Patterns of Inheritance Chapter 12 You are not

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Patterns of Inheritance Chapter 12 You are not responsible for the section on epistasis

Patterns of Inheritance Chapter 12 You are not responsible for the section on epistasis Read on your own the sections on -- Environmental influences -- Pleiotrophy Chromosomes & Inheritance Chapter 13 Gregor Mendel, 1862 you are not responsible for: Section 13. 4 (genetic mapping) Genomic imprinting (in section 13. 5) Genetic test – we’ll cover this later Mendelian Inheritance 1

Genetics: the study of heredity What is the mechanism of inheritance? There have been

Genetics: the study of heredity What is the mechanism of inheritance? There have been earlier theories… Homunculus – 1695 “little man in a sperm” Blending of traits But how could traits “skip a generation” ? ? Gregor Mendel solved the riddle Mendelian Inheritance 2

What was Mendel’s contribution to Biology? Mendel uncovered ‘rules’ of heredity Augustinian Monk (Czech

What was Mendel’s contribution to Biology? Mendel uncovered ‘rules’ of heredity Augustinian Monk (Czech republic) 1856 -1865 Gregor Mendel, 1862 Why pea plants? Variation in traits Can control pollination Peas normally selfpollinating Mendelian Inheritance Convent Garden, 1920 s 3

What were some typical results of Mendel’s experiments? Worked with “true breeding” varieties P

What were some typical results of Mendel’s experiments? Worked with “true breeding” varieties P P F 1 cross-pollinated true breeding: P, F 1 & F 2 generations F 1 x F 1 F 2 some traits “skip” a generation Why? 3 : 1 ratio Mendelian Inheritance 4

Why do traits sometimes ‘skip’ a generation? P P F 1 Mendel deduced: True

Why do traits sometimes ‘skip’ a generation? P P F 1 Mendel deduced: True breeding: hold information for only a single trait F 1: possess information for both traits ‘Dominant’ trait is one that appears ‘Recessive’ trait is suppressed Mendel saw many such relationships Mendelian Inheritance 5

How can an organism possess information for two different traits? 1) organisms possess 2

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 Round ‘R’ Wrinkled ‘r’ 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 RR Rr rr A dominant allele Is designated with uppercase Letter. A recessive allele with the corresponding lower case letter Phenotype round wrinkled Question Mendelian Inheritance 6

How can a Punnett square be used to predict the outcomes of crosses? r

How can a Punnett square be used to predict the outcomes of crosses? r r RR P X rr = F 1 all Rr True breeding traits must have a homozygous genotype R Rr Rr R F 1 X Rr F 1 X = F 2 Rr Only the homozygous recessive genotype will yield the recessive phenotype Genotypes of gametes are placed on borders r R RR Rr Round r Rr rr Round wrinkled Ratio of genotypes= 1: 2: 1 Ratio of phenotypes= 3: 1 Crosses that examine 1 gene at a time are called “monohybrid” Mendelian Inheritance 7

What are the characteristics of Autosomal Recessive traits and disorders? Carried on non-sex chromosomes

What are the characteristics of Autosomal Recessive traits and disorders? Carried on non-sex chromosomes Phenotype can skip generations People can be a carrier And many disorders, e. g. Cystic fibrosis & Tay Sachs Question hexadactyly freckles Mendelian Inheritance 8

What are the characteristics of Autosomal Dominant traits and disorders? Also on non-sex chromosomes

What are the characteristics of Autosomal Dominant traits and disorders? Also on non-sex chromosomes Phenotype does not skip generations 50 or 100% of children will get trait And Various disorders, e. g. Huntingtons Question Cleft chin Mendelian Inheritance 9

How does meiosis explain Mendel’s Principle of Segregation? Key questions: 1) Where do a

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 Mendelian Inheritance 10

What is Mendel’s ‘Principle of Independent Assortment’? Alleles for one trait sort independently of

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 Gamete genotypes Question 1 Question 2 Mendelian Inheritance 11

How can the outcomes of ‘multiple-hybrid’crosses be predicted mathematically? How does meiosis explain Mendel’s

How can the outcomes of ‘multiple-hybrid’crosses be predicted mathematically? How does meiosis explain Mendel’s Principle of Independent Assortment? Ratios of ‘multiple-hybrid’ crosses are the product of the ratios of monohybrid crosses Rr. Yy x Rr. Yy ¾ Round : ¼ wrinkled Yy x Yy = ¾ Yellow : ¼ green Rr x Rr = (¾R_ : ¼rr) x (¾Y_ : ¼yy)___ 9/16 R_Y_ Rnd Yel : 3/16 : R_yy rr. Y_ Rnd Grn 3/16 rryy Wrk Yel : 1/16 Wrk Grn In a cross of Rr. Yy x Rr. YY what is the expected frequency of “rr. Yy”? ¼x½=⅛ Question Mendelian Inheritance 12

Why do inheritance patterns sometimes not follow normal Mendelian ratios? How is Incomplete Dominance

Why do inheritance patterns sometimes not follow normal Mendelian ratios? How is Incomplete Dominance different from ‘normal’ dominance? HS -- heterozygote has intermediate phenotype HC e. g. Human hair form -- two alleles: HS – straight HS HC HS HS HS HC Straight Wavy HS HC HC HC Wavy Curly and HC – 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 Mendelian Inheritance 13

What is Codominance? Tabby -- heterozygote has distinctive phenotype -- combination of alleles yields

