Download 1 Chapter 14: Mendel and the Gene Idea Mendelian Genetics

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Chapter 14: Mendel and the Gene Idea
Mendelian Genetics
Mendel’s Experiment
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Mendel worked with garden peas, a good choice of study organism because:
1. They’re available in many varieties
2. Their fertilization is easily controlled
3. The characteristics of their many offspring can be quantified
Mendel studied 7 characters, or heritable features, that occurred in alternative forms called
traits. These 7 different traits turned out to be 7 different alleles on 7 different chromosomes.
He worked exclusively with true-breeding pea plants. This means the plants he used were
genetically pure and consistently produced the same traits.
- For example, tall plants always produced tall plants; short plants always produced short
plants
To follow the transmission of these well-defined traits, Mendel performed hybridizations in
which he cross-pollinated contrasting true-breeding varieties, and then allowed the next
generation to self-pollinate.
 P generation (parental): the parental organisms involved in the first genetic cross; the truebreeding parental plants in Mendel’s experiment
 F1 generation (first filial): the offspring of the first cross
 F2 generation (second filial): the next generation from the self-cross of the F1
Inheritance of Traits
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Every trait – or expressed characteristic – is produced by hereditary factors known as genes.
- A gene is a segment of a chromosome. There are many genes within a chromosome that
control the inheritance of a particular gene.
- The position of a gene on a chromosome is called a locus.
Diploid organisms have 2 copies of a gene – or 2 alleles – one on each homologous
chromosome.
- Alleles = alternate forms of the same gene
- An organism has 2 alleles for each inherited trait, one received from each parent
- For example, there can be 2 alleles for the height of a plant: tall and short
An allele can be dominant or recessive.
- Dominant: an allele that masks the presence of a recessive allele of the same gene in a
heterozygous organism  determines the organism’s appearance
- Recessive: an allele that’s masked and not expressed in a heterozygous organism  has no
observable effect on its appearance
- A Punnett square can be used to predict the results of simple genetic crosses.
- The dominant allele receives a capital letter and the recessive allele receives a lowercase.
An organism is homozygous when an organism has 2 identical alleles for a given trait
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- For example, TT is homozygous dominant and tt is homozygous recessive
An organism is heterozygous when an organism has two different alleles for a given trait
- For example, Tt is heterozygous dominant
Genotype is the genetic makeup of an organism
Phenotype is the physical appearance of an organism
How Mendel discovered his law of segregation:
- For each of the 7 traits that Mendel studied, only one trait of each pair was visible in the F1
generation. He found that the F1 offspring didn’t show a blending of the parental traits.
- However, in the F2 generation, the missing parental trait reappeared in the ratio of 3:1 –
three offspring with the dominant trait to one offspring with the reappearing recessive trait
- Allele pairs separate (or segregate) during the formation of gametes, so an egg or sperm
carries only one allele for each inherited character.
Mendel’s 3 Laws of Inheritance
1. Law of Segregation
Bb
B
or
b
**Alleles are inherited separately and can segregate and recombine.
-During meiosis: the alleles of each gene segregate during meiosis I when homologous
chromosomes are divided among gametes
2. Law of Independent Assortment
BB
Bb
Bb
bb
**Alleles can segregate and recombine independently of other alleles
- During meiosis: the alleles for one trait randomly assort and divide among the gametes
during meiosis I, independently of alleles for other traits
- -When fertilization occurs, chromosomes – along with the alleles they carry – separate and
get paired up in a new random combination
-For example, gametes from an F1 hybrid generation (AaBb) contain four combinations
of alleles in equal quantities (AB, Ab, aB, ab)
Bb x Bb
3. Law of Dominance
**One trait masks the effects of another trait
- The presence of dominant alleles masks recessive alleles (except during incomplete and
codominance)
Laws of Probability
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General rules of probability apply to the laws of segregation and independent assortment.
The probability scale goes from 0 to 1: the probabilities of all possible outcomes add up to 1.
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The probability of an event occurring is the number of times that event could occur over all the
possible events.
The outcome of independent events is not affected by previous or simultaneous trials.
