Concepts Summary

• Constraints on the genetic study of human traits include the inability to conduct controlled crosses, long generation time, small family size, and the difficulty of separating genetic and environmental influences. Pedigrees are often used to study the inheritance of traits in humans.
• Autosomal recessive traits typically appear with equal frequency in both sexes and tend to skip generations. When both parents are heterozygous for a particular autosomal recessive trait, approximately one-fourth of their offspring will have the trait. Recessive traits are more likely to appear in families with consanguinity (mating between closely related persons).
• Autosomal dominant traits usually appear equally in both sexes and do not skip generations. When one parent is affected and heterozygous for an autosomal dominant trait, approximately half of the offspring will have the trait. Unaffected people do not normally transmit an autosomal dominant trait to their offspring.
• X-linked recessive traits appear more frequently in males than in females. When a woman is a heterozygous carrier for an X-linked recessive trait and a man is unaffected, approximately half of their sons will have the trait and half of their daughters will be unaffected carriers. X-linked traits are not passed on from father to son.
• X-linked dominant traits appear in males and females but more frequently in females. They do not skip generations. Affected men pass an X-linked dominant trait to all of their daughters but none of their sons. Heterozygous women pass the trait to half of their sons and half of their daughters.
• Y-linked traits appear only in males and are passed from father to all sons.
• A trait’s higher concordance in monozygotic than in dizygotic twins indicates a genetic influence on the trait; less than 100% concordance in monozygotic twins indicates environmental influences on the trait.
• Similarities between adopted children and their biological parents indicate the importance of genetic factors in the expression of a trait; similarities between adopted children and their genetically unrelated adoptive parents indicate the influence of environmental factors.
• Genetic counseling provides information and support to people concerned about hereditary conditions in their families.
• Genetic testing includes prenatal diagnosis, screening for disease-causing alleles in newborns, the detection of people heterozygous for recessive alleles, and presymptomatic testing for the presence of a disease-causing allele in at-risk people.
• The interpretation of genetic tests may be complicated by the presence of numerous causitive mutations, incomplete penetrance, and the influence of environmental factors.
• The availability of direct-to-consumer genetic tests has raised concerns about the adequacey of information provided, the absence of genetic counseling, acccuracy, privacy, and the practical uses of some tests.
• Genetic testing has raised concerns about genetic descrimination and privacy regarding test results. The Genetic Information Nondiscrimination Act prohibits the use of genetic information in deciding health insurability and employment.