the above two Punnett squares, and ask them to vote on which shows a clearer pattern.

It would be an interesting and, I suspect, revealing experiment.