Monohybrid Crosses
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The multiplication rule states that the probability that a certain combination of independent
events will occur together is equal to the product of the separate probabilities of the
independent events.
The probability of a particular genotype being formed by fertilization = the probabilities of
forming each type of gamete needed to produce that genotype
If a genotype can be formed in more than one way, then the addition rule states that its
probability equals the sum of the separate probabilities of the different, mutually exclusive ways
the event can occur.
For example, a heterozygote offspring can occur if the egg contains the dominant allele and the
sperm the recessive (½ x ½ = ¼ probability) and vice versa ( ¼ ). Therefore, a heterozygote
offspring would be the predicted result from a monohybrid cross half the time ( ¼ + ¼ = ½ )
Complex Genetic Crosses
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Fairly complex genetics problems can be solved by applying the multiplication and addition
rules.
The probability of a particular genotype arising from a cross can be solved by considering each
gene involved as a separate monohybrid cross and then multiplying the probabilities of all the
independent events involved in the final genotype.
When more than one outcome is involved, the addition rule is also used.
The larger the sample size, the more closely the results will conform to statistical predictions.
Genetic Crosses
 Animals used in crosses include rabbits, mice, and fruit flies (=Drosophila Melanogaster)
1.
2.
3.
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5.
High reproduction rate
Easily maintained
Many generations
Many visible traits
Indiscriminate mates
Monohybrid Cross
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Monohybrid cross: genetic cross between monohybrids, or individuals that only differ in one
character
- For example, Mendel crossed plants that differed in only seed shape
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A monohybrid heterozygous cross will always result in the 3:1 ratio.
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For example: Tt x Tt (T = tall; t = short)
The ratio of phenotypes is 3 : 1 (3 tall: one short)
The ratio of genotypes is 1 : 2 : 1 (one TT: 2 Tt: one tt)
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Dihybrid Cross
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Dihybrid cross: a genetic cross between dihybrids, or individuals that differ in 2 characters
- Mendel made dihybrid crosses to follow the inheritance of 2 characters and determine
whether the 2 characters were transmitted independently from the parent plants
- Parents have 4 alleles and games have 2 alleles:
P = Bbll  4 alleles
It’s gametes are: Bl and bl  2 alleles
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A dihybrid heterozygous cross will always result in the 9:3:3:1 ratio.
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For example: TtGg x TtGg (T = tall; t = short; G = green; g = yellow)
Just memorize the ratio of phenotypes:
 9 tall and green (Dominant-Dominant)
 3 tall and yellow (D-R)
 3 short and green (R-D)
 1 short and yellow (R-R)
Law of Probability: For dihybrid crosses, the probability that two or more independent events
will occur simultaneously is equal to the product of the probability that each will occur
independently
- For example, to find the probability of a tall, yellow plant, just multiply the probabilities of
each event.
-If the probability of being tall is ¾ and the probability of being yellow is ¼, then the
probability of being tall and yellow is ¾ x ¼ = 3/16
- For example: If you cross AaBbCCdd x AABbccDd, what is the probability of offspring with
AABbCcdd?
-Multiply the probabilities of each
AA
BB
Cc
dd
½ x ¼
1
x ½
= 1/16
Test Cross
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A testcross uses a recessive organism to determine the genotype of an organism that expresses
a dominant phenotype.
- It’s used to determine if a plant appears dominant because it is homozygous (TT) or
heterozygous (Tt). The only way to determine its genotype is to cross it with a recessive
organism.
Using the recessive plant, there are only 2 possibilities:
1) TT x tt
- F1 = all Tt
- So if none of the offspring is short, the original plant must be homozygous, TT.
2) Tt x tt
- F1 = ½ Tt and ½ tt
- But if even 1 short plant appears in the bunch, the original plant must be
heterozygous, Tt. So it isn’t a purebred.
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Beyond Mendelian Genetics
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In complete dominance, the phenotype of the heterozygote (Tt) is the same as the dominant
homozygote (TT).
But not all patterns of inheritance obey the principles of Mendelian genetics.
