Download Chapter 16 – Genetics

Chapter 16 – Genetics
Pg’s 526 - 544
Genetics – the branch of biology dealing with the
principles of variation and inheritance in animals
and plants.
Who is Gregor Mendel?
LINK
- Mendel is known as the
‘father of genetics’.
- He was a monk in an Austrian
monastery, who had studied
botany and mathematics. This
background helped him
formulate the basics behind
today’s study of genetics, using
experiments performed on
common pea plants.
Why was Mendel’s use of the
Garden Pea ideal?
• They were commercially
available.
• Easy to grow and matured
quickly.
• Enclosed sex organs in the
flower.
• 7 Easily identifiable traits
with ONLY two possible
variations.
Mendel’s First Crosses
• Mendel first developed true
(pure) breeding lines for
each of the 7 pea plant traits
by cross pollinating plants
with the same characteristics
over many generations until
they only yielded on form of
a trait.
• Pure Bred = Homozygous
Dominance & Recessive
• Crossing these parents he
created the First Filial
(F1) generation, that all
possessed the same trait
variation.
• What did this tell him
about yellow vs. green
color?
Principle of Dominance
• Dominant forms of a trait will always be
expressed when it is inherited.
– Found in hybrids ( heterozygous individuals)
– Found in pure dominant ( homozygous
dominant )
– GG = Yellow
– Gg = Still Yellow
– gg = green (recessive)
Genes, Alleles, and Chromosomes
Crossing the F1’s
• His second experiment:
• He took 2 of the plants produced during his first
test (F1 Generation), and breed them together to
make another generation of offspring (F2
Generation).
• When two of these F1 hybrids (dominant
looking heterozygous plants) are crossed, the
result is an F2 generation showing 3 dominant
for every 1 recessive...a “Mendelian Ratio”
(expressing 75% dominant, 25% recessive)
• Mendelian Ratio:
– 3:1 phenotype ratio - dominant to recessive
– Phenotype (what you can Physically see)
Genotype – the GEnetic
makeup of the alleles.
1 homozygous dominant :
2 heterozygous :
1 homozygous recessive
1:2:1 Genotype Ratio
What do the
numbers tell???
• Based on the repeated
pattern of results after
completing thousands
of crosses, Mendel
derived the 1st Law of
genetics.
• Law of Segregation
Mendel’s Conclusions – Law of
Segregation
• Each parent in any generation starts with
two hereditary factors.
– (Either both dominant, both recessive, or a
combination of dominant & recessive.)
• Only one factor from each parent is
contributed to the offspring.
Cont….
• Each offspring inherits only one factor from
each parent. If the dominant factor is
inherited, it will be expressed. However, the
recessive factor will only be expressed if the
dominant trait is not present
– (Principle of dominance)
Unit Theory
• The genes / factors for one trait are
inherited as a unit ....the “Unit Theory of
Inheritance”.
• Why Units??
• Remember the inner workings of a cell, i.e.
chromosomes/DNA have not been identified
yet!
Law of Segregation – Demystified using the
understanding of chromosome behavior.
Segregation
NOW WE KNOW:
The Law of segregation is explained by the behavior of
homologous chromosomes at meiosis.
Genes and Environment Determine Characters
The final phenotype expressed in many cases involves the
interaction between your genotype and the environment conditions
together.
Ex. For humans:
Skin color,
Eye color,
Height,
Weight
Genetically identical hydrangeas
growing in soils of different acidity
(different environments).
A Punnett Square is a Handy Way of Analyzing Crosses
Possible offspring
In a Punnett square for a monohybrid cross, the possible
gametes of the parents are put on the side of the table and
the Principle of Segregation is applied.
Let’s try a cross using two of the
heterozygous plants from Mendel’s F1
generation
Always show the parents genotype first:
P - ____ _____ x ____ _____
• Phenotype =
• Genotype =
Consistency is Good
No matter what
physical trait
Mendel observed a
3:1 phenotypic
ratio of
characteristics was
always found in the
F2.
Characters
investigated by
Mendel
Monohybrid Crosses Yielded Consistent Results
Therefore, the Law of Segregation indeed is a general Law of genetics.
