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Mendelian Inheritance1 Gregor Mendel, 1862 Convent Garden, 1920s You are not responsible for the section on epistasis Read on your own the sections on.

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Presentation on theme: "Mendelian Inheritance1 Gregor Mendel, 1862 Convent Garden, 1920s You are not responsible for the section on epistasis Read on your own the sections on."— Presentation transcript:

1 Mendelian Inheritance1 Gregor Mendel, 1862 Convent Garden, 1920s You are not responsible for the section on epistasis Read on your own the sections on -- Environmental influences -- Pleiotrophy Patterns of Inheritance Chapter 12

2 Mendelian Inheritance2 Gregor Mendel, 1862 Convent Garden, 1920s What was Mendel’s contribution to Biology? Mendel uncovered ‘rules’ of heredity Augustinian Monk (Czech republic) Why pea plants? Variation in traits Can control pollination Peas normally self- pollinating

3 Mendelian Inheritance3 What were some typical results of Mendel’s experiments? Worked with “true breeding” varieties cross-pollinated true breeding: P, F1 & F2 generations some traits “skip” a generation Why? P P F1 F1 x F1 F2 3 : 1 ratio

4 Mendelian Inheritance4 Why do traits sometimes ‘skip’ a generation? Mendel deduced: True breeding: hold information for only a single trait F1: possess information for both traits ‘Dominant’ trait is one that appears ‘Recessive’ trait is suppressed Mendel saw many such relationships P P F1

5 Mendelian Inheritance5 How can an organism possess information for two different traits? 1) organisms possess 2 ‘genes’ for a trait 2) specific information is called an ‘allele’ Mendel’s “Principle of Segregation” Each trait is inherited as a pair of alleles, which separate in the gametes and recombine upon fertilization What are three possible combinations of alleles? 2 dominants = ‘homozygous dominant’ 1 dom & 1 rec = “heterozygous’ 2 recessives = ‘homozygous recessive’ What is Phenotype vs Genotype? -- genetic information vs its physical expression Genotype Phenotype RR round Rr round rr wrinkled Round ‘R’ Wrinkled ‘r’ A dominant allele Is designated with uppercase Letter. A recessive allele with the corresponding lower case letter Question

6 Mendelian Inheritance6 How can a Punnett square be used to predict the outcomes of crosses? RrRrRrRr RrRrRrRr r RRRR Genotypes of gametes are placed on borders P F 1 RR X rr = all Rr True breeding traits must have a homozygous genotype F 1 X F 1 = F 2 Rr X Rr r wrinkled Rr Round Rr RoundR R r RrRr Ratio of genotypes= 1:2:1 Ratio of phenotypes= 3:1 Only the homozygous recessive genotype will yield the recessive phenotype Crosses that examine 1 gene at a time are called “monohybrid”

7 Mendelian Inheritance7 Question What are the characteristics of Autosomal Recessive traits and disorders? Carried on non-sex chromosomes Phenotype can skip generations People can be a carrier hexadactyly freckles And many disorders, e.g. Cystic fibrosis & Tay Sachs

8 Mendelian Inheritance8 What are the characteristics of Autosomal Dominant traits and disorders? Question Also on non-sex chromosomes Phenotype does not skip generations 50 or 100% of children will get trait Cleft chin And Various disorders, e.g. Huntingtons

9 Mendelian Inheritance9 How does meiosis explain Mendel’s Principle of Segregation? Key questions: 1)Where do a pair of alleles exist in the cell? 2) When does separation of alleles occur? 3) When does recombination occur? Probability yields 3:1 ratio in phenotypes

10 Mendelian Inheritance10 What is Mendel’s ‘Principle of Independent Assortment’? Alleles for one trait sort independently of the alleles for a different trait --illustrated by a Dihybrid Cross Question 1 Question 2 Gamete genotypes