Incomplete dominance:
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Incomplete dominance occurs when traits blend and are diluted
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For example, if you cross a white snapdragon plant (dominant) with a red snapdragon plant
(recessive), the resulting offspring will be pink.
Codominance:
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Codominance occurs when alleles are equally expressed
For example, a palomino calf is speckled brown (dominant) and white (recessive).
For example, hypercholesterolemia is codominant.
For example, an individual can have an AB blood type. In this case, each allele – IA (the A allele)
and the IB (the B allele) – is equally expressed.
Blood Types:
Phenotype
Genotype
A
AA (IA), AO (IAi)
B
BB (IB), BB (IBi)
AB
AB (IAB)
O
OO (ii)
Polygenetic/Multigene inheritance:
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Polygenetic inheritance is when a trait results from the interaction of many genes. Each
gene has a small effect on that particular trait.
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A polygenic character may result in a normal distribution of the character within a population.
For example, quantitative characters such as height, skin color, eye color, and weight are all
examples of polygenetic traits.
Multiple alleles:
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Multiple alleles occur when traits are determined by many different alleles that occupy
the same specific gene locus.
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For example, the ABO blood group system is where 3 alleles determine blood type: IA, IB, and i
-Each allele codes for an enzyme that adds a specific carbohydrate to the red blood cell.
Epistasis
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Epistasis is when the interaction between two gene products affects the expression of a
trait. A gene at one locus may influence the expression of another gene at another
locus.
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For example, 2 gene loci affect the coat color of mice. In one case, black (B) is dominant to
brown (b). Yet at another gene locus, a pair of alleles (C and c) also affects coat color.
 If the mouse is homozygous recessive for the second locus (cc) then the coat is white
(albino), regardless of the genotype of the black/brown locus. In this example, the recessive
albino genotype is epistatic to the brown/black one – it influences it.
F2 ratios that differ from the typical 9:3:3:1 often indicate epistasis.
Pleiotropy
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Pleiotropy is when a single allele exerts multiple effects on the genotype and can affect
a number of characteristics of an organism.
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For example, albinism in mice results in white fur and blindness.
For example, in sickle-cell anemia, cystic fibrosis, and other hereditary symptoms, multiple
symptoms are caused by a single pair of alleles.
Environmental Impact on Phenotype
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The phenotype of an individual is the result of complex interactions between its genotype and
the environment.
Genotypes have a phenotypic range called a norm of reaction within which the environment
influences phenotypic expression.
Polygenic characters are often multifactorial, meaning that a combination of genetic and
environmental factors affects phenotype.
Pedigree Analysis
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A family pedigree is a family tree with the history of a particular trait shown across generations.
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Circles = females; squares = males
Solid filled in symbols = individuals that have the trait (like attached earlobes)
Parents joined by horizontal line; offspring listed below parents from left to right in order of birth
The genotype of individuals in the pedigree can be deduced by following the patterns of
inheritance.
It looks like this:
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Chapter 15: The Chromosomal Basis of Inheritance
Mendelian inheritance has its physical basis in the behavior of
chromosomes.
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Mendel’s laws led to the chromosome theory of inheritance:
-Genes occupy specific positions (loci) on chromosomes, and chromosomes undergo segregation
and independent assortment during the process of meiosis in gamete formation.
Morgan’s Experiment
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T.H. Morgan worked with fruit flies, Drosophila Melanogaster.
Fruits flies are prolific and rapid breeders. They have only 4 pairs of chromosomes; the sex
chromosomes occur as XX in females and XY in males.
The wild type is the normal phenotype found most commonly in nature for a character. Mutant
phenotypes are alternative traits, assumed to have arisen as mutations.
Morgan discovered a mutant white-eyed male fly that he mated with a wild-type red-eyed
female.
The F1 were all red-eyed.
In the F2, however, all female flies were red-eyed, whereas half of the males were red-eyed and
half were white-eyed.
Morgan deduced that the gene for eye color was located on the X chromosome. Males have
only one X, so their phenotype is determined by the eye-color allele they inherit from their mom
Sex-Linked Traits
Sex Chromosomes
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Humans contain 23 pairs of chromosomes: 22 pairs of autosomes and 1 pair of sex
chromosomes.
