A guinea pig with short orange fur is crossed with a guinea pig with long orange fur. The offspring are also crossed, and the F₂ generation contains 17 guinea pigs with long orange fur, 2 guinea pigs with short beige fur, 6 with short orange fur, and 7 with long beige fur.
Which of the following phenotypes are dominant?

Question

Quizplus · Accepted Answer

According to Mendel's law of independent assortment, alleles of two different genes get sorted into gametes (sex cells) independently of one another. Therefore, the allele one gamete receives for a gene does not influence the allele received for another gene.
In this example, we see a cross between two dihybrids. A dihybrid is an organism that is heterozygous for two genes. This means that the organism has two different alleles for each gene (e.g. AaBb). A cross between two dihybrids is known as a dihybrid cross.
When Mendel did a dihybrid cross with his pea plants, he found that there were four different categories of pea seeds in the offspring: yellow and round, yellow and wrinkled, green and round, and green and wrinkled. These categories, which were defined by observable traits (phenotypes) appeared in a ratio of approximately 9:3:3:1.
We can use this approximate ratio to determine which of the phenotypes are dominant.
If we mark fur color with the gene Y and y (for the recessive allele) and fur length with R and r (for the recessive allele), the genotypes that would be produced are approximately in the ratio of 9:3:3:1 (if you divide the total offspring by 2, you get approximately the same numbers as Mendel). *Note: When a genotype is written as Y_ it is to indicate that it could be either YY or Yy. This can be used when discussing phenotypes since the dominant allele will be the one that is expressed.
17 long orange (Y_R_):  6 short orange (Y_rr): 7 long beige (yyR_): 2 short beige (yyrr)
= approximately 9:3:3:1
This leads us to the understanding that long and orange fur is dominant.

A Guinea Pig with Short Orange Fur Is Crossed with a Guinea