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The Inheritance of Traits Mendel noticed that certain varieties of garden pea plants produced specific forms of a trait, generation after generation. For instance, he noticed that some varieties always produced green seeds and others always produced yellow seeds. In order to understand how these traits are inherited, Mendel performed cross-pollination by transferring male gametes from the flower of a true-breeding green-seed plant to the female organ of a flower from a true-breeding yellow-seed plant. To prevent self-fertilization, Mendel removed the male organs from the flower of the yellow-seed plant. Mendel called the green-seed plant and the yellow-seed plant the parent generation-also known as the P generation. F 1 and F_{2} generations When Mendel grew the seeds from the cross between the green-seed and yellow-seed plants, all of the resulting offspring had yellow seeds. The offspring of this P cross are called the first filial (F_{1}) generation. The green-seed trait seemed to have disappeared in the F_{2} generation, and Mendel decided to investigate whether the trait was no longer present or whether it was hidden, or masked. Mendel planted the F_{2} generation of yellow seeds, allowed the plants to grow and self-fertilize, and then examined the seeds from this cross. The results of the second filial (F_{2}) generation-the offspring from the F_{1} cross-are shown in Figure 2. Of the seeds Mendel collected, 6022 were yellow and 2001 were green; almost a perfect 3:1 ratio of yellow to green seeds. Mendel studied seven traits-seed or pea color, flower color, seed pod color, seed shape or texture, seed pod shape, stem length, and flower position-and found that the F_{2} generation from these crosses also showed a 3:1 ratio.
Probability The inheritance of genes can be compared to the probability of flipping a coin, as shown in Figure 9. The probability of the coin landing on heads is 1 out of 2, or 1/2. If the same coin is flipped twice, the probability of it landing on heads is 1/2 each time or 1/2 x 1/2, or 1/4 both times. Actual data might not perfectly match the predicted ratios. You know that if you flip a coin twice you might not get heads 1 out of the 2 times. You might get heads twice, or you might get tails twice. However, the more times you flip the coin, the closer your results will be to the predicted ratio of heads to tails. Mendel's experi-mental results were not exactly a 9:3:3:1 ratio. However, the larger the number of offspring involved in a cross, the more likely it will match the results predicted by the Punnett square.
FOCUS QUESTION What are examples of selective breeding? Selective Breeding You might be familiar with different breeds of dogs, such as Saint Bernards, huskies, and German shepherds. Observe some of the phenotypic traits of these breeds in Figure 13. All three have strong, muscular bodies. Saint Bernards have traits such as a keen sense of smell that make them good rescue dogs. Huskies are endurance runners and pull sleds long distances. German shepherds are highly trainable for special ser-vices. Since ancient times, humans have bred animals with certain traits to obtain offspring that have desired traits. As a result, these traits become more common. Breeding for desired traits is not restricted to animals alone. Plants also are bred to produce desired traits, such as larger fruits and shorter growing times. The process by which desired traits of certain plants and animals are selected and passed on to their future generations is called selective breeding. Through the processes of hybridization and inbreeding, desired traits can be passed on to future generations.
Inferring genotypes Pedigrees are used to infer genotypes from the observation of phenotypes. By knowing physical traits, genealogists can determine what genes an individual is most likely to have. Phenotypes of entire families are analyzed in order to determine family genotypes. Pedigrees help genetic counselors determine whether inheritance patterns are domi-nant or recessive. Once the inheritance pattern is determined by analyzing the available information, the genotypes of the individuals can largely be resolved, or identified, through pedigree analysis. To analyze pedigrees, one particular trait is selected to be studied, and a determination is made as to whether that trait is dominant or recessive. Dominant traits are easier to recognize than recessive traits because dominant traits are exhibited in the phenotype of individuals. A recessive trait will not be expressed unless the person is homozygous recessive for the trait. That means that a recessive allele is passed on by each parent. When recessive traits are expressed, the ancestry of the person expressing the trait is followed for several generations to determine which parents and grandparents were carriers of the recessive allele. Get It? Analyze What can be determined about the genotypes of the parents of an individual who expresses a recessive trait? Predicting disorders If good records have been kept within families, the possibility of disorders occurring in future offspring can be predicted. However, more accuracy can be expected if several individuals within the family can be evaluated. Having information about several generations of family members can provide valuable information. The study of human genetics can be difficult, because scientists are limited by time, ethics, and and availabil-ity of information. For example, it takes decades for each generation to mature and then to have offspring when the study involves humans. Therefore, good record keeping, where it exists, helps scientists use pedigree analysis to study inheritance patterns, to determine phenotypes, and to ascertain genotypes within a family.
