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Mendelian Genetics
Using a six-sided die, what is the probability of
rolling either a 5 or a 6?
A) 1/6 x 1/6 = 1/36
B) 1/6 + 1/6 = 1/3
C) 1/6 + 1/6 = 2/3
D) 1/6 + 1/6 = 1/12
E) 1/6
People knew that traits were inherited
• Selective breeding is
based on empirical
knowledge that
offspring resemble their
parents
• “Like begets like”
• Mechansim unknown
• Possible blending of
traits, possible
environmental factors
Mendel studied the mechanism of
inheritance
• Particulate theory of
inheritance
• Mendel got low scores
in biology
Mendel’s model organism- the pea plant
• Monoecious plant- both
male + female
reproductive parts on
same flower
• Pistil- contains
unfertilized seeds
• Stamen- hold pollen
Mendel studied characters for which 2
traits existed
•
•
•
•
Purple vs. White flowers
Axial vs. Apical flowers
Yellow vs. green peas
Round vs. wrinkled
seeds
• Etc.
Mendel crossed true-breeding pea
plant strains
• True-breeding plants were generated from
plants by repeatedly inbreeding like traits
• When purple flowers only begot purple
flowers, and white only begot white, they
were said to be true-breeding
F1 generation showed dominance of
one of the traits over the other
• But when the F1 plants
were allowed to selfpollinate, the lost trait
returned
• In the F2 generation,
25% of the plants had
the recessive trait (3:1
ratio)
Mendel’s Model
• Mendel developed a hypothesis to explain the
3:1 inheritance pattern he observed in F2
offspring
• Four related concepts make up this model
• These concepts can be related to what we now
know about genes and chromosomes
1. There are different versions of a gene
determining a trait (alleles)
2. Alleles are inherited in pairs- each parent
donates one allele of a gene
3. When individuals inherit different alleles
from each parent, some will be manifest
(dominant) and others will hide (recessive)
4. Law of segregation- gamete formation
involves placing a single allele for each trait
into a gamete
What Mendel did not know
• The genes are on DNA
• The DNA is on
chromosomes
• Alleles exist at specific
loci on chromosomes
• Chromosomes are
segregated into
gametes
LE 14-4
Allele for purple flowers
Locus for flower-color gene
Allele for white flowers
Homologous
pair of
chromosomes
Useful Genetic Vocabulary
• An organism with two identical alleles for a character is
said to be homozygous for the gene controlling that
character
• An organism that has two different alleles for a gene is
said to be heterozygous for the gene controlling that
character
• Unlike homozygotes, heterozygotes are not true-breeding
• Physical apperance is called the phenotype
• Genes which influence that appearance is called the
genotype
LE 14-6
3
Phenotype
Genotype
Purple
PP
(homozygous
Purple
Pp
(heterozygous
1
2
1
Purple
Pp
(heterozygous
White
pp
(homozygous
Ratio 3:1
Ratio 1:2:1
1
LE 14-5_2
P Generation
Appearance:
Genetic makeup:
Purple
flowers
PP
White
flowers
pp
P
p
Gametes
F1 Generation
Appearance:
Genetic makeup:
Purple flowers
Pp
1
Gametes:
2
1
P
2
p
F1 sperm
P
p
PP
Pp
Pp
pp
F2 Generation
P
F1 eggs
p
3
:1
Gamete formation and fertilization are
influenced by chance
• For heterozygotes, half
of gametes will have a
given allele
• 50% chance that a
fertilizing gamete will
contain a particular
gene
Making predictions about offspring
with a Punnett square
• If genotypes of parents
are known, predictions
can be made about
offspring
• The Punnett square is
used to calculate odds
• Notation: Capitals for
dominant alleles, lowercase for recessive
Like algebra, doing Punnett squares correctly
depends on setting up the problem correctly
• Setting up the problem correctly depends on
generating gametes correctly
• Recall, gametes are generated by meiosis
Generating gametes are generated only for
the genes being considered
• Living things have
thousands of genes
• Punnett squares only
look at one or two
(maybe three)
• Aa
• AA
• A, a
• ___?
• Aa
• AA
• AaBB
• A, a
• A
• ____?
•
•
•
•
Aa
AA
AaBB
AaBb
•
•
•
•
A, a
A
AB, aB
_____?
•
•
•
•
Aa
AA
AaBB
AaBb
•
•
•
•
A, a
A
AB, aB
AB, Ab, aB, ab
•
•
•
•
Aa
AA
AaBB
AaBb
•
•
•
•
A, a
A
AB, aB
AB, Ab, aB, ab
• ≠ AA, BB, aa, bb
• Why are these gametes
not formed?
Gamete formation and fertilization are
influenced by chance
• For heterozygotes, half
of gametes will have a
given allele
• 50% chance that a
fertilizing gamete will
contain a particular
gene
The Test Cross
• For determining
Genotype of a
dominant phenotype
• Crossed with a
recessive, a
homozygote yields all
domiant phenotype,
with a heterozygote, a
1:1 ratio
The Testcross
• How can we tell the genotype of an individual
with the dominant phenotype?
