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If you have some extra time, or especially interested students, they might enjoy learning about where Punnett square comes from. Here's a brief summary of their history and man who developed them. The Punnett square, developed by Reginald Crundall Punnett (1875 - 1967), is used by biologists to predict the probability of possible genotypes. This allows the outcome of plant crossings or animal breedings to be predicted with better than chance accuracy. Punnett became interested in science and genetics as a boy. He suffered from appendicitis and began reading a series of books called Naturalist's Library during one of his recoveries. This led to him pursuing a degree at Cambridge University, he graduated in 1898 with a degree in zoology. The classic pea plant Punnett square comes from Punnett's research at Cambridge. During his time there he collaborated with William Bateson. Together they investigated gene linkage in sweet peas, resulting in the first publication on the subject.
It's always good for students to see a practical application of what they're learning. Even though Punnett squares are not especially difficult to do or tricky to remember they're no exception! They're generally more enthusiastic and willing to apply themselves if they have a practical application attached to the topic.
The practical application for Punnett squares comes from Punnett's work during World War I. During the war he applied his knowledge to the problem of determining the gender of baby chicks. It takes a talented and experienced eye to determine the gender of standard, all-yellow, chicks. To the average person they're indistinguishable until about three months, when the feathers in front of the tail (called saddle feathers) begin to change shape depending on the breed. It's a sad fact of war time that the roosters needed to be identified as early as possible to minimize the use of animal feed.
By crossing the Barred Rock with the Golden Campine, Punnett and Michael Pease, developed the Cambar breed. This was the first example of an auto-sexing chicken breed. The sex of the chick can be determined by its color at hatching with nearly 100% accuracy.
If you have an advanced class, or student, you can tie Punnett squares to statistics with the Hardy-Weinberg Law. This law was developed as a result of Punnett being dissatisfied with his ability to explain why recessive phenotypes continue from generation to generation.
The Hardy-Weinberg law states that alleles and genotypes remain constant through generations in the absence of deviating influences. The derivation is rather mathematically heavy but an interesting read. Ask your student to evaluate the Punnett square for females: A(p)a(q) and males: A(p)a(q) and determine what the sum of the three genotypes should be. That is, what should p2 + 2pq + q2 equal? You can show them the graph below to help point them in the right direction.