Alleles are randomly donated from parents to

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Alleles Are Randomly Donated from Parents to Offspring Using all seven of the traits he studied in peas, Mendel found that the generation averaged about three offspring exhibiting the dominant trait for each one exhibiting the recessive trait. Punnett squares illustrate that underlying the ratio for phenotypes is a ratio of genotypes—one homozygous dominant to two heterozygotes to one homozygous recessive.Mendel recognized that the ratio of phenotypes from so many different crosses, and the ratio of genotypes, imply that heterozygous parents are equally likely to donate either of their two different alleles to their offspring.Were this not the case—if, for example, only the dominant allele could be passed on—then 100% of the offspring would exhibit the dominant phenotype. Likewise, if the recessive allele were favored, offspring would all exhibit the recessive trait. The allele donated by a heterozygous parent is random.This simple but elegant insight is the best explanation of the experimental results—the ratio of phenotypes and the ratio of genotypes— from a cross between hybrids.This is called Mendel’s law of segregation .Formally stated, it says that a parent contributes only one of its two alleles for a trait to each offspring, and if the parent is heterozygous for that trait, the particular allele that is donated to the offspring is random. Thus each embryo has exactly the same chance of receiving a particular allele from each parent.Assuming all combinations of alleles are equally likely to survive, the offspring of crosses between heterozygous parents will exhibit a phe- notypic ratio of dominant/recessive. If the phenotypic ratio resulting from a hybrid cross differs significantly from (as we shall see in some special situations explained in Section 3-3), other forces must be operating. 3 : 1 3 : 1 1 : 2 : 1 3 : 1 1 : 2 : 1 3 : 1 1 : 2 : 1 3 : 1 F 2 HbA HbS/HbS HbA/HbA HbA/HbS HbA/HbA HbA/HbS Mother's Alleles Father's Alleles HbA HbA HbS (a) (b) HbA/HbS HbA/HbA HbA/HbS Mother's Alleles Father's Alleles HbA HbS HbS HbA Figure 3-7 Punnett squares illustrating inheritance of the Hb gene. (a) The offspring of a normal mother and a father who is a carrier for sickle cell anemia have a 50% chance of being carriers themselves. (b) When both parents are sickle cell carriers, the offspring have a 25% chance of inheriting no alleles for the disease ( HbA HbA ), a 25% chance of inheriting the disease ( HbS HbS ), and a 50% chance of being sickle cell carriers. > >
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3-1 How Are Traits Passed from Generation to Generation? 73 Mendel’s Factors Can Act Independently . . . Sometimes Thus far, we have considered the inheritance of a single characteristic, seed texture in peas or the gene for hemoglobin in humans, much as Mendel did at the beginning of his plant breeding experiments. But organisms are combinations of many traits, all of which are derived from the genes of their parents. Do Mendel’s laws apply if two or more traits are considered simultaneously? When Mendel performed crosses in which he followed two traits at a time, he found that all of his original conclusions applied to both traits.
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