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Transcript
Lecture 2 2013 Prof Jane Farrar Campbell Biology by Reece Taylor Simon Dickey Or any Genetics textbook Chapter 9: Patterns of Inheritance Dr Sophia Millington-Ward Next lecture in the series from Prof Farrar is 4pm Tues Oct 15th Genetics and Human Health/Welfare 1. Production of safer vaccines: recombinant single subunit vaccines e.g. hepatitis B vaccine 2. Production of recombinant human therapeutic proteins e.g. insulin, growth hormone, clot dissolving proteins 3. Inherited disorders can be diagnosed prenatally 4. Prenatal genotyping – in vitro fertilisation & pre implantation diagnosis 1 5. Populations and individuals could be genotyped for Quantitative disease alleles e.g. predisposition to mental health problems, heart disease, high blood pressure, cancer, Alzheimer s disease etc. Issues of privacy: who owns the information and who has access Implications for health insurance and employment 6. Pharmacogenomics: using genomics to genotype populations and individuals for alleles that determine responsiveness to drug therapies. Aim: individualization of drug therapies 7. Gene therapy for inherited diseases (What about other traits?) designer babies 2 3 Some Basics First! What is Genetics? - the study of inheritance - the flow of biological information from one generation to the next Information about what? - How to construct a living organism - How the organism s physical characteristics are determined The Information Flow: Inheritance - Is it governed by rules? - If so, what are these rules? - How can the rules be discovered? Uncovering the Rules of Inheritance: A General Strategy Mate or cross 2 individuals that show different forms of the same physical trait Trait Height Hair color Eye color Disease X Parent 1 X Tall Dark Blue Disease X Parent 2 Short Blond Brown Healthy Ask: 1. How many distinct types of offspring? 2. How frequent is a given type? 3. Is there a predictable pattern of inheritance of the traits? (Refer to the dominant, recessive & sex-linked patterns of inheritance in humans outlined in Lecture 1) 4 Mendel studied 7 traits Flower color Gregor Mendel (1822 – 1884) Flower position Seed color Seed texture Pod texture Pod color Ref: You Tube Gregor Mendel Rap Plant height 5 Before Mendel s work (1860 s), no consistently predictable patterns of inheritance were observed. Mendel s successful experimental strategy: 1. Choose an organism that is known to breed true i.e. when crossed with itself it gives only offspring that are the same as itself 2. Choice of experimental organism: pea (Pisum sativum) Why? a. Many physically distinct varieties available b. Varieties with alternative forms of a given trait Additional reasons for Mendel s success 1. He investigated the inheritance of only one or two traits at a time 2. He kept accurate quantitative data about: (a) how many different types of offspring were obtained from a cross (b) the frequency of occurrence of the various types 6 F1 Pp x Pp F2 PP Pp pP pp Mendel s First Experiments Inheritance of a single trait = monohybrid cross Parental generation P1 purple X P2 white F1 offspring generation all purple Let F1 plants self-fertilize = F1 X F1 F2 offspring generation purple: 705 white: 224 Ratio: purple : white = 3 : 1 7 Each of the 7 individual traits that Mendel studied was inherited in the same way! P1 X P2 F1 offspring show one parent s phenotype only said to be the DOMINANT phenotype F1 X F1 F2 reappearance of the second phenotype called the RECESSIVE phenotype Ratio of DOMINANT : RECESSIVE = 3 : 1 Mendel s explanation – some lateral thinking! 1. Each trait is determined by a pair of unit factors which we now call genes Why a pair? Because each parent supplies one 2. There are two alternative forms = alleles of each gene that determines a given trait: a dominant allele a recessive allele 3. The two alleles segregate randomly during gamete formation i.e. the male and female gametes each contain one allele This is known as Mendel s law of segregation 4. During fertilization, when an egg cell fuses with a sperm cell, the resulting zygote contains one allele from each parent 8 5. Since there are two alleles of each gene, there are three ways in which pairs of allele can be combined e.g. Flower colour: Dominant P allele + Recessive p allele GENOTYPE 1 PHENOTYPE PP homozygous Pp x Pp Pp heterozygous F1 3 2 Pp heterozygous 1 pp homozygous PP Pp pP pp F2 1 What about the inheritance of two traits? A di-hybrid cross Will the inheritance of one influence the inheritance of the other? i.e. dependent or independent inheritance: which? Seed colour – yellow (Y) or green (y) Seed shape – Round (R) or wrinkled (r) This doesn t happen! Just 2 sperm & ova types: YR and yr (Dependent inheritance) 9 Mendel s second law: Independent Assortment Each trait is inherited independently of the other yellow (Y) green (y) Round (R) wrinkled (r) 4 types sperm 4 types egg cells 9: 3: 3: 1 ratio Where does the 9:3:3:1 ratio come from? From the independent probabilities of inheriting a given allele Single probabilities: probability of Yellow 3/4 Seed color 3:1 ratio probability of green 1/4 probability of Round 3/4 Seed shape 3:1 ratio probability of wrinkled 1/4 Combined probabilities: Probability of Yellow AND Round Yellow AND wrinkled green AND Round green AND wrinkled = 3/4 X 3/4 = 9/16 = 3/4 X 1/4 = 3/16 = 1/4 X 3/4 = 3/16 = 1/4 X 1/4 = 1/16 10 Dominant Recessive 11