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PATTERNS OF HEREDITY AND HUMAN GENETICS Chapter 12 Objectives Interpret a pedigree Determine human genetic disorders that are caused by inheritance of recessive alleles Predict how a human trait can be determined my a simple dominant allele SCS: Mendelian Inheritance of Human Traits How can we trace traits in our family? Geneticists use a tool called a pedigree (graphic representation of genetic inheritance) Male Female Marriage Heterozygous or carrier I, II, III – represent generations pedigrees are used by genetic counselors to help trace genetic disorders and other traits Simple Recessive Heredity most genetic disorders are caused by recessive alleles (rare) – you have to receive 2 alleles, 1 from mom and 1 from dad in order to inherit the disorder Examples Cystic Fibrosis (CF) 1/2000 are affected by CF, 1/200 are carriers characterized by a defective protein that causes mucas to build up in the lungs Tay Sachs Disease common in Ashkenazi Jews characterized by the absence of an enzyme that breaks down fat in the brain most children die by the age of 5 Simple Dominant Heredity only need a single dominant allele for the trait to be expressed examples of dominant traits in humans: unattached ear lobes, tongue rolling, hitchhikers thumb, mid digit hair Examples of dominant genetic disorders Huntington’s Disease doesn’t affect individuals until their 30-50 years of age characterized by the breakdown of areas of the brain 12-2 Whey Heredity follows different rules Distinguish between incompletely dominant and codominant alleles Compare multiple allelic and polygenic inheritance Analyze the pattern of sex-linked inheritance Summarize how internal and external environments affect gene expression SCS: Complex Patterns of Heredity most traits observed are not a result of simple dominant/ recessive inheritance Types Incomplete dominance- heterozygous condition results an intermediate phenotype or “mixing”; neither allele is completely dominant over the other Example: Snap dragons. Homozygous Red and white flowers are crossed, and they produce all pink offspring Codominance- both alleles are expressed equally Example: Cows. Homozygous Red and white cows are crossed (mated), and they produce offspring that appear pink from a distance because they both red and white hairs Multiple Alleles- traits controlled by two or more alleles Sex linked inheritancetraits controlled by genes located on the sex chromosomes Thomas Morgan was the first to discover Worked with fruit flies Problems involving sex linkage In fruit flies, red eyes are dominant over white example of a cross: XRXr x XrY Polygenic Inheritancetraits that are controlled by more than 1 gene Genes might be on the same or different chromosomes Other factors influence the expression of genes Environmental Influences External Environmental (temp. nutrition, light, chemicals, infectious agents) Internal Environment (hormones, age) genetic makeup determines the potential to develop and function 12-3 Complex Inheritance of Human Traits Compare codominance, multiple alleleic, sex- linked, and polygenic patterns of inheritance in humans Distinguish among conditions in which extra autosomal or sex chromosomes exist SCS: Complex Inheritance of Human Traits Codominance Sickle Cell Anemia most common in black Americans Characterized by the sickling of red blood cells, which hampers circulation and affects oxygen levels of the blood In malaria prone areas people with this gene are protected against malaria Homozygous conditions results in full blown SC, and medication has to be taken Heterozygous condition results in a mild case, may not need medication Multiple Alleles Blood types There are 4 blood types in humans (A, B, AB, and 0) the type of marker (protein on your red blood cells determine your blood type) Phenotypes Type A Type B Type AB Type O Genotypes IAIA or IA i IBIB or IBi IAIB ii knowing your blood type is important: blood transfusions, child birth, blood donations Type Blood Can Receive A A, O B B, O AB AB, O, A, B Universal recipient: AB Universal Donor: O Sex Linkage Some human traits are determined by genes found on the sex chromosomes (mostly X) Examples of sex linked traits: Red/Green Color Blindnesscan’t differentiate red from green recessive disorder More males have it because they only need one allele Hemophilia bleeders disease; blood does not clot naturally so a person can bleed to death 1/10,000 males are afflicted, only 1/ 100 million females are affeced Polygenic Traits example include skin color, eye color, Changes in chromosome number Normally humans have 46 chromosomes, but sometimes during replication, chromosomes do not separate correctly Geneticist use a tool called a karyotype to determine if an extra chromosome is present Take a sample of a person’s DNA and line up the metaphase chromosomes Examples of Disorders Downs Syndrome (Trisomy 21) a result of an extra chromosome in the 21st pair some degree of mental retardation Turners Syndrome XO, female, infertile Kleinfelter’s XXY, Male, infertile