They were heterozygous peas with a wrinkled allele

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. They were heterozygous peas, with a wrinkled allele from one parent and a round allele from the other. Yet in Mendel’s experiments, all the hybrids produced round seeds.There was no sign of wrinkled seeds, even though one of the parents had wrinkled seeds. The seven traits that Mendel studied all showed the same pattern. Mendel called the trait that appeared in the generation the dominant trait and the trait that was hidden the recessive trait.The allele for the hidden trait was there,whole and unchanged,in the het- erozygous individuals, but it was completely masked by the presence of the dominant allele.The recessive form of a trait is only seen when both of the alleles are recessive. Because some alleles are dominant and some are recessive, you cannot always tell the genetic makeup of an individual by looking at its traits.There may be recessive alleles in the genes masked by their dominant counterparts. For example, a round pea with two R alleles for round seeds would look the same as a round pea with one R allele and one r allele for wrinkled seeds.We call the forms of traits that an individual expresses—those we can see or otherwise detect—its phenotype . The combination of the expressed and hidden alleles in an individual’s genes we refer to as its genotype . While we cannot always tell by looking,it is sometimes possible to learn the genotype of an individual by using a testcross . In a testcross, an individual of unknown genotype is mated with one that is homozygous for the recessive trait. In our example, a pea plant grown from a round seed whose genotype is R ? is mated with a plant grown from a wrin- kled seed, an rr individual. If all the offspring have round seeds, the parent of unknown genotype is most likely RR . What would happen if that parent were heterozygous for the seed texture gene? Try a testcross on the BioInquiry web site. Try describing your own phenotype with regard to some traits.Do you have a widow’s peak or a dimple in your chin? Are your earlobes attached, or are they free? Now try to describe your genotype.You will find that you need more information to accurately assess F 1 F 1 F 1 F 1 F 2 ? F 1 F 1 Testcross: Experiment with phenotypes and genotypes.
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3-1 How Are Traits Passed from Generation to Generation? 69 Figure 3-4 Actors (a) Leonardo DiCaprio, (b) Matt Damon, and (c) Parker Posey illustrate some examples of human traits governed by single genes.A widow’s peak, seen in all three, is a dominant genetic trait. Compare Leonardo’s detached earlobes with Matt’s attached earlobes. Detached earlobes are the dominant form of the trait. Other single gene traits in humans include the ability to roll the tongue, hair on middle finger segments, dimples, and the ability to wiggle one’s ears.
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