BIOLOGY FOR AP^® COURSES

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Chapter 12 MENDEL’S EXPERIMENTS AND HEREDITY

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COLLEGE PHYSICS

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Historical misconceptions about heredity

Commonly held beliefs:

• Homunculus: sperm contained fully formed “little people”
• Like begets like: offspring are like their parents
• Everything from the egg: females controlled all traits
• Paternal heredity: males control all traits
• Blending inheritance: offspring were the average between both parents
• Inheritance of acquired characters: changes happening to parents could be passed on to offspring
• Pangenesis: the idea that particles called “gemmules” carry the traits we inherit.

Example: Homunculus

Homunculus from a 1640 Swedish text

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Note the “little man”.

Some earlier editions even clothed little men and women in the fashions of their day.

MENDEL’S HISTORY LEADING TO HIS WORK

• Gregor Johann Mendel (1822–1884)
• Considered the father of genetics
• Family farm in Austria (now Brno) gardened and studied beekeeping - Too sickly to farm – his older brother took over
• Studied philosophy – University of Olomouc and became a friar to pay for his education.
• Entered the Augustinian Saint Thomas Abbey - trained to become a priest and taught science
• Sent to the University of Vienna to study under the sponsorship of Abbot C. F. Napp 
• Returned to the abbey - taught while studying heredity
• Eventually became Abbot and gave up science

WHY PEAS?

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• Many simple dichotomous traits – flower color, seed shape, seed color etc.
• Many such traits were true breeding
• Monoecious (male and female in one flower – easy to manipulate matings)
• Each pea was a new individual – could evaluate thousands of offspring
• Short generation times
• Cheap ornamental readily available

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Mendel’s Notation System

• P: parent generation of true breeding plants
• f1: first filial generation of offspring from a cross between parents (brothers and sisters)
• f2: second filial generation of offspring produce by crossing 2 f1 individuals with each other
• f3 , f4, f5 ……. Succeeding generations
• Siblings: brothers and sisters
• Cross sibs: mating between brothers and sisters
• Backcross: mating an offspring back to a parent of other ancestor
• Self cross: crossing a plant with itself (selfing)
• True breeding: when parents having the same traits always produce offspring having the same traits

Formal Cross for One Trait

P

X

True breeding parent generation – both true breeding for purple flower color trait

f1

X

Cross f1 siblings

f2

F2 siblings

Mendel crossed parent plants that differed in one trait to produce hybrid strains in order to see if what was inherited could be detected.

Formal monohybrid cross

RESULTS FROM F2 TYPES

Background:

In one experiment, Mendel crossed plants that were true-breeding for violet flower color with plants true-breeding for white flower color (the P generation).

Results:

• F₁ generation all had violet flowers.
• F₂ generation, approximately three quarters of the plants had violet flowers, and one quarter had white flowers.
• 3:1 f2 ratio from a formal monohybrid cross held true for other monohybrid crosses with different traits

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CONCLUSIONS FROM MONOHYBRID CROSSES:

• Blending inheritance was not a viable explanation
• Inheritance appeared to be particulate
• Some versions of a trait masked other versions of a trait - Dominant
• Other versions of a trait were masked – Recessive
• The recessive trait appeared to have passed through the f1 monohybrids unchanged to the f2
• f2 ratios of 3:1 dominant to recessive versions of the trait suggested that the f1 monohybrids must have carried one hereditary particle for purple and one hereditary particle for white versions of the trait
• Ratios further suggested that the hereditary particles segregated into gametes and then came back together at fertilization

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CONCLUSIONS FROM MONOHYBRID CROSSES:

Question:

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Mendel had no notion of the physical basis of these “particles.”

What would the “particles” turn out to be?

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SOME USEFUL, MODERN TERMS

• Trait : characteristic like flower color, eye color etc.
• Phenotype: the characteristic version of a trait we actually see. Example: purple flower or blue eyes
• Alleles: specific versions of a “hereditary particle” (today= versions of a gene)
• Dominant: alleles that mask others – often designated with capital letters. Example: P for purple flower
• Recessive: alleles that are masked by others – often designated with lower case letters Example p for white flower
• Genotype: the specific combination of “hereditary particles” carried by an individual that cause a phenotype
• Homozygote: when both alleles for the same trait are the same. Example: PP or pp (Note – homozygotes are always true breeding)
• Heterozygote: when alleles for the same trait differ. Example Pp (Note – heterozygotes are always non-true breeding).
• Zygote: first diploid cell produced by fertilization

SOME USEFUL, MODERN TERMS

• Gene: place in the DNA strand that encodes information causing a trait. (Mendel’s “hereditary particles”
• Locus: place of interest on a chromosome, usually a gene
• Alleles(again): versions of a gene
• Reciprocal cross: A forma mating cross where a previous cross is repeated, but the parents exhibiting versions of a phenotypic trait are reversed by sex
• Wildtype allele: the most common allele in a population (so called “normal allele”
• Mutant allele: A rare allele in a population. Reasoned to be the most recently formed allele by mutation (so called “non-normal allele”)

Defining monohybrid crosses

P

X

True breeding purple

True breeding white

X

f1

f2

Cross F1sib hybrids heterozygotes Each is Pp – these were obviously non-true breeding!

F2 sibs both white and purple

All purple No blending!

