What is the phenotypic ratio of a cross between heterozygous and heterozygous?
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Dihybrid cross is a cross between two individuals with two observed traits that are controlled by two distinct genes. The idea of a dihybrid cross came from Gregor Mendel when he observed pea plants that were either yellow or green and either round or wrinkled. Crossing of two heterozygous individuals will result in predictable ratios for both genotype and phenotype in the offspring. The expected phenotypic ratio of crossing heterozygous parents would be 9:3:3:1.[1] Deviations from these expected ratios may indicate that the two traits are linked or that one or both traits has a non-Mendelian mode of inheritance. Show
Mendelian History[edit]Gregor Mendel was an Austrian monk who bred peas plants in his monastery garden and compared the offspring to figure out inheritance of traits form 1856-1863.[2] He first started looking at individual traits, but began to look at two distinct traits in the same plant. In his first experiment, he looked at the two distinct traits of pea color (yellow or green) and pea shape (round or wrinkled).[3] He applied the same rules of a monohybrid cross to create the dihybrid cross. From these experiments, he determined the phenotypic ratio (9:3:3:1) seen in dihybrid cross for a heterozygous cross.[1] Through these experiments, he was able to determine the basic law of independent assortment and law of dominance. The law of independent assortment states that traits controlled by different genes are going to be inherited independently of each other.[3] Mendel was able to determine this law out because in his crosses he was able to get all four possible phenotypes. The law of dominance states that if one dominant allele is inherited then the dominant phenotype will be expressed.[3] Expected genotype and phenotype ratios[edit]The phenotypic ratio of a cross between two heterozygotes is 9:3:3:1, where 9/16 of the individuals possess the dominant phenotype for both traits, 3/16 of the individuals possess the dominant phenotype for one trait, 3/16 of the individuals possess the dominant phenotype for the other trait, and 1/16 are recessive for both traits.[1] Valid only for Angiosperms or similar sexually reproducing organisms. This is assuming that Mendel's laws are followed. The expected phenotypic ratio of 9:3:3:1 can be broken down into:
In the example pictured to the right, RRYY/rryy parents result in F1 offspring that are heterozygous for both R and Y (RrYy).[4] This is a dihybrid cross of two heterozygous parents. The traits observed in this cross are the same traits that Mendel was observing for his experiments. This cross results in the expected phenotypic ratio of 9:3:3:1. Another example is listed in the table below and illustrates the process of a dihybrid cross between pea plants with multiple traits and their phenotypic ratio patterns. Dihybrid crosses are easily visualized using a 4 x 4 Punnett square. In these squares, the dominant traits are uppercase, and the recessive traits of the same characteristic is lowercase. These findings were incompatible with the idea of blending. Each parental character was recovered intact in the F2, rather than being “lost” in the F1. Mendel reasoned that the yellow peas in the P1 were not identical to the yellow peas in the F1 because the P1 yellows were true breeding and the F1 yellows were not. He proposed that the trait which appeared in the F1 was dominant and that the trait which disappeared in the F1 but reappeared in the F2 was recessive. But what accounted for the reproducible 3 : 1 ratio? Mendel proposed—in an astonishingly prescient way—that each plant carried two copies of a unit of inheritance for each trait, one inherited from the male, one from the female. He proposed further that each unit comes in alternative forms that give rise to the differentiating characteristics he studied (yellow–green, round–wrinkled, etc.). Today, we call his “units” “genes” and his “alternative forms” “alleles.” He went on to propose that the two alleles found in cells of a mature plant segregate (separate) during germ cell formation and reunite, one from each parent, at fertilization. Mendel set out to find laws of inheritance. This was his first: the law of gene segregation. The law explains the 3 : 1 ratio in the F2 as follows (Figure 5.4), using the visually accessible Punnett square (a diagram that is used to predict an outcome of a particular cross or breeding experiment). The true-breeding yellow pea plants (P1) have two copies of the dominant allele, denoted Y; the plants yielding only green peas have two copies of the recessive allele, denoted y. (Capital letters generally depict the dominant allele, small letters the recessive one.) Gametes of these P1 plants (YY and yy, referred to as “homozygotes”) are either Y or y. At fertilization, all zygotes are Yy (heterozygotes). Because Y is dominant, all plants are yellow. When these plants are self-fertilized, the male and female each produce gametes that are either Y or y. In the F2, then, 1/4 of the progeny are YY, 1/4 are Yy, 1/4 are yY, and 1/4 are yy. Given that Y is dominant, and that Yy and yY are equivalent, the ratio between yellow and green peas is 3 : 1. View chapterPurchase book Read full chapter URL: https://www.sciencedirect.com/science/article/pii/B9780123852120000056 Transmission GeneticsJ.R. Fabian, in Encyclopedia of Genetics, 2001 Monohybrid Cross and Test CrossMendel's cross-hybridization studies involved purebred plants that differed with regard to a single contrasting trait. Purebred, homozygous, parental stocks were crossed and the offspring of this cross are called F1 hybrids, or monohybrids. In the F1 generation, all of the hybrids resembled the parent with the dominant trait. The genotype of these monohybrid, or heterozygous, plants can be represented as genotype Aa, with the uppercase letter representing the dominant allele and the lowercase letter representing the recessive allele. The F1 hybrid plants were next self-fertilized (Aa×Aa) and this cross is known as a monohybrid cross. In the offspring of monohybrid crosses, or F2 generation, Mendel repeatedly observed a phenotype ratio of three plants with the dominant phenotype to one plant with the recessive phenotype (3:1 phenotype ratio) in the F2 generation. Mendel