Download lecture 10, patterns of inheritance, 042109c

Patterns of Inheritance
Lecture 10
http://www.southwesternexposure.com
Much of the text material in the lecture notes is from our textbook,
“Essential Biology with Physiology” by Neil A. Campbell, Jane B.
Reece, and Eric J. Simon (2004 and 2008). I don’t claim authorship.
Other sources were sometimes used, and are noted.
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Outline
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Pollinators and sexual reproduction
In an abbey garden
Pea plant experiments
Mendel’s hypotheses
Principles of inheritance
Probability and its applications
Inheritance of human traits
Family pedigrees
Single-gene recessive and dominant disorders
Words and terms to know
Possible test items
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Sexual Reproduction in Flowering Plants
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Pollinators are known as biotic agents that move pollen from the male to
the female parts of flowers to enable sexual reproduction.
The gametes produced in meiosis are contained in the pollen grains and
plant ovaries.
The most prolific pollinators are bees, especially domestic honey bees.
Bees have fuzzy bodies and an electrostatic charge that attracts and
carries pollen as they search for nectar in travelling from flower-to-flower.
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Honeybee and Pollen
http://earthobservatory.nasa.gov
Tiny hitchhikers on a domestic honeybee.
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Wide Range of Pollinators
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While many species of wild bees exist, domestic honeybees perform
much of the pollination for agriculture.
Other pollinators include wasps, bats, hummingbirds, some reptiles,
mammals, and some species of flies especially at higher elevations.
Bats in flight at nightfall
http://www.hillcountryadventaure.com
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http://www.jesuiscultive.com
Waggle Dance
‘Language of the bees’—how honeybees
communicate the location of a nectar
source.
http://www.zbp.univie.ac.at
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Yesterday, Today, and Tomorrow
A more pastoral time
http://www.newmediaexplorer.org
Beekeeping today
http://www.architecturalantiques.com
What does the future hold with
colony collapse disorder?
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“The Buzz on Honeybees”
Domestic honeybees: Bred from European stock introduced in the 1600s
Inhabitants per hive: Up to 60,000 bees
Agricultural crops tended: About 100
Effort to produce one pound of clover honey: Over 7,000 bee-hours
U.S. honey yield (2006): 155 million pounds
U.S. diet tied to honeybee services: 33 percent
Annual value of pollination: $14.6 billion
U.S. beekeepers reporting colony collapse disorder: At least 25 percent
Possible causes: Insecticides, parasites, diseases, or a mix of stressors
Native bees: They do tend a few crops including apple trees and alfalfa
National Geographic Magazine, October 2007
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We now turn to patterns of inheritance.
http://www.retinaaustraliansw.com.au
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Parakeets
http://www.upload.wikimedia.org
Parakeets are also known as budgies.
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Parakeets
Parakeets are small, long-tailed parrots native to Australia, and kept as
pets in many countries.
• Wild parakeets typically have green underparts and yellow upperparts
mixed with black stripes.
• Geneticists call the physical traits most commonly found in the natural
world, ‘wild-types.’
• Parakeets of other colors are raised in captivity—breeders can predict
which colors will result because feather color is an inherited characteristic.
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Breeding
Parents: Wild-type x Wild-type
Offspring: Wild-type
http://whatbird.wildbird.com
Parents: Wild-type x Sky-blue
First-generation offspring: Wild-Type
Second-generation offspring: 3/4 Wild-Type and 1/4 Sky-blue
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In an Abbey Garden
Gregor Mendel
http://kentsimmons.uwinnipeg.ca
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Gregor Mendel
Gregor Mendel was the first person to analyze patterns of inheritance in
a systematic, scientific manner.
• During the 1860s, he deduced the fundamental principles of genetics by
breeding garden peas in an abbey garden in Brunn, Austria (now part of
the Czech Republic).
• He was strongly influenced by physics, mathematics, and chemistry in
applying experimental techniques and mathematics to the study of pea
plants.
