1. 11-1 The Work of Gregor Mendel
photo credit: W. Perry Conway/CORBIS
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2. Genetics is the scientific study of heredity.
Gregor Mendel’s Peas
Gregor Mendel’s Peas
Genetics is the scientific study of heredity.
Gregor Mendel was an Austrian monk. His work was important to the understanding of heredity.
Mendel carried out his work with garden peas.
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3. Gregor Mendel’s Peas
During sexual reproduction, sperm and egg cells join in a process called fertilization.
Fertilization produces a new cell called a zygote.
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4. Genes and Dominance
A trait is a specific characteristic that varies from one individual to another.
Mendel studied seven pea plant traits, each with two contrasting characteristics.
He crossed plants with each of the seven contrasting characteristics and studied their offspring.
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5. Genes and Dominance The 7 traits of pea plants:
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6. Genes and Dominance
Each original pair of plants is the P (parental) generation.
The offspring are called the F1, or “first filial,” generation.
The offspring of crosses between parents with different traits are called hybrids.
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7. Today, scientists call the “factors” that determine traits genes.
Genes and Dominance
Mendel’s 2 Laws
1. Law of Dominance
Mendel's first conclusion was that biological inheritance is determined by “factors” that are passed from one generation to the next.
Today, scientists call the “factors” that determine traits genes.
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8. The different forms of a gene are called alleles.
Genes and Dominance
Each of the traits Mendel studied was controlled by one gene that occurred in two contrasting forms that produced different characteristics for each trait.
The different forms of a gene are called alleles.
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9. Genes and Dominance
The principle of dominance states that some alleles are dominant and others are recessive.
An organism with a dominant allele for a trait will always exhibit that form of the trait.
An organism with the recessive allele for a trait will exhibit that form only when the dominant allele for that trait is not present.
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10. 1. In humans, hairy knuckles are dominant over hairless knuckles
PRACTICE…… For each trait below, assign the symbols for the dominant and recessive alleles
1. In humans, hairy knuckles are dominant over hairless knuckles
DOMINANT = _____ = _____________________
RECESSIVE = _____ = _____________________
2. In tomatoes, fuzzy stems are dominant over smooth stems
In watermelons, green skin is dominant over striped skin

11. Segregation
2. Mendel’s Law of Segregation
When any organism produces gametes, the two alleles segregate from each other so that each gamete carries only a single copy of each gene.
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12. Segregation Copyright Pearson Prentice Hall
During gamete formation, alleles segregate from each other so that each gamete carries only a single copy of each gene. Each F1 plant produces two types of gametes—those with the allele for tallness and those with the allele for shortness. The alleles are paired up again when gametes fuse during fertilization. The TT and Tt allele combinations produce tall pea plants; tt is the only allele combination that produces a short pea plant.
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13. 11-2 Probability and Punnett Squares
photo credit: W. Perry Conway/CORBIS
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14. Genetics and Probability
The likelihood that a particular event will occur is called probability.
The principles of probability can be used to predict the outcomes of genetic crosses.
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15. Punnett Squares
The gene combinations that might result from a genetic cross can be determined by drawing a diagram known as a Punnett square.
Punnett squares can be used to predict and compare the genetic variations that will result from a cross.
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16. Punnett Squares
A capital letter represents the dominant allele for tall.
A lowercase letter represents the recessive allele for short.
In this example,
T = tall
t = short
The principles of probability can be used to predict the outcomes of genetic crosses. This Punnett square shows the probability of each possible outcome of a cross between hybrid tall (Tt) pea plants.
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17. Punnett Squares
Gametes produced by each F1 parent are shown along the top and left side.
Possible gene combinations for the F2 offspring appear in the four boxes.
The principles of probability can be used to predict the outcomes of genetic crosses. This Punnett square shows the probability of each possible outcome of a cross between hybrid tall (Tt) pea plants.
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18. Punnett Squares
Organisms that have two identical alleles for a particular trait are said to be homozygous.
Organisms that have two different alleles for the same trait are heterozygous.
Homozygous organisms are true-breeding for a particular trait.
Heterozygous organisms are hybrid for a particular trait.
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19. Punnett Squares
All of the tall plants have the same phenotype, or physical characteristics.
The tall plants do not have the same genotype, or genetic makeup.
One third of the tall plants are TT, while two thirds of the tall plants are Tt.
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20. S = _____________ s = ______________
PRACTICE……. For the following gene and trait, fill in the table with the missing information below.
In guinea pigs, the allele for smooth coat is dominant over rough coat.
S = _____________ s = ______________
Genotype Description
Genotype
Phenotype
Homozygous Dominant
Homozygous Recessive
Heterozygous

