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Dominant and Recessive Physical Traits www.BioEdOnline.org Pedigree Diagrams: I Basic Symbols BioEd Online www.BioEdOnline.org normal male normal female affected male affected female male carrier female carrier www.BioEdOnline.org Pedigree Diagrams: II Basic Symbols for offspring and the expression of a trait. Offspring are depicted below the parents. Filled in symbol indicates the expression of the studied trait. Roman numerals – Generations Parents Arabic numerals – Individuals in a certain generation Generation Siblings BioEd Online www.BioEdOnline.org www.BioEdOnline.org Factors to Consider in Pedigrees Is the trait located on a sex chromosome or an autosome? Autosomal – not on a sex chromosome #1-22 Sex Linkage – located on one of the sex chromosomes Y-linked - only males carry the trait. X-linked (recessive) - sons inherit the disease from normal parents How is the trait expressed? Dominant - the trait is expressed in every generation. Recessive - expression of the trait may skip generations. BioEd Online www.BioEdOnline.org Autosomal Dominant Disorders Requires only ONE allele for the disorder to be passed Probability of having offspring with the disorder: 50% if one parent has the defective allele www.BioEdOnline.org Huntington’s Disease Results in damage to the brain Symptoms generally occur after the affected reaches 30 years old Many have children before they show symptoms Rate of Occurrence: 1 in 15,000 Life expectancy: 10-20 years after onset of symptoms Genetic screening can identify those affected before the onset of symptoms www.BioEdOnline.org Marfan’s Syndrome: An Example Expressed in both sexes. Thus, autosomal or sex linked?? Expressed in every generation. Thus, dominant or recessive? BioEd Online Marfan Syndrome www.BioEdOnline.org Marfan’s: Genotype the Normal Individuals Assign codes for the alleles. Code “m” for the recessive normal allele. Code “M” for the dominant allele for Marfan’s syndrome. Normal individuals must be “_____.” BioEd Online www.BioEdOnline.org Marfan’s: Genotype the Affected Individuals Affected individuals must have at least one “____.” BioEd Online www.BioEdOnline.org Marfan’s: Parent-Offspring Relationships Possibilities for #1 and #2: Heterozygote (Mm) or homozygous for “M?” If “MM,” all offspring from a normal mate should be affected. Therefore, both must be heterozygotes. BioEd Online www.BioEdOnline.org Marfan’s: Parental Genotypes Known “M” must have come from the mother. The father can contribute only “m.” Thus, the remaining genotypes are “Mm.” BioEd Online www.BioEdOnline.org Autosomal Recessive Disorders Autosomes: homologous chromosomes 1-22 many disorders are autosomal recessive Requires defective recessive allele to be passed by BOTH parents Probability of offspring having the disorder: 25% if both parents are carriers/ 0 if only one parent is a carrier. www.BioEdOnline.org Cystic Fibrosis (CF) 1 in 20 Caucasians are carriers Rate of occurrence: 1 in 2000 Characteristics: accumulation of mucus in lungs and digestive tract. Life expectancy: ~30 years (before treatment became available), but now depends heavily on the age at when treatment is started. www.BioEdOnline.org Phenylketonuria (PKU) Metabolic disorder in which phenylalanine cannot be broken down Rate of occurrence: 1 in 15,000 Can result in mental retardation All infants in US are screened Reducing intake of phenylalanine until puberty can prevent retardation www.BioEdOnline.org PKU www.BioEdOnline.org Sickle cell anemia Disorder of the Red Blood Cells Causes sickling of cells preventing normal function Lack of oxygen to organs can cause tissue damage resulting in intense pain Affects mostly those of African descent 1 in 12 African-Americans are carriers >70,000 have the disease www.BioEdOnline.org Albinism: An Example Expressed in both sexes at approximately equal frequency. Thus, autosomal. Not expressed in every generation. Thus, recessive. BioEd Online www.BioEdOnline.org Albinism: Genotype the Affected Individuals Assign codes for the alleles. Code “A” for the dominant normal allele. Code “a” for the recessive allele for albinism. Affected individuals must be homozygous for “a.” First generation parents must be “Aa” because they have normal phenotypes, but affected offspring. BioEd Online www.BioEdOnline.org Albinism: Genotype the Normal Individuals Normal individuals must have at