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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 dominantrecessive 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 sickleshaped 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 lowphenylalanine diet, ensuring that the baby will get enough phenylalanine to make proteins, but not enough to cause enough damage.