Sec2-Is+it+geneticB - Mendel's principle of equal...

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Mendel's principle of equal segregation 10/11/07; 2:59 PM J.L.Marsh Page 1 SECTION 2 SEGREGATION [MENDEL I] & its application THE PROBLEMS/QUESTIONS: HOW DO WE KNOW A TRAIT IS GENETICALLY CONTROLLED? WHAT THINGS ARE GENETICALLY INFLUENCED AND HOW DOES GENETIC VARIATION AFFECT HOW we might deal with these things e.g. ADHD attention deficit hyperactivity disorder, homosexuality, addictive behavior, birth defects e.g. thalidomide I] HOW DO WE KNOW A TRAIT IS GENETICALLY DETERMINED?? Imagine the following scenarios: You deliver a baby that has short arms and legs. The parents are planning on having more children and ask for advice. Would your answer be different if the problem were genetic or caused by other factors (e.g. thalidomide)? You have a patient who is suffering cognitive and motor loss. Eventually, this patient dies of severe emaciation, with cognitive loss and loss of motor control. What do you advise the family? (e.g. HD) You find that many frogs in a pond have abnormal limbs. Is this due to a genetic trait or environmental causes? What do we mean by this question? #1- Is it heritable? Is it multigenic or single gene? #2- Is the problem caused by a change in the genetic material (heritable or not)? Is it somatic or germ line e.g. cancer vs birth defect – both mutations e.g. Is it a somatic genetic problem? #3- Is it an environmentally caused pathogenesis How many ways can genes change? 1 a base can change - point mutation 2 DNA can rearrange - chromosomal aberrations where do you put LOH due to mitotic XO? cytologically non-detectable chromosome rearrangement (like a reciprocal translocation between homologous arms? 3 Chromosomes can misbehave - Segregation problems 4 Other! imprinting Focus 1st on heritability: How do we determine if a trait is genetic? How do we approach the problem? We start eliminating possibilities. What is the simplest possibility that can be tested? We hypothesize that the trait is caused by a simple
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Mendel I & Ascertainment 2006 ©J.L. Marsh Bio D137 Page 2 Mendelian gene. This hypothesis has very precise quatitative predictions about segregation and behavior that can be tested. So we ask if it segregates as a single simple Mendelian trait . This is the working hypothesis or null hypothesis (ie. the traits behavior is NOT different from a simple single locus Mendelian trait). First we must consider how many possibilities there are for the behavior of Mendelian traits or stated another way, we must consider how the types of alleles will affect the quantitative predictions of a Mendelian trait. The fraction of offspring affected vs unaffected would be quite different if the trait were dominant vs recessive; hence, determining dominance relationships is a first requirement for this task. Consider an example. You find a cat with 6 toes? Is this genetic? How will you tell?
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