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12.4 Bellringer Study the animal shown in figure 11 (page 284) List possible mechanisms that allow the arctic fox to change its fur color with changing seasons 12.4 – Beyond Mendelian Heredity **Complex control of characters A horse with red hair mates with a horse with white hair, and their offspring has both red and white hair. If characters are controlled by single genes with simple dominant and recessive alleles, the colt’s hair should be on color or the other How can this be? NOT ALWAYS! Most of the time, characters, such as hair color in horses, display more complex patterns of heredity than the simple dominant-recessive patterns discussed so far…. Characters influenced by several genes Polygenic inheritance – when several genes influence a character Can be scattered along the same chromosome or located on different chromosomes entirely Determining the effect of these genes is difficult because of random fertilization and crossing-over Many different combinations appear Examples Eye color, height, weight, and hair and skin color All have degrees of intermediate between one extreme and the other Intermediate characters Some organisms display a trait that is intermediate between the two parents Known as incomplete dominance SNAPDRAGONS When a red snapdragon is crossed with a white snapdragon, a pink snapdragon is produced **What if a plant breeder crossed a pink snapdragon with a white snapdragon? Characters controlled by genes with three or more alleles Multiple alleles – genes controlled by three or more alleles BLOOD TYPE Even for traits controlled by three or more genes, an individual can still only have two of the possible alleles for that gene Characters with two forms displayed at the same time For some traits, two dominant alleles are expressed a the same time CODOMINANCE Different from incomplete dominance because both traits are displayed Practice Characters Influenced by the Environment Phenotype often depends on conditions in the environment HYDRANGEA Range in color from blue to pink Plants in acidic soil bloom blue, while those in neutral to basic soil will bloom pink flowers ARCTIC FOX Color is affected by temperature During summer, the fox produces enzymes that darken its coat to a reddish brown, enabling it to blend in During winter, the pigment-producing genes do not function because of the cold temps resulting in a white coat, enabling it to blend **Environment, cont. More examples FUR COLOR IN SIAMESE CATS Influenced by temperature Fur on ears, nose, paws, and tail is darker than the rest of its body • Has a genotype that results in dark fur at locations on its body that are cooler than the normal body temperature Darkened parts have a lower body temp than lighter parts HUMANS Height – influenced by nutrition Color of skin – exposure to sun Human personality – aggression – strongly influenced by environment Identical twins are often used to study environmental differences because their genes are identical to each other Downfall…. Twins must be separated. **Genetic Disorders In order to be “normal” a persons genes must encode and function precisely Sometimes they are damaged or copied incorrectly resulting in fault proteins Changes in genetic material are called mutations Rare because cells have efficient systems for correcting errors Many mutations are carried by recessive alleles in heterozygous individuals Two phenotypic ally normal people who are heterozygous could produce children who are homozygous recessive Some Human Genetic Disorders Genetic Disorders explained… Sickle Cell Anemia The recessive allele that causes sickle-shaped blood cells, helps protect heterozygous individuals from malaria Malaria – disease caused by protozoan in dirty water Cystic Fibrosis (CF) Most common fatal, hereditary, recessive disorder among Caucasians One in 25 Caucasians carry at least one copy of defective gene Disorders, cont. Hemophilia More than a dozen genes code for genes involved in blood clotting Receive the defective gene from your mother, because it is located on the X chromosome Huntington’s Disease (HD) First appears between ages 30-40. Loss of muscle control, uncontrollable physical spasms, severe mental illness, and eventual death Most people don’t know they have it until they have had children Unknowingly passed from generation to generation Treating Genetic Disorders Most cannot be cured In some cases, can be treated if diagnosed early enough People with a family history of severe genetic disorders will often undergo genetic counseling to understand risks and probabilities Gene Therapy Could soon allow scientists to correct recessive genetic disorders by replacing defective genes with copies of healthy ones Gene therapy The first and essential step to gene therapy is to isolate the gene Mendel’s Hypotheses The four hypotheses Mendel developed as a result of his experiments now make up the Mendelian theory of heredity—the foundation of genetics. For each inherited trait, an individual has two copies of the gene—one from each parent. There are alternative versions of genes. Today the different versions of a gene are called its alleles. When two different alleles occur together, one of them may be completely expressed, while the other may have no observable effect on the organism’s appearance. When gametes are formed, the alleles for each gene in an individual separate independently of one another. Thus, gametes carry only one allele for each inherited trait. When gametes unite during fertilization, each gamete contributes one allele.