Chapter 14—Mendel and the Gene Idea

Concept 14.1: Mendel used the scientific approach to identify two laws of inheritance.

I. Gregor Mendel’s Discoveries Pre-Mendelian theory of heredity proposed that

hereditary material from each parent mixes in the offspring; once blended like two liquids in solution, the hereditary material is inseparable and the offspring’s traits are some intermediate between the parental typesblending theory of heredity

Individuals of a population should reach a uniform appearance after many generations.

Once traits were blended, they can no longer be separated out again.

Gregor Mendel came up with the theory that parents transmit their genes as separate factors from one generation to another Particulate theory of heredity

A. Mendel brought an experimental and quantitative approach to genetics Bred garden peas Peas were available in many easily

distinguishable varieties Strict control over mating to ensure the

parentage of new seeds—no cross pollination Mendel chose characters in peas that differed in

a relatively clear-cut manner. Chose seven characteristics:

1. flower color2. flower position3. seed color4. seed shape5. pod shape6. pod color

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7. stem length Mendel used true breeding—always producing

offspring with the same traits as the parents—which he hybridized in experimental crosses.

a. Parental plants—P generationb. Offspring of the P generation—F1 (first filial)c. Allowing F1 plants to self-fertilize produces the

next generation—F2 Mendel observed the transmission of selected

traits for at least three generations and arrived at two principles of heredity that are now known as the law of segregation and the law of independent assortment.

B. Law of Segregation Mendel believed that traits did not blend—

experiments proved this. Hypothesis : If the inheritable factor for white flowers

had been lost, then a cross between F1 plants should produce only purple flowered plants.Experiment: Allowed F1 plants to self-pollinateResults: 705 purple flowered and 224 white flowered plants in the F2—ratio of 3:1Hypothesis rejectedConclusion: Mendel concluded that since the inheritable factor for white flowers was not lost in the F1 generation, it must have been masked by the presence of the purple flower factorpurple flowers are the dominant trait and white are recessive/

Repeated these experiments with six other characteristics with similar results.

Developed a hypothesis that can be divided into four parts:

1. Alternative forms of genes are responsible for variations in inherited characteristicsalleles

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2. For each character, an organism inherits two alleles, one from each parent creating a gene.

3. If the two alleles differ, one is fully expressed (dominant) and the other is completely masked (recessive).

Dominantcapital letter (P)Recessivelowercase letter (p)

4. The two alleles for each character segregate during gamete production.

Gametes carry only one allele for each characteristic

Gametes of true-breeding plants will all carry the same allele.

The sorting of alleles into separate gametes is known as Mendel’s law of segregation.

This law predicts the 3:1 ratio observed in the F2 generation of a monohybrid cross.

The combinations resulting from a genetic cross may be predicted using a Punnett square.

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P generationPP pp

Gametes P p

F1 generation Pp (purple flowers)

Gametes P pP p

P PP Ppp Pp pp

F2 generation: 3 purple: 1 white

2. Genetic Vocabularya. Homozygoushaving 2 identical alleles for a

given trait. All gametes carry that allele and are true-breeding (pure bred)

b. Heterozygous having 2 different alleles for a trait

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c. Phenotype Expressed traits—what is showingd. Genotype genetic make up (2 alleles expressing

the phenotype)

3. The testcross—used to determine whether an organism with a dominant phenotype is homozygous dominant or heterozygous.

Unknown (PP or Pp) Xpp—known (homozygous recessive)

p p p pP Pp Pp P Pp PpP Pp Pp p pp pp

If PP then all the flowers will be purple If Pp then flowers will be ½ white and ½ purple.

C.Law of Independent AssortmentEach pair of alleles segregates into gametes independently Mendel used a dihybrid cross (cross between two

parents that are heterozygous for two characteristics)Hypothesis 1: If the two characters segregate together, the F1 hybrids can only produce the same two classes

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of gametes that they received from the parents, and the F2 progeny will show a 3:1 phenotypic ratio.

