Download Genetics after Mendel

Genetics after Mendel
Sutton-Boveri
Hypothesis
 Chromosome Theory
of Heredity
 Genes are carried on
chromosomes
 Segregation and
independent
assortment due to
meiosis
 Physical basis for
Mendel’s rules
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Mendel focused on
probability and
having high validity
He had no knowledge
of the process of
meiosis
Meiosis and Mendel
Law of Segregation is division of genetic
information during the first meiotic
division
 The location each gene on the
chromosome is called the locus
 Since the gamete can only contain one of
each pair of chromosomes it can have
only one allele
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Chromosomal Basis of Heredity
Each homologous chromosome has replicated
forming the tetrad, each tetrad has two
chromosomes each with two chromatids
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To identify an
unknown genotype
Cross with a known
homozygous
recessive
Homozygote only
produces one type
of gamete
ex ttrr crossed with
heterozygous
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tr
Test Cross
To identify an
unknown genotype
TR
Tr
tR
tr
TrRr
¼
Ttrr
¼
ttRr
¼
Ttrr
¼
1
1
1
1
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Mendel crossed
different plants and
found his original
ideas didn’t work
He crossed a red
snap dragon and
white
The results were pink
F1
Pink is the
intermediate
phenotype
Red RR
 White rr
 Pink Rr
 Rr doesn’t produce
enough protein to
make it red
 In the F2 generation
Red and White
flowers reappear
 Humans hair, skin
and eyes are
incomplete
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Incomplete Dominance
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Two alleles are
expressed at the
same time
No one dominates
over the other
Either two capital
letters RW or CRCW
Co-dominance
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]]
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The real question is
does it produce milk
or chocolate milk?
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When there are
more than two
alleles possible for a
given gene
We can still only
carry two alleles
ABO blood type is
an example
A and B are both
dominant over O
but not each other
Multiple Allelism
Genotype
Phenotype
Ii
Type O
IAIA or IAi
Type A
IBIB or Ibi
Type B
IAIB
Type AB
Discontinuous
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One pair of alleles
involved
Ex Red or White
Ex Tall or Dwarf
One shows
dominance
Lower organisms
Multifactorial
Continuous
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Multifactorial – genes
found at many loci
Ex Height
We have a range
Humans and higher
organisms
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Some alleles
produce the same
phenotype
consistently
Others depend on
the environment
Poverty may lead to
genes not fully
being expressed
Deprived food
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Drugs and alcohol
Chemicals
Nature Vs Nurture
debate
Is genetics behind
intellegence
Possible 50% comes
from the genes
Chromosome 4 links
to high IQ
Nature Vs. Nurture
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Crossing over
Homologous chromosomes exchange
information
Occurs during meiosis
Accounts for recombinants
This new F1 generation had different
combinations of genes from its parents
The further apart the genes are the more likely
they will be crossed over
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We have 27000 to 40000 genes on 46
chromosomes
Gene Linkage – genes occur on the same
chromosome
They will not assort independently as Mendel
proposed
Linkage Groups – package of genes inherited
together
Gene Maps – location of genes on specific
chromosomes
So What?