Download Mendel`s Legacy

Mendel’s Legacy
Genetics is everywhere these days – and it will continue as a dominant
force in biology and society for decades to come.
Wouldn’t it be nice if people understood it better?
The Fundamental Question
What is the relationship between genes (genotype) and
observable characteristics (phenotype)?
The answer?
Phenotype = Genotype + Environment.
Genes and Environment Determine Characters
Genetically identical hydrangeas growing in soils of different acidity
(different environments).
The phenotype = genotype + environment principle applies equally
to human traits.
Dangerously Ahead of the Game - A Eugenics Exhibit at the
1920 Kansas State Fair
Can history be
repeated?
A Mendelian Genetic Primer
Genes come in pairs that separate in the formation of
gametes.
The members of the pair may be identical (homozygous) or
non-identical (heterozygous).
Each form of a particular gene is an allele.
A Mendelian Genetic Primer
Only two alleles of a given gene are possible in an
individual although many alleles of a gene are possible
within a population.
One allele is dominant over another (or so Mendel
believed).
Genes, Alleles, and Chromosomes
Mendel’s Model
Organism – the
Garden Pea
The Reality of “Round and Wrinkled” – Two Alternative Traits of the
Seed Shape Character
Note that each
of seed is a
new individual
of a different
generation –
seeds are not
of the same
generation as
the plant that
bears them.
Mendel’s Monohybrid
Cross – P to F1
A Punnett
square,
something
we’ll cover in
a moment.
Staying the Course –
Mendel Continued
Crosses to the F2 (the
grandchildren)
What was learned?
The green trait was not lost
or altered, even though it
disappeared in the F1.
One trait is dominant to
the other in its expression.
The reappearance of the recessive trait in ¼ of the F2,
suggests genes come in pairs that separate in the formation
of sex cells.
Monohybrid Crosses and the Principle of Segregation
A cross between individuals differing
in single character is a monohybrid
cross.
The analysis of monohybrid crosses
allowed Mendel to deduce the
Principle of Segregation ....
Genes come in pairs that separate
in the formation of sex cells (and
these sex cells unite randomly at
fertilization).
Principle of Segregation Demystified
Segregation
The principle of segregation is explained by the behavior of
homologous chromosomes at meiosis.
A Punnett Square is a Handy Way of Analyzing Crosses
In a Punnett square for a monohybrid cross, the Principle of
Segregation is applied.
Different Genotypes Can Produce the Same Phenotype
Consistency is
Good
No matter what the
character, Mendel
observed a 3:1 ratio
of characters in the
F2.
Characters
investigated by
Mendel
Monohybrid Crosses Yielded Consistent Results
Therefore, the Principle of Segregation indeed is a general
principle of genetics.
What Works for Peas Also Works for Humans
An albino woman
In the cross Aa x Aa, where A
is a dominant allele for wild
type (standard) pigmentation
and a is a recessive allele for
no pigmentation (albinism), ¾
of offspring will be wild type
and ¼ will be albino.
Do this monohybrid cross
• In pea plants, round seeds are dominant to
wrinkled seeds. If a heterozygous round seed
plant is crossed with a heterozygous round
seed plant, what is the expected phenotypic
and genotypic ratios in the F1 ?
Answer
P – Rr x Rr
Alleles produced for gametes are R and r from each
parent
R
r
R
RR
r
Rr
Rr
rr
Phenotypic Ratio: 3 round : 1 wrinkled
Genotypic ratio: 1 RR : 2 Rr : 1 rr
Try This one !
Fruit fly wing length is controlled by a
dominant allele for long wings (L). If a
heterozygous long winged fly is mated to a
homozygous long winged fly, what is the
expected phenotypic and genotypic ratio in
their offspring?
Extensions to Mendel: Complexities in Relating
Genotype to Phenotype
Outline of extensions to
Mendel’s analysis
• Single-gene inheritance
– In which pairs of alleles show deviations from complete dominance and
recessiveness
– In which different forms of the gene are not limited to two alleles
– Where one gene may determine more than one trait
• Multifactorial inheritance in which the phenotype
arises from the interaction of one or more genes
with the environment, chance, and each other
Dominance is not always complete
• Crosses between true-breeding strains can
produce hybrids with phenotypes different
from both parents
– Incomplete dominance
• F1 hybrids that differ from both parents express an intermediate
phenotype. Neither allele is dominant or recessive to the other
• Phenotypic ratios are same as genotypic ratios
– Codominance
• F1hybrids express phenotype of both parents equally
• Phenotypic ratios are same as genotypic ratios
Three Different Forms of Dominance
Incomplete Dominance for Flower Color in Snapdragon
Codominance of Spotted and Dotted Coat Pattern Alleles
Codominance of IA and IB Blood Group Alleles
There Are Often More Than Two Alleles of a Gene
The ABO blood group system is determined by one gene with three alleles.
There Are Often More Than Two Alleles of a Gene
Note that the ABO blood system shows both complete dominance and codominance.
Multiple Alleles Can be Grouped in a Dominance Series
Dominance series for lentil bean coat color.
Do variations on dominance relations
negate Mendel’s law of segregation?