• Such an individual must have one dominant
allele, but the individual could be either
homozygous dominant or heterozygous
• The answer is to carry out a testcross: breeding
the mystery individual with a homozygous
recessive individual
• If any offspring display the recessive phenotype,
the mystery parent must be heterozygous
Mendel studied two traits at a time
• Punnett squares can be
used in crosses of two
genes
• Mendel found that the
two genes were
inherited independently
of each other
• The law of independent
assortment
LE 14-8
P Generation
YYRR
yyrr
Gametes YR
yr
YyRr
F1 Generation
Hypothesis of
dependent
assortment
Hypothesis of
independent
assortment
Sperm
1
Sperm
1
2
YR
1
2
yr
1
1
2
2
1
4
Yr
1
4
yR
1
4
yr
4
YR
4
Yr
YYRR
YYRr
YyRR
YyRr
YYRr
YYrr
YyRr
Yyrr
YyRR
YyRr
yyRR
yyRr
YyRr
Yyrr
yyRr
yyrr
YR
YYRR
1
YR
Eggs
Eggs
F2 Generation
(predicted
offspring)
4
YyRr
1
yr
YyRr
3
4
yyrr
1
1
yR
4
4
1
Phenotypic ratio 3:1
yr
4
9
16
3
16
3
16
3
16
Phenotypic ratio 9:3:3:1
Meiosis accounts for Independent
Assortment of genes
• The probablility of one
chromosome being
passed to a gamete is
50%
• Different genes are on
different chromosomes
Mendel’s data was extremely good
• Improbably good, in
fact
• R.A. Fisher analyzed
the odds of his dataextremely remote!
• Did Mendel fudge his
data?
Mendel’s data was extremely good
• Improbably good, in
fact
• R.A. Fisher analyzed
the odds of his dataextremely remote!
• Did Mendel fudge his
data?
• Did Mendel suffer
from confirmation
bias?
Other patterns of inheritance
•
•
•
•
•
•
Incomplete dominance
Codominance
Multiple alleles
Pleiotropy
Polygenic inheritance
Epistasis
Incomplete dominance
• Neither gene is
dominant of the other
• Result is a blending of
phenotypes
• Dominance:
Genotype ≠ Phenotype
• Incomplete dominance:
Genotype = phenotype
Codominance
• Both genes
expressed
simultaneously
Codominance in rhododendrons
• Red and white
genes expressed
simultaneously
Many human diseases exhibit Mendelian (and
non-Mendelian) inheritance
Mendelian Ineritance in Man
• Inheritance of certain
alleles can be analyzed
using pedigrees
• Many genetic diseases
exhibit Mendelian
inheritance
LE 14-14a
Ww
ww
ww
Ww ww ww Ww
WW
or
Ww
Widow’s peak
Dominant trait (widow’s peak)
Ww
Ww
First generation
(grandparents)
Second generation
(parents plus aunts
and uncles)
ww
Third
generation
(two sisters)
ww
No widow’s peak
Sickle cell anemia
• Round cells- normal
• Crescent-shaped cellssickle cell
• Caused by a single
mutation in the human
gene for hemoglobin
Cystic Fibrosis and Sickle Cell Anemia
exhibit Mendelian Inheritance
• Both are recessive
disorders
• Both exhibit
multiple
phenotypic traits
for a single gene
defect (pleiotropy)
Pleiotropy
Individuals heterozygous for sickle cell
exhibit resistance to malaria
• Incidence of sickle
cell correlates to
the prevalence of
malria-carrying
mosquitoes in the
environment
• What is the
inheritance pattern
of sickle celldominance?
INHERITANCE OF HD
Completely
Genetic
Autosomal
Dominant
100% Lethal
Why has Huntington’s not been totally eradicated from
the population?
Genetic testing raises many ethical
questions
• Alro Guthrie’s dad, Woody,
had HD
• What were the chances Arlo
would have HD?
• What if he could have been
tested?
• What about insurance?
• Should he be allowed to
have children?
Polygenic inheritance
• One trait is affected by many genes
• Human height is an example
Many traits have multiple alleles
Sex Linkage
• Genes on sex chromosomes are
not necessarily involved in sex
determination
• Genes for color blindness and
hemophilia are found on
human X-chromosomes
• Sex-linkage (or X-linkage) shows
different patterns of
inheritance
• In males, Recessive traits
cannot be masked by domiannt
ones
• Sex is not always determined
by presence of a Y chromosome
LE 15-9
Parents
Ova
Sperm
Zygotes
(offspring)
The X-Y system
The X-0 system
The Z-W system
The haplo-diploid system
Epistasis
• One gene nullifies the
effects of another
• Bombay O blood type is
an example of recessive
epistasis
Alleles of a gene are found at __________
chromosomes.
