Studied in pea plants and in every case he found that

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studied in pea plants, and in every case, he found that the ratio of different phenotypes of the offspring approximated the predicted For each pair of genes, the alleles segregated to the offspring independently of one another. More Combinations What general rule about the ratio of genotypes can you infer from a dihybrid cross? What would happen if three traits were considered simultaneously in a trihy- brid cross? Is there a mathematical rule that can predict the number of possible genotypes and phenotypes that can occur when any number of traits are considered simultaneously? 9 : 3 : 3 : 1. 9 : 3 : 3 : 1 9 : 3 : 3 : 1, v Vv Ee * Vv Ee F 1 F 1 F 1 . Vv Ee F 1 F 1 * vv ee * P 1 ( vv ) Exploration 1 In this particular example, the mutant forms are recessive to the wild-type forms. It is important to note, however, that there are many examples of mutant forms that are actually dominant to the more common wild-type alleles.
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3-1 How Are Traits Passed from Generation to Generation? 75 Female Gametes V E Vv E e v E V e v e VV EE VV Ee Vv EE Vv Ee VV Ee VV ee Vv Ee Vv ee Vv EE Vv Ee vv EE vv Ee Vv Ee Vv ee vv Ee vv ee V E V e v E v e Male Gametes Vv E e Figure 3-10 Genotypes of the generation of a dihybrid cross. Each parent fly is heterozygous for two traits, wing shape and body color.This kind of cross creates nine distinct genotypes. F 2 VE VE Ve vE ve Ve vE ve Wild type Wild type Wild type Wild type Wild type Broad wing ebony body Broad wing ebony body Wild type Broad wing ebony body Wild type Wild type Vestigial wing striped body Vestigial wing striped body Vestigial wing striped body Vestigial wing ebony body Wild type Male Gametes Female Gametes Figure 3-11 Phenotypes of the generation in a dihybrid cross. Four phenotypes result from this cross between flies heterozygous for both wing shape and body color.The ratio of phenotypes is nine wild-type flies (broad wings, striped body), to three flies with vestigial wings and striped bodies, to three flies with broad wings and ebony bodies, to one fly with vestigial wings and an ebony body. F 2
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76 C HAPTER 3 Mendelian Genetics: How Are Traits Inherited? Chance Determines Which Alleles an Individual Inherits A young couple anxiously awaits the arrival of their first child.Among the many things they wonder about before that child is born is whether their new baby will be healthy. One concern of prospective parents is cystic fibrosis, a genetic disease caused by a re- cessive allele of the CF gene.The recessive allele is carried by about 1 of every 25 white North Americans, making cystic fibrosis the most common genetic disorder among this group. It affects about 1 in 2,500 babies in this population.The disease is characterized by production of a thick, sticky mucus in the lungs that is difficult to propel from the air- ways (Figure 3-12). Victims suffer from chronic infections that progressively destroy the lungs. It is not surprising that the disease is a concern of prospective parents.
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