• Dominance relations affect only the
relationship between genotype and
phenotype
• Alleles still segregate randomly and unite
randomly
• Gene products control expression of
phenotypes
• Interpretation of phenotype/genotype
relationship can be complex
Pedigree Charts
The family tree of genetics
What is a Pedigree?
• A pedigree is a chart of the genetic history of
family over several generations.
• Scientists or a genetic counselor would find out
about your family history and make this chart to
analyze.
Constructing a Pedigree
• Female
• Male
Connecting Pedigree Symbols
Examples of connected symbols:
• Married Couple
• Children
Example
• What does a pedigree chart look like?
Symbols in a Pedigree Chart
•
•
•
•
Affected
X-linked
Autosomal carrier
Deceased
Shade in half please
Interpreting a Pedigree Chart
1. Determine if the pedigree chart shows an
autosomal or X-linked disease.
– If most of the males in the pedigree are
affected the disorder is X-linked
– If it is a 50/50 ratio between men and
women the disorder is autosomal.
Example of Pedigree Charts
• Is it Autosomal or X-linked?
Answer
• Autosomal
Interpreting a Pedigree Chart
2. Determine whether the disorder is dominant
or recessive.
– If the disorder is dominant, one of the
parents must have the disorder.
– If the disorder is recessive, neither parent
has to have the disorder because they can
be heterozygous.
Example of Pedigree Charts
• Dominant or Recessive?
Answer
• Dominant
Example of Pedigree Charts
• Dominant or Recessive?
Answer
• Recessive
Summary
• Pedigrees are family trees that explain your
genetic history.
• Pedigrees are used to find out the probability
of a child having a disorder in a particular
family.
• To begin to interpret a pedigree, determine if
the disease or condition is autosomal or Xlinked and dominant or recessive.
SAMPLE Public exam questions
Human Genetics
Changes in Chromosomes
• Other than crossing over, changes can occur in the
actual chromosomes code that can have profound
affects on the possible outcomes!!!
• Changes will usually occur spontaneously when a
cell becomes irradiated or exposed to certain
chemicals.
• There are four main types of changes:
- deletion
- inversion
- duplication
-translocation
• Deletion – is when a portion of
the chromosome is lost or
removed due to irradiation,
viruses or various chemicals.
These pieces coded for genes,
so when they are lost so is the
genetic trait it coded for.
• Example when a piece of
chromosome #5 is lost, the
child is born with a mentally
handicapped and a different
facial appearance.
• Inversion – occurs when a piece of the
chromosome becomes free momentarily
before being reinserted in the reverse order.
This completely changes the genes that this
chromosome coded for.
Example - Autism is believed to be linked to a
chromosomal inversion.
• Duplication – is
when multiple
copies of a gene
sequence occur. For
the most part this
can have no affect
on a human, but in
some cases to many
repeats can be
detrimental.
• Example – Duplication on the X chromosome
may become too excessive and lead to a
condition known as fragile X syndrome. Most
people have about 29 repeats of this
particular region, but a person with fragile X
syndrome will have around 700 repeats.
• Translocation –
occurs when part
of one
chromosome
changes places
with part of a nonhomologous
chromosome.
• Example – If part of chromosome 14
exchanges places with #8 then cancer can
occur.
• Down syndrome – linked to translocation
between chromosome 14 & 21.
• A type of Leukemia has been traced to
translocation between #22 and #9.
Many other defects occur due to
Nondisjunction
• Nondisjunction – is the result of
chromosomes not separating during meiosis.
As a result the chromosomes will either have
too many or not enough chromosomes.
• inheriting an extra chromosome is referred to
as trisomy
• inheriting only one chromosome is referred to
as monosomy
• Human embryos with either condition rarely
survive to birth. Many miscarriages can be
linked to these conditions.
• Down Syndrome – is probably one of the
most common nondisjunction syndromes. It
occurs when an individual receives 3 copies of
chromosome 21.
• -Symptoms include: mild to moderate mental
impairment and a thick tongue that can create
speech defects. Skeleton may not develop properly
resulting in a short stocky body type.
• Turner Syndrome – results when a person
only has one X sex chromosome. The woman
will have external female genitalia, but will
lack ovaries. They are therefore infertile and
not mature sexually. Other defects include
heart, kidney and skeletal defects.
• A single Y chromosome individual is not
possible. This embryo would not survive
where they would be lacking vital genetic
information.
• Klinefelter syndrome – occurs when an extra
X chromosome occurs in a male (XXY). This
individual will have immature sex organs and
will not grow facial hair. They are also likely to
develop some breasts. XXX females do not
show any at all symptoms.
• Jacobs syndrome – occurs in males with an
extra Y (XYY). These individuals show speech
and reading problems and have persistent
acne. A study once found that there seemed
to be an extremely high occurrence of this
condition amongst prisoners compared to the
rest of society.
• Do page 554 - # 1,2,3,5,8 and 9
The Human
• 46 Chromosomes that occur in 23 pairs of
homologous chromosomes.
- 1 pair of sex chromosomes (X and Y)
- 22 pairs of autosomes
Autosome – non-sex determining chromosomes,
responsible for containing the remaining traits of
the human being.