• Mendel’s studies, like the work of his contemporary Charles Darwin, are
a classic in science.
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Inherited Characteristics
Mendel postulated in a paper published in 1866 that parents pass on
factors to their offspring that are responsible for inherited their characteristics.
• He thought that these heritable factors retain their uniqueness from
generation to generation—these factors are now known as genes.
• Mendel’s work on inherited characteristics is a foundation of modern
biology and genetics.
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Postulate = to make a claim; to assume or assert
the truth.
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Pea Plants
Pea pods
http://upload.wikimedia.org
Pea flowers
http://upload.wikimedia.org
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Why Study Pea Plants?
Mendel chose garden peas for his experiments because they are easy
to grown and have readily distinguishable varieties.
• Just as important—he could exercise strict control over the transfer of
pollen.
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Pea Flower
Stigma
Anther
Petals
Filament
Pistil
Stamen
Ovary
The stamens produce
pollen, which contain sperm
cells.
The stigma, pistil, and
ovary make-up the carpel.
http://www.mendelweb.org
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Fertilization
Pea plants usually self-fertilize since the pollen from the stamens settle
on the stigma of the same flower.
• Mendel assured self-fertilization by covering the flower with a small bag
so that no pollen grains from other pea plants could reach the stigma.
• He also cross-fertilized by pollinating other pea plants using a small
brush.
• The precise parentage of the offspring could be controlled through the
fertilization process.
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True-Breeding Varieties
Mendel worked with his pea plants until he was sure he had true breeding
varieties.
• These are varieties in which many generations of offspring are all identical
to their parents.
• Mendel chose seven characteristics of pea plants to study, including flower
color.
• Each of the characteristics occurs in only two forms—for example, purple
or white flowers.
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Seven Characteristics
Seed shape
Seed color
Flower
position
Pod shape
Pod color
Stem
heighth
http://mac122.cu.ac.jp
Flower color
Dominant to the left and recessive to the right for each
characteristic.
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Hybrids
The offspring of two different true-breeding varieties are called hybrids.
• This cross-fertilization is referred to as hybridization, or simply a cross.
• The parent plants are the P generation, and their offspring are the F1
generation.
• When F1 plants self-fertilize or cross-fertilize, their offspring are the F2
generation.
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Monohybrid Cross
A monohybrid cross occurs between parent plants that differ in only one
characteristic.
• The outcome between a pea plant with purple flowers and one with white
flowers is a monohybrid cross.
• In crossing pea plants with purple and white flowers, Mendel found that the
F1 generation had purple flowers.
• This is exactly what we saw for a wild-type parakeet bred with a parakeet
of another color.
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Disappearance and Reappearance
Was the inheritance factor for white flowers lost in the F1 generation of
purple flowers?
• By mating F1 plants, the answer was ‘no,’ one-fourth of the F2 plants had
white flowers.
• Mendel concluded that the inheritance factor for white flowers did not
disappear in the F1 plants, although only the factor for purple flowers was
expressed.
• He deduced that the F1 plants must be carrying two inheritance factors for
flower color (purple and white).
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Observed Cross
P generation: purple flowers x white flowers
P x P fertilization
F1 generation: purple flowers
F1 x F1 fertilization
F2 generation: 3/4 purple flowers and 1/4 white flowers
A diagram of Mendel’s most basic experiment with
the color of pea flowers.
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Genotype and Phenotype
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Mendel’s experiment led to a conclusion that have been confirmed
many times by other biologists and geneticists.
An organism’s appearance does not always reveal its inherited
traits, or genetic composition.
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The physical traits of an organism are its phenotype, and its genetic
makeup is its genotype.
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Mendel’s Hypothesis
1. Alternative forms of genes, known as alleles, determine an organism’s
inherited characteristics.
2. An organism has two genes—one from each parent—for each inheritable
characteristic.
3. An egg or sperm carries only one allele for each inherited characteristic,
which are then paired during fertilization.
