Gregor Mendel and Heredity
R. H. Beeman
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2008-03-01
Gregor Mendel1822 - 1884
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Gregor Mendel and Heredity
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.
Gregor Mendel and Heredity
R. H. Beeman
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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.
Gregor Mendel and Heredity
R. H. Beeman
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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.
Gregor Mendel and Heredity
R. H. Beeman
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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?
Gregor Mendel and Heredity
R. H. Beeman
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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.
Gregor Mendel and Heredity
R. H. Beeman
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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.
Gregor Mendel and Heredity
R. H. Beeman
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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
Gregor Mendel and Heredity
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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).
Gregor Mendel and Heredity
R. H. Beeman
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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.
Gregor Mendel and Heredity
R. H. Beeman
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2008-03-01
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
Gregor Mendel and Heredity
R. H. Beeman
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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
!!!
Gregor Mendel and Heredity
R. H. Beeman
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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.
Gregor Mendel and Heredity
R. H. Beeman
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2008-03-01
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
Gregor Mendel and Heredity
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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
Gregor Mendel and Heredity
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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
Gregor Mendel and Heredity
R. H. Beeman
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Mendel’s HypothesesMendel thought about this and developed 4 hypotheses that appeared to explain the data.
Gregor Mendel and Heredity
R. H. Beeman
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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.
Gregor Mendel and Heredity
R. H. Beeman
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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.
Gregor Mendel and Heredity
R. H. Beeman
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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).
Gregor Mendel and Heredity
R. H. Beeman
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2008-03-01
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.
Gregor Mendel and Heredity
R. H. Beeman
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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)
Gregor Mendel and Heredity
R. H. Beeman
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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.
Gregor Mendel and Heredity
R. H. Beeman
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2008-03-01
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.
Gregor Mendel and Heredity
R. H. Beeman
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2008-03-01
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
Gregor Mendel and Heredity
R. H. Beeman
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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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R. H. Beeman
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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
Gregor Mendel and Heredity
R. H. Beeman
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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
Gregor Mendel and Heredity
R. H. Beeman
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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.
Gregor Mendel and Heredity
R. H. Beeman
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2008-03-01Vocabulary
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
Gregor Mendel and Heredity
R. H. Beeman
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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.
Gregor Mendel and Heredity
R. H. Beeman
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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.
Gregor Mendel and Heredity
R. H. Beeman
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2008-03-01Vocabulary
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.
Gregor Mendel and Heredity
R. H. Beeman
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2008-03-01Vocabulary
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.
Gregor Mendel and Heredity
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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.
Gregor Mendel and Heredity
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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.
Gregor Mendel and Heredity
R. H. Beeman
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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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