Predicting Results of Dihybrid Crosses • A cross in which two characteristics are tracked is a dihybrid cross. • The two characteristics are assumed to have genes located on different chromosomes. Alleles for each characteristic will independently assort when forming gametes. • In guinea pigs, dark fur color (D) is dominant to light fur color (d). Rough coat texture (R) is dominant; while smooth coat (r) is recessive. • What is the genotype of a guinea pig that is heterozygous for both characteristics? • What types of gametes will this guinea pig form?
Dihybrid Crosses • Example #1 • In garden peas, tallness (T) is dominant to shortness (t) and purple flowers (P) are dominant to white flowers. Determine the genotypic and phenotypic ratios of the offspring produced by the crossing of a heterozygous tall, heterozygous purple plant with a heterozygous tall, white plant.
Example 2 • For humans, there is a disease called phenylketonuria (PKU) which is caused by a recessive allele. People with this disease cannot break down the amino acid phenylalanine. Let “E” represent the normal gene. Also, in humans there is a condition called galactosemia, which is also caused by a recessive allele. Let “G” represent the normal allele for galactose digestion. If two adults were heterozygous for both traits, • What are the chances of having a child that is completely normal? • Has just PKU? • Has just galactosemia? • Has both diseases?