Section 2

Human Genetics

• Geneticists use pedigrees to trace diseases or traits through families. Pedigrees reveal inheritance patterns of genes.

• A carrier has one copy of a recessive allele but does not express the trait.

• Polygenic characters, such as skin color, are controlled by two or more genes.

• Complex characters, such as height, are influenced by both genes and environment.

• Multiple-allele characters, such as ABO blood groups, are controlled by three or more alleles of a gene.

• The gene for colorblindness, an X-linked recessive, is found on the X chromosome.

• A sex-influenced trait, such as pattern baldness, is expressed differently in men than in women even if it is on an autosome and both sexes have the same genotype.

• Genetic screening examines a person's genetic makeup and potential risks of passing disorders to offspring. Amniocentesis and chorionic villi sampling help physicians test a fetus for the presence of genetic disorders.

• Genetic counseling informs screened individuals about problems that might affect their offspring.

• Genetic disorders are treated in various ways. Among the treatments are symptom-relieving treatments and symptom-prevention measures, such as insulin injections for diabetes.

• Gene therapy is a type of treatment under development. In gene therapy, a defective gene is replaced with a copy of a healthy gene.

• Somatic cell gene therapy alters only body cells. Germ cell gene therapy attempts to alter eggs or sperm.

Vocabulary pedigree (p. 241) carrier (p. 242) genetic disorder (p. 242) polygenic (p. 242)

complex character (p. 243) multiple allele (p. 244) codominance (p. 244)

incomplete dominance (p. 244) sex-influenced trait (p. 245) Huntington's disease (p. 245)

amniocentesis (p. 245) chorionic villi sampling (p. 246) genetic counseling (p. 247) gene therapy (p. 248)

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