Question

What is the fraction of individuals with double h********* condition in a mendelian dihybrid cross?

Answer 1

Mendel'slaw of independent assortmentstates that the alleles of two (or more) different genes get sorted into gametes independently of one another. In other words, the allele a gamete receives for onegene does not influence the allele receivedfor another gene.Example: Pea color and pea shape genesLet's look at a concrete example of the lawof independent assortment. Imagine that we cross two pure-breeding pea plants: one with yellow, round seeds (YYRR) and one with green, wrinkled seeds (yyrr). Because each parent is homozygous, the law of segregation tells us that the gametes made by the wrinkled, green plant all arery, and the gametes made by the round, yellow plant are allRY. That gives usF1F1​offspring that are allRrYy.The allele specifying yellow seed color is dominant to the allele specifying green seed color, and the allele specifying round shape is dominant to the allele specifying wrinkled shape, as shown by the capital and lower-case letters. This means that theF1F1​plants are all yellow and round. Because they are heterozygous for two genes, theF1F1​plants are calleddihybrids(di-= two,-hybrid= heterozygous).A cross between two dihybrids (or, equivalently, self-fertilization of a dihybrid)is known as adihybrid cross. When Mendel did this cross and looked at the offspring, he found that there were four different categories of pea seeds: yellow and round, yellow and wrinkled, green and round, and green and wrinkled. Thesephenotypiccategories (categories definedby observable traits) appeared in a ratio of approximately9:3:3:19:3:3:1.