P generation YYRR yyrrGametes

YR yrF1 generation

YyRr

YR yrYR YYRR YyRryr YyRr yyrr

F2 generation: 3 Yellow Round: 1 green wrinkled

Hypothesis 2: If the two characters segregate independently, the F1 hybrids will produce 4 classes of gametes, and the F2 progeny will show 9:3:3:1 ratio.Experiment: Performed a dihybrid cross of the F1 plants. (RrYy X RrYy)

YR Yr yR yr

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YR YYRR YYRr YyRR YyRrYr YYRr YYrr YyRr YyrryR YyRR YyRr yyRR yyRryr YyRr Yyrr yyRr yyrr

Result: Mendel categorized the F2 progeny and determined a ratio of 315:108:101:32 9:3:3:1Conclusion: The experimental results support the hypothesis that each allele pair segregates independently during gamete formation.

Concept 14.2: The laws of probability govern Mendelian inheritance.

D. Mendelian inheritance reflects rules of probabilitySegregation and independent assortment of alleles during gamete formation and fusion of gametes at

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fertilization are random events. Thus, if we know the genotypes of the parents, we can predict the most likely genotypes of their offspring by using the simple laws of probability: The probability scale ranges from 0 to 1; an event

that is certain to occur has a probability of 1, and an event that is certain not to occur has a probability of 0.

The probabilities of all possible outcomes for a specific event must add up to 1.

Event Probability Tossing heads with a two

headed coin Tossing tails with a two

headed coin

Tossing heads with a normal coin

Tossing tails with a normal coin

Rolling a 3 on a six sided die

Rolling anything else other than 3

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0

½

½

1/6

5/6

1 + 0 = 1

½ + ½ = 1

1/6 + 5/6 = 1

Random events are independent of one another The outcome of a random event is unaffected by the

outcome of previous such eventsExample:If is possible that five successive tosses of a normal coin will produce five heads; however, the probability of heads on the sixth try is still ½.

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1. Rule of multiplicationQuestion: In a Mendelian cross between pea plants that are heterozygous for flower color (Pp), what is the probability that the offspring will be homozygous?Answer:Homozygous recessiveppProbability that an egg from the F1 will receive a p allele is ½

Pp

P p½ ½

Probability that a sperm will receive a p allele is ½Pp

P p½ ½

The overall probability that two recessive alleles will unite at fertilization ½ x ½ = ¼

2. Rule of addition The probability of an event that can occur in two or more independent ways is the sum of the separate probabilities of the different ways.Question: In a Mendelian cross between pea plants that are heterozygous for flower color (Pp), what is the probability of the offspring being heterozygous?Answer: There are two ways in which a heterozygote may be produced: Egg P Spermp

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or Eggp Sperm P

Egg (P)½ x Sperm(p) ½ = ¼Egg (p) ½ x Sperm(P) ½ = ¼

Therefore ¼ + ¼ = ½ chance of producing a heterozygote.

3. Using rules of probability to solve genetic problemsQuestion: What is the probability that a trihybrid cross between two organisms with the genotypes AaBbCc and AaBbCc will produce an offspring with the genotype aabbcc?

Answer: Treat this as three separate monohybrid crosses:

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Aa x Aa Bb x Bb Cc x Cc

A a B b C cA AA Aa B BB Bb C CC Cca Aa aa b Bb Bb c Cc cc

Probability for offspring:aa= ¼ bb=¼ cc=¼

The probability that these independent events will occur simultaneously is the product of their independent probabilities (rule of multiplication). So to be aabbcc ¼aa x ¼bb x ¼cc = 1/64

Try this:

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CharacterFlower Color

Seed Color

Seed Shape

Trait and GenotypePurple: Pp or PPWhite: pp

Yellow: YY or YyGreen: yy

Round: RR or RrWrinkled: rr

Question: Recessive phenotypes for at least two of the three traits?

PpYyRr x PpyyrrAnswer:

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Concept 14.3: Inheritance patterns are often more complex than predicted by simple Mendelian genetics

II. Extending Mendelian GeneticsA. The relationship between genotype and phenotype

is rarely simple.1. Incomplete dominance

One allele is not completely dominant over the other, so the heterozygote has a phenotype that is intermediate between the phenotypes of the two homozygotes

P generation RR rrGametes

R rF1 generation

Rr (pink)

GametesR r

R rR RR Rrr Rr rr

F2 generation: 1: 2: 1Genotypes (RR : Rr : rr)Phenotypes (Red: Pink: white)

Incomplete dominance is not support for the blending theory of inheritance, because

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Incomplete Dominance

(A is incompletely dominant)

alleles maintain their integrity in the heterozygote and segregate during gamete formation. Red and white phenotypes reappear in the F2 generation.

