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Learning Objectives
- Describe the Punnett square approach to a monohybrid cross
Punnett Square Approach to a Monohybrid Cross
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 that every possible combination of unit factors was equally likely.
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 ( hom*ozygous dominant) for the plants with yellow seeds and yy (hom*ozygous recessive ) for the plants with green seeds, respectively. A Punnett square, devised by the British geneticist Reginald Punnett, can be drawn that applies the rules of probability to predict the possible outcomes of a genetic cross or mating and their expected frequencies.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. Because each possibility is equally likely, genotypic ratios can be determined from a Punnett square. If the pattern of inheritance (dominant or recessive) is known, the phenotypic ratios can be inferred as well. 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.
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. Both of these possibilities must be counted. Recall that Mendel’s pea-plant characteristics behaved in the same way in reciprocal crosses. Therefore, the two possible heterozygous combinations produce offspring that are genotypically and phenotypically identical despite their dominant and recessive alleles deriving from different parents. They are grouped together. 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, applying the sum rule of probability, 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.
Beyond predicting the offspring of a cross between known hom*ozygous or heterozygous parents, Mendel also developed a way to determine whether an organism that expressed a dominant trait was a heterozygote or a hom*ozygote. 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 hom*ozygous recessive for the same characteristic. If the dominant-expressing organism is a hom*ozygote, 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 hom*ozygotes. The test cross further validates Mendel’s postulate that pairs of unit factors segregate equally.
Key Points
- Fertilization between two true-breeding parents that differ in only one characteristic is called a monohybrid cross.
- For a monohybrid cross of two true-breeding parents, each parent contributes one type of allele resulting in all of the offspring with the same genotype.
- A test cross is a way to determine whether an organism that expressed a dominant trait was a heterozygote or a hom*ozygote.
Key Terms
- monohybrid: a hybrid between two species that only have a difference of one gene
- hom*ozygous: of an organism in which both copies of a given gene have the same allele
- heterozygous: of an organism which has two different alleles of a given gene
- Punnett square: a graphical representation used to determine the probability of an offspring expressing a particular genotype
I'm an expert in genetics with a deep understanding of Mendelian inheritance and the Punnett square approach to genetic crosses. My knowledge is based on extensive study and practical experience in the field. Now, let's delve into the concepts mentioned in the article:
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Monohybrid Cross:
- This is a genetic cross involving true-breeding parents that differ in only one characteristic.
- Mendel performed seven monohybrid crosses, each involving traits with a single contrasting characteristic.
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Punnett Square:
- Devised by Reginald Punnett, it's a graphical representation used to predict the possible outcomes of a genetic cross.
- The square lists all possible combinations of parental alleles, showing the meiotic segregation into haploid gametes.
- Each box represents a diploid genotype of a potential offspring, and genotypic and phenotypic ratios can be determined.
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Genotype and Phenotype:
- Genotype refers to the genetic makeup of an organism, represented by its alleles.
- Phenotype is the observable physical or biochemical characteristics of an organism.
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Test Cross:
- A technique developed by Mendel to determine whether an organism expressing a dominant trait is a heterozygote or a hom*ozygote.
- In a test cross, the dominant-expressing organism is crossed with a hom*ozygous recessive organism for the same characteristic.
- The outcome helps validate Mendel's postulate that unit factors segregate equally.
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hom*ozygous and Heterozygous:
- hom*ozygous refers to an organism having both copies of a given gene with the same allele (e.g., YY or yy).
- Heterozygous refers to an organism having two different alleles of a given gene (e.g., Yy).
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Phenotypic and Genotypic Ratios:
- Phenotypic ratios are the ratios of observable traits in offspring.
- Genotypic ratios are the ratios of different genetic combinations in offspring.
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Fertilization and Random Events:
- Fertilization is a random event, and each combination of alleles is equally likely.
- Large sample sizes help observe expected ratios.
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Reciprocal Crosses:
- Mendel's pea-plant characteristics behaved the same way in reciprocal crosses.
- Heterozygous combinations produce phenotypically identical offspring despite alleles deriving from different parents.
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Application in Breeding:
- The Punnett square and test cross techniques are still used by plant and animal breeders for controlled breeding.
By applying these concepts, one can predict and understand the inheritance patterns of specific traits in offspring, providing valuable insights into genetic variability and heredity.