predicted that the plants with a dominant phenotype in the F2 generation were of mixed genotypes with some being homozygous dominant genotype AA and others being heterozygous genotype Aa. In order to determine the genotypes of plants with dominant phenotypes in the F2 generation Mendel devised the test cross. The test cross takes the organism with a dominant phenotype but unknown genotype and crosses it to a homozygous recessive individual with a known genotype aa. In a test cross with a plant of genotype AA all offspring will have the dominant phenotype and will have the heterozygous genotype Aa. However, if a plant with genotype Aa is used in a test cross, then the genotypes of 50% of the offspring will have the genotype Aa and display the dominant trait. The other 50% will be display the recessive phenotype since they will have the homozygous recessive genotype aa. Mendel's test cross method is still used today in breeding procedures with plants and animals in order to determine the genotype of plants with dominant phenotypes. View chapterPurchase book Read full chapter URL: https://www.sciencedirect.com/science/article/pii/B0122270800013112 Mendel’s LawsR. Lewis, in Brenner's Encyclopedia of Genetics (Second Edition), 2013 The First Generation from the Hybrids, and BeyondThe fifth section of Mendel’s paper shows, repeatedly, that the dominant and recessive forms of each trait appear in a 3:1 ratio in the progeny of hybrids crossed to each other. The numbers speak for themselves in Table 3. Mendel showed the classic 3:1 phenotypic ratio of a monohybrid cross (one trait present in two forms, or alleles), although the terms ‘phenotype’ (an individual’s appearance) and ‘genotype’ (the gene variants present) were not yet in use. This observation would become known as Mendel’s first law, or the law of segregation, years later (Figure 1). The ratios that Mendel chronicled were actually the result of meiosis, the type of cell division that gives rise to gametes. When a sperm or egg forms, the chromosome pairs (homologous pairs), whose DNA has been replicated, separate. Likewise, the pairs of genes that comprise the chromosomes separate and are distributed into different gametes. The part of meiosis that determines the gene combinations that will enter gametes, and eventually be expressed in organisms, is called metaphase, when chromosomes align down the center of the cell. Table 3. The ‘first generation from the hybrids’ experiments reveal a 3:1 dominant-to-recessive phenotypic ratio ExperimentTotalDominantRecessiveRatioSeed form7324547418502.96:1Seed color8023602220013.01:1Seed coat color9297052243.15:1Pod form11818822992.95:1Unripe pod color5804281522.82:1Flower position8586512073.14:1Stem length10647872772.84:1Average2.98:1  Figure 1. Mendel derived what would become known as his first law, the law of segregation, by crossing plants that ‘bred true’ for tall to plants that bred true for short, in the parental or P1 generation. All the plants of the first filial (F1) generation were tall. Allowing the F1 plants to self-fertilize yielded an F2 generation of plants in which tall plants outnumbered short plants three to one. By conducting further crosses of the F2 plants to short plants, Mendel deduced the genotypic ratio in the F2 generation to be one short to two tall non-true-breeding hybrids to one tall true-breeding. Reproduced with permission from Lewis R (2010) Human Genetics: Concepts and Applications, 9th edn. New York: McGraw-Hill.Mendel followed crosses beyond the third generation, determining that the dominant-appearing individuals among the progeny of the hybrids had ‘double signification’, meaning that they were of two types. He wrote, “… of those forms which possess the dominant character in the first generation, two-thirds have the hybrid character, while one-third remains constant with the dominant character.” One type bred true, always yielding the dominant phenotype in further crosses. The second type, when crossed to hybrids, produced both the dominant and recessive phenotypes. The plants that did not breed true outnumbered the other plants two to one. Today, we call the dominant-appearing plants that are ‘constant’ homozygous dominant. They have two copies of the dominant allele. The hybrids, called heterozygotes, have one dominant and one recessive allele. Individuals expressing the recessive trait constitute the homozygous recessive class, and they too breed true. That is, when crossed among themselves, they yield only homozygous recessive individuals. A monohybrid cross results in a phenotypic ratio of 3:1 (dominant to recessive), and a genotypic ratio of 1:2:1 (homozygous dominant to heterozygous to homozygous recessive). Mendel carried out crosses for four to six generations for each of the seven traits, each time self-crossing the individuals that ‘bred true’ (the homozygous dominants and homozygous recessives) as well as self-crossing the hybrids. When he did this repeatedly, the proportion of hybrids decreased by 50% at each generation. By the 10th generation, only two hybrids would remain for every 1023 individuals of each homozygous class. What is the phenotypic ratio of a cross between two heterozygous parents?The phenotypic ratio of a cross between two heterozygotes is 9:3:3:1, where 9/16 of the individuals possess the dominant phenotype for both traits, 3/16 of the individuals possess the dominant phenotype for one trait, 3/16 of the individuals possess the dominant phenotype for the other trait, and 1/16 are recessive for ...
What does a 9 3 3 1 phenotypic ratio mean?This 9:3:3:1 phenotypic ratio is the classic Mendelian ratio for a dihybrid cross in which the alleles of two different genes assort independently into gametes. Figure 1: A classic Mendelian example of independent assortment: the 9:3:3:1 phenotypic ratio associated with a dihybrid cross (BbEe × BbEe).
What does a 1 2 1 phenotypic ratio mean?A cross of two F1 hybrids, heterozygous for a single trait that displays incomplete dominance is predicted to give a 1:2:1 ratio among both the genotypes and phenotypes of the offspring.
What type of cross gives you a 3 1 phenotypic ratio?A monohybrid cross results in a phenotypic ratio of 3:1 (dominant to recessive), and a genotypic ratio of 1:2:1 (homozygous dominant to heterozygous to homozygous recessive).
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