What Works for Peas Also Works for Humans
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):
Legend: A –
a-
P - ____ _____ x ____ _____
¾ of offspring
will be wild
type and ¼
will be albino
An albino woman
Do this monohybrid cross
• In pea plants, round seeds are dominant to
wrinkled seeds. If a homozygous round seed plant
is crossed with a heterozygous round seed plant,
what is the expected phenotypic and genotypic
ratios in the F1 ?
• R• rP - ____ _____ x ____
Results -
_____
Answer
P – RR x Rr
Alleles produced for gametes are R or R from one parent
and R or r from the other parent
R
r
R
R
RR RR
Rr
Rr
Phenotypic Ratio: All Round
Genotypic ratio: 2 RR : 2 Rr
Try This one !
Fruit fly wing length is controlled by a
dominant allele for long wings (L), short is
recessive. If a heterozygous long winged fly
is mated to a homozygous short winged fly,
what is the expected phenotypic and
genotypic ratio in their offspring?
Legend:
P - ____ _____ x ____
_____
• Do both sheets from webpage.
– Monohybrid Practice
– Spongebob genetics
Then Pg, 535 # 10,11,12 and 14.
Review - Understand these concepts
pg 529-530
• Principle of Dominance –
• Law of Segregation –
• Unit Theory of Inheritance –
Probability & Product Rule
• Probability – is the chance of an event
occurring, usually represented by a ratio.
• Product Rule – states the chance that two
or more independent events will occur
TOGETHER.
– This is calculated by finding the product of
their individual probabilities occurring alone.
Flip a coin
• Probability of Heads?? - - ½ (50%)
• Probability of flipping Two (2) heads Together?
Try this
• What is the probability of rolling a regular die
and getting a 6?
• What is the probability of rolling a pair of dice
and getting “snake-eyes” (a pair of 1’s)?
Test Cross – pg 534
• To determine the UNKNOWN genotype of
an organism that shows the dominant
phenotype, we perform a TEST CROSS.
• This is done by taking the organism with
the unknown genotype (TT or Tt) and cross
it with a purebred (homozygous) recessive
and look at the results.
• If any of the
offspring show
the recessive
condition then
the unknown
parent MUST
have been
heterozygous.
Extensions to Mendel: (The Exceptions)
Complexities in Relating Genotype to Phenotype
Extensions to Mendel’s analysis
• In Single-gene inheritance:
– Sometimes pairs of alleles show deviations from complete
dominance and recessiveness
– Sometimes more than two different variations (alleles) exist
for a gene
– Sometimes one gene may determine more than one trait
• In Multifactorial inheritance:
• the phenotype expressed arises from the
interactions between one or more genes, the
environment, and chance events.
Dominance is not always complete
• Crosses between true-breeding strains can
produce hybrids with phenotypes different
from both parents
• Ex.
• Incomplete Dominance
• Co-Dominance
• Multiple Alleles
Incomplete dominance
• This occurs in the case where two pure breeding
(homozygous) parents that show two completely
different phenotypes have F1 hybrids that show a
completely different phenotype from either
parent.
• The F1 expresses an intermediate phenotype.
Neither allele is dominant or recessive to the
other.
• Phenotypic ratios are same as genotypic
ratios
Incomplete Dominance for Flower Color in Snapdragon
Codominance
• Co-Dominance occurs when parents have
different traits and the alleles for these traits
are equally dominant to the other. The F1
hybrid offspring of a cross between two
purebreds will express the phenotype of
both parents equally.
• Phenotypic ratios are same as genotypic
ratios
Codominance
Three Different Forms of Dominance
Multiple Alleles
• In many species a gene may have more than
two possible alleles for any given gene.
– Ex. Human Blood Groups (A, B, and O)
– (A & B are co-dominant and O is recessive)
Codominance of IA and IB Blood Group Alleles
There Are Often More Than Two Alleles of a Gene
Blood groups are determined by one gene with three alleles.
Also that the blood groups show both complete dominance and
codominance.
Multiple Alleles Can be Grouped into a Dominance Series
Dominance series for lentil bean coat color.
Try:
-Practice Problem from Pg 542
-Thinking Lab pg 543.
- Then pg 544 - #2, 5, 6 a & b,
Do variations on dominance relations
negate Mendel’s law of segregation?
• Dominance relations only affect the relationship
between genotype and phenotype
• Alleles still segregate randomly and unite
randomly
• Gene products (plus external factors) control the
expression of phenotypes
What about Multiple traits?
• Does being tall (dominant) give you more
of a chance at also being yellow (dominant).