4. In each pair, the gene that is fully expressed is the dominant allele and
the gene that has no noticeable effect is the recessive allele.
Dominant alleles are represented by uppercase letters and
recessive alleles by lowercase letters.
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Punnett Square
The Punnett square is a visual
tool for showing all combinations
of alleles of an inherited
characteristic.
Parent 2
P
p
P
PP
Pp
p
Pp
pp
Parent 1
Clockwise rotation by 45o
P
Parent 1
P
PP
p
Pp
Pp
pp
Reginald C. Punnett
http://www.epidemiology.ch
p
Parent 2
PP—purple flowers
Pp—purple flowers
pp—white flowers
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Crossing True Breeding Varieties
P
Parent 1
p
Pp
P
Pp
p
Parent 2
Pp
Pp
All offspring will have a genotype of Pp and a phenotype
of purple flowers.
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Color Cross Revisited
P generation: purple flowers x white flowers
P x P fertilization
F1 generation: purple flowers
F1 x F1 fertilization
F2 generation: 3/4 purple flowers and 1/4 white flowers
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Three Generations of Pea Flowers
PP
P plants
(true-breeding
varieties)
P
X
pp
Allelles of parents
p
Gametes produced
by meiosis
Pp, Pp… Pp
Alleles of F1 offspring
F1 plants
(hybrids)
P
or
p
X
P
F2 plants
P
P
PP
p
p
or
Pp
p
Pp
pp
Gametes produced
by meiosis
Phenotypic ratio
3 purple : 1 white
Genotypic ratio
1 PP : 2 Pp : 1 pp
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Principle of Segregation
Mendel found the same type of inheritance pattern occurred for all seven
characteristics of peas that he studied.
• For true-breeding varieties, one parental trait disappears in the F1 generation and then reappears in one-fourth of the F2 generation.
• The underlying mechanism is known as Mendel’s principle of segregation.
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Principle of Segregation
The principle states that pairs of alleles separate—or segregate—during
meiosis, and that the fusion of gametes at fertilization creates allele pairs
once again.
• Research over the past 140+ years has shown the principle of segregation
applies to all sexually-reproducing organisms.
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Homologous Chromosomes
A pair of chromosomes, as we discussed last week, are said to be homologous.
• In each pair, one homologous chromosome is from the female parent and
the other is from the male parent.
• Each homologous pair contains the same genes (such as for flower color)
at the same locations, or loci.
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Homologous Chromosome Fragments
P
s
C
Dominant allele (C)
P
s
c
Recessive allele (c)
PP
ss
Cc
Homozygous
for the
dominant allele
Homozygous
for the
recessive allele
Heterozygous
Genotypes:
Homo- = same
Hetero- = different
Genes are shown as three banded colors on the chromosome fragments.
The letters for the three gene loci are arbitrary and are only used to convey the
concept.
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Other Pea Plant Characteristics
Mendel also studied seed shape and seed color, among other characteristics.
• He found the allele for round seed shape (S) is dominant to the allele for
wrinkled shape (s).
• And he found the allele for yellow seed color (C) is dominant to the allele
for green color (c).
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Principle of Independent Assortment
What would result from a dihybrid cross, the mating of parents differing in
seed shape and seed color?
• Mendel found the yield ratios in the F2 generation were the same as the
they would have been if seed shape and color were studied as separate
monohybrid crosses.
• Mendel’s principle of independent assortment states that each pair of
alleles segregates independently of other pairs of alleles during gamete
formation.
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Testcross
A testcross is a mating between an individual of an unknown genotype and
a homozygous recessive individual (Pp).
• The purpose is to determine the unknown genotype by observing the F2
yields.
• Mendel used testcrosses to determine if he had true-breeding varieties of
pea plants.
• Testcrosses are still used by geneticists in determining unknown genotypes
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P
Parent 1
p
?
Observe
Observe
?