Autosomes code for many different traits.
Sex chromosomes determine the sex of an individual.
Females = XX chromosomes  produce gametes (ova) with 1 X chromosome
Males = X and Y chromosomes  produce gametes (sperm) with either an X or Y
There are some genes found only on the Y chromosome, like SRY, whose protein product
regulates many other genes
Inheritance of Sex-linked Genes
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Sex-linked traits are traits that are carried on sex-chromosomes
Males inherit sex-linked alleles from their mothers.
If a male has a defective X chromosome, he’ll express the sex-linked trait no matter what
because he has one X and one Y chromosome.
Recessive sex-linked traits are seen more often in males, since they are hemizygous for sexlinked traits.
Females inherit sex-linked alleles from both parents.
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A female with one defective X is a carrier. Although she doesn’t exhibit the trait, she can still
pass it to her children.
For her to express the trait, she has to inherit 2 defective X chromosomes.
Sex-Linked Diseases
 Duchenne muscular dystrophy
 Colorblindness
 Hemophilia
Hemophiliagene for blood clotting protein
female
H H
X X
XHXh
XhXh
male
normal
XHY
XhY
hemophilia
-it’s very rare for a girl to have hemophilia
-must have a mom that’s a carrier or has the disease and a father with the disease
Barr Bodies
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Only one of the X chromosomes is fully active in most mammalian female somatic cells.
The other X chromosome is condensed into a Barr body located inside the nuclear membrane.
This means that both males and females have an equal dosage of X chromosome genes.
- Females don’t have twice the amount of X chromosome genes (even though they have 2
Xs) because one of their X chromosomes is inactive in the form of a Barr body.
Surprisingly, the X chromosome destined to be inactivated is randomly chosen in each cell.
- A gene called XIST is active on the X chromosome that forms the Barr body.
- Its RNA product may trigger DNA methylation and X-inactivation.
Therefore, in every tissue in the adult female one X chromosome remains condensed and
inactive. However, this X chromosome is replicated and passed on to a daughter cell.
Linked Genes and Mapping Chromosomes
 Linked genes are genes located on the same chromosome that tend to be inherited together.
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They stay together during assortment and move as a group.
For example, the genes for blue eyes and blond hair are linked on the same chromosomes and
show up together.
How Linkage Affects Inheritance
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Morgan found the concept of linked genes:
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He performed a testcross of F1 dihybrid wild-type flies that were homozygous recessive for
black bodies and vestigial wings.
Wild type = traits found in nature; usually dominant
He found that the offspring were not in the predicted 1:1:1:1 phenotypic classes.
Instead, most of the offspring were the same phenotypes as the P generation parents – either
wild type (gray, normal wings) or double mutant (black, vestigial).
Morgan deduced that these traits were inherited together because their genes were located on
the same chromosome.
Genetic recombination results in offspring with combinations of traits that differ from those of
the parents.
Genetic Recombination and Linkage
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Parental types – have phenotypes like one or the other of the parents
Recombinant types (recombinants) – have combinations of the 2 traits that are unlike the
parents.
A frequency of recombination greater than 50% occurs when 2 genes ARE NOT LINKED! They
must be located on different chromosomes.
In these cases, recombination simply results from the random alignment of homologous
chromosomes at metaphase I and the resulting independent assortment of alleles.
For example: a cross between a dihybrid heterozygote (YyRr) and a recessive homozygote (yyrr):
- ½ will be parental types (YyRr, yyrr)
- ½ of the offspring called recombinant types (Yyrr, yyRr)
- This frequency of recombination is 50%  the genes are not linked!
Recombination of linked genes does occur, due to crossing over
- Crossing over is the exchange of genes between nonsister (a maternal and a paternal)
chromatids of synapsed homologous chromosomes during prophase of meiosis I.
Linked genes do not assort independently, so crossing over is necessary for recombination.
The percentage of recombinant offspring is called the recombination frequency.
