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Laws of Heredity - Single Gene Disorders
Mendel (1865), Experiments in Plant Hybridization - no
impact, paper essentially ignored, rediscovery in 1900
Mendel’s key idea: “elements” – now called genes are
the basic units of heredity. Laws based on observation.
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Mendel’s conclusions
Mendel’s experiments:
1.
In cross-pollinating plants that either produce
yellow or green peas exclusively, Mendel found
that the first offspring generation (f1) always has
yellow peas. However, the following generation
(f2) consistently has a 3:1 ratio of yellow to green.
2.
3.
Inheritance of each trait is determined by "units"
that are passed on to descendents unchanged
(these units are now called genes)
An individual inherits one such unit from each
parent for each trait
A trait may not show up in an individual but can still
be passed on to the next generation.
In this experiment, the starting parent plants were
homozygous for pea color. The plants in the f1
generation were all heterozygous
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Practice …
Family tree (pedigree)
Male
Female
Draw the family tree for a YY male and a GG female!
Genes …
Alleles …
Mating
f1: all
heterozygotes!!
heterozygotes
Parents
Children
Affected
Draw the family tree for a YG male and a YG female!
Homozygous YY or GG
Heterozygous YG
Plomin, 2000 Fig 2.1 (Pg. 6)
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Mendel’s second law: principle of
independent assortment
Mendel’s first law: the principle of
segregation
For any particular trait, the pair of
alleles of each parent separate and
only one allele passes from each
parent on to an offspring. Which
allele in a parent's pair of alleles is
inherited is a matter of chance.
We now know that this segregation of
alleles occurs during the process of sex
cell formation (meiosis).
Different pairs of alleles are passed to
offspring independently of each other.
The result is that new combinations of
genes present in neither parent are possible.
Today, we know this is due to the fact that
the genes for independently assorted traits
are located on different chromosomes.
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Practice …
The concept of dominance
Draw the family tree for a AB male and a BB female!
AB
BB
AB BB AB BB
Draw the family tree for a Cc male and a Cc female!
Cc
Cc
CC Cc
Cc
cc
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Mendel’s Laws: SUMMARY
The concept of dominance
What will be the color if the Y allele is dominant?
Genotype
–
With all of the seven pea plant traits that Mendel
examined, one form appeared dominant over the
other. Which is to say, it masked the presence of
the other allele. For example, when the genotype
for pea color is YG (heterozygous), the
phenotype is yellow.
However, the dominant yellow allele does not
alter the recessive green one in any way. Both
alleles can be passed on to the next generation
unchanged.
Law of Segregation: …alleles separate,
Law of Independent Assortment alleles
one allele passes from each parent …
Phenotype
pass independently of each other
Principles:
1.
2.
YG appears as yellow!!!
f2: 3:1 ratio! Only GG will be green!!!
3.
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Inherited features governed by a pair of
“elements”
One element inherited from each parent
Elements can dominate in their expression
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Examples from psychiatry:
Genetic disorders with
Mendelian Patterns of
Inheritance
Terminology TODAY
Mendel’s “elements” are now called genes
Genes come in alternative forms, called alleles
Genotype – an individual’s combination of alleles
Phenotype – the observable trait
Homozygous – two copies of the same allele (AA, aa)
Heterozygous – one copy of each allele (Aa)
Mendelian diseases are diseases that are the result of a
single gene, they generally have a large effect on
behavior & distinctive patterns of familial transmission
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Heredity of Huntington disease
Huntington’s Disease
Male
Onset: mid-adulthood
Prevalence: 1 in every 20,000
Starts with: personality changes,
Draw the pedigree of a Huntington
disease family with one HD parent!
Female
Mating
forgetfulness
Next 15-20 years: complete loss of motor
No treatment has been found to stop or
delay the decline
Consistent pattern of Heredity:
AUTOSOMAL DOMINANT
Parents
function and intellect
Children
Affected
Plomin, 2000 Fig 2.1 (Pg. 6)
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Heredity of Phenylketonuria
Phenylketonuria
Male
Onset: early childhood
Prevalence: 1 in every 10,000
Female
Severe problems in early neural development
leading to mental retardation
Mating
Due to disturbance in the metabolism of
phenylalanine (an essential amino-acid)
Treatment: special diet in early childhood lacking
phenylalanine
Affected
“Runs in families”, but typically NO affected
parents: AUTOSOMAL RECESSIVE
Carriers
Affected individuals have one parent
with the disease, in the above case, the
affected parent is heterozygous. In this
case approx. half of the children
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develop the disease.
Draw the pedigree of a PKU family!
Parents
Children
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Plomin, 2000 Fig 2.1 (Pg. 6)
PKU individuals do not typically have
parents with PKU. If one child has
PKU, the risk for other siblings is
25%. Parents are carriers in such cases.
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Other examples of Autosomal
Dominant Diseases
Mendelian Patterns of Inheritance
Autosomal dominant
FH Familial Hypercholesterolemia
Autosomal recessive
X-linked recessive (next class…)
Polydactyly
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Other examples of Autosomal
Dominant Diseases
Neuro-
From alleles to genotypes
A – is the normal allele (wild type),
fibromatosis
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a – is the mutant allele
Marfan’s syndrome
Achondroplasia
AA is the homozygous wild genotype
aa is the homozygous mutant genotype
Aa is the heterozygous genotype
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Allele & genotype frequencies
A – is the normal allele (wild type),
a – is the mutant allele
IF frequency of A allele is 50% = 0.5
IF frequency of a allele is 50% = 0.5
What is the frequency of the following genotypes:
AA = 0.5 x 0.5 = 0.25
Aa = (both Aa & aA) = 2 x 0.5 x 0.5 = 0.5
aa = 0.5 x 0.5 = 0.25
Another example…
R – a rolling tounge dominant allele
NR – a non-rolling tongue recessive allel
R = 60%
NR = 40%
Can roll: R-R,
R-NR, NR-R
Cannot roll: NR
homozygotes
84%
16%
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http://udel.edu/~mcdonald/mythtongueroll.html
Not all mendelian traits are
„true” autosomal… ☺
• Mendel's laws of heredity
(Law of segregation, Principle of independent assortment,
Mendelian diseases)
• Family tree (pedigree)
(gene, allele, homozygous, heterozygous)
• Huntington Disease & Phenylketonuria
(features and heredity)
• Allele and genotype frequencies (calculations)
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Selected readings
Practice for exam:
Textbook: Ch 2. pg. 6-12.
The outline from this lecture presentation
will be available at the course website!
Calculate the percentage of carriers for an
autosomal recessive trait, if the allele
frequency of the dominant allele is 10%.
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