Parent 2
Observe
Observe
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Probability
The segregation of allele pairs during gamete formation (meiosis) and the
re-forming of allele pairs at sexual fertilization follow the rules of probability.
• The same rules apply to tossing a coin, rolling a die, and drawing playing
cards.
• Just as in probability experiments, Mendel found he needed to obtain large
sample sizes of F1 and F2 offspring to determine inheritance patterns since
variation exists.
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Morgan Silver Dollar, 1895 (tails)
http://z.about.com
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Rule of Multiplication
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In coin tossing, the probability of heads is 1/2 and the probability of tails is
1/2.
The two events sum to a probability of 1.0 since only two outcomes exist.
Each toss is an independent event since its outcome is unaffected by what
happened during previous events.
When two coins are tossed simultaneously, the outcome for each coin is an
independent event unaffected by the other coin.
The probability of such a simultaneous event (two heads with two coins) is
1/2 times 1/2, or 1/4.
The rule of multiplication also applies to independent events that occur in
genetics.
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Probability Visualized
Pp female
Pp male
Meiosis
F1 genotypes
Egg (P or p)
Sperm (P or p)
(random chance in fertilization)
http://www.rpi.edu
P 1/2
1/2 P
1/2
F2 genotypes
PP
1/4
p
Pp
1/4
p
Pp
1/4
pp
1/4
1/2
Yields:
PP = 1/4
Pp = 1/4 + 1/4 = 1/2
pp = 1/4
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Complex Genetic Problems
The results of the rule of multiplication are the same as for a Punnett
square.
• Complex genetic problems can be solved by applying rules of probability
to segregation and independent assortment.
• The outcomes of trihybrid crosses involving three different characteristics
can be readily determined using probability.
• To analyze such a problem with the Punnett tool, would require 64 cells in
an 3 x 3 x 3 cube.
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Inheritance of Human Traits
Mendel’s principles apply to the inheritance of some human traits including
those we will discuss next.
• Each of these traits is the result of simple dominant-recessive inheritance
at one gene locus.
• The genetic basis of many human characteristics, such as eye color and
hair color, are not as well-understood.
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Dominant and Recessive Alleles
For the sake of this discussion, the dominant allele is indicated by A and
the recessive allele by a.
• Recall that phenotype represents the expressed or physical appearance.
• The dominant phenotype results from the homozygous genotype, AA, or
the heterozygous genotype, Aa.
• The recessive phenotype results only from the homozygous genotype,
aa.
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Dominant Phenotype
‘Dominant’ does not imply that a phenotype is necessarily more common
in a population than a recessive phenotype.
• Freckles, for example, are the result of a dominant allele, but they are not
very common in the general population.
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Earlobes
http://www.jbhs.k12.nf.ca
Which is dominant and which
is recessive?
http://nps.k12.nj.us
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http://www.moe.gov.sg
Hairline
Which is dominant and which is recessive?
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http://www.moe.gov.sg
Tongue Roll
Which is dominant and which is recessive? What about right- and
left-handedness?
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Handedness and Cerebral Lateralization
Right-handed—the left hemisphere
contains the processing areas for
verbal and math abilities.
Left-handed—the right hemisphere
often contains the areas for verbal
and math abilities.
Handedness is not the result of
a single gene, and is not fullyunderstood.
From the medical journal, The Lancet
http://www.answers.com
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Albert Einstein
At Cal Tech in the early-1930s
http://www.ligo.caltech.edu
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Family Pedigree
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For obvious reasons, geneticists cannot control the matings of humans,
unlike researchers working with pea plants, parakeets, or other organisms.
Instead, they must analyze results of matings that have already occurred.
The geneticist collects as much information as possible about a family’s
history for a specific trait.
The information is assembled into a tree structure known as the family
pedigree.
The geneticist uses the concepts of dominant and recessive alleles and
the principle of segregation to analyze the family pedigree to determine if
patterns of inheritance exist.
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Case Study
A classic study is the family pedigree for a rare type of deafness found on
Martha’s Vineyard in Massachusetts.