2. What is a dominant allele?

Complete dominance inheritance characterized by an allele that is fully expressed in the phenotype of a heterozygote and that masks the phenotypic expression of the recessive allele. Phenotype of homozygous dominant and heterozygous are indistinguishable.

Codominance Characterized by full expression of both alleles in the heterozygote.

Blood Types GenotypeA AAB BB

AB AB

Dominance/recessive relationships among alleles: Are a consequence of the mechanism that

determines phenotypic expression, not the ability of one allele to subdue another at the level of the DNA

Do not determine the relative abundance of alleles in a population

Dominant alleles are not necessarily more common and recessive alleles more rare.

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Complete Dominance(A is dominant)

Codominance(no dominance)

AA and Aa have the same phenotype

Aa= Both alleles are equally expressed in

phenotype

Aa= Intermediate phenotype.Phenotype is intermediate between the two homozygotes (AA & aa)

Example: The allele for polydactyly is quite rare in the US, yet it is caused by a dominant allele. (condition of extra digits)

3. Multiple Alleles more than just two alternative forms of a gene.--Blood type A, B, AB or O--A and B refer to two genetically determined polysaccharides--There are three alleles for this gene: IA, IB and i.

IA and IB are codominant IA and IB are dominant to allele I, which is recessive. Even though there are three possible alleles, every person carries only two alleles which specify their ABO blood type; one allele is inherited from each parent.

Blood Type

PossibleGenotypes

Antigens on the Red Blood Cell

Antibodiesin theSerum

A IAIA

IAiA Anti-B

B IBIB

IBiB Anti-A

AB IAIB A, B --O ii --- --

Foreign antigens usually cause the immune system to respond by producing antibodies, globular proteins that bind to the foreign molecules causing a reaction that destroys or inactivates it.

The antigens are located on the red blood cell and the antibodies are in the serum

A person produces antibodies against foreign blood antigens. These antibodies react with the foreign

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antigens causing the blood cells to clump or agglutinate, which may be lethal

For blood transfusion to be successful, the red blood cell antigens of the donor must be compatible with the antibodies of the recipient.

4. Pleiotropy the ability of a single gene to have multiple phenotypic effects. Many hereditary diseases in which a single

defective gene causes complex sets of symptoms

One gene can also influence a combination of seemingly unrelated characteristics.

5. Epistasisdifferent genes can interact to control the phenotypic expression of a single trait. In some cases, a gene at one locus alters the phenotypic expression of a second gene. If one gene suppresses the phenotypic

expression of another, the first gene is said to be epistatic to the second.

C have melaninc no melanin; therefore, creating no pigment to be expressed.BbCc X BbCc

BC Bc bC bcBC BBCC BBCc BbCC BbCcBc BBCc BBcc BbCc BbccbC BbCC BbCc bbCC bbCcbc BbCc Bbcc bbCc bbcc

B_C_=Black9 bbC_=Brown3__cc=Albino4 Phenotypic ratio 9:3:4

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6. Polygenic inheritanceCharacters that vary by degree in a continuous distribution rather than by discrete qualitative differences. Usually continuous variation is determined not by

one, but by many segregating loci or polygenic inheritance.

Three genes with the dark-skin allele (A, B, C) contribute one “unit” of darkness to the phenotype. These alleles are incompletely dominant over the other alleles (a, b, c)

An AABBCC person would be very dark and an aabbcc person would be very light.

An AaBbCc person would have skin of an intermediate shade.

Because the alleles have a cumulative effect, genotypes AaBbCc and AABbcc make the same genetic contribution to skin darkness.

Environmental factors also affect the phenotype

7. Nature vs. Nurture: environment impacts phenotype

a. Environmental conditions can influence the phenotypic expression of a gene, so that a single genotype may produce a range of phenotypes.

Norm of reaction range of phenotypic variability produced by a single genotype under various environmental conditions

May be quite limited, so that a genotype only produces a specific phenotype, such as the blood group locus that determines ABO blood type.

May also include a wide range of possibilities—blood cell count varies with environmental factors such as altitude, activity level or infection.

Are generally broadest for polygenetic characters—most polygenic traits, such as skin, is multifactorial;

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that is, it depends upon many factors—a variety of possible genotypes, as well as a variety of environmental influences.