Recombination frequency = # of recombinant offspring
Total # of offspring
-For example: A wild type fruit fly (gray, normal wings) and a recessive fruit fly (black,
vestigial wings) are mated
-The offspring are:
-965 wild type (gray-normal)  IDENTICAL TO PARENT
-944 black-vestigial  IDENTICAL TO PARENT
-206 gray-vestigial  RECOMBINANT
-185 black-normal  RECOMBINANT
-So the recombinant frequency = 206 + 185 x 100% = 17% or 17 map units
2300
Mapping Chromosomes
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A genetic map is an ordered list of genes on a chromosome.
Because linked genes are found on the same chromosome, they can’t segregate independently.
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The frequency of crossing over between any 2 linked alleles is proportional to the distance
between them.
The farther apart 2 linked genes are on a chromosome, the higher the frequency of crossing
over. The closer together 2 genes are, the lower the frequency of crossing over.
A recombination/linkage mapping is a mapping of linkage groups.
***1 map unit = 1% recombination.
Genes may be located on the same chromosome, but too far apart to be linked genes.
- Linkage can’t be determined if genes are so far apart that crossovers between them are
almost certain. They would then have the 50% recombination frequency typical of
unlinked genes.
- Such genes are physically linked but genetically unlinked.
- Distant genes on the same chromosome may be mapped by adding the recombination
frequencies determined between them and intermediate genes.
The sequence of genes on a chromosome can be determined by finding the recombination
frequency.
FOR EXAMPLE: if 2 linked genes, A and B, recombine with a frequency of 15% and B and C
recombine with a frequency of 9%, and A and C recombine with a frequency of 24%, what is the
sequence and the distance between them?
- The sequence is A-B-C:
A______15 map units______B___9 map units____ C
The frequency of crossing over may vary along the length of a chromosome. A linkage map
provides the sequence but not the exact location of genes on chromosomes.
Cytogenetic maps locate gene loci in reference to visible chromosomal features.
Inheritance Patterns
Genomic Imprinting
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A few dozen traits in mammals depend on which parent supplied the alleles for the trait.
Genomic imprinting determines whether an allele will be expressed or not in the offspring.
- It causes the activation or inactivation of certain genes, which depends on the gene’s
location on a chromosome and its parental origin.
- It occurs during gamete formation.
- For example: prader-willi syndrome results when genes on the paternal chromosome are
deleted. Angelman syndrome results when genes on the maternal chromosome are
deleted.
When the next generation makes gametes, old maternal and paternal imprints are removed,
and alleles are again imprinted according to the sex of each parent.
- Most of the mammalian genes subject to imprinting identified so far are involved in
embryonic development.
The addition of methyl groups may inactivate the imprinted gene, assuring that the developing
embryo has only one active copy
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Inheritance of Organelle Genes
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Exceptions to Mendelian inheritance are found in the case of extranuclear (or cytoplasmic)
genes located on small circles of DNA in mitochondria and plant plastids, which are
transmitted to offspring in the cytoplasm of the ovum.
Some rare human disorders can be caused by mitochondrial mutations, and maternally
inherited mitochondrial defects may contribute to diabetes, heart disease, and Alzheimer’s.
Chi Square Problems
Chi square = statistical analysis of date that allows the experimenter to determine if the results are due
to change (random events) or another factor.
Chi square formula: (**We have to memorize this!!**)
X2 = £ (o – e)2 + (o – e)2 + …
e
e
# of degrees of freedom = one less than the # of cases
Null hypothesis = assumes there is no difference
- Nonsignificant = accept Null hypothesis
- Significant = reject Null hypothesis
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Genetic Disorders
Disorders due to gene mutations
 A gene mutation results in a defective protein (a change in the sequence of genes
causes a change in the shape of a protein)
Autosomal-Recessive Disorders
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Only homozygous recessive individuals express the phenotype for the thousands of genetic
disorders that are inherited as simple recessive traits
Carriers of the disorder are heterozygotes, who are phenotypically normal but may transmit
the recessive allele to their offspring.
The likelihood of two mating individuals carrying the same rare deleterious allele increases
when the individuals have common ancestors.
Consanguineous matings, between close relatives, are indicated on pedigrees by double lines.
Genetic diseases are unevenly distributed among groups of humans.
