Given the principles revealed in a monohybrid cross, Mendel hypothesized that the result of two characters segregating simultaneously (a dihybrid cross) would be the product of their independent occurrence. Consider two characters, seed color and seed shape. As previously shown, Y dominates y to determine seed color, and R factor for "round" dominates the r factor for "wrinkled" to determine seed shape. He then proceeded to test his hypothesis experimentally. The P (Parental) cross is between true-breeding lines of wrinkled yellow peas (rrYY) and round green peas (RRyy). The F1 offspring are therefore all RrYy, and are all round and yellow. In forming the F2 plants, the alleles at the two loci segregate independently. That is, the chance of getting an R allele and a Y allele is 1/2 x 1/2, of getting an R and a y 1/2 x 1/2, and so on. Thus, all four possible diallelic combinations occur with an equal probability of 1/4. The same is true for both parents. Given four possible gamete types in each parent, there are Show
Alternatively, recall that the phenotypic ratio expected for either character is 3:1, either 3 "Y" : 1 "y", or 3 "R" : 1 "R". Then, the expected phenotypic ratios of the two traits together can be calculated algebraically as a binomial distribution:(3Y + 1y) x (3R +
1r) = 9YR + 3Yr + 3Ry + 1 ry That is, we expect a characteristic 9:3:3:1 phenotypic ratio of round-yellow : wrinkled-yellow : round-green : wrinkled-green pea seeds. To predict the genotypic ratios, recall that for each gene the ratio is 1 : 2 : 1 :: AA : Aa : aa . Then, algebraically (1YY + 2Yy + 1yy) x (1RR + 2Rr + 1rr) = 1 YYRR + 2 YYRr + 1 YYrr + 2YyRR + 4YyRr + 2 Yyrr + 1yyRR + 2yyRr + 1yyrr That is, we expect a characteristic 1:2:1:2:4:2:1:2:1 ratio of the nine possible genotypes. These nine genotypes can be grouped into four phenotypes, for example 1 YYRR + 2 YYRr + 2 YyRR + 4 YyRr = 9Y-R- round, yellow peas. The ratio of these phenotypes is of course9:3:3:1. Mendel reported the results of some but not all of the "7 choose 2" = (7)(7-1)/(2) = 21 possible dihybrid crosses with seven characters. He performed several trihybrid crosses as well. Homework : (1) Repeat the analysis above with a cross of RRYY x rryy. (2) Predict the phenotypic and genotypic ratios of a trihybrid cross. Pea plants may be tall or short: use T for the tall allele, which is dominant to the t allele for short plants. How would you diagram such a cross?(3) Suppose one character is semi-dominant (Aa intermediate between AA and aa). Predict the phenotypic and genotypic ratios in the offspring of a dihybrid cross between AaBb x AaBb where A is semidominant to a, and B is dominant to b. 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. 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.
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What is a cross between two heterozygous individuals?A dihybrid cross describes a mating experiment between two organisms that are identically hybrid for two traits. A hybrid organism is one that is heterozygous, which means that is carries two different alleles at a particular genetic position, or locus.
What is the ratio of a heterozygous cross?A testcross to a heterozygous individual should always yield about a 1:1 ratio of the dominant to recessive phenotype.
When 2 heterozygous individuals are crossed what is the phenotypic ratio of the offspring?Answer and Explanation: If two heterozygous individuals are crossed, then the phenotypic ratio of the offspring would be 3:1(dominant:recessive).
What is the genotypic ratio of a monohybrid cross between two individuals who are heterozygous for an allele?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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