Download Mendelian Genetics

Mendelian Genetics
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The science of modern genetics had its humble beginning in the garden of an
Austrian monk in the 1860s.
The monk was Gregor Mendel, and his contribution to the study of heredity is
discussed in every biology textbook.
Ironically, his work was ignored and not rediscovered until the beginning of the 20 th
century.
What did Mendel do??
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He studied the characteristics of
pea plants, and how they were
passed from generation to
generation.
The seven characteristics that he
observed can be seen in the
illustration to the right.
Fortunately these traits were
carried on different pairs of
chromosomes.
By controlling the transfer of
pollen, he was able to conduct
cross-fertilization experiments.
Did you know that although
Mendel did his work in the 1860’s,
it was ignored and not
rediscovered until 1901?
Monohybrid Cross
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A monohybrid cross looks at a
single characteristic. In this case
Mendel examined flower color
Analysis lead to the following four hypothesis
These ideas are very important!
• There are alternative forms of genes that account for variations in
inherited of characteristics. We call these alternative forms
alleles.
• For each characteristic, an organism inherits two alleles, one from
each parent. If the alleles are the same they are said to be
homozygous. If the two alleles are different they are said to be
heterozygous.
• If the two alleles of an inherited gene pair differ, then one
determines the organism’s appearance and is called the dominant
allele. The other has no effect and is called the recessive allele.
• A sperm or egg carries only one allele for each inherited trait
because allele pairs separate from each other during meiosis and
the production of gametes. This is the Law of Segregation.
The Law of Independent Assortment
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A dihybrid cross looks at 2 characteristics located on different pairs of
chromosomes.
Each pair of alleles segregates independently of the other pair of alleles
during meiosis. (during gamete formation)
The Law of Independent Assortment
(cont.)
• When you make a dihybrid
cross between two parent who
are heterozygous for both
traits (AaBb X AaBb) The
resulting cross will give a
9:3:3:1 ratio of phenotypes.
• 9/16 of the individuals will
exhibit both dominant traits.
• 3/16 of the individuals will
exhibit one dominant and one
recessive trait.
• 3/16 of the individuals will
exhibit the other combination
of dominant and recessive
traits
• 1/16 of the individuals will
exhibit both recessive traits.
• A Punnett square is the
easiest way to determine
genotypes and phenotypes
when doing dihybrid crosses
Don’t know the genotypes? Use a Testcross!
Cross the unknown genotype with an individual that is
homozygous recessive.
What are the chances?
• Mendel’s experiments
follow the laws of
probability.
• Review the rule of
multiplication and the rule
of addition!
• You can use these rules
to calculate the
probability of any
genotype resulting from a
cross of known parents.
You may have a pedigreed dog, but do you have a
pedigree?
• Yes, all of us do.
• A pedigree is a family tree.
• Geneticists use pedigrees to
track inherited traits and
conditions.
• The tree at the right traces
Martha’s Vineyard deafness in
a family.
• Pedigrees such as this can be
helpful in determining both
how a condition is inherited
and the probability of an
individuals having a trait.
Inherited genetic disorders
• Genetic disorders may be
recessive or dominant.
• Examples of dominant
inherited genetic disorders
include Huntington’s disease
and Achondroplastic dwarfism.
• Albanism, Cystic fibrosis, and
Sickle-cell anemia are
recessive genetic disorders.
• Inbreeding (mating between
closely related individuals) can
lead to an increase in
individuals that are
homozygous for recessive
genes.
• Many breeds of domestic
dogs have a high incidence of
disorders, such as hip
displasia, due to inbreeding.
• The breeding of endangered
animals in zoos is carefully
controlled to try to prevent
problems related to
inbreeding. That is why
animals are exchanged or on
temporary loan for breeding
purposes
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How can I know if my baby will have a genetic
disorder?
• This question is one that many parent have, especially if there
is a family history of genetically related disease or if the
mother is over 35.
• Techniques available for genetic testing include:
– Amniocentesis
– Chorionic villus sampling
– Ultrasound
– Newborn screening
• Many of these tests open a host of ethical considerations.
What can/should one do with the information available?
These issues are still being debated by society.
It may not be all or nothing!
Incomplete dominance
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When the offspring of a cross
show phenotypes that are
intermediate between those of the
parents, it is called incomplete
dominance.
Unlike Mendel’s pea plants,
snapdragons can have F1 flowers
that are neither red or white, but
pink.
In this case the Rr genotype
produces less pigment than does
the RR genotype, hence the pink
color.
Multiple alleles
• You can only have two alleles for a gene (Remember there are
only two chromosomes that carry that gene).
• In a population there can be a number of alleles for a gene (though
no one individual has more than two of them.
• In the ABO blood grouping system there are 3 alleles: A,B, and O
• That means persons will have one of the following genotypes:
AA,AO,AB,BB,BO, or OO
A and B are carbohydrates found on the surface of the red blood cell,
while O is an absence of either A or B. When both A and B are
present, they are said to be codominant.
Environment can effect the expression of the
genes.
• Crotons (Codiaeum variegatum) are common landscape plants
that can be greatly effected by their environment.
• Leaf size can be influenced by the amount of water and fertilizer.
• Color of the leaf is determined by light conditions.
• Clones of the exact same plant will look totally different depending
upon these environmental conditions.
• On the next slide you can see leaves from plants grown under
different light conditions.
All these leaves came from clones of the same plant
grown under different light conditions
When genes are on the same chromosome
Mendel’s laws don’t work!
• When genes are on the same chromosome they tend to be
inherited together.
• The closer two genes are located on a chromosome, the greater
the chance of them being inherited together.
• Genes located close together on a chromosome are called linked
genes.
• During meiosis, when pairs of chromosomes line up together
forming tetrads, crossing over can occur between adjacent strands.
Linkage and Crossing over
• Crossing over can lead to the
formation of new combinations
of genes.
• Thomas Hunt Morgan and his
students were leaders in the
study of crossing over and its
role in genetic diversity.
• Determining the frequency of
recombinant genotypes
allowed researcher to
determine the relative position
of genes on a chromosome.
Sex Determination
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Probably all of you know that the X and Y chromosomes determine sex in
humans (and other mammals), however there are other
systems for sex determination that can be found in the animal kingdom.
Other systems for sex determination include the X-O system of
grasshoppers and some other insects, the Z-W system of birds and a few
other animals.
Sounds fairly simple…but…
Often it is a bit more complicated than this. Take a few minutes and go to the
following website. Take the role of selection committee member and
determine the eligibility of Jane Doe to compete. You may be surprised!
http://www.hhmi.org/biointeractive/gender/index.html and go to gender testing of
female athletes
Sex-linked Genes
• In fruit flies and humans traits carried on the X chromosome are
said to be sex-linked.
• A recessive gene on the X chromosome will always be expressed
in the male, since there is a single X present.
• A female with the recessive gene on one of her two X
chromosomes will be able to pass the trait to her male offspring,
but will not express the trait herself. She is a carrier.
• White eyes in Drosophila is one such sex-linked trait.
Sex-Linked Disorders
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Sex-linked disorders primarily
affect males since the defective
gene is carried on the X
chromosomes, and males have a
single X chromosome.
Females would have to be
homozygous for a sex-linked
condition for it to be expressed,
however many females are
heterozygous carriers for these
recessive traits.
Three well-known sex-linked
disorders are:
– Hemophilia
– Duchenne muscular dystrophy
– Red-Green color blindness