21. Punnett Squares
The plants have different genotypes (TT and Tt), but they have the same phenotype (tall).
Although these plants have different genotypes (TT and Tt), they have the same phenotype (tall). TT
Homozygous Tt
Heterozygous
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22. Probability and Segregation
One fourth (1/4) of the F2 plants have two alleles for tallness (TT).
2/4 or 1/2 have one allele for tall (T), and one for short (t).
One fourth (1/4) of the F2 have two alleles for short (tt).
The principles of probability can be used to predict the outcomes of genetic crosses. This Punnett square shows the probability of each possible outcome of a cross between hybrid tall (Tt) pea plants.
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23. Probability and Segregation
Because the allele for tallness (T) is dominant over the allele for shortness (t), 3/4 of the F2 plants should be tall.
The ratio of tall plants (TT or Tt) to short (tt) plants is 3:1.
The predicted ratio showed up in Mendel’s experiments indicating that segregation did occur.
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24. Probabilities predict the average outcome of a large number of events.
Probabilities Predict Averages
Probabilities predict the average outcome of a large number of events.
Probability cannot predict the precise outcome of an individual event.
In genetics, the larger the number of offspring, the closer the resulting numbers will get to expected values.
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25. 11-3 Exploring Mendelian Genetics
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26. Independent Assortment
To determine if the segregation of one pair of alleles affects the segregation of another pair of alleles, Mendel performed a two- factor cross.
The alleles for round (R) and yellow (Y) are dominant over the alleles for wrinkled (r) and green (y).
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27. The Two-Factor Cross: F1
Mendel crossed true-breeding plants that produced round yellow peas (genotype RRYY) with true- breeding plants that produced wrinkled green peas (genotype rryy).
RRYY x rryy  RrYy
All of the offspring were RrYy. Next, he crossed 2 of these plants (RrYy X RrYy)
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28. Here are the results of that cross:
When Mendel crossed plants that were heterozygous dominant for round yellow peas, he found that the alleles segregated independently to produce the F2 generation.
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29. The alleles for seed shape segregated independently of those for seed color. This principle is known as independent assortment.
Genes that segregate independently do not influence each other's inheritance.
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30. Independent Assortment
The principle of independent assortment states that genes for different traits can segregate independently during the formation of gametes.
Independent assortment helps account for the many genetic variations observed in plants, animals, and other organisms.
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31. A Summary of Mendel's Principles
Genes are passed from parents to their offspring.
If two or more forms (alleles) of the gene for a single trait exist, some forms of the gene may be dominant and others may be recessive.
In most sexually reproducing organisms, each adult has two copies of each gene. These genes are segregated from each other when gametes are formed.
The alleles for different genes usually segregate independently of one another.
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32. Beyond Dominant and Recessive Alleles
Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or multiple genes.
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33. 1. Incomplete Dominance 
When one allele is not completely dominant over another it is called incomplete dominance.
In incomplete dominance, the heterozygous phenotype is between the two homozygous phenotypes.
Black x White  Gray
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34. RR
A cross between red (RR) and white (WW) four o’clock plants produces pink flowers (RW). WW
Some alleles are neither dominant nor recessive. In four o’clock plants, for example, the alleles for red and white flowers show incomplete dominance. Heterozygous (RW) plants have pink flowers—a mix of red and white coloring.
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35. Practice Problems
Flower color in snapdragons shows incomplete dominance. A genotype of RR results in a red flower. A homozygous WW plant is white. The intermediate trait is pink flowers. Give the genotype and phenotype ratios for the following crosses:
RR x WW
RW x RW

36. 2. Codominance 
In codominance, both alleles contribute to the phenotype. Both are dominant, both are shown.
In certain varieties of chicken, the allele for black feathers is codominant with the allele for white feathers. Heterozygous chickens are speckled with both black and white feathers. The black and white colors do not blend to form a new color, but appear separately.
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37. A roan cow or horse has both red and white hairs, not pink ones:
Codominance 
A roan cow or horse has both red and white hairs, not pink ones:
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38. Labs can be yellow, black, or chocolate
3. Multiple Alleles 
Genes that are controlled by more than two alleles are said to have multiple alleles.
An individual can’t have more than two alleles. However, more than two possible alleles can exist in a population.
Labs can be yellow, black, or chocolate
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39. Humans show multiple alleles in their blood type: A, B, O
A rabbit's coat color is determined by a single gene that has at least four different alleles.
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40. Different combinations of alleles result in the colors shown here.
KEY
C = full color; dominant
to all other alleles
cch = chinchilla; partial defect in pigmentation; dominant to ch and c alleles
ch = Himalayan; color in certain parts of the
body; dominant to
c allele
c = albino; no color;
recessive to all other
alleles
Coat color in rabbits is determined by a single gene that has at least four different alleles. Different combinations of alleles result in the four colors you see here. photo credits: 1. ©John Gerlach/Visuals Unlimited 2.Animals Animals/©Richard Kolar 3. ©Jane Burton/Bruce Coleman, Inc. 4. ©Hans Reinhard/Bruce Coleman, Inc.
AIbino: cc
Chinchilla: cchch, cchcch, or cchc
Himalayan: chc, or chch
Full color: CC, Ccch, Cch, or Cc
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41. 4. Polygenic Traits  
Traits controlled by two or more genes are said to be polygenic traits.
Skin color in humans is a polygenic trait controlled by more than four different genes.
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42. 5. Genetics and the Environment
Characteristics of any organism are determined by the interaction between genes and the environment.
we can alter the color of our skin by tanning
Siamese cats have dark hairs on the colder parts of their bodies
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