Concept 5 Genetic inheritance follows rules.

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Reginald Punnett and William Bateson explain Mendel's ratios.

I'm Reginald Punnett. William Bateson and I were very keen on Mendel's laws of inheritance. Herr Mendel was correct about how parental genes are passed to offspring. He was also correct about the 3 to 1 ratio of dominant to recessive traits in a hybrid cross. Let me show you what happens in these crosses. Each trait is defined by a pair of genes. Each parent can produce two types of gametes: one allele is "randomly separated" into each gamete. This is Mendel's law of segregation. I invented the Punnett square to keep track of the alleles in the gametes. So, in Herr Mendel's Yy heterozygous cross, the alleles of one parent are represented along the top, the other along the side of the square. See how they pair up in the empty boxes that represent the offspring. Offspring result from pairing of the parental gametes; one from each parent. The offspring genotypes are YY, Yy, and yy. Of course, yy peas are green. And while YY and Yy peas have different genotypes, they have the same yellow phenotype. As you can see, there are three yellow peas to one green pea. This is Mendel's 3 to 1 ratio. My squares are especially useful when tracking the inheritance of more than one trait at a time. Try the problem in this section and you'll see what I mean.

factoid Did you know ?

Some genes in two copies are lethal to the organism.


The Manx breed of cats is known for being tailless, though some are born with tails. A dominant gene shortens the spine and is the cause for no tail. In a cross between two tailless Manx, you get a litter of kittens where for every 2 tailless kittens, there is one with a tail. What happened to Mendel's 3 to 1 ratio?