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NAME___________________________ DATE___________________ HOUR___________________ BIOLOGY I - DIHYBRID CROSSES HEREDITY - 5 INTRODUCTION – Crosses involving two traits can be followed with a Punnett square just like a Monohybrid. It just requires a larger square. These types of crosses are called Dihybrids. The major difference is that when following one trait, the parent can produce a maximum of two gametes. With two triats, a dihybrid, the parent could produce as many as four gametes. For example, look at a plant that is heterozygous for tallness and heterozygous for pod color. Its genotype would be TtGg. It can produce four different gametes. Any one of these gametes could be passed on: TG, Tg, tG, and tg. Here is an example of a Dihybrid cross: TtGg X TtGg TG Tg tG tg TG TTGG TTGg TtGG TtGg THE RESULTS: Tg TTGg TTgg TtGg Ttgg tG TtGG TtGg ttGG ttGg tg TtGg Ttgg ttGg ttgg 9 TALL GREEN PLANTS 3 TALL YELLOW PLANTS 3 SHORT GREEN PLANTS 1 SHORT YELLOW PLANT PROBLEMS – Use the table below to solve the following problems. Determine the gametes that will be produced. Then using a Punnet square, predict the genotypic ratio and phenotypic ratio for the following crosses. Always write down the genotype of the parents if it is not supplied for you. TRAIT Seed coat color Seed coat shape Pod color Height of plant HOMOZYGOUS DOMINANT SYMBOL LOOKS LIKE BB Brown RR Round GG Green TT Tall HETEROZYGOUS SYMBOL LOOKS LIKE Bb Brown Rr Round Gg Green Tt Tall HOMOZYGOUS RECESSIVE SYMBOL LOOKS LIKE bb White rr Wrinkled gg Yellow tt Short 1. Cross these two plants: TTBB X ttbb What gametes can the first plant produce? _________ _________ _________ _________ What gametes can the second plant produce? _________ _________ _________ _________ What is the genotypic ratio _________________ What is the phenotypic ratio__________________ 2. Cross two offspring from the cross in problem number 1. What gametes can the first plant produce? _________ _________ _________ _________ What gametes can the second plant produce? _________ _________ _________ _________ What is the phenotypic ratio__________________ 3. Cross two plants that are heterozygous for both round seed coats and green pod color. What gametes can the first plant produce? _________ _________ _________ _________ What gametes can the second plant produce? _________ _________ _________ _________ What is the phenotypic ratio__________________
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