• Mendel performed a series of experiments
like before, but tracked two different traits
at the same time.
• Two purebred
parents when
crossed still
produced a
heterozygous F1
• Note: Notice the
numbers for the
phenotype and
genotype of the
F2
• Anything similar?
Law of Independent
Assortment
• Phenotype: 9:3:3:1
• Genotype 1:1:2:2:4:2:2:1:1
• These ratios were obtained repeatedly which
helped Mendel in the statement of the Law of
Independent Assortment
– The genes/factors for one trait are inherited as a single
unit and are inherited separately (independently) from
other traits.
Step by step!!
• #1 - Make a legend
• #2 – Show parents 4 dashes times 4 dashes.
• ____ ____ ____ ____
x ____ ____ ____ ____
– NOTE- Remember two dashes for each trait!!
• #3 Perform “FOIL” on each parent to make
gametes.
• #4 make your Punnett square.
• Tip- Keep Same letter together in the boxes
and always put the capital letter first (when
possible)
Try it
• Tall is dominant over short.
• Yellow is dominant over green.
• Cross a homozygous tall, homozygous
green plant with a plant that is heterozygous
for both traits.
• REVIEW LINK
Chromosomes & Heredity
Pg’s 545 - 555
Chromosome Theory of Inheritance
• 1902 - Walter Sutton & Theodor Boveri
– Observed that the behavior of the chromosomes
during Meiosis was similar to the behavior that
Mendel predicted of his ‘inheritance factors’.
• Three Key Observations:
1. Chromosomes occur in pairs.
2. Paired chromosomes segregate during Anaphase 1.
3. Chromosomes align independently at the cell’s
equator.
Definition
• The Chromosome Theory of Inheritance
– Genes (Mendel’s factor’s) are carried on
chromosomes and it is the segregation and
independent assortment of these chromosomes
during the stages of meiosis that accounts for the
patterns of inheritance shown in offspring.
Think about this – do the cross
• Color-blindness in humans is recessive.
When you cross two (2) parents with
normal color vision you produce an
offspring that is color-blind.
– So far so good…….???????
• Now if you were to take a normal color vision
female (from the offspring above) and cross
her with a male having normal color vision and
produced the following:
– offspring where ALL the females have normal
color vision, but the males are 50% normal
vision: 50% color blind.
– ?????? EXPLAIN ???????????
Review - 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.
X-linked Inheritance
• Red-green colorblindness is a sex linked (often
called X-linked) condition.
• The allele for color vision 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 colorblind, her father had to be
colorblind and her mother had to be colorblind or a carrier
of the recessive allele.
• For a man to be colorblind the mother only has to provide
one recessive allele on her X chromosome. A male will get
a Y from his father.
Morgan’s Discoveries
• In 1910, an American
scientist called
Thomas Morgan made
a very important
discovery from his
work with fruit flies
Fruit Flies
• Normal fruit flies have red
eyes
• Morgan crossed two red
eyed parent flies and
obtained a white eyed male.
In other crosses, he obtained
red eyed females, red eyed
males and white eyed males.
• Since the white eye color
was only present in the male
flies, Morgan concluded that
eye color was linked to an
organisms sex.
Gene Linkage
• Thomas Morgan also found out that some other
genes (other than sex chromosomes genes) did
not seem to follow Mendel’s Laws.
– some genes would tend to be inherited together.
• Further understanding of the genechromosome theory (pg 546) described that
genes found very close together on the same
chromosome were rarely separated by crossing
over and would therefore be inherited together.
(called LINKED)
Gene Linkage explained
• Note: all genes are located in a
linear fashion from one end of
the chromosome to the other.
• Crossing over occurs during
Prophase 1, however if two
genes are very close together
they have a high probability of
not being separated.
– (Red hair and freckles)
Mendel’s Law of Independent
Assortment - Restated
• If Crossing over does not occur, genes that
are located on different chromosome will
assort independently while genes that are
located on the same chromosome will be
inherited together.
• Try #4 (NOT sex-linked) & 8 (Sex-link) on
Pg 554.
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.
Do Karyotype lab
Chromosomes and Gene
Expression
Exceptions to the Rule… other than
sex linked traits
Chromosome Inactivation
• Where a girl has two X chromosomes, do
they produce more proteins than boys with
only one X chromosome???
• No – One of the X chromosomes from the
female pair ‘turns off’, or becomes inactive.
