In this lesson, you will learn about how to predict the possible outcomes of a genetic cross or mating by using Punnett squares. Specifically, this lesson will cover:
As you have learned, specific alleles for traits may be dominant or recessive in terms of how they are expressed. Natural variation in humans, such as the presence of new dominant or recessive alleles, is caused by mutations.
Recall that mutations can arise spontaneously from errors during DNA replication, or they can result from environmental insults such as radiation, certain viruses, or exposure to tobacco smoke or other toxic chemicals. Genes encode for the assembly of proteins, and a mutation in the nucleotide sequence of a gene can change the amino acid sequence and, in some cases, a protein’s structure and function (a nonsynonymous mutation). Although mutations are largely responsible for the natural variation of organisms, they can also cause disease.
Comparison of the Products of a Normal Gene With a Mutated Gene
think about it
How might you be able to predict the probability that your potential future offspring will have a known genetic abnormality that causes disease?
A Punnett square is a grid that can be drawn that applies the rules of probability to predict the possible outcomes of a genetic cross or mating and their expected frequencies. Genotypic ratios of offspring can be predicted from a Punnett square, and if the pattern of inheritance (dominant or recessive) is known, the phenotypic ratios can be inferred as well.
term to know
Punnett Square
A grid used to display all possible combinations of alleles transmitted by parents to offspring and predict the mathematical probability of offspring inheriting a given genotype.
2. Punnett Squares
The Punnett square is named after its creator, the British geneticist Reginald Punnett. To prepare a Punnett square, all possible combinations of the parental alleles are listed along the top (for one parent) and side (for the other parent) of a grid, representing their meiotic segregation into haploid gametes. Then, the combinations of egg and sperm are made in the boxes in the table to show which alleles are combining. Each box then represents the diploid genotype of a zygote, or fertilized egg, that could result from this mating.
IN CONTEXT
Predicting Hairline Probability With Punnett Squares
Below is a Punnett square for hairline showing whether offspring of two parents are predicted to have a straight hairline or a widow's peak. A widow's peak is a dominant trait; if a person has at least one dominant allele (W), they will have the widow's peak.
A Punnett square crossing the alleles for hairline.
The mom in this example is homozygous dominant for a widow's peak. This means that she has the same alleles (W and W), and both are dominant. Her phenotype would be the dominant trait, which in this case means she has a widow’s peak.
The father is heterozygous for the trait, meaning that he has one dominant allele (W) and one recessive allele (w). Since he has at least one dominant allele, it masks that recessive allele. The dad will also have a widow's peak as his phenotype.
How many of the offspring from this example are going to have a widow’s peak?
We can see that because each box has at least one big W, or dominant allele, 100% of their offspring will have a widow's peak.
It is important to note that Punnett squares demonstrate probability. Therefore, if two out of four offspring (represented by boxes in the Punnett square) are predicted to have a trait, there would be a 50% of offspring inheriting a trait; if three out of four are predicted to have a trait, there would be a 75% probability of offspring inheriting a trait.
2a. Monohybrid Crosses
When fertilization occurs between two true-breeding parents that differ in only one characteristic, the process is called a monohybrid cross, and the resulting offspring are monohybrids. Mendel performed seven monohybrid crosses involving contrasting traits for each characteristic. On the basis of his results in F1 and F2 generations, Mendel postulated that each parent in the monohybrid cross contributed one of two paired unit factors to each offspring, and every possible combination of unit factors was equally likely (we now know these factors are alleles).
To demonstrate a monohybrid cross, consider the case of true-breeding pea plants with yellow versus green pea seeds. The dominant seed color is yellow; therefore, the parental genotypes were YY for the plants with yellow seeds and yy for the plants with green seeds, respectively.
Recall that to prepare a Punnett square, all possible combinations of the parental alleles are listed along the top (for one parent) and side (for the other parent) of a grid, representing their meiotic segregation into haploid gametes. Then, the combinations of egg and sperm are made in the boxes in the table to show which alleles are combining. Each box then represents the diploid genotype of a zygote, or fertilized egg, that could result from this mating.
