His technique employs what we now call a Punnett square. This is a simple graphical way of discovering all of the potential combinations of genotypes that can occur in children, given the genotypes of their parents.
It also shows us the odds of each of the offspring genotypes occurring. Setting up and using a Punnett square is quite simple once you understand how it works. You begin by drawing a grid of perpendicular lines:. Next, you put the genotype of one parent across the top and that of the other parent down the left side. For example, if parent pea plant genotypes were YY and GG respectively, the setup would be:. Note that only one letter goes in each box for the parents. It does not matter which parent is on the side or the top of the Punnett square.
Next, all you have to do is fill in the boxes by copying the row and column-head letters across or down into the empty squares. This gives us the predicted frequency of all of the potential genotypes among the offspring each time reproduction occurs. These will be the odds every time a new offspring is conceived by parents with YG genotypes. An offspring's genotype is the result of the combination of genes in the sex cells or gametes sperm and ova that came together in its conception.
One sex cell came from each parent. Sex cells normally only have one copy of the gene for each trait e. Each of the two Punnett square boxes in which the parent genes for a trait are placed across the top or on the left side actually represents one of the two possible genotypes for a parent sex cell. Which of the two parental copies of a gene is inherited depends on which sex cell is inherited--it is a matter of chance.
If you are not yet clear about how to make a Punnett Square and interpret its result, take the time to try to figure it out before going on. Why is it important for you to know about Punnett squares? The answer is that they can be used as predictive tools when considering having children. Let us assume, for instance, that both you and your mate are carriers for a particularly unpleasant genetically inherited disease such as cystic fibrosis.
Of course, you are worried about whether your children will be healthy and normal. For this example, let us define "A" as being the dominant normal allele and "a" as the recessive abnormal one that is responsible for cystic fibrosis. As carriers, you and your mate are both heterozygous Aa. This disease only afflicts those who are homozygous recessive aa. B is dominant to b , so offspring with either the BB or Bb genotype will have the purple-flower phenotype. Only offspring with the bb genotype will have the white-flower phenotype.
Therefore, in this cross, you would expect three out of four 75 percent of the offspring to have purple flowers and one out of four 25 percent to have white flowers.
These are the same percentages that Mendel got in his first experiment. A Punnett square can also be used to determine a missing genotype based on the other genotypes involved in a cross. Suppose you have a parent plant with purple flowers and a parent plant with white flowers. Because the b allele is recessive, you know that the white-flowered parent must have the genotype bb. The purple-flowered parent, on the other hand, could have either the BB or the Bb genotype.
The Punnett square in Figure below shows this cross. The question marks? This Punnett square shows a cross between a white-flowered pea plant and a purple-flowered pea plant. Can you fill in the missing alleles? What do you need to know about the offspring to complete their genotypes? Can you tell what the genotype of the purple-flowered parent is from the information in the Punnett square?
No; you also need to know the genotypes of the offspring in row 2. What if you found out that two of the four offspring have white flowers? Now you know that the offspring in the second row must have the bb genotype. The other b allele must come from the purple-flowered parent. Therefore, the parent with purple flowers must have the genotype Bb. When you consider more than one characteristic at a time, using a Punnett square is more complicated.
This is because many more combinations of alleles are possible. For example, with two genes each having two alleles, an individual has four alleles, and these four alleles can occur in 16 different combinations. This is illustrated for pea plants in Figure below. In this cross, known as a dihybrid cross , both parents are heterozygous for pod color Gg and pod form Ff. Punnett Square for Two Characteristics.
This Punnett square represents a cross between two pea plants that are heterozygous for two characteristics. G represents the dominant allele for green pod color, and g represents the recessive allele for yellow pod color.
F represents the dominant allele for full pod form, and f represents the recessive allele for constricted pod form. Draw a Punnett square of an Ss x ss cross. The S allele codes for long stems in pea plants and the s allele codes for short stems. If S is dominant to s , what percentage of the offspring would you expect to have each phenotype?
What letter should replace the question marks? Explain how you know. How do the Punnett squares for a monohybrid cross and a dihybrid cross differ? Mendel carried out a dihybrid cross to examine the inheritance of the characteristics for seed color and seed shape.
The dominant allele for yellow seed color is Y , and the recessive allele for green color is y.
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