Each chromosome contains anywhere from
hundreds to thousands of genes for particular
traits.
•
Autosomal
Recessive
Inheritance
Disorders that are carried on the
autosomes and are not related to the
sex of the individual.
- Tay-Sachs disease – a disease where
the body lacks the ability of producing
a vital enzyme within the lysosomes of
the nervous system. These individuals
are normal at birth, but by 8 months
the lysosomes rupture and break down
the brain cells. By their 1st birthday
they will usually be blind, mentally
handicapped, and display little muscle
activity. Most die before the age of 5.
A.R.I.’s cont.
• Phenylketonuria (PKU) – this condition again
affects children. In PKU an enzyme that converts
phenylalanine to tyrosine is defective or missing.
A child with PKU will breakdown phenylalanine
abnormally creating products that damage the
nervous system. Luckily there is a routine test
and treatment in place for babies with this
condition.
If the condition is not detected the baby with
become severely mentally handicapped within a
few months.
A.R.I.’s cont
• Albinism – A genetic condition
where the hair, skin and eyes have
no pigment. A normal individual is
capable of producing different
colours in our bodies due to a
varying amounts of the brown
pigment called melanin.
Where an albino individual lacks this
pigment they lack the ability to tan,
thereby lacking the ability to
naturally protect their bodies from
the sun’s powerful UV rays.
Codominant Inheritance
• Sickle Cell Anemia – where an individual carries
2 different copies of an allele, but both are
observed. Affected individuals have a defect in
the hemoglobin of their red blood cells, causing
an irregular shape that can clog up capillaries
and lead to blood clots.
These individuals tend to lack energy, suffer from
various illnesses and are in constant pain.
• This recessive allele is believed to have
originated from Africa.
Heterozygous Advantage
• Until recently homozygous recessive
individuals never lived to adulthood.
Therefore the presence of the allele should
have decreasing each generation. However in
some African regions nearly half of an entire
population would be heterozygous for the
condition.
• How could this be possible????
The answer
• Malaria – Yes, the answer was found while
studying the leading cause of illness and death in
Africa. It appeared that children that were
heterozygous for the sickle cell gene were less
likely to contract malaria and survive to
adulthood.
• Heterozygous Advantage – where an individual
with two different alleles for the same trait have
a better chance of survival.
Autosomal Dominant Inheritance
• Due to the behaviour of dominant traits in
Mendelian genetics, we can trace dominant
disorders 2 ways:
- since both heterozygous and homozygous
individuals show a trait, the trait should be
seen in every generation.
- if one parent is heterozygous and crosses
with a homozygous recessive individual, then
the trait should still be present 50% of the
time.
Although rare they do exist!
• Some can be caused be random mutations of
a gene sequence.
• In most cases the condition does not become
prevalent until the individual has already
conceived and potentially passed on the gene
to their offspring.
Two examples
Progeria – a rare disorder that causes an
individual to age rapidly. It occurs in newborns
at a rate of 1 in 8 million, and does not run on
families. It is therefore linked to a rare
random mutation.
Huntington’s Disease – a lethal disorder where
the brain progressively deteriorates over time.
Symptoms generally begin around age 35,
after the individual has already had children.
Incomplete Dominance
• A condition where having even one copy of the
affected gene (dominant or recessive) leads to the
condition.
- Familial hypercholesterolemia (FH) – is a
condition that affects heterozygotes (1:500).
The cell produces less receptors for LDL (lipids)
that are required to take these lipids into the cell.
Without them these lipids build up in the arteries
and lead to a heart attack or stroke by around the
age of 35. Homozygous individuals for this
recessive condition can die of a heart attack by the
age of 2
X-linked Inheritance
• Red-green colourblindness is a sex linked
condition due to the allele for detecting the
red- green pigment is located on the X
chromosome.
• This explains why it is so much more common
in men (8%) than women (0.04%).
- For a woman to be colourblind, her father had
to be colourblind and her mother had to be
colourblind or a carrier.
- For a man to be colourblind only the mother
had to possess the allele.
Analysis of Human Genetics
• Two techniques have been used to examine
human genetics:
- Karyotypes
- Pedigrees
The Human Karyotype
• The human karyotype is an illustration or
photograph of the chromosomes in the
nucleus of a somatic cell in an organism.
• Creating a Karyotype involves growing cells
and stopping the division process during the
metaphase stage. The chromosomes are then
separated, stained and photographed. The
chromosomes are then cut out and arranged
in pairs according to size, shape and
appearance.
Pedigrees
• As described earlier, a pedigree shows the genetic
relationships between individuals in a family,
• Through many years of tracing a family history and
applying this data to Mendelian genetics,
scientists can determine if a condition is
dominant, recessive, autosomal, or sex-linked.
• Pedigrees can also be used to predict the possible
inheritance of a disorder.
Ex. Page 560 shows a pedigree tracing Hemophilia
throughout Queen Victoria’s family.
• Do Page 559 – Mini Lab Karyotype
• Page 561 – Solve the Case of the Caped Murderer
• Pg 562 - #1, 7, 9, 10