1. Introduction to Genetics: The Work of Gregor Mendel
Ch. 11
Ms. Haut

2. 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 ordinary garden peas.

3. Modern Theory of Heredity
Based on Gregor Mendel’s fundamental principles of heredity
Parents pass on discrete inheritable factors (genes) to their offspring
These factors remain as separate factors from one generation to the next

4. Gregor Mendel’s Peas Mendel knew that
the male part of each flower produces pollen, (containing sperm).
the female part of the flower produces egg cells.
To cross-pollinate pea plants, Mendel cut off the male parts of one flower and then dusted it with pollen from another flower.
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5. Gregor Mendel’s Peas
During sexual reproduction, sperm and egg cells join in a process called fertilization.
Fertilization produces a new cell.

6. Gregor Mendel’s Peas Pea flowers are self- pollinating.
Sperm cells in pollen fertilize the egg cells in the same flower.
The seeds that are produced by self- pollination inherit all of their characteristics from the single plant that bore them.

7. Gregor Mendel’s Peas
Mendel had true-breeding pea plants that, if allowed to self-pollinate, would produce offspring identical to themselves.
Mendel wanted to produce seeds by joining male and female reproductive cells from two different plants.
He cut away the pollen-bearing male parts of the plant and dusted the plant’s flower with pollen from another plant.
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8. Gregor Mendel’s Peas This process is called cross-pollination.
Mendel was able to produce seeds that had two different parents.
To cross-pollinate pea plants, Mendel cut off the male parts of one flower and then dusted it with pollen from another flower.
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9. 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 characters.
He crossed plants with each of the seven contrasting characters and studied their offspring.

10. 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.
The F1 hybrid plants all had the character of only one of the parents.
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11. Genes and 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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12. Genes and Dominance
Each of the traits Mendel studied was controlled by one gene that occurred in two contrasting forms that produced different characters for each trait.
The different forms of a gene are called alleles.
Mendel’s second conclusion is called the principle of dominance.
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13. Principal of Dominance
The principle of dominance states that some alleles are dominant and others are recessive.
For each characteristic, an organism inherits two alleles, one from each parent.
If 2 alleles differ, one is fully expressed (dominant allele); the other is completely masked (recessive allele)
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14. Useful Genetic Vocabulary
Homozygous—having 2 identical alleles for a given trait (PP or pp)
Heterozygous—having 2 different alleles for a trait (Pp); ½ gametes carry one allele (P) and ½ gametes carry the other allele (p)
Phenotype—an organism’s expressed traits (purple or white flowers)
Genotype—an organism’s genetic makeup (PP, Pp, or pp)

15. Law of Segregation
Mendel crossed the F1 generation with itself to produce the F2 (second filial) generation.
The traits controlled by recessive alleles reappeared in one fourth of the F2 plants.
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16. Law of Segregation 1st Law of genetics
The two members of an allele pair segregate (separate) from each other during the production of gametes.

17. Segregation
When each F1 plant flowers and produces gametes, the two alleles segregate from each other so that each gamete carries only a single copy of each gene.
Therefore, each F1 plant produces two types of gametes— those with the allele for tallness, and those with the allele for shortness.
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18. 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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19. 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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20. Punnett Square tt t Tt x tt T t Tt Tt Tt tt tt
Cross a heterozygous tall pea plant with a dwarf pea plant.
T = tall, t = dwarf tt t
Tt x tt T t Tt Tt Tt tt tt

21. Monohybrid Cross
Genetic cross between individuals that are identical in all traits but one.

22. Practice Monohybrid Cross
In pea plants, spherical seeds (S) are dominant to dented seeds (s). In a genetic cross between two plants that are heterozygous for the seed shape trait, what are the genotypic and phenotypic ratios? Ss Ss X S s
S= spherical seed
s = dented seeds S SS Ss Ss ss s
Genotypic Ratio=
Genotypic Ratio= 1 SS: 2 Ss: 1 ss
Phenotypic Ratio=
Phenotypic Ratio= 3 spherical seed plants: 1dented seed plant

23. 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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24. Genetics
Quiz
Wiz