PP purple homozygote

pp white homozygote

Pp heterozygote purple phenotype

Pp

heterozygote purple phenotype

PP

Pp

Pp

pp

Mendel’s First Law

• Heredity is particulate
• Each true breeding parent carries 2 hereditary particles for each trait.
• Particles come apart during gamete formation such that each parent passes only one particle or the other of a pair into each of their gametes.
• Hereditary particles are then recombined randomly during fertilization to produce offspring

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To test this hypothesis, he reasoned that each f1 monohybrid should have one of each of both types of hereditary particle and he devised a formal test cross to prove it. It also became a useful means of detecting unknown heterozygotes.

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A Formal Test Cross

Predict a 1:1 ratio of dominant to recessive offspring in the f2 – why?

P

X

Begin with true breeding parents

f1

X

Cross the f1 monohybrid with a recessive homozygote

f1

Punnett Squares

P generation white homozygote pp

P generation purple homozygote PP

P

P

p

p

Parental gametes

Pp

Pp

Pp

Pp

f1 generation genotypes

P generation white homozygote pp

X

Pp

Pp

X

Cross two f1 sibs

P

p

P

p

f2 generation genotypes

TEST CROSS

Rationale: Determine whether an organism expressing a dominant trait is a homozygote or a heterozygote.

• Mendel verified these results for test crosses with numerous different traits in monohybrids.
• Each parent carries 2 alleles for each trait.
• Alleles segregate equally and randomly into gametes.
• Gametes randomly recombine alleles in zygotes during fertilization.

PUNNETT SQUARES CAN PREDICT MENDELIAN CROSS OUTCOMES

Probability Basics

• Chance events are described by probability
• Probability is given as a fraction or decimal equivalent over a range of zero to one. (also 0% to 100%)
• A probability of 0 means an event will not happen and therefore is not due to chance at all.
• A probability of 1 means an event will happen – it is certain and therefore not due to chance at all
• Anything that is chance ranges between those values
• For example, the chance of a flipped coin landing with heads up is ½ ( or 0.5, or 50%)

MULTIPLICATIVE LAW OF SIMPLE PROBABILITY

• The overall chance for the occurrence of two or more independent random events in exact order is equal to the product of their individual probabilities.
• For Example:
• The chance of rolling exactly a five on a die is 1/6,
• The chance of rolling exactly a four on a die is 1/6,
• So, the overall chance of rolling exactly a five on the first die and then a four on the second die is :
• 1/6 X 1/6 = 1/36

ADDITIVE LAW OF SIMPLE PROBABILITY

When there are multiple ways or tries for a chance outcome to occur, the overall probability is the sum of the individual events.

For Example:

• If one can win by rolling a 4 or a 5 with one die, then the overall chance of winning is:
• The chance of rolling a four is 1/6
• Or the chance of rolling a five is also 1/6
• So, the overall chance is 1/6 +1/6 = 2/6 = 1/3

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OR

BOTH LAWS CAN BE COMBINED FOR COMPLEX RANDOM EVENTS

• The chance of rolling a 5 and a 4 in any order is:
• Chance of a five first (1/6) times the chance of a 4 second (1/6) = 1/36
• But, if order doesn’t matter we might also roll a four first (another 1/6) and a five second (another1/6) or, 1/36
• So, the overall chance is 1/36 + 1/36 = 2/36= 1/18.

Symbols For Pedigrees

Probability rules can be used in conjunction with pedigrees to make genetic inferences.

ALKAPTONURIA

Formal Dihybrid Cross

P generation: cross parents that were true breeding for different phenotypes of two different traits. Here traits were seed shape (round or wrinkled) and seed color (yellow or green).

P generation homozygotes

f1 double heterozygotes These are dihybrids

YYRR X yyrr

YyRr X YyRr

f2 genotype and phenotype categories

9:3:3:1 f2 dihybrid cross Mendelian ratio

If you count for each trait individually you will see each still gives a 3:1 ratio as in monohybrid crosses. This led Mendel to conclude that traits were inherited independently from one another.

Segregation and recombination of alleles for one trait has no effect on segregation and recombination of alleles for other traits.

Mendel’s Second Law: The Law of Independent Segregation

Another example of f2 genotype and phenotype categories from a formal dihybrid cross

This figure shows all possible combinations of offspring resulting from a dihybrid cross of pea plants that are heterozygous for the tall/dwarf and inflated/constricted alleles.

INCOMPLETE DOMINANCE

Consider Snapdragons.

• Cross a true breeding red flower parent with a true breeding white flowered parent
• Result: all pink f1 heterozygotes.
• If you then cross pink f1 sibs you get an f2 that has the following ratio of phenotypes:

1 red: 2 pink: 1 white

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MULTIPLE ALLELES FOR A SINGLE TRAIT �SHOWING AN ORDER OF DOMINANCE

Four different alleles exist for the rabbit coat color (C) gene.

C is dominant to c^ch

C^ch is dominant to c^h

C^h is dominant to c

Homeotic mutants affect development and potentially many characters. Remember Mendel’s traits were caused by single genes that affected only one trait.

As seen in comparing the wild-type Drosophila (left) and the Antennapedia mutant (right), the Antennapedia mutant has legs on its head in place of antennae.

MULTIPLE ALLELES CONFER DRUG RESISTANCE IN THE MALARIA PARASIT

The (a) Anopheles gambiae, or African malaria mosquito, acts as a vector in the transmission to humans of the malaria-causing parasite (b) Plasmodium falciparum, here visualized using false-color transmission electron microscopy. (credit a: James D. Gathany; credit b: Ute Frevert; false color by Margaret Shear; scale-bar data from Matt Russell)