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Download Chapter 14 Mendelian Genetics Notes
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1/4/2016 Inheritance Mendel and the Gene Idea Gregor Mendel The passing of traits from parents to offspring Genetics—the scientific study of inheritance Helpful Vocabulary 1822-1884 Homozygous—when the two alleles are the same (TT/tt) Scientist and monk (“Father of Genetics”) Heterozygous—when the two alleles are different (Tt) He chose the pea plant because of short lifespan, bisexual, many traits known and they are capable of cross and self-pollinating True-breeding means homozygous. Trait—a heritable feature (ex. Height) Allele—a variant of a character (ex. Tall or short) Mendel’s Experimental Design Mendel’s Experimental Design Mendel began with true-breeding varieties that contained only one type of gene for each character. Mendel called the trait expressed in the F1 plants the dominant trait and the trait not expressed was recessive. As first, he only looked at one character at a time—Monohybrid cross. When F1 were allowed to self-fertilize, Mendel found a 3:1 dominant to recessive phenotype in the F2 generation. The first generation of crosses is the P or parental generation. Their offspring are the F1 generation or first filial generation. When F2 plants were allowed to self-fertilize, Mendel found a 1:2:1 of true breeding dominant to not true-breeding dominant to true-breeding recessive (genotypes). Offspring of two F1 members comprise the F2 generation. 1 1/4/2016 Today we use these terms: Each individual has two genes for each trait. Alternate types of genes for each trait are alleles. Phenotype refers to the outward expression of the genes. The actual genetic makeup of an individual is the genotype. Punnett Squares—easy way to express the probabilities of genotypes. 1. One trait at a time= monohybrid cross 2. Two traits at a time= dihybrid cross Mendel’s Laws Mendel’s 1 st Law: Law of Segregation Only one of a pair of alleles is passed to a gamete Mendel’s 2 nd Law: Law of Independent Assortment Mendel determined this when working with dihybrid crosses. Law states that genes located on different chromosomes are inherited independently. Ex. Inheritance of height is independent of the inheritance of flower color 2 1/4/2016 Exceptions to Mendel’s Laws Incomplete dominance: when offspring exhibit a phenotype intermediate to that of both parents. Ex. Red X White Snapdragons Exceptions to Mendel’s Laws Multiple alleles: more than two alleles exist for a given trait in a population of individuals. Exceptions to Mendel’s Laws Codominance: two alleles affect the phenotype in separate distinguishable ways Ex. Tabby cat Black cat (BB) X Tan cat (TT) Exceptions to Mendel’s Laws Pleiotropic Effects: when an allele affects more than one trait Ex. PKU and CF Ex. ABO blood groups Exceptions to Mendel’s Laws Exceptions to Mendel’s Laws Epistasis: Interaction between the products of two genes in which one of the genes modified the phenotypic expression produced by the other. Polygenic inheritance: when one trait is determined by the action of several genes. Ex. Gerbils Ex. Skin color, height 1st gene: C=color, c=albino --skin color is likely controlled by atleast 4 genes. 2nd gene: B=brown, b=black CcBb X CcBb (Brown X Brown) 3 1/4/2016 Exceptions to Mendel’s Laws Environmental Effects: the degree to which an allele is expressed can sometimes depend on the environment Genetic Studies of Humans Often done by pedigree charts Male Ex. Nature vs. Nurture, Heart Disease Female PHENOTYPE= GENOTYPE + ENVIRONMENT Person with trait Sample Pedigree Dominant Trait Autosomal Recessive Disorders Recessive Trait Autosomal Recessive Disorders Both parents must have the allele to have a child born with the condition. Cystic Fibrosis Even if both parents are carriers, they only have a 1-in-4 chance of having an offspring with the condition -most common genetic disorder in Caucasian Americans -1 in 25 is a carrier; 1 in 2500 is affected. -Causes accumulation of mucus in lungs, pancreas and other organs. Tay Sachs -most common in Jews from Eastern & Central Europe -1 in 30 are carriers, 1 in 3600 are affected 4 1/4/2016 Autosomal Recessive Disorders Sickle-cell anemia -prevalent in population in or from Africa (malariaravaged) -single amino acid substitution results in malformed hemoglobin -Heterozygotes are more resistant to malarial infection Autosomal Dominant Disorders Single “faulty” allele of a gene causes damage, even with a “good” allele present, because the “faulty” allele is dominant. Huntington’s Disease -dominant lethal condition that doesn’t express itself until later in life -degenerative neural disease ALS (Lou Gehrig’s Disease) -degenerative neural disease Genetic Counseling and Therapy Genetic counseling—helps couples predict risk of bearing children with genetic defects. Amniocentesis—minute amount of amniotic fluid surround fetus is removed and checked for genetic defects; happens in 4th month Chorionic villus sampling—removal of small portion of the chorionic villi of the placenta for genetic testing; earlier in pregnancy and less invasive Ultrasound—allows for viewing of the fetus without harming it. 5 1/4/2016 6