Download Lecture 2 2013 Genetics and Human Health/Welfare

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
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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
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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)
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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
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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
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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
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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
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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
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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)
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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
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Dominant
Recessive
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