Chapter 12: Inheritance Patterns and Human Genetics

Pedigrees Geneticists can study human genetic traits and trace genetic diseases from one generation to the next by studying the phenotypes of family members in a pedigree. A pedigree is a diagram that shows how a trait is inherited over several generations.

Pedigrees

Patterns of Inheritance Biologists learn about genetic diseases by analyzing patterns of inheritance. For Example: If a trait is autosomal, it will appear in both sexes equally. If a trait is sex-linked it is usually only seen in males. Most sex-linked traits are recessive. If a trait is autosomal dominant, every individual with the trait will have a parent with the trait. If the trait is recessive, an individual with the trait can have one, two or neither parent exhibit the trait.

Pedigrees Pedigrees can be analyzed to determine whether a trait is autosomal or sex-linked. Pedigrees can be analyzed to determine whether a trait is dominant or recessive. Individuals on a pedigree that have the allele but do not express the trait are known as carriers.

Genetic Traits and Disorders Genes controlling human traits show many patterns of inheritance. Some of these genes cause genetic disorders. GENETIC DISORDERS are diseases or disabling conditions that have a genetic basis

Polygenic Inheritance Most human characteristics (traits) are polygenic. Polygenic characters (traits) are influenced by several genes. Polygenic traits include: height, nose length, size of foot, hair color, skin color, and eye color.

Complex Characters Many human conditions are complex characters. Complex characters (traits) are influenced strongly by both the environment and by genes. Skin color and height are both polygenic and complex. Explain why. Heart disease, diabetes, cancer are also thought to be complex.

Complex Characters Biologists hope that by identifying environmental components that contribute to a disease, they can educate people in ways that minimize their risk of developing the disease.

Multiple Alleles Many genes have more than two alleles. Genes with three or more alleles are said to have multiple alleles. The ABO blood groups are governed by multiple alleles.

Codominance Alleles that are codominant are expressed equally. The IA and IB alleles in the ABO blood group gene are both codominant since they both produce a corresponding antigen (protein) on the surface of erythrocytes (red blood cells).

Incomplete Dominance Sometimes an individual displays a trait that is intermediate between two parents, a condition known as incomplete dominance. Example: In Caucasians, the child of a straight-haired parent and a curly-haired parent would have wavy hair.

X-linked Traits X-linked traits are carried on the X chromosome of the 23rd pair. This means that X-linked traits are carried on a sex chromosome. Colorblindness, Hemophilia are both X-linked traits.

Sex-Influenced Traits Sex-influenced traits are involved in other complex characters. Males and females can show different phenotypes even when they share the same genotype. Sex-influenced traits are usually autosomal. For example, an allele that is dominant in males but recessive in females controls pattern baldness, the type of baldness usually found in men. The difference is due to higher levels of the hormone testosterone in men, which interacts with the genotype to produce pattern baldness.

Detecting Genetic Disease Many people with a family history of genetic disease seek genetic screening before having children. Genetic Screening is an examination of a person’s genetic makeup. It may involve karyotypes, blood tests for certain proteins, or direct test of DNA. Doctors can now also detect more than 200 genetic disorders in the fetus.

Amniocentesis Performed between the 14th and 16th week of pregnancy. Allows a physician to remove some amniotic fluid from the amnion, the sac that surrounds the fetus. Geneticists can analyze fetal cells for genetic disease by examining chromosomes and proteins in the fluid.

Amniocentesis

Chorionic Villi Sampling (CVS) Performed between the 8th and 10th week of pregnancy. The physician takes a sample of the chorionic villi derived from the zygote that grow between the mother’s uterus and the placenta. Technicians analyze the cells, chromosomes and proteins to detect genetic disease.

Chorionic Villi Sampling (CVS)

Genetic Counseling Many people with a family history of a genetic disease also undergo genetic counseling. Genetic counseling is the process of informing a person or couple about their genetic makeup. Genetic counseling is a form of medical guidance that informs individuals about problems that might affect their offspringl

Treating Genetic Disease Physicians treat genetic diseases in several ways. For some diseases, physicians can implement symptom-prevention measures. Example: insulin injections for diabetes. Physicians can even do some types of surgery to correct genetic defects in a fetus before birth.

Gene Therapy Gene Therapy is a technique that replaces a defective gene with a healthy copy of the gene. A virus is used to inject the gene into the cells. The gene functions until the cells die. Gene Therapy needs to be repeated. Gene therapy, in which only body cells are altered, is called somatic cell gene therapy. Germ-cell gene therapy is the attempt to alter sperm or eggs. This form of gene therapy is highly controversial.

Critical Thinking Review A woman with cystic fibrosis marries a man who is heterozygous for cystic fibrosis. What is the likelihood (probability) that their children will have cystic fibrosis? (Cystic fibrosis is autosomal recessive) (Give yourself a key, determine the genotypes of parents, perform the cross)

Critical Thinking Review Why is colorblindness less common among females?

ABO Blood Group A man with blood type B marries a woman with blood type A. Their first child is blood type O. What is the probability their next child will be blood type AB? Blood Type B?

Analyzing a Pedigree

Analyzing a Pedigree