• The form of deafness results from a homozygous recessive genotype
(dd).
• Family members with the heterozygous dominant genotype (Dd) are not
deaf, but they are carriers of the disorder.
• Members with the homozygous dominant genotype (DD) are neither deaf
nor carriers.
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Martha’s Vineyard
Cape Cod
Martha’s
Vineyard
Nantucket
Island
http://ma.water.usgs.gov
The isolation of Martha’s Vineyard fostered marriages between close
relatives between 1700 and 1900. The frequency of deafness was high
since there was very little exchange of alleles with outsiders.
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Punnett Square for Deafness
The appearance of deafness from generation-to-generation on Martha’s
Vineyard can be solved using a 2 x 2 Punnett square or the rule of
multiplication.
D
Parent 1
D
DD
d
Dd
d
Parent 2
Dd
dd
Which offspring for two heterozygous parents will be deaf
and which will be carriers?
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Family Pedigree for Deafness
http://www.myops.org
A sketch of family pedigree showing inheritance of deafness.
Females are shown by circles and males by squares.
Deafness is indicated by dark symbols representing an allele of dd.
Hearing is indicated by light symbols representing an allele of DD or Dd.
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Inheritance Patterns
The hereditary deafness observed on Marta’s Vineyard is one of over a
thousand genetic disorders from dominant or recessive traits controlled
by single genes.
• The disorders show simple inheritance patterns just like the traits Mendel
studied in pea plants.
• The genes are all located on the autosomes, that is chromosomes other
than X and Y.
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Recessive Disorders
Most human genetic disorders are recessive—they can range in severity
from relatively harmless to life-threatening.
• The vast majority of people afflicted with recessive disorders are born to
parents that are heterozygous; that is, they are carriers but don’t have the
disorder.
• As with family deafness on Martha’s Vineyard, the percentage of affected
offspring can be predicted as a result of the mating between two carriers.
• Examples of single-gene recessive disorders include albinism, sickle-cell
disease, Tay-Sachs, phenylketonuria, galactosemia, and cystic fibrosis.
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Cystic Fibrosis
Cystic fibrosis is the most common lethal genetic disorder in the United
States.
• It affects about one in 1,800 European-Americans and is carried as a
recessive allele in about one in 25.
• It affects about one in 17,000 African Americans and about one in about
90,000 Asian Americans.
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Physical Effects
Cystic fibrosis is characterized by the secretion of thick mucus from the
lungs, pancreas, and other body organs.
• The mucus can interfere with breathing, digestion, and liver function, and
can make the person more vulnerable to pneumonia and other infections.
• The lives of children afflicted with the disorder can be prolonged through
special diets, antibiotics, frequent pounding of the chest and back to clear
the lungs, and other treatments.
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Public Awareness Campaign
http://images.cff.org
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Likelihood of Genetic Disorders
Due to the increase mobility in modern societies, it is relatively unlikely
that two carriers of a rare and harmful allele will meet and have children.
• People with recent common ancestors are more likely to carry the same
recessive alleles than unrelated people.
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Inbreeding
Mating of close relatives—known as inbreeding—can produce offspring
who are homozygous for a harmful recessive trait.
• Many societies have taboos and laws forbidding marriages between close
relatives.
• Legal prohibitions may have formed from observations that still-births and
birth defects are more common when parents are closely-related.
• Some very small groups of endangered animals, such as cheetahs, show
the detrimental effects of inbreeding.
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Cheetahs
http://www.whozoo.org
Inbreeding has led to low sperm counts and birth defects,
among other problems. Zoos are being increasingly used
as a biological ‘ark’ to maintain genetic variability.
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Dominant Disorders
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Some human disorders are the result of dominant alleles, which are less
common than those resulting from recessive alleles.
One reason for the difference is that lethal dominant alleles also affect the
carrier.
Lethal dominant alleles may kill the embryo, or the afflicted individual may
not live long enough to reproduce.