For a monohybrid cross of two true-breeding parents, each parent contributes one type of allele. In this case, only one genotype is possible. All offspring are Yy and have yellow seeds.
In the P generation, pea plants that are true-breeding for the dominant yellow phenotype are crossed with plants with the recessive green phenotype. This cross produces F1 heterozygotes with a yellow phenotype. Punnett square analysis can be used to predict the genotypes of the F2 generation.
A self-cross of one of the Yy heterozygous offspring can be represented in a 2 × 2 Punnett square because each parent can donate one of two different alleles. Therefore, the offspring can potentially have one of four allele combinations: YY, Yy, yY, or yy. Notice that there are two ways to obtain the Yy genotype: a Y from the egg and a y from the sperm, or a y from the egg and a Y from the sperm. Therefore, the two possible heterozygous combinations produce offspring that are genotypically and phenotypically identical despite their dominant and recessive alleles deriving from different parents.
Because fertilization is a random event, we expect each combination to be equally likely and for the offspring to exhibit a ratio of YY:Yy:yy genotypes of 1:2:1. Furthermore, because the YY and Yy offspring have yellow seeds and are phenotypically identical, we expect the offspring to exhibit a phenotypic ratio of 3 yellow:1 green. Indeed, working with large sample sizes, Mendel observed approximately this ratio in every F2 generation resulting from crosses for individual traits.
Mendel validated these results by performing an F3 cross in which he self-crossed the dominant- and recessive-expressing F2 plants. When he self-crossed the plants expressing green seeds, all of the offspring had green seeds, confirming that all green seeds had homozygous genotypes of yy. When he self-crossed the F2 plants expressing yellow seeds, he found that one-third of the plants bred true, and two-thirds of the plants segregated at a 3:1 ratio of yellow:green seeds. In this case, the true-breeding plants had homozygous (YY) genotypes, whereas the segregating plants corresponded to the heterozygous (Yy) genotype. When these plants self-fertilized, the outcome was just like the F1 self-fertilizing cross.
watch
View the video to learn more about Punnett Squares.
term to know
Monohybrid Cross
The result of a cross between two true-breeding parents that express different traits for only one characteristic.
2b. Test Crosses
Beyond predicting the offspring of a cross between known homozygous or heterozygous parents, Mendel also developed a way to determine whether an organism that expressed a dominant trait was a heterozygote or a homozygote. Called the test cross, this technique is still used by plant and animal breeders. In a test cross, the dominant-expressing organism is crossed with an organism that is homozygous recessive for the same characteristic. If the dominant-expressing organism is a homozygote, then all F1 offspring will be heterozygotes expressing the dominant trait. Alternatively, if the dominant expressing organism is a heterozygote, the F1 offspring will exhibit a 1:1 ratio of heterozygotes and recessive homozygotes. The test cross further validates Mendel’s postulate that pairs of unit factors segregate equally.
Test Cross—A test cross can be performed to determine whether an organism expressing a dominant trait is a homozygote or a heterozygote.
try it
Directions: In pea plants, round peas (R) are dominant to wrinkled peas (r). You do a test cross between a pea plant with wrinkled peas (genotype rr) and a plant of unknown genotype that has round peas. You end up with three plants, two of which have round peas and one of which has wrinkled peas. Answer the following questions about your pea plants.
The round pea parent would be heterozygous because if they were homozygous dominant, they would not be able to produce any wrinkled offspring.
Many human diseases are genetically inherited. A healthy person in a family in which some members suffer from a recessive genetic disorder may want to know if they have the disease-causing gene and what risk exists of passing the disorder on to their offspring. Of course, doing a test cross in humans is unethical and impractical. Instead, geneticists use pedigree analysis to study the inheritance pattern of human genetic diseases.
think about it
Alkaptonuria is a recessive genetic disorder in which two amino acids, phenylalanine and tyrosine, are not properly metabolized. Affected individuals may have darkened skin and brown urine, and may suffer joint damage and other complications. In this pedigree, individuals with the disorder are indicated in blue and have the genotype aa. Unaffected individuals are indicated in yellow and have the genotype AA or Aa. Note that it is often possible to determine a person’s genotype from the genotype of their offspring. For example, if neither parent has the disorder but their child does, they must be heterozygous. Two individuals on the pedigree have an unaffected phenotype but unknown genotype. Because they do not have the disorder, they must have at least one normal allele, so their genotype gets the “A?” designation.
