This Punnett square shows a cross between two heterozygotes, Bb. Do you know where each letter allele in all four cells comes from? Two pea plants, both heterozygous for flower color, are crossed. The offspring will show the dominant purple coloration in a ratio. In the cross shown in Figure above , you can see that one out of four offspring 25 percent has the genotype BB , one out of four 25 percent has the genotype bb , and two out of four 50 percent have the genotype Bb.
These percentages of genotypes are what you would expect in any cross between two heterozygous parents. Of course, when just four offspring are produced, the actual percentages of genotypes may vary by chance from the expected percentages. However, if you considered hundreds of such crosses and thousands of offspring, you would get very close to the expected results, just like tossing a coin.
You can predict the percentages of phenotypes in the offspring of this cross from their genotypes. 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. 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. Probability is the likely outcome a given event will occur from random chance. Examine the picture below that shows how sex chromosomes are inherited from parents. Stop and Think: Which parent determine the sex of a baby?
Is it the father or the mother? Answer: The father. The father contributes either an X or a Y chromosome. The mother only contributes an X. Look at the diagram above again. Stop and Think: Look at the family above. Mom and dad already have three girls in their family. What is the probability that the fourth child with be a boy? What is the probability of rolling two fives with a pair of dice? Punnett is a chart drawn to determine the probable results of a genetic cross.
It will show you every possible combination of offspring that result from a cross. Therefore, a Punnett square is a prediction that estimates what we should see in nature. The genes of the parents represented by assigned letters are written on the left-hand side and the top of the Punnett Square. Alleles of each gene are separated above each column on the top or into rows on the bottom.
For example if the letter Y represents the gene for pea pod color where yellow is dominant to green. One parent is heterozygous Yy and the other is homozygous yy, the completed Punnett Square would look like the one on the right. A completed Punnett square gives the probable outcome of a given cross. In this case, the probability of each phenotype green vs. We can write these offspring probabilities as ratios to simplify our results:.
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