least one “A.” BioEd Online www.BioEdOnline.org Albinism: Parent-Offspring Relationships #1 must transmit “a” to each offspring. The “A” in the offspring must come from the father. Normal father could be either heterozygous or homozygous for an “A.” ** BioEd Online www.BioEdOnline.org Albinism: Parental Genotypes are Known Both parents are heterozygous or homozygous? Normal offspring could have received an “A” from either parent, or from both. BioEd Online www.BioEdOnline.org Albinism: One Parental Genotype is Known Only the genotype of the offspring expressing albinism are known. Normal offspring must have received an “a” from their affected father. BioEd Online www.BioEdOnline.org Tay-Sachs Disease Metabolic disorder that affects the nervous system More common in those of Central and Eastern European Jewish descent 1 in 30 are carriers Life expectancy: ~5 years www.BioEdOnline.org Sex-Linked Inheritance Alleles that are inherited on the sex chromosomes Written as superscripts on the X and Y chromosomes XªX or XªY Most sex-linked traits are associated with the female (X) chromosome www.BioEdOnline.org Red-Green Color Blindness Disorder in which a person cannot differentiate between red and green Allele passed on the X chromosome Disorder more common in males Males: one copy results in color blindness Females: one copy (carrier) two copies result in color blindness www.BioEdOnline.org Red-Green Color Blindness www.BioEdOnline.org Hairy Ears: An Example Only males are affected. All sons of an affected father have hairy ears. Thus, hairy ears is Y-linked. BioEd Online www.BioEdOnline.org Hairy Ears: Female Sex Determination All females are XX. BioEd Online www.BioEdOnline.org Hairy Ears: Male Sex Determination All males are XY. BioEd Online www.BioEdOnline.org Hairy Ears: Gene on the Y Chromosome Code “H” indicates the allele on the Y chromosome for hairy ears. BioEd Online www.BioEdOnline.org Hemophilia Disorder that prevents blood from clotting properly Allele passed on the X chromosome Males more commonly affected males: one copy results in disorder Females: one copy (carrier) two copies results in disorder www.BioEdOnline.org Hemophilia: An Example In this pedigree, only males are affected, and sons do not share the phenotypes of their fathers. Thus, hemophilia is linked to a sex chromosome–the X. Expression of hemophilia skips generations. Thus, it is recessive. BioEd Online Extensive bruising of the left forearm and hand in a patient with hemophilia. www.BioEdOnline.org Hemophilia: Expression of the Female Sex Chromosomes All females are XX. BioEd Online www.BioEdOnline.org Hemophilia: Expression of Male Sex Chromosomes All males are XY. BioEd Online www.BioEdOnline.org Hemophilia: Genotype the Affected Individuals Assign codes for the alleles. Code “h” for the recessive hemophilia allele. Affected individuals must have an “h” on an X chromosome. BioEd Online www.BioEdOnline.org Hemophilia: Father-Daughter Relationship All daughters of an affected father receive an X chromosome with the “h” allele. BioEd Online www.BioEdOnline.org Hemophilia: Homozygous or Heterozygous? Only males affected Not Y-linked Skips a generation: recessive X-linked BioEd Online www.BioEdOnline.org Incomplete Dominance When traits exhibit incomplete dominance, heterozygotes exhibit intermediate characteristics Ex. Snapdragons Red flowers (RR)X White flowers (R´R’) F1: Pink flowers (RR’) www.BioEdOnline.org Codominance Heterozygotes exhibit both characteristics ex. Chickens Black rooster (BB) X White hen (WW) F1 : black & white checkered pattern (BW) www.BioEdOnline.org Polygenic Inheritance when traits are controlled by two or more genes ex. Hair color, eye color, skin color, height, fingerprint patterns www.BioEdOnline.org Multiple Alleles Many traits are controlled by more than two alleles ex. Fur color in many animals www.BioEdOnline.org Blood Types An example of several different hereditary patterns Multiple alleles: A, B, and O Codominance: A and B Recessive and Dominant:O is recessive to both A and B www.BioEdOnline.org Blood Types *Notice how the genotype is written for each type www.BioEdOnline.org Blood Transfusion Facts People with type O- blood can donate to anyone (universal donor), but can only receive type O- blood. People with type AB+ blood can receive blood from anyone (universal recipient), but can only donate to someone who is type AB+. www.BioEdOnline.org