1. Whose genes are you wearing?
Ch 10 Section 2
Genetics

2. Probability & Punnett Squares
VOCABULARY:
Probability
Punnett Square
Homozygous
Heterozygous
Phenotype
Genotype
Dominant
Recessive
Allele
Law of segregation
Hybrid
Law of independent assortment
KEY CONCEPTS:
Describe technique used by Mendel to understand how traits are inherited.
Relate meiosis to the law of segregation.
Determine the probability of the phenotype and genotype for monohybrid and dihybrid Punnett square crosses.

3. Cross-pollinated pea plants, which normally self- fertilize
How Genetics Began
The passing of traits to the next generation is called inheritance, or heredity.
Gregor Mendel published his findings on the method of inheritance in garden pea plants:
Cross-pollinated pea plants, which normally self- fertilize
Rigorously followed various traits in the pea plants he bred
Began the study of genetics, the science of heredity.
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Mendelian Genetics

4. Mendel cross-bred the green and yellow seed plants.
The Inheritance of Traits
One trait Mendel noticed was seed color – some plants always produced green seeds, others always produced yellow seeds.
Mendel cross-bred the green and yellow seed plants.
Mendel called the green-seed and yellow-seed plants the parent, or P, generation.
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Mendelian Genetics

5. The Inheritance of Traits F1 and F2 generations
The offspring of this P cross are called the first filial (F1) generation.
The second filial (F2) generation is the offspring from the F1 cross.
In Mendel’s peas, the green-seed trait disappeared in the F1 generation, but reappeared in the F2 generation.
The F2 generation showed a 3:1 ratio of yellow: green seeds
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Mendelian Genetics

6. The Inheritance of Traits F1 and F2 generations
Mendel studied seven different traits.
Seed or pea color
Flower color
Seed pod color
Seed shape or texture
Seed pod shape
Stem length
Flower position
In all cases, Mendel found the F2 generation plants showed a 3:1 ratio of traits.
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Mendelian Genetics

7. The Inheritance of Traits
Genes in Pairs
Mendel concluded that there must be two forms of the seed trait in the pea plants, and that each was controlled by a factor.
An allele is an alternative form of a single gene.
The gene for yellow seeds and the gene for green seeds are different alleles for the same gene.
Dominant alleles controlled the traits that appeared in the F1 generation.
Recessive alleles were masked in the F1 generation.
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Mendelian Genetics

8. Genes are on chromosomes

9. Alleles

10. The Inheritance of Traits
Dominance
When modeling inheritance, the dominant allele is represented by a capital letter (Y), and a recessive allele is represented with a lower case letter (y).
An organism with two of the same alleles for a particular trait is homozygous for that trait (YY or yy).
An organism with two different alleles for a particular trait is heterozygous for that trait (Yy).
In heterozygous individuals, the dominant trait will be observed.
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Mendelian Genetics

11. Genotype and phenotype
The Inheritance of Traits
Genotype and phenotype
The appearance of an organism does not always indicate which pair of alleles it possesses.
An organism’s allele pairs are called its genotype. (the 2 alleles the organism inherited)
Ex: TT; Tt: tt
The observable characteristic or outward expression of an allele pair is called the phenotype. (what the organism looks like)
Ex: Tall; short
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Mendelian Genetics

12. 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

13. The Inheritance of Traits Mendel’s law of segregation
The law of segregation states that the two alleles for each trait separate during meiosis.
During fertilization, two alleles for that trait unite.
Heterozygous organisms are called hybrids.
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Mendelian Genetics

14. Segregation of gametes

15. A little background info…
Every time Mendel performed a cross with his pea plants, he carefully counted the offspring
Out of the over 20,000 plants he counted he noticed there was a pattern
Whenever he crossed 2 plants that were hybrid for stem height (Tt), about ¾ of the resulting plants were Tall and ¼ were short (3:1 ratio)
He realized that the principles of probability could be used to explain his results!

16. The Inheritance of Traits Monohybrid cross
A cross that involves hybrids for a single trait is called a monohybrid cross.
Dihybrid cross
The simultaneous inheritance of two or more traits in the same plant is a dihybrid cross.
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Mendelian Genetics

17. The Inheritance of Traits Law of independent assortment
The law of independent assortment states that random distribution of alleles occurs during gamete formation.
Genes on separate chromosomes sort independently during meiosis.
Each allele combination is equally likely to occur.
Copyright © McGraw-Hill Education
Mendelian Genetics

18. Punnett Squares
The gene combinations that MIGHT result from a genetic cross can be determined by drawing a Punnett Square
The parents go on the outside of the square along the side and top
The possible outcomes of the cross are on the inside of the square
The letters in the Punnett Square represent alleles

19. Alleles in a Punnett Square
“T” represents the Dominant allele: TALL
“t” represents the recessive allele: short
Organisms with 2 identical alleