Patterns of Heredity Can Be Complex Section 8-4

Most Traits Are Not Controlled by Simple Dominant-Recessive Alleles Some traits display more complex patterns of heredity than the simple dominant-recessive patterns discussed

Traits Influenced by Several Genes Polygenic trait – when several genes influence a trait The genes for a polygenic trait may be scattered along the same chromosome or located on different chromosomes. Determining the effect of any one of these genes is difficult. Due to independent assortment and crossing-over, many combinations appear in offspring. Human polygenic traits – eye color, height , weight, hair color, and skin color

Intermediate Traits Incomplete Dominance – when an individual displays a trait that is intermediate between two parents Ex: Red snapdragon crossed with white snapdragon – not red nor white, but pink

Traits with Two Forms Displayed at the Same Time Codominance – when two dominant alleles are expressed at one time Different from incomplete dominance in that both traits are displayed, not the two traits being mixed Ex: Roan color in horses: Red hair parent and white hair parent, offspring has red and white hair or roan color hair

Traits controlled by Genes with Three or More Alleles Multiple Alleles – genes with three or more alleles Ex: ABO blood groups/blood types Determined by 3 alleles – IA, IB, i Letters A and B refer to a carbohydrate on the surface of the red blood cell. The letter i means there is no carbohydrate present. A and B are dominant over i, but neither A nor B is dominant over the other. When they are both present, they are codominant. Only 4 blood types – A, B, AB, O

Traits Influenced by the Environment Phenotype can be influenced by the environment Hydrangeas – blue if in acidic soil, pink if in basic soil Arctic Fox – changes coat color in seasons (temperature)

Some traits are Caused by Mutations In order for a person to develop and function normally, the proteins encoded by his or her genes must function precisely. Sometimes genes are damaged or copied incorrectly, resulting in faulty proteins. Changes in genetic material are called mutations. Mutations are rare because most of the time the body can correct them, but sometimes they may have harmful effects.

The harmful effects produced by inherited mutations – genetic disorders Many mutations are carried by recessive alleles Two phenotypically normal people who are heterozygous carriers of a recessive mutation can produce children who are homozygous for the recessive allele.

Sickle Cell Anemia Condition caused by a mutated allele that produces a defective form of the protein hemoglobin Causes red blood cells to bend into a sickle shape Red blood cells that are sickle rupture easily Results in less oxygen being carried by the blood Tend to get stuck in blood vessels, so can cut off supply of blood to organs

Sickle Cell Anemia The recessive allele that causes sickle-shaped RBC also helps protect cells of heterozygous individuals from effects of malaria. Cause death of the parasite that causes malaria

Hemophilia A condition that impairs the blood’s ability to clot Sex-linked trait

Huntington’s Disease Caused by a dominant allele located on an autosome Begins in thirties and forties with mild forgetfulness and irritability Causes loss of muscle control, uncontrollable physical spasms, severe mental illness, and eventually death

Detecting and Treating Genetic Disorders Most genetic disorders cannot be cured, although progress is being made Can undergo genetic counseling – form of medical guidance that informs people about genetic problems that could affect them or their offspring

Phenylketonuria (PKU) Lack an enzyme that converts the amino acid tyrosine Phenylalanine builds up in the body and causes severe mental retardation If newborn is diagnosed soon after birth, the newborn is placed on a low-phenylalanine diet, ensuring that the baby will get enough phenylalanine to make proteins, but not enough to cause enough damage.