While neither researcher is willing to speculate on

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While neither researcher is willing to speculate on how human intelligence and behavior might be affected by imprinted genes, some human genetic disorders hint that we are not entirely dissimilar to the mouse. For example, children suffering from Prader- Willi syndrome have brain disorders that cause overeating, obesity, a placid personality, mild retardation, and a reduced sexual drive.These behaviors are under the control of
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88 C HAPTER 3 Mendelian Genetics: How Are Traits Inherited? the brain’s primitive regions, the areas whose development can be traced to paternal alleles. Maternal alleles for these brain areas are normally silenced. Prader-Willi syndrome sufferers lack the paternal allele due to a deleterious mutation. Their abnormal phenotype implies that the maternal alleles for these brain areas cannot make up for the absence of the alleles from the father. In addition,another human genetic disorder called Angelman syndrome occurs when a baby is born lacking a certain maternal allele that resides on chromosome number 15. Children with Angelman syndrome have defects in activities controlled by the higher brain centers. These defects include mental retardation, speech deficiencies, and jerky movements. The symptoms occur even though the chromosome 15 of paternal origin is present and intact.The abnormal phenotype of children with Angelman syndrome implies that the paternal alleles on chromosome 15 cannot substitute for the absence of those of maternal origin. While it is too soon to say whether human intelligence is maternal in origin, these exciting discoveries may give new insights into why we are the way we are. R EVIEW Q UESTIONS 1. How did Mendel’s approach to answering scientific questions dif- fer from that of his contemporaries? How did his novel approach contribute to his success in describing how traits are inherited? What advantages did he gain by choosing to study the garden pea? 2. When Mendel crossed plants that bred true for round pea seeds with those that bred true for wrinkled seeds, what was the phe- notype of the generation? What was the genotype of the generation? 3. What is a gamete? In terms of the number of alleles for each gene, what general statement can you make about how gametes differ from other cells in a mature pea plant? 4. In a cross between a pea plant that breeds true for purple flowers and one that breeds true for white flowers,the generation has all purple flowers. Draw a Punnett square showing how the alleles for flower color are combined in the Then draw a Punnett square showing the genotypes that can result in the generation. 5. The presence of freckles is a dominant trait in humans. What can you say about the genotype of a person with freckles? How could you find out for sure what that person’s genotype is? 6. You are a genetic counselor, and a married couple comes to you for advice. The man’s father suffers from sickle cell anemia. No one in the family history of the woman has ever had the disease.
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