What is Codominance? Tabby -- heterozygote has distinctive phenotype -- combination of alleles yields new trait Tabby gene (T) affects patterning of cat fur TSTS – striping of colored and white hairs = ‘Tabby’ TATA – no striping (hairs are mixed) = ‘Agouti’ TSTA – ‘chinchilla-like’ hair: color-tipped hairs Agouti chinchilla Question Mendelian Inheritance 14

How can “multiple alleles” for a gene influence inheritance? -- When more than 2

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 Blood Group Antigen on cells A A B B O neither AB A&B Alleles code for antigens -- 3 alleles in species -- each person can possess only 2 Allele & antigen Expression Associated Blood group Possible genotype IA yields A antigen Codominant A IA IA or IA i IB yields B antigen Codominant B IB IB or IB i i Recessive O ii yields no antigen [Given tables such as these, you should be able to fill in missing values] Mendelian Inheritance 15

Inheritance of blood type, con’t. What is theoretical frequency of genotypes and phenotypes among

Inheritance of blood type, con’t. What is theoretical frequency of genotypes and phenotypes among children of heterozygous-A and a heterozygous-B parents? IB If a child is heterozygous for B-type blood, what are the possible blood types of the parents? i IA IB IB i AB B IA i ii A O What causes blood type incompatibility? -- role of antibodies Blood Group Antigen on cells Antibodies in serum A A anti-B B B anti-A O U-donor Neither anti-a & anti-b AB U-recipient A&B neither Blood type and Paternity If a child has type-O blood, could a man With type-B blood be the father? Explain. Question 1 Question 2 Mendelian Inheritance 16

ONON What is a Lethal Allele? OOON -- homozygous recessive is fatal Overo gene

ONON What is a Lethal Allele? OOON -- homozygous recessive is fatal Overo gene affects hair color pattern in horses Two alleles: ON = normal OO = white OOON X OOON yields ratio of 2 patterned : 1 solid Why? OOOO : lethal aganglionic colon -- absence of nerves in colon ‘Creeper’ gene in birds; CA = ‘Creeper’ allele causes deformed wings and legs; heterozygotes (CNCA) display creeper trait OOOO This is impossible: Why? C N CA X CA CA Mendelian Inheritance 17

Sickle-cell anemia: incomplete dominance and lethal allele (Read about this topic in Chapter 13)

Sickle-cell anemia: incomplete dominance and lethal allele (Read about this topic in Chapter 13) Cause SC anemia Genetics Hb. A: normal allele Hb. S: abnormal Effects -- ‘SC-anemia’: early death -- ‘SC-trait’: moderate symptoms Consider cross of heterozygotes Hb. A Why does a lethal allele persist? Hb. S Hb. A Hb. S Normal SC trait Hb. A Hb. S SC trait SC anemia Question 1 Question 2 Mendelian Inheritance 18

How does Sickle cell allele cause the phenotype? Changes in DNA code alter protein

How does Sickle cell allele cause the phenotype? Changes in DNA code alter protein structure Chromosomes and Human Genetic Disorders 19

What are the characteristics of Polygenic Inheritance? -- when more than 1 gene contributes

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 – A “Gene Dosage” effect 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 Mendelian Inheritance 20

Chapter 13 What is a Karyotype? How is a karyotype prepared? 1. Collect &

Chapter 13 What is a Karyotype? How is a karyotype prepared? 1. Collect & culture cells 2. Treat with mitotic inhibitor 3. Spread out cells and stain 4. Photograph & pair up chromosomes Chronic myeloid leukemia A reciprocal chromosome translocation has occurred between Chromosomes 9 & 22 Chromosomes and Human Genetic Disorders 21

How is sex determined? Does egg or sperm determine the sex of the offspring?

How is sex determined? Does egg or sperm determine the sex of the offspring? What genes are on the sex chromosomes? -- > 1000 on X; many different functions -- < 100 on the Y Why does Y chromosome contain few genes? Study ‘X chromosome inactivation’ in the textbook Chromosomes and Human Genetic Disorders 22

What is unusual about the inheritance of hemophilia? The Romanov family Czar Nicholas and

What is unusual about the inheritance of hemophilia? The Romanov family Czar Nicholas and Alexandra Chromosomes and Human Genetic Disorders Rasputin 23

Genes on the X chromosome are said to be ‘sex linked’ Abnormal genes often

Genes on the X chromosome are said to be ‘sex linked’ Abnormal genes often act as dominants in males and recessive in females. Why? Designation of X-linked genes What is theoretical frequency of hemophilia among the children of a normal father and a carrier mother? … From a hemophiliac mother? Red-Green Color Blindness ~10% of males; <1% of females XC = normal allele Xc = recessive abnormal Question Ishihara test Chromosomes and Human Genetic Disorders Color responsiveness of different types of cone cells 24

How do chromosomal abnormalities affect traits? Nondisjunction and aneuploidy Nondisjunction Autosomal aneuplopidy e. g.

How do chromosomal abnormalities affect traits? Nondisjunction and aneuploidy Nondisjunction Autosomal aneuplopidy e. g. , Down’s syndrome (trisomy 21) -- non-disjunction of chromosomes Sex-chromosome aneuploids X-chromosome nondisjunction -- XO (Turner syndrome) -- XXY (Klinefelter syndrome) Y-chromosome nondisjunction -- XYY male Chromosomes and Human Genetic Disorders 25