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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)1856-1865 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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