- Tay-Sachs: most common in Jews
- Cystic fibrosis: most common in people of European descent
- Sickle cell disease: most common in African Americans.
Autosomal-Dominant Disorders
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A few human disorders are due to dominant genes.
In achondroplasia, dwarfism is due to a single copy of a mutant allele.
Dominant lethal alleles are more rare than recessive lethals because the harmful allele cannot
be masked in the heterozygote.
A late-acting lethal dominant allele can be passed on if the symptoms don’t develop until after
reproductive age.
An example of this is Huntington’s disease
Mutlifactoral Disorders
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Many diseases have genetic (usually polygenic) and environmental components.
These multifactorial disorders include heart disease, diabetes, cancer, and others.
Genetic Testing
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The probability of a child having a genetic defect may be determined by considering the family
history of the disease.
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A karyotype is a picture of what the chromosomes in a cell look like.
Tests to identify carriers have been developed:
1) Amniocentesis – extracting amniotic fluid from the sac surrounding the fetus.
- Allows you to get a karyotype after cells are cultured for several weeks
- Biochemical tests can be performed immediately
- Less invasive and easier, but less reliable
2) Chorionic villi sampling (CVS) - taking a sampling of placenta tissue
- Allows immediate karyotyping and biochemical testing
- More invasive, but more reliable
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An ultrasound is simple, noninvasive procedure that can reveal major abnormalities.
Fetoscopy is the insertion of a needle thing viewing scope and light into the uterus which allows
the fetus to be checked for anatomical problems.
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PGD (pregestational diagnosis) is another genetic test for the fetus.
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It involves a long process:
Scientists take eggs from a female and fertilize them in a petri plate and wait until the
zygote develops into an embryo of 8 cells
They then extract one of the cells (which doesn’t harm the embryo)
They perform a DNA test to look for any genetic disorders
They then implant the embryos that don’t carry disorders or mutations back into the female
and destroy the ones with mutations or disorders
Disorders due to chromosome mutations
 A chromosome mutation results in an abnormal number of chromosomes, either too
many or too few.
 Chromosome mutations cause multiple things to go wrong, which is why these disorders are
called syndromes.
 It is usually lethal.
Abnormal Chromosome Number
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Nondisjunction is when homologous chromosomes don’t separate during meiosis.
- As a result, a gamete receives either 2 or no copies of that chromosomes.
Aneuploidy is a chromosome alteration where cells have too many or too few chromosomes.
- Monosomy: only one chromosome of a pair is present (i.e. too few)
- Trisomy: 3 chromosomes are present for one homologous pair, instead of 2 (i.e. too many)
- Aneuploid organisms usually have a set of symptoms caused by the abnormal number of
genes.
- A disjunction during mitosis early in embryonic development is also likely to be harmful.
Polypoidy is a chromosomal alteration where an organism has more than 2 complete
chromosomal sets.
- Triploidy (3n) is 3 chromosome sets; Tetraploidy is 3 chromosome sets.
- Polyploidy is common in the plant kingdom and has played an important role in the
evolution of plants.
Alterations of Chromosome Structure
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Chromosome breakage can lead to four types of changes in chromosome structure.
1) Deletion is when chromosome fragments are lost
2) Duplication is when chromosome fragments join to a sister chromatid (or a nonsister
chromatid)
3) Inversion is when fragments rejoin to the original chromosome in the reverse
orientation
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4) Translocation is when chromosome fragments join a nonhomologous chromosome, and
one chromosome gets stuck on another
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A nonreciprocal crossover can result in a deletion and duplication in nonsister chromatids,
caused by nonequal exchange between chromatids
Human Disorders due to Chromosomal Alterations
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The frequency of aneuploid zygotes many be failry high in humans, but development is usually
so disrupted that embryos spontaneously abort and die.
Some genetic disorders, expressed as syndromes of characteristic traits, are the result of
aneuploidy.
1. Down Syndrome (trisomy 21)
2. Turners Syndrome (XO)
3. Klinefelters Syndrome (XXY)
Males with XYY and females with XXX usually do not exhibit any syndromes.
Why do genetic disorders persist in a population?