25. Gametes are also known as
genes.
sex cells.
alleles.
hybrids.
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26. The offspring of crosses between parents with different traits are called
alleles.
hybrids.
gametes.
dominant.
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27. In Mendel’s pea experiments, the male gametes are the
eggs.
seeds.
pollen.
sperm.
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28. In a cross of a true-breeding tall pea plant with a true- breeding short pea plant, the F1 generation consists of
all short plants.
all tall plants.
half tall plants and half short plants.
all plants of intermediate height.
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29. If a particular form of a trait is always present when the allele controlling it is present, then the allele must be
mixed.
recessive.
hybrid.
dominant.
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30. Organisms that have two different alleles for a particular trait are said to be
hybrid.
heterozygous.
homozygous.
recessive. 6
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31. Two F1 plants that are homozygous for shortness are crossed
Two F1 plants that are homozygous for shortness are crossed. What percentage of the offspring will be tall?
100%
50% 0%
25% 7
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32. 8 The Punnett square allows you to predict
only the phenotypes of the offspring from a cross.
only the genotypes of the offspring from a cross.
both the genotypes and the phenotypes from a cross.
neither the genotypes nor the phenotypes from a cross. 8
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33. 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.
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34. Independent Assortment
The Dihybrid Cross:  
Mendel crossed true-breeding plants that produced round yellow peas (genotype RRYY) with true-breeding plants that produced wrinkled green peas (genotype rryy).
All of the F1 offspring produced round yellow peas (RrYy).
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35. Independent Assortment
The alleles for round (R) and yellow (Y) are dominant over the alleles for wrinkled (r) and green (y).
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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36. Independent Assortment
Mendel crossed the heterozygous F1 plants (RrYy) with each other to determine if the alleles would segregate from each other in the F2 generation.
RrYy × RrYy
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37. Gamete formation AB Ab aB AaBbCc AaBb ab ABC aBC ABc aBc AbC abC Abc

38. Independent Assortment
The Punnett square predicts a 9 : 3 : 3 :1 ratio in the F2 generation.
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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39. Law of Independent Assortment
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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40. 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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41. 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.
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42. A Summary of Mendel‘s Principles
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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43. 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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44. Beyond Dominant and Recessive Alleles
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.
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45. Beyond Dominant and Recessive Alleles
A cross between red (RR) and white (WW) four o’clock plants produces pink-colored flowers (RW).
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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46. Beyond Dominant and Recessive Alleles
Codominance 
In codominance, both alleles contribute to the phenotype.
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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47. Codominance
In chickens, black feather color (BB) is codominant to white feather color (WW). Both feather colors show up in a checkered pattern in the heterozygous individual (BW). Cross a checkered hen with a checkered rooster. What are the genotypic and phenotypic ratios?

48. Codominance B W B W Feather colors in chickens
Black (BB) x White (WW) = Black and White checkered Chicken B W BB BW B BW WW W

49. Multiple Alleles
Some genes may have more than just 2 alternate forms of a gene.
Example: ABO blood groups
A and B refer to 2 genetically determined polysaccharides (A and B antigens) which are found on the surface of red blood cells (different from MN blood groups)
A and B are codominant; O is recessive to A and B

50. Multiple Alleles for the ABO Blood Groups
3 alleles: IA, IB, i
Figure 9.18

51. Blood Types The immune system produces blood proteins
That may cause clotting when blood cells of a different type enter the body.
Figure 9.19

52. Example of ABO Blood Groups
A man with type A blood whose father had type O marries a woman with type AB. What are the possible blood types of their children? IA IB IA
IA IA
IA IB
IA i
IB i i
IAi
IAIB X
½ type A : ¼ type AB : ¼ type B

53. Beyond Dominant and Recessive Alleles
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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54. Applying Mendel's Principles
Thomas Hunt Morgan used fruit flies to advance the study of genetics.
Morgan and others tested Mendel’s principles and learned that they applied to other organisms as well as plants.
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55. Applying Mendel‘s Principles
Mendel’s principles can be used to study inheritance of human traits and to calculate the probability of certain traits appearing in the next generation.
Genetics and the Environment
Characteristics of any organism are determined by the interaction between genes and the environment.
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56. In a cross involving two pea plant traits, observation of a 9 : 3 : 3 : 1 ratio in the F2 generation is evidence for
the two traits being inherited together.
an outcome that depends on the sex of the parent plants.
the two traits being inherited independently of each other.
multiple genes being responsible for each trait.
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57. Traits controlled by two or more genes are called
multiple-allele traits.
polygenic traits.
codominant traits.
hybrid traits.
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58. In four o'clock flowers, the alleles for red flowers and white flowers show incomplete dominance. Heterozygous four o'clock plants have
pink flowers.
white flowers.
half white flowers and half red flowers.
red flowers.
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59. A white male horse and a tan female horse produce an offspring that has large areas of white coat and large areas of tan coat. This is an example of
incomplete dominance.
multiple alleles.
codominance.
a polygenic trait.
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60. Mendel's principles apply to
pea plants only.
fruit flies only.
all organisms.
only plants and animals.
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