11 Mendelian Inheritance11 How can the outcomes of ‘multiple-hybrid’crosses be predicted mathematically? Ratios of ‘multiple-hybrid’ crosses are the product of the ratios of monohybrid crosses RrYy x RrYy Rr x Rr = ¾ Round : ¼ wrinkled Yy x Yy = ¾ Yellow : ¼ green ( ¾ R_ : ¼rr) x ( ¾ Y_ : ¼ yy)___ 9/16 : 3/16 : 3/16 : 1/16 R_Y_ R_yy rrY_ rryy Rnd Yel Rnd Grn Wrk YelWrk Grn In a cross of RrYy x RrYY what is the expected frequency of “rrYy”? ¼ x ½ = ⅛ How does meiosis explain Mendel’s Principle of Independent Assortment? Question

12 Mendelian Inheritance12 How is Incomplete Dominance different from ‘normal’ dominance? -- heterozygote has intermediate phenotype e.g. Human hair form -- two alleles: H S – straight and H C – curly In a mating of heterozygotes, what is expected frequency of hair styles among children? Other examples: Pea flower color (see book) Chicken “Naked neck” allele homozygote NA NA -- normal neck feathers homozygote Na Na -- lack neck feathers heterozygote NA Na -- reduced # of feathers H C Curly H S H C Wavy H S H C Wavy H S Straight H S H C HSHCHSHC Why do inheritance patterns sometimes not follow normal Mendelian ratios?

13 Mendelian Inheritance13 What is Codominance? -- heterozygote has distinctive phenotype -- combination of alleles yields new trait Tabby gene (T) affects patterning of cat fur T S T S – striping of colored and white hairs = ‘Tabby’ T A T A – no striping (hairs are mixed) = ‘Agouti’ T S T A – ‘chinchilla-like’ hair: color-tipped hairs Tabby Agouti chinchilla Question

14 Mendelian Inheritance14 How can “multiple alleles” for a gene influence inheritance? -- When more than 2 alleles for a trait exist in the population Inheritance of blood type -- involves codominance and multiple alleles Antigens: present on the cells Alleles code for antigens -- 3 alleles in species -- each person can possess only 2 Blood Group Antigen on cells AA BB Oneither ABA & B Allele & antigenExpression Associated Blood group Possible genotype I A yields A antigen CodominantA I A I A or I A i I B yields B antigen CodominantB I B I B or I B i i yields no antigen RecessiveOi [Given tables such as these, you should be able to fill in missing values]

15 Mendelian Inheritance15 Inheritance of blood type, con’t. What is the theoretical frequency of genotypes and phenotypes among children of heterozygous-A and a heterozygous-B parents? If a child is heterozygous for B-type blood, what are the possible blood types of the parents? What causes blood type incompatibility? -- role of antibodies Blood type and Paternity If a child has type-O blood, could a man With type-B blood be the father? Explain. Blood Group Antigen on cells Antibodies in serum AAanti-B BBanti-A O U-donor Neitheranti-a & anti-b AB U-recipient A & Bneither I A I B AB I B i B I A i A i O I A i I B i Question 1 Question 2

16 Mendelian Inheritance16 What is a Lethal Allele? -- homozygous recessive is fatal Overo gene affects hair color pattern in horses Two alleles: O N = normal O O = white O O O N X O O O N yields ratio of 2 patterned : 1 solid Why? O O O O : lethal aganglionic colon -- absence of nerves in colon ‘Creeper’ gene in birds; C A = ‘Creeper’ allele causes deformed wings and legs; heterozygotes (C N C A ) display creeper trait This is impossible: Why? C N C A X C A C A ONONONON OOONOOON OOOOOOOO

17 Mendelian Inheritance17 Sickle-cell anemia: incomplete dominance and lethal allele (This topic is in Chapter 13) Cause Genetics Hb A : normal allele Hb S : abnormal Effects -- ‘SC-anemia’: early death -- ‘SC-trait’: moderate symptoms Consider cross of heterozygotes Why does a lethal allele persist? Hb A Normal Hb A Hb S SC trait Hb A Hb S SC trait Hb S SC anemia Hb A Hb S Hb A Hb S Question 1 SC anemia Question 2

18 Mendelian Inheritance18 What are the characteristics of Polygenic Inheritance? -- when more than 1 gene contributes to a phenotype e.g., human height, intelligence, eye color, skin color, etc Skin color Melanin production -- dominant Why variation? No longer reflect long-term phylogenic relationships May be 9 genes involved -- 3 shown Note: this is not albinism Question


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