http://www.southwesternexposure.com

Patterns of InheritanceLecture 10

2

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.

3

Outline

• 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

• 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.

5

Honeybee and Pollen

http://earthobservatory.nasa.gov

Tiny hitchhikers on a domestic honeybee.

6

Wide Range of Pollinators

• 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 nightfallhttp://www.hillcountryadventaure.com

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Waggle Danceht

tp:/

/ww

w.je

suis

culti

ve.c

om

‘Language of the bees’—how honeybees communicate the location of a nectar

source.

http://www.zbp.univie.ac.at

8

Yesterday, Today, and Tomorrow

http://www.architecturalantiques.com

A more pastoral time

http://www.newmediaexplorer.org

Beekeeping today

What does the future hold with colony collapse disorder?

9

“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 character-istic.

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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 Mendelhttp://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 charac-teristics.

• 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.

Postulate = to make a claim; to assume or assert the truth.

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Pea Plants

http://upload.wikimedia.org

Pea pods

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

http://www.mendelweb.org

The stigma, pistil, and ovary make-up the carpel.

StigmaAnther

Petals

Filament

Ovary

PistilStamen

The stamens produce pollen, which contain sperm

cells.

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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

Dominant to the left and recessive to the right for each characteristic.

http

://m

ac12

2.cu

.ac.

jp

Seed shape

Seed color

Flower color

Pod shape

Pod color

Flowerposition

Stemheighth

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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

A diagram of Mendel’s most basic experiment with the color of pea flowers.

P generation: purple flowers x white flowers

F1 generation: purple flowers

F2 generation: 3/4 purple flowers and 1/4 white flowers

F1 x F1 fertilization

P x P fertilization

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Genotype and Phenotype

• Mendel’s experiment led to a conclusion that have been confirmed many times by other biologists and geneticists.

• The physical traits of an organism are its phenotype, and its genetic makeup is its genotype.

An organism’s appearance does not always reveal its inherited traits, or genetic composition.

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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

PP

Pp Pp

pp

P P

pp

P

PP

Pp

p

PpP

p ppParent 1

Parent 2

Parent 1 Parent 2

PP—purple flowersPp—purple flowerspp—white flowers

Clockwise rotation by 45o

The Punnett square is a visual tool for showing all combinations

of alleles of an inherited characteristic.

Reginald C. Punnetthttp://www.epidemiology.ch

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Crossing True Breeding Varieties

Pp

Pp Pp

Pp

P p

pPParent 1 Parent 2

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

F1 generation: purple flowers

F2 generation: 3/4 purple flowers and 1/4 white flowers

F1 x F1 fertilization

P x P fertilization

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Three Generations of Pea Flowers

PP

Pp Pp

pp

P p

P P

pp

Pp, Pp… Pp

F2 plants

F1 plants(hybrids)

Phenotypic ratio3 purple : 1 white

Genotypic ratio1 PP : 2 Pp : 1 pp

P p

P plants(true-breeding

varieties)

PP pp Allelles of parents

X

X

Gametes produced by meiosis

Gametes produced by meiosisPp oror

Alleles of F1 offspring

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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 genera-tion 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 homo-logous.

• 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

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.

P s C

P s c

Dominant allele (C)

Recessive allele (c)

Genotypes: PP ss Cc

Homozygousfor the

dominant allele

Homozygousfor the

recessive allele

Heterozygous

Homo- = sameHetero- = different

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Other Pea Plant Characteristics

• Mendel also studied seed shape and seed color, among other character-istics.

• 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

Observe

P ?

?pParent 1 Parent 2

Observe

Observe Observe

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Probability

Morgan Silver Dollar, 1895 (tails)http://z.about.com

• 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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Rule of Multiplication

• 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

Yields:

PP = 1/4Pp = 1/4 + 1/4 = 1/2pp = 1/4

PP1/4

Pp1/4

Pp1/4

pp1/4

P P

pp

1/2

1/21/2

1/2

F2 genotypes

F1 genotypes

Pp female

Egg (P or p)

Pp male

Sperm (P or p)

Meiosis

(random chance in fertilization)http://www.rpi.edu

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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://nps.k12.nj.us

http://www.jbhs.k12.nf.ca

Which is dominant and which is recessive?

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Hairline

Which is dominant and which is recessive?

http

://w

ww

.moe

.gov

.sg

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Tongue Roll

http

://w

ww

.moe

.gov

.sg

Which is dominant and which is recessive? What about right- and left-handedness?

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Handedness and Cerebral Lateralization

From the medical journal, The Lancethttp://www.answers.com

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 fully-

understood.

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Albert Einstein

At Cal Tech in the early-1930shttp://www.ligo.caltech.edu

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Family Pedigree

• For obvious reasons, geneticists cannot control the matings of humans, unlike researchers working with pea plants, parakeets, or other organ-isms.

• 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

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.

http://ma.water.usgs.gov

Martha’s Vineyard

Nantucket Island

Cape Cod

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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.

DD

Dd Dd

dd

D D

ddParent 1 Parent 2

Which offspring for two heterozygous parents will be deaf and which will be carriers?

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Family Pedigree for Deafness

A sketch of family pedigree showing inheritance of deafness.

http://www.myops.org

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

• 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-to-generation 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

• 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

http://www.icongrouponline.com

TopLittle People of America

http://www.flickr.com

LeftConference gatheringhttp://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 landFrom 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.

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.

http://www.adliterate.com

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We Return to Inheritance Next Week

Austrian postage stamphttp:/library.utem.edu

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Words and Terms to Know

• 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.