Biology 8.4 Complex Patterns of Heredity

Complex Control of Character Traits A flower all red is bred with another flower with a white appearance. The result is both red and white in appearance. Sometimes we find crosses can be more complex than simple dominant-recessive genes. When several genes influence a character, the character is said to be polygenic inheritance.

Complex Control of Character Traits The genes for some polygenic characters may be located on different chromosomes or scattered on the same chromosome. Examples of polygenic characters include eye color, hair color, height, weight, and skin color. All these characters have degrees of intermediate conditions. They are a blend of the genes.

Complex Control of Character Traits Intermediate Characters: Remember that in Mendel’s pea plant experiments, one allele was always dominant over another. In some organisms an individual displays a phenotype that is a blend of the two parents. This condition is known as incomplete dominance.

Complex Control of Character Traits For example, when a rose with red flowers is crossed with a rose with white flowers. Neither the red or white allele is completely dominant over the other. A rose that is a pink blend may be produced, somewhere between each of the parent’s color.

Complex Control of Character Traits In Caucasians, the child of a straight-haired parent and a curly-haired parent will have wavy hair. Straight and curly hair are both homozygous dominant. Wavy hair is heterozygous and is a blend between straight and curly hair.

Complex Control of Character Traits Genes with three or more alleles are said to have multiple alleles. In humans, the ABO bloodtype has three alleles Two of the alleles both overpower the third but they are equal to each other. These two equally dominant alleles are said to be codominant over the third when they are both present.

Complex Control of Character Traits For some characters, two dominant alleles are expressed at the same time. In this case, when both forms of the character are displayed we call it codominance. In codominance, both traits are displayed.

Complex Control of Character Traits The situation of human ABO blood groups is an example of codominance. In the genotype of a person with blood AB, neither A nor B are dominant over each other. They both are codominant together.

Complex Control of Character Traits Characters Influenced by the Environment: An individuals phenotype often depend on conditions in the environment. In plants, hydrangea plants in acidic soil bloom blue flowers. Those in normal to base soil bloom pink flowers. Their phenotype is influenced by the content of the soil they are in. In a similar way, the color of the arctic fox changes with the seasons. During summer, the fox produces enzymes that darken it’s fur. During winter, the pigment production genes shut down. In either season, the trait helps the fox blend into it’s environment.

Complex Control of Character Traits Genetic Disorders: In order for a person to develop and function normally, the proteins encoded by his or her genes must function precisely. Unfortunately, sometimes genes are damaged or are copied incorrectly. Changes in genetic material are called mutations. The harmful effects produced by inherited mutations are called genetic disorders.

Complex Control of Character Traits Many mutations are carried by recessive alleles in heterozygous individuals. This means that two normal persons who are heterozygous carriers of a recessive mutation can produce children who are homozygous for the recessive allele. In these cases, the affects of the alleles can not be avoided. Several human genetic disorders occur in this group.

Complex Control of Character Traits Sickle Cell Anemia: An example of a recessive genetic disorder is sickle cell anemia, a condition caused by a mutated allele that produces a defective gene for the production of the protein hemoglobin. Hemoglobin is found within red blood cells, where it binds oxygen and transports it through the body. In sickle cell anemia, the defective form of the hemoglobin causes many hemoglobin to bend into a sickle shape.

Complex Control of Character Traits Cystic Fibrosis: Cystic fibrosis is the most common fatal, hereditary disorder among Caucasians. One in 25 individuals has at least one copy of a defective gene that makes a protein necessary to move chloride into and out of cells. About 1 in 2,500 Caucasian infants in the U.S. is homozygous for the cf allele. The airways of the lungs become clogged with thick mucus, and the ducts of the liver and pancreas become blocked. While treatments can relieve symptoms, there is no known cure.

Complex Control of Character Traits Hemophilia: Another recessive disorder is hemophilia, a condition that impairs the blood’s ability to clot. Hemophilia is a sex-linked trait. More than a dozen genes code for the proteins involved in clotting of the blood.

Complex Control of Character Traits Huntington’s Disease: Huntington’s disease is a genetic disorder caused by a dominant allele located on an autosome . The first symptoms of Huntington’s disease, a mild irritability and forgetfulness, appear in victims in their thirties or forties. In time, HD causes loss of muscle control, uncontrollable physical spasms, severe mental illness, and eventually death. Unfortunately, most people who have the HD allele do not know they have the disease until after they have children.

Complex Control of Character Traits Treating Genetic Disorders: Most genetic disorders cannot be cured, although progress is being made. A person with a family history of genetic disorders may want to undergo genetic counseling. Genetic counseling is a form of medical guidance that informs people about genetic disorders that could affect them or their offspring. In some cases, a genetic disorder can be treated before it becomes a problem.

Complex Control of Character Traits Gene Therapy: Gene technology may soon allow scientists to correct certain recessive genetic disorders by replacing defective genes with copies of healthy genes, a process called gene therapy.

Computer Lab: Today’s assignment is to go to the computer lab, use the internet to research and short report describing the following: What gene therapy is: Who and where was gene therapy developed What are the benefits of gene therapy What current diseases are treatable by gene therapy What potential diseases may eventually be cured by gene therapy in the future