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Gregor Mendel and Heredity

R. H. Beeman

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TitleObjectivesVocabulary

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2008-03-01

Gregor Mendel1822 - 1884

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R. H. Beeman

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BackgroundGregor Mendel was an Augustinian priest who lived in a monastery in Austria and discovered the basic laws of inheritance and genetics.

He was educated at the University of Vienna, and entered the Augustinian Abbey of St. Thomas in Brno where he was the physics instructor.

In addition, the abbot assigned him to work in the garden, where he worked from 1856 - 1863.


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HumilityMendel, a university-educated physics instructor, could have taken this gardening assignment as a waste of time for an educated person like himself.

Had he done so, he would have been just an obscure monk who died a long time ago in a far-away country.

Fortunately, he saw the opportunity in this relatively menial task, and he became one of the founders of modern biology. Now, 150 years later we are still studying his work.


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PeasMendel began his work on pea plants. He had a number of different varieties of peas, with many different characteristics. Among these, he identified 7 different characteristics, each of which had two possible traits.

He found that by self-pollinating plants with particular traits, he obtained hereditary lines which always had offspring with that particular trait.


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Self-PollinationMendel could force plants to self-pollinate by shielding each flower from other flowers. Then the pollen (male cells) made in the stamen of a flower could only reach the pistil (female part) of the flower which contains the eggs.

Because all the flowers have

pistils!

Why are gardens dangerous?


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The Offspring of Self-PollinationHe found that by self-pollinating plants with particular traits for a number of generations, he obtained hereditary lines which always had offspring with that particular trait.


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True-Breeding PeasMendel called these True-Breeding lines of plants, because their offspring always had the same trait of one particular characteristic.

He continued his work with 7 particular easily-distinguished characteristics, each of which had two traits. He assigned a letter to each of these characteristics.


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Mendel’s True-BreedingPea Characteristics

Characteristic Trait 1 Trait 2

Stem Height Tall Short

Flower Color

Seed Shape

Seed Color

Pod Shape

Pod Color

Flower Position

Purple

Round

Yellow

Smooth

Green

Axial

White

Wrinkled

Green

Constricted

Yellow

Terminal

T

P

R

Y

S

G

A

LTR


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Cross-Pollinating PeasOnce Mendel had established these True-Breeding lines of plants, he tried cross-pollinating them.

That is he took the pollen (male cells) from a plant with one trait of one of the 7 characteristics (e.g. Stem Height - Tall) and applied it to the female part of a plant with the other trait of that characteristic (e.g. Stem Height - Short).


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Cross-Pollinating Peas (2)He called the original True-Breeding generation the P or parent generation, and the offspring resulting from the cross-pollination the F1 or First Filial Generation.

Remember, although we will look at small numbers of plants, the results are RANDOM, so you only get accurate ratios when you do this to hundreds of plants.


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Cross-Pollinating Peas (3)Surprisingly, he found that the F1 generation of pea plants showed only one of the two traits. The other trait had seemingly disappeared.

X

P F1


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Cross-Pollinating Peas (4)This was puzzling, so he again cross-pollinated plants from the F1 generation to produce an F2

or second filial generation.

X

F1 F2

!!!


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Cross-Pollinating Peas (5)In the F2 or second filial generation, the missing trait re-appeared, but only in one-quarter of the plants in the F2 generation.

Realizing that each plant inherited from both the male and female side, and that it must carry characteristics from both, he visualized the inheritance of any characteristic as being like a double-coin flip.


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Double Coin-FlipIf you flip two coins there are 4 equally probable outcomes: HH, HT, TH, TT, as illustrated below:

Coin 1

Coin 2

H

T

H T

TTHT

THHH


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Double Coin-Flip (2)Each of the 4 possibilities has equal probability, but HT and TH are not distinguishable unless you see the actual coins.

Coin 1

Coin 2

H

T

H T

TTHT

THHH


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Double Coin-Flip (3)Interestingly, the chance of flipping either HH or TT is exactly 1/4. The same as the probability of a short plant in the F2 Generation.

Coin 1

Coin 2

H

T

H T

TTHT

THHH


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Mendel’s HypothesesMendel thought about this and developed 4 hypotheses that appeared to explain the data.


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Hypothesis 1For each characteristic, each plant inherits two copies of each gene, one from each parent.

Hypothesis 2There are alternative versions of genes. For example, Tall and Short.

In modern biology the two versions of each gene are called alleles.


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Hypothesis 3When a plant inherits both alleles, one may be expressed, & one hidden.

In these cases, the expressed allele is called dominant and the hidden allele is called recessive.

Each of Mendel’s 7 characteristics (e.g. Tall, Short) turned out to have a dominant and recessive form.


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Hypothesis 4When gametes (pollen or eggs) are formed, the two alleles which an individual carries for each characteristic separate, and each gamete randomly gets only one of them.

When fertilization occurs, forming a zygote or fertilized egg, the two alleles from the male (pollen) and female (egg) gametes are combined, and these become the two alleles carried by the new individual (or seed, at this point).


