“Dihybrid cross is the cross between two different genes that differ in two observed traits.” Gregor Johann Mendel was the first person who discovered the basic principles of heredity during the mid-19th century. Hence, he is known as the “Father of Modern Genetics”. He conducted experiments in his garden on pea plants and observed their pattern of inheritance from one generation to the next generation. Mendel laid the basic groundwork in the field of genetics and eventually proposed the laws of inheritance. Law of Segregation, Law of Independent Assortment and Law of Dominance are the three laws of inheritance proposed by Gregor Mendel. These laws came into existence from his experiments on pea plants with a variety of traits. Mendel first studied the inheritance of one gene in the plant through monohybrid cross. He considered only a single character (plant height) on pairs of pea plants with one contrasting trait. Later, he studied the inheritance of two genes in the plant through dihybrid cross. Mendel studied the following seven characters with contrasting traits: Also Read: Difference between Monohybrid and Dihybrid Cross A dihybrid cross is a breeding experiment between two organisms which are identical hybrids for two traits. In other words, a dihybrid cross is a cross between two organisms, with both being heterozygous for two different traits. The individuals in this type of trait are homozygous for a specific trait. These traits are determined by DNA segments called genes. In a dihybrid cross, the parents carry different pair of alleles for each trait. One parent carries homozygous dominant allele, while the other one carries homozygous recessive allele. The offsprings produced after the crosses in the F1 generation are all heterozygous for specific traits. Visualisation of dihybrid cross using a Punnett square Mendel took a pair of contradicting traits together for crossing, for example colour and the shape of seeds at a time. He picked the wrinkled-green seed and round-yellow seed and crossed them. He obtained only round-yellow seeds in the F1 generation. This indicated that round shape and yellow colour of seeds are dominant in nature. Meanwhile, the wrinkled shape and green colour of seeds are recessive traits. Then, F1 progeny was self-pollinated. This resulted in four different combinations of seeds in the F2 generation. They were wrinkled-yellow, round-yellow, wrinkled-green seeds and round-green in the phenotypic ratio of 9:3:3:1. During monohybrid cross of these traits, he observed the same pattern of dominance and inheritance. The phenotypic ratio 3:1 of yellow and green colour and of round and wrinkled seed shape during monohybrid cross was retained in dihybrid cross as well. Consider “Y” for yellow seed colour and “y” for green seed colour, “R” for round shaped seeds and “r” for wrinkled seed shape. Thus, the parental genotype will be “YYRR” (yellow-round seeds) and “yyrr” (green-wrinkled seeds). Also Read: Mendel’s Laws of Inheritance Explore BYJU’S biology to learn more about dihybrid cross and its examples. Further Reading: Gregor Mendel is known as the father of modern genetics. He was awarded this honour for his experiments which laid the groundwork for genetics and inheritance. Mendel’s experiment with peas is a classic example of a dihybrid cross. The experiment was done to highlight if any relationship exists between various pairs of alleles.
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A Punnett Square* shows the genotype*s two individuals can produce when crossed. To draw a square, write all possible allele* combinations one parent can contribute to its gametes across the top of a box and all possible allele combinations from the other parent down the left side. The allele combinations along the top and sides become labels for rows and columns within the square. Complete the genotypes in the square by filling it in with the alleles from each parent. Since all allele combinations are equally likely to occur, a Punnett Square predicts the probability of a cross producing each genotype. A single trait Punnett Square tracks two alleles for each parent. The square has two rows and two columns. Adding more traits increases the size of the Punnett Square. Assuming that all traits exhibit independent assortment, the number of allele combinations an individual can produce is two raised to the power of the number of traits. For two traits, an individual can produce 4 allele combinations (2^2). Three traits produce 8 combinations (2^3). Independent assortment typically means the genes are on different chromosome*s. If the genes for the two traits are on the same chromosome, alleles for each trait will always appear in the same combinations (ignoring recombination). With one row or column for each allele combination, the total number of boxes in a Punnett Square equals the number of rows times the number of columns. Multi-trait Punnett Squares are large. A three trait square has 64 boxes. A four trait square has 256 boxes. The genotype in each box is equally likely to be produced from a cross. A two-trait Punnett Square has 16 boxes. The probability of a cross producing a genotype in any box is 1 in 16. If the same genotype is present in two boxes, its probability of occurring doubles to 1/8 (1/16 + 1/16). If one of the parents is a homozygote for one or more traits, the Punnett Square still contains the same number of boxes, but the total number of unique allele combinations is 2 raised to the power of the number of traits for which the parent is heterozygous.
A commonly discussed Punnett Square is the dihybrid cross. A dihybrid cross tracks two traits. Both parents are heterozygous, and one allele for each trait exhibits complete dominance*. This means that both parents have recessive alleles, but exhibit the dominant phenotype. The phenotype ratio predicted for dihybrid cross is 9:3:3:1. Of the sixteen possible allele combinations:
A simpler pattern arises when one of the parents is homozygous for all traits. In this case, the alleles contributed by the heterozygous parent drives all of the variability. A two trait cross between a heterozygous and a homozygous individual generates four phenotypes, each of which are equally likely to occur. More complicated patterns can be examined. In an extreme case when more than two alleles exists for each trait and the parents do not possess same alleles, the total number of genotypes equals the number of boxes in the Punnett Square. It is possible to generate Punnett squares for more that two traits, but they are difficult to draw and interpret. A Punnett Square for a tetrahybrid cross contains 256 boxes with 16 phenotypes and 81 genotypes. A third allele for any one of the traits increases the number of genotypes from 81 to 108. Given this complexity, Punnett Squares are not the best method for calculating genotype and phenotype ratios for crosses involving more than one trait. Test your understanding with the Punnett Square Calculator Problem Set.
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What type of cross is a meeting between two individuals that are both heterozygous for the same two genes?
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