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Dihybrid crosses involve two traits. It is essential to recognize dihybrid (two-trait) crosses in order to solve them. Dihybrid crosses are predictions of how two traits will show up in the offspring produced by a mating between two parent organisms. If two traits were studied in dihybrid crosses, how many alleles would be involved in a Mendelian dihybrid cross: one, two, three, or four? (Remember, there are two alleles per trait.) The inheritance of two traits is studied in dihybrid crosses. For example, will the offspring of two guinea pigs have long black fur, short black fur, long brown fur, or short brown fur? DIHYBRID CROSSES A dihybrid cross involves two alleles per trait for two traits, for a total of four alleles. Gamete formation must occur for guinea pigs and other sexually reproducing organisms to reproduce. Thus, the first step to solving a dihybrid cross is to see how the various alleles (forms of the two traits) combine in the gametes. The principle of independent assortment explains this combining of alleles. Independent assortment means that each allele of a particular trait has an equal chance of being in a gamete with each of the alleles of the other trait; that is, B has an equal chance of ending up with S or s during gamete formation. This is allele pairing—alleles for the different traits pair. Independent assortment occurs during meiosis. For example, the male guinea pig's alleles for this dihybrid cross are BbSs. The possible allele combinations in the male guinea pig's gametes are: BS Bs bS bs In this example, the same possible allele combinations apply to the female, and to both guinea pigs' gametes. F1 guinea pigs are crossed (mated). These guinea pigs are heterozygous for black, short hair (BbSs). Next, all possible gamete combinations (with their alleles) from the two F1 guinea pigs must be considered. A Punnett square is an easy way to determine these combinations. PUNNETT SQUARE FOR DIHYBRID CROSS This Punnett square illustrates the predicted results of combining each of the gametes in the mating of the parent guinea pigs. One possible gamete combination is highlighted: male gamete BS; female gamete BS; possible offspring BBSS. The Punnett square represents all the possible gamete combinations of a mating between the two guinea pigs. The traits of the offspring can be determined by filling in the Punnett square based on the alleles (in each gamete) that intersect in each of the squares. PHENOTYPIC RATIO The gamete combinations predict which offspring (F2 generation) can be produced from these F1 parents. The phenotypic ratio (relative count of trait combinations) can now be determined. It is recommended that phenotypes be written in each square when working a Punnett square problem. Remember that phenotype is determined by genotype. Therefore, the phenotypes can be determined by the alleles in each offspring square: an uppercase letter represents a dominant allele. Any offspring with one uppercase letter for a particular trait will have that trait. For example, a B means the guinea pig will have black fur, and an S means the guinea pig will have short fur. Any trait represented by two lowercase letters, ss (long fur) and bb (brown fur) in this case, will be expressed. The purpose of determining the phenotypic ratio is to reveal an inheritance pattern. If the pattern is known, then the type of inheritance can often be determined. For example, the phenotypic ratio for this cross (two heterozygous individuals) is 9:3:3:1. This pattern is characteristic of a cross between two heterozygous parents with traits that exhibit complete dominance. The 9:3:3:1 ration would NOT be produced by any other combination of alleles, nor would this ratio result from traits exhibiting incomplete or codominance. UMMARY The inheritance patterns of two traits is studied in dihybrid crosses. The first step to solving a dihybrid cross is to see how the alleles of each trait could possibly combine during gamete formation. The next step is to use a Punnett square to determine how these gametes could combine to form offspring (F2 generation). The phenotypes (black, short, and so on) can be determined after the genotypes of the offspring (BbSs, and so on) are determined. A 9:3:3:1 phenotypic ratio was determined for this particular cross. The 9:3:3:1 ratio is typical for a dihybrid cross between two heterozygous individuals (BbSs X BbSs). #geneExpression #gene #prokaryotes #GeneticsExamQuestionsSolutions #alleles #eukaryotes #Anaphase #genotype #aminoAcid #gregorMendel #enzyme #genes #lawOfProbability #locus #RNA #cytoplasm #phenotype #GeneticsLecture #GeneticExamQuestionsSolutions #mitosis #codons #genotypes #viruses #eukaryotic #chromosomes #genetics #genomes #HowDoYouFindPhenotypeRatios #punnettSquareGenotypeAndPhenotype #dihybridCrossPunnettSquareTutorial #punnettSquareClass12 #punnettSquareClass10