This is in contrast to lethal recessive alleles passed from generation-togeneration by heterozygous carriers who did not exhibit the disorder.
Some dominant disorders, such as for extra or webbed fingers and toes,
are not lethal.
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Achondroplasia
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Achondroplasia is characterized by very short stature, with arms and legs
too short for the torso.
About one in 25,000 people have this disorder.
Only individuals with a single copy of the dominant allele (Aa) have the
disorder because the homozygous genotype (AA) results in the death of
the embryo.
A person with achondroplasia has a 50 percent probability of passing the
dominant allele to his or her children.
This pattern can be demonstrated using a Punnett square or the rule of
multiplication.
The late David Rappaport, an actor, was a well-known case of people
having the disorder.
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David Rappaport
http:/upload.wikimedia.org
http://www.time-bandits.be
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Resources and Support
Top
Little People of America
http://www.flickr.com
http://www.icongrouponline.com
Left
Conference gathering
http://www.ksginfo.org
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Huntington’s Disease
A lethal dominant allele can escape timely detection if it does not result in
death until later in life.
• Huntington’s disease, which causes degeneration of the nervous system,
is not apparent until middle age.
• Symptoms include uncontrolled movements, memory loss and impaired
judgment, depression, and in later stages, inability to swallow and speak.
• Death usually occurs 10 to 20 years after the onset of the first symptoms.
•
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Late Appearance of Symptoms
By the time symptoms are evident, the afflicted individual may have had
children—about half will have received the lethal dominant allele.
• A famous case involved the folk singer-songwriter, Woody Guthrie, who
died from the disease in 1967 at the age of 55.
• He has well-known children, Nora and Arlo, who have been at potential
risk.
•
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Woody Guthrie
This Land Is Your Land
This land is your land this land is my land
From California to the New York island;
From the red wood forest to the Gulf Stream waters
This land was made for you and Me.
As I was walking that ribbon of highway,
I saw above me that endless skyway:
I saw below me that golden valley:
This land was made for you and me.
I've roamed and rambled and I followed my footsteps
To the sparkling sands of her diamond deserts;
And all around me a voice was sounding:
This land was made for you and me.
When the sun came shining, and I was strolling,
And the wheat fields waving and the dust clouds rolling,
As the fog was lifting a voice was chanting:
This land was made for you and me.
As I went walking I saw a sign there
And on the sign it said "No Trespassing."
But on the other side it didn't say nothing,
That side was made for you and me.
http://www.adliterate.com
In the shadow of the steeple I saw my people,
By the relief office I seen my people;
As they stood there hungry, I stood there asking
Is this land made for you and me?
Nobody living can ever stop me,
As I go walking that freedom highway;
Nobody living can ever make me turn back
This land was made for you and me.
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We Return to Inheritance Next Week
Austrian postage stamp
http:/library.utem.edu
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Words and Terms to Know
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•
•
•
•
•
•
•
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Achondroplasia
Allele
Carrier
Cross-fertilization
Cystic fibrosis
Dihybrid cross
Dominant allele
Dominant disorder
Family pedigree
Genotype
Heterozygous
Homozygous
Huntington’s disease
Hybrid
Inbreeding
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Monohybrid cross
P, F1, and F2 generations
Phenotype
Principle of independent assortment
Principle of segregation
Probability
Punnett square
Recessive allele
Recessive disorder
Rule of multiplication
Self-fertilization
Testcross
True-breeding variety
Wild-type
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Possible Test Items
1. Describe how pea plants have been used in defining the laws of heredity.
2. Describe color crosses in pea plants, and how Punnett squares are used
in determining genotype and phenotype.
3. Describe how probability can be applied to reaching the same result as
Punnett squares.
4. Describe how family pedigrees can be used in determining generational
patterns of genetic disorders.
5. Describe one recessive and one dominant genetic disorders that can
occur in humans.
6. How could society help individuals and their families cope with genetic
disorders? Please include biological and social perspectives.
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