Pedigree Analysis of Alkaptonuria
Based on the pedigree above, what are the genotypes of the individuals labeled 1, 2, and 3?
term to know
Test Cross
A cross between a dominant-expressing individual with an unknown genotype and a homozygous recessive individual; the offspring phenotypes indicate whether the unknown parent is heterozygous or homozygous for the dominant trait.
2c. Dihybrid Crosses
As you have learned, Mendel’s principle of independent assortment states that genes do not influence each other with regard to the sorting of alleles into gametes, and every possible combination of alleles for every gene is equally likely to occur. Independent assortment of genes can be illustrated by the dihybrid cross, a cross between two true-breeding parents that express different traits for two characteristics. Consider the characteristics of seed color and seed texture for two pea plants, one that has wrinkled, green seeds (rryy) and another that has round, yellow seeds (RRYY). Because each parent is homozygous, the law of segregation indicates that the gametes for the wrinkled, green plant all are ry, and the gametes for the round, yellow plant are all RY. Therefore, the F1 generation of offspring all are RrYy.
The gametes produced by the F1 individuals must have one allele from each of the two genes. For example, a gamete could get an R allele for the seed shape gene and either a Y or a y allele for the seed color gene. It cannot get both an R and an r allele; each gamete can have only one allele per gene. The law of independent assortment states that a gamete into which an r allele is sorted would be equally likely to contain either a Y or a y allele. Thus, there are four equally likely gametes that can be formed when the RrYy heterozygote is self-crossed, as follows: RY, rY, Ry, and ry. Arranging these gametes along the top and left of a 4 × 4 Punnett square gives us 16 equally likely genotypic combinations. From these genotypes, we find a phenotypic ratio of 9 round-yellow:3 round-green:3 wrinkled-yellow:1 wrinkled-green. These are the offspring ratios we would expect, assuming we performed the crosses with a large enough sample size.
Dihybrid Cross—A dihybrid cross in pea plants involves the genes for seed color and texture. The P cross produces F1 offspring that are all heterozygous for both characteristics. The resulting 9:3:3:1 F2 phenotypic ratio is obtained using a Punnett square.
try it
Directions: Answer the following questions based on what you learned about genotypes and phenotypes.
Possible genotypes are RrYY, RrYy, rrYY, and rrYy. Possible phenotypes are round and yellow (RrYY, RrYy), and wrinkled and yellow (rrYY, rrYy).
term to know
Dihybrid Cross
The result of a cross between two true-breeding parents that express different traits for two characteristics.
summary
In this lesson, you learned about how a Punnett square can be used to display possible combinations of alleles that offspring can inherit from their families. First, you learned about how predicting trait inheritance can be used to better understand disease heritability. You then explored how to construct Punnett squares to display possible genotypes and phenotypes of potential offspring. Specifically, you learned about how monohybrid crosses display possible genotype and phenotype combinations of offspring from a cross between two true-breeding parents with one different trait. Then, you explored how to predict a parent’s genotype using test crosses. Finally, you examined how dihybrid crosses display the result of crossing two true-breeding parents with different traits for two characteristics.
Terms to Know
Dihybrid Cross
The result of a cross between two true-breeding parents that express different traits for two characteristics.
Monohybrid Cross
The result of a cross between two true-breeding parents that express different traits for only one characteristic.
Punnett Square
A grid used to display all possible combinations of alleles transmitted by parents to offspring and predict the mathematical probability of offspring inheriting a given genotype.
Test Cross
A cross between a dominant-expressing individual with an unknown genotype and a homozygous recessive individual; the offspring phenotypes indicate whether the unknown parent is heterozygous or homozygous for the dominant trait.