Concept 14.4: Many human traits follow Mendelian patterns of inheritance

III. Mendelian Inheritance in Humans

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A. Pedigree analysis reveals Mendelian patterns in human inheritance.Mendelian inheritance in humans is difficult to study because: The human generation time is about 20 years Humans produce relatively few offspring

compared to most other species Well-planned breeding experiments are

impossible.Pedigree a family tree that diagrams the relationships among parents and children across generations and that shows the inheritance pattern of a particular phenotypic character.

Squares symbolize males and circles represent females.

A horizontal line connecting a male and female indicates a mating; offspring are listed below in birth order; from left to right.

Shaded symbols indicate individuals showing the trait being traced.

Following a dominant traitEx. widow’s peakdominant trait

1st generation(grandparents)

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2nd generation(parents & auntsand uncles)

3rd generation(two sisters)

Following a recessive trait—attached earlobes

1st generation(grandparents)

2nd generation

3rd generation

Pedigree analysis can also be used to: Deduce whether a trait is determined by a recessive or

dominant allele. Predict the occurrence of a trait in future generations.

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This type of analysis is important to geneticists and physician, especially when the trait being analyzed can lead to a disabling or lethal disorder.

1. Recessively inherited disorders Recessive alleles that cause human disorders

are usually defective versions of normal alleles. Recessively inherited disorders range in

severity from nonlethal traits to lethal diseases. Since these disorders are caused by recessive alleles:a. The phenotypes are expressed only in

homozygotes (aa) who inherit one recessive allele from each parent.

b. Heterozygotes (Aa) can be phenotypically normal and act as carriers, possibly transmitting the recessive allele to their offspring.

Most people with recessive disorders are born to normal parents, both of whom are carriers.

Human genetic disorders are not usually evenly distributed among all racial and cultural groups due to the different genetic histories of the world’s people.a. Cystic fibrosis 1 out of 2500 Caucasiansb. Tay-Sachs 1 out of 3600; highest in

central European Jewsc. Sickle-cell disease 1 out of 400 African

Americans born in the US Consanguinity a genetic relationship that

results from shared ancestry. The probability is higher that consaguinous matings will result in homozygotes for harmful recessives, since parents with recently shared ancestry are more

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likely to inherit the same recessive alleles than unrelated persons.

2. Dominantly inherited disorders Some human disorders are dominantly inherited. Achondroplasia (dwarfism) affects 1 in 10,000

people who are heterozygous for this gene. Homozygous dominant condition results in

spontaneous abortion of the fetus, and homozygous recessives are of normal phenotype

Lethal dominant alleles are much rarer than lethal recessives, because they: Are always expressed Usually result from new genetic mutations that

occur in gametes and later kill the developing embryo.

Late-acting lethal dominants can escape elimination if the disorder does not appear until an advanced age after afflicted individuals may have transmitted the lethal gene to their children. Huntington’s disease a degenerative disease

of the nervous system; usually does not show up until 25 to 40 years of age.a. Molecular geneticists have recently located

the gene for Huntington’s near the tip of chromosome #4.

b. Children of an afflicted parent have a 50% chance of inheriting the lethal dominant allele. A newly developed test can detect Huntington’s allele before disease symptoms appear; therefore, parents do not unknowingly pass this allele on.

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3. Multifactorial disorders diseases that have both genetic and environmental influences

The hereditary component is often polygenic and poorly understood

Examples: heart disease, diabetes, cancer and alcoholism

B. Technology is providing new tools for genetic testing and counseling

1. Carrier recognition—tests available to determine if prospective parents are carriers of genetic disorders

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2. Fetal Testing—a test to determine if the fetus has the disease carried by both parents. Amniocentesis removal of amniotic fluid for

testing—done between the 14th and 16th week of pregnancy.

Chorionic villus sampling (CVS)a newer technique during which a physician suctions off a small amount of fetal tissue form the chorinic villi of the placenta—performed at 8 to 10 weeks of pregnancy

Ultrasoundexamines fetus for abnormalities by using sound waves to create an image of the fetus.

Fetoscopyuses a thin fiber-optic scope into the uterus to examine the developing fetus.

3. Newborn screening—simple tests routinely done at birth to detect genetic disorders.

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