A) the same locus on homologous
mitochondrial
B) the same locus on heterologous
C) different loci on homologous
D) different loci on heterologous
E) the same locus on homologous
MENDEL’S THREE LAWS
The Law of Dominance
Dominant alleles can phenotypically mask the effect of
recessive alleles.
The Law of Segregation of Alleles
The two alleles in a gene pair segregate into gametes so
that half of the gametes carry one allele and the other
half carry the other allele.
The Law of Independent Assortment
Allelic segregation at one gene during gamete formation
is independent of allelic segregation at another gene.
Exceptions to Mendel’s Laws
• The Law of Dominance
• The Law of Segregation
• The Law of independent
assortment
Which one of the following is false?
A) Sickle-cell disease is common in tropical Africa.
B) Persons who are heterozygous for sickle-cell
disease are also resistant to malaria.
C) Sickle-cell disease causes white blood cells to be
sickle shaped.
D) All of the symptoms of sickle-cell disease result
from the actions of just one allele.
E) About one in 10 African Americans is a carrier of
sickle-cell disease.
Exceptions to Mendel’s Laws
• The Law of Dominance
• The Law of Segregation
• The Law of independent
assortment
• Incomplete dominance,
codominance
Exceptions to Mendel’s Laws
• The Law of Dominance
• The Law of Segregation
• The Law of independent
assortment
• Incomplete dominance,
codominance
• Nondisjunction
• ______?
Reginald Punnett found two traits in peas
that did not assort independently
What could have caused this?
LINKAGE
• Physical association between 2 genes on
the same chromosome
• Linkage is the exception to Mendel’s
Law of Independent Assortment
T. H. Morgan found the particles of
inheritance
• Mendel’s work had just
been rediscovered
(1900)
• He found them on
chromosomes through
the study of Drosophila
melanogaster, fruit flies
• Fruit flies are an ideal
model organism for a
variety of reasons
Morgan treated flies with mutagens
• Ultimately, he obtained
a fly with white eyes
• Mendelian ratios were
observed in the
offspring
• Only male flies
exhibited the recessive
trait in the F2
generation
Male flies also had different
chromosome sets
• This linked individual
traits with the particles
that carried them- the
chromosomes
• It also showed a new
pattern of inheritancesex linkage
Two-gene crosses of linked genes were
studied
• Morgan generated a
few more mutants
• Offspring did not
sort independently
• Yet the genes did not
sort completely
together
• Higher incidence of
parental phenotypes
What determines the rate at which
linked genes cross over?
LINKAGE
• Physical association between 2 genes on
the same chromosome
•Linkage Correlates with physical
proximity of genes on a chromosomes
•Genes can be unlinked through
recombination, or crossing over
RECOMBINATION
• Reassortment of genes or alleles in new
combinations
• Physical crossing over of two non-sister
chromatids from homologous chromosomes
during Prophase I
**The closer
the genes, the
lower the
recombination
frequency**
Since recombination correlates with
physical proximity, genes can be
mapped
• Genes can thus be
located on chromosomes
relative to other genes
• Map distance is measured
with recombination rates
in a two-point cross
• Distance can be
quantified in
CentiMorgans
RECOMBINATION
FREQUENCY
#
of
recombinants
R.F. =
x 100%
total # of progeny
1 map unit = 1 centiMorgan (cM) = 1% crossover
THEORETICAL LIMIT
OF RECOMBINATION
Why is the maximum recombination frequency
between two genes 50%?
Four haploid cells
One diploid cell
25%
25%
Recombination
25%
25%
Drosophila labs are full of weirdos
Changes in chromosomes cause
aberrant phenotypes
• Abnormal chromosome
number is called
aneuploidy
• can be caused by
nondisjunction of
chromosomes in either
Meiosis I or II
• Complete chromosomal
nondisjunction causes
polyploidy
Polyploidy is common in plants
• Gigantic strawberries
are polyploid
• Polyploidynondisjunction of an
entire genome
• Double diploid genomes
= tetraploid
Polyploidy usually results in larger
structures
Chromosomes can be damaged
• Deletion- a segment of
a chromosome is lost
• Duplication
• Inversion
• Translocation
• Causes of disease (cri
du chat) and evolution
• Error often occur during
crossover
Chromosome alterations can drive
speciation
• Comparative analysis of chimpanzee
and human Y chromosomes unveils
complex evolutionary pathway
• ..Here we report the finished
sequence of the chimpanzee Y
chromosome... We also identified
lineage-specific changes, including
deletion of a 200-kb fragment
…expansion of young Alu families…
Reconstruction of the common
ancestral Y chromosome reflects the
dynamic changes in our genomes in
the 5–6 million years since
speciation.
• Nature, January 2006
Y-chromosome similarities help trace
human migration and evolution
Cytoplasmic Inheritance
• Includes carriers of
cytoplasmic DNA
• Cytoplasm is completely
maternally inherited
• Also includes
cytoplasmic proteins,
epigenetic
programming
Inheritance patterns of mtDNA also
help trace human migrations
All human mtDNA is traceable to a
single human female origin