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Genotypes and PhenotypesA genotype is the combination of alleles that an individual carries in its genes. For example a plant may have a genotype of Tall, Short.

A phenotype is the way a plant looks externally. In the example above, the phenotype of the plant would be Tall, because it carries both Tall and Short alleles, and Tall is dominant while Short is recessive.Mendel represented the dominant trait with an upper-case letter and the recessive trait with a lower-case letter as in the following chart.


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Pea Characteristics

Gene Dominant Trait RecessiveTrait

Stem Height Tall (T) Short (t)

Flower Color

Seed Shape

Seed Color

Pod Shape

Pod Color

Flower Position

Purple (P)

Round (R)

Yellow (Y)

Smooth (S)

Green (G)

Axial (A)

White (p)

Wrinkled (r)

Green (y)

Constricted (s)

Yellow (g)

Terminal (a)


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Tall and Short PhenotypesWe know from the previous chart that for the Stem Height characteristic the Tall (T) allele is dominant and the Short (t) allele is recessive.

Lets look at the P, F1 and F2 generations in light of this knowledge, our coin-flip experiment, and Mendel’s 4 hypotheses.


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Cross-Pollination Genotypes

P Generation F1 Generation F2 Generation

TT

tt

Tt

Tt

Tt

Tt

TT

Tt

tT

tt


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Confirmation!So we see that Mendel’s four hypotheses and his chart of dominant and recessive traits explains the data from the P, F1, and F2 generations very nicely.


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Punnet SquaresA Punnet Square is just our coin-flipping diagram relabeled with alleles (traits) instead of heads and tails. This one illustrates going from the F1 to the F2 generation.

Male Ancestor

Female Ancestor

T

t

T t

tttT

TtTT


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Homozygous & HeterozygousThe pink and green genotypes are homozygous (both alleles the same) while the yellow genotypes are heterozygous (different alleles).

T

t

T t

tttT

TtTT

Male Parent

Female Parent


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Incomplete DominanceIn tulips there are red, pink, and white flowers. The red (R) allele has incomplete dominance, so tulips with heterozygous color are pink.

Male Parent

Female Parent

R

r

R r

rrrR

RrRR


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Complex Punnet SquaresA Punnet Square with 2 or more characteristics.

RY Ry rY ry

RY

Ry

rY

ry

RRYY RRYy RrYY RrYy

RRYy RRyy RrYy Rryy

RrYY RrYy rrYY rrYy

RrYy Rryy rrYy rryy


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2008-03-01Vocabulary

Heredity:The passing of genetic traits from parent to offspring

Genetics:The science of heredity and of the mechanisms by which traits are passed from parents to offspring.

Monohybrid Cross:A cross between individuals that involves one pair of contrasting traits.


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True-Breeding:Organisms or genotypes that are homozygous for a specific trait and thus always produce offspring that have the same phenotype for that trait

P Generation:Parental generation, the first two individuals that mate in a genetic cross.

F1 Generation:The first generation of offspring obtained from an experimental cross of two organisms


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F2 Generation:The second generation of offspring, obtained from an experimaental cross of two organisms; the offspring of the F1 generation

Alleles:Parental generation, the first two individuals that mate in a genetic cross.

Dominant:The trait or allele that is expressed when two different alleles for the same characteristic are inherited.


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Recessive:The trait or allele that is expressed only when two recessive alleles for the same characteristic are inherited.

Homozygous:An individual that identical alleles for a trait on both homologous chromosomes.

Heterozygous:An individual that has two different alleles for a trait.


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Genotype:The entire genetic makeup of an organism; also the combination of genes for one or more specific traits.

Phenotype:An organism’s appearance or other detectable characteristic that results from the organism’s genotype and the environment.


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Law of Segregation:Mendel’s law that states that the pairs of homologous chromosomes separate in meiosis so that only one chromosome from each pair is present in each gamete.

Law of Independent Assortment:The law that states that genes separate independently of one another in meiosis.

Punnet Square:A graphic used to predict the results of a genetic cross.


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Test Cross:The crossing of an individual of unknown genotype with a homozygous recessive individual to determine the unknown genotype.

Probability:The liklihood that a possible future event will occur in any given instance of the event; the mathematical ratio of the number of times one outcome of any event is likely to occur to the number of possible outcomes of the event.


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Pedigree:A diagram that shows the occurrence of a genetic trait in several generations of a family.

Sex-linked gene:A gene found on one of the sex chromosomes, such as the X chromosome or Y chromosome in humans.

Polygenic Inheritance:A characteristic of an organism that is determined by many genes.


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Incomplete Dominance:A condition in which a trait in an individual is intermediate between the phenotype of the individual’s two parents because the dominant allele is unable to express itself fully.

Multiple Alleles:More than two alleles (versions of the gene) for a genetic trait.

Codominance:A condition in which both alleles for a gene are fully expressed.


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Ω


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