• The inactivated chromosome becomes
called the BARR BODY.
Barr Body
• The inactivation of the X chromosome in
different cells is random. So different cells
may have a different X chromosome
activated.
• This is responsible for producing the
tortoiseshell coat color in female cats.
Polygenic Inheritance
• Literally – Many genes control the one
product.
– Ex. The ear length of corn, height and skin
color of humans.
• Where multiple genes are contributing to
the final product we arrive at a continuous
variation.
Modifier Genes
• Genes that work with other genes to influence
the final product. - Eye color in humans.
• Arguably, there should be only two types of
eye colors:
– Brown (dominant) and Blue (recessive)
• However modifying genes act together to
create the final range of colors.
Major Changes - Mutation
• Other than crossing over, changes can occur in
the actual chromosomes code (mutations)
causing profound effects on the possible
outcomes!!!
• Changes can occur:
1. Spontaneously
2. When a cell becomes irradiated
3. When exposed to certain chemicals.
Deletion
• Deletion – 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.
• Ex. – deletion in a specific area of chromosome #5 causes the
condition Cri du chat.
Inversion
• Inversion – a piece of the chromosome
becomes free momentarily before being
reinserted in the reverse order. This
completely changes the order of genes that this
chromosome coded for.
Example - Autism has been linked to
chromosomal inversions.
Duplication
• 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 too many repeats can be detrimental.
– Ex. Fragile X - syndrome
Translocation
• 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
Anaphase 1 or 2 of meiosis. As a result the
daughter cells will either have too many or
not enough chromosomes.
– inheriting an extra chromosome is referred to as
trisomy (3 copies)
– inheriting only one chromosome is referred to
as monosomy (only 1 copy)
• Embryos with either condition rarely
survive to birth due to the large number of
genes being affected.
• Many miscarriages can be linked to
nondisjunction events taking place.
• 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 from a monosomy
where a person has only 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 – is a trisomy condition
where 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. Why?????
• 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, and 9
• What is the
fastest way to
determine the
sex of a
chromosome?
• Pull down its
genes.
• LOL
Human Genetics
Distinguish among the different types
of inheritance patterns
Pg’s 555 - 565
Autosomal Recessive Inheritance
Disorders carried on autosomes and are not related to
the sex of the individual.
 Tay-Sachs disease – body lacks the
enzymes required to break specialized
lipids of the nervous system.
– These individuals are normal at birth,
but by 8 months cells are swollen with
undigested lipids, rupture and kill the
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) – PKU is caused by the
lack of an enzyme that converts phenylalanine to
tyrosine.
– A child with PKU will breakdown phenylalanine to
abnormal products which damage the nervous system
causing mental retardation and seizures.
– 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 condition where the
hair, skin and eyes have no pigment.
– A normal individual is capable of
producing different colors 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
• where an individual carries 2 different copies of
an allele, but both are expressed (Co-dominant).
• Sickle Cell Anemia – Affected individuals have
a defect in the hemoglobin of their red blood cells
causing an irregular shape which 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.
Sickle Cell
• Until recently homozygous recessive
individuals never lived to adulthood. Therefore
you would think the presence of the allele
should have decreasing each generation.
However in some African regions nearly half
of an entire population is heterozygous for the
condition.
• How could this be possible????
The answer - Heterozygous Advantage
• The answer was found while studying the leading
cause of illness and death in Africa. (Malaria)
• Children that were heterozygous for the sickle
cell gene were less likely to contract malaria and
survived 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.
Dominant Disorders - Although
rare they do exist!
• Some can be caused by random mutations
of a gene sequence.
• In most cases these conditions do not
become prevalent until the individual has
already reached reproductive age and have
already conceived, potentially passing on
the gene to their offspring.
Two examples
• Progeria – a rare disorder
that causes an individual to
age rapidly. Usually death
occurs by age 10 - 15
• 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.
• 15 year old male
• 16 year old female
Huntington’s Disease
• 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
Analysis of Human Genetics
• Two techniques have been used to examine
human genetics:
- Karyotypes
- Pedigrees
Pedigrees
• 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.
Go to “Pedigree Presentation”
Different powerpoint
• Do Page 559 – Mini Lab Karyotype
• Page 561 – Solve the Case of the Caped Murderer
• Pg 562 - #1, 7, 9, 10, 11 and 12