1) Can’t distinguish a carrier from a homozygous normal
2) For humans, there are small numbers of offspring
3) Heterozygotes often have an advantage that allows them to survive
 Heterozygote advantage
- Heterozygous carriers can sometimes be immune to or resistant to another disease
- For example: heterozygote carriers for sickle cell anemia are resistant to malaria
 Genetic anticipation
- Age of onset: when the severity of symptoms of a genetic disorder increases with
age and symptoms show up earlier with each generation
- For example: in Huntington’s disease, symptoms don’t appear till late 30s or 40s
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Human Genetic Disorders
Disorder
Gene/
Abnormality
Chromosome
mutation
Most
common
in
Sickle cell
Gene
African
-sickle shaped
Americans red blood cells
Hemoglobin
Autosomal
recessive
Hemophilia
Gene
*Sex-linked
recessive
Duschene
Muscular
Dystrophy
Gene
Tay-Sachs
Gene
*Sex-linked
recessive
Autosomal
recessive
Cystic
Fibrosis
Gene
Autosomal
recessive
Characteristics Treatment
of disorder
Blood
transfusions
-can’t carry
enough oxygen,
clots blood
vessels
Life
span
Slightly
less
than
normal
Protein
clotting
factors
Males
-excessive
bleeding and
bruising
Injection of
normal
clotting factor
(rDNA
=recombinant
DNA)
Dystrophin
(muscle)
Males
-atrophy
(=wasting away)
of skeletal
muscles
Physical
therapy
(minimal
effects)
20s
Defective
enzyme (that
breaks down
lipids)
Jewish
-Lipid buildup in
brain
None
Less
than 4
years
Defective
transport
protein
(affects
chloride ions)
-western
European
descent –
Caucasian
Antibiotics
Late
20s
and
older
-seizures,
blindness,
degenerate
motor skills
-thick mucus
that clogs lungs,
liver pancreas
-difficulty
breathing and
digestive
problems
(=pleiotrophy)
Lung
transplant
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PKU
Gene
Autosomal
recessive
Huntington’s Gene
Disorder
Defective
enzyme
-mental
retardation
Diet free of
Slightly
phenylalanine less
than
normal
Unknown
Equal for
all
-significant
deterioration of
the nervous
system
None
*symptoms
don’t show
up till 40s or
50s
Less
than
normal
*Autosomal
dominant*
Down
syndrome
(trisomy 21)
Chromosome
nondisjunction
– an extra
chromosome
21 (=trisomy)
Children
-characteristic
facial features,
short statute,
heart defects,
mental
retardation
Regular
checkups,
medicine,
surgery,
counseling
Less
than
normal
Turner’s
syndrome
(XO)
Chromosome
nondisjunction
– monosomy X
(=only one X
chromosome)
Females
only
-still feminine
characteristics,
webbed neck,
reduced IQ
Estrogen
replacement
therapy
normal
Klinefelter’s
syndrome
(XXY)
Chromosome
nondisjunctionan extra X
chromosome in
males
Males
only
-tall and skinny,
male sex
organs, female
characteristics,
reduced IQ
Fragile X
syndrome
Chromosome
a change in a
single gene,
the Fragile X
Mental
Retardation 1
(FMR1) gene,
which is found
on the X
chromosome
Males
-mental
retardation
Deletion of
genes on
paternal
chromosome
(genome
Infancy or
at birth
Prader-Willi
syndrome
Chromosome
normal
-therapy
normal
Therapy
normal
-intellectual
disabilities
-This means that
while most
people have a
single working
copy of these
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17
genes, people
with PWS have
no working copy.
imprinting)
The paternal
genes are
deleted, but
the maternal
copy is silenced
(=genomic
imprinting).
Angelman
Syndrome
Chromosome
Deletion of
genes on
maternal
chromosome
(genome
imprinting)
-learning
difficulties,
obesity
Equal for
all
-no working
copy of the
deleted
maternal genes
-intellectual and
developmental
delay, sleep
disturbance,
seizures,
frequent
laughter
Medical,
physical,
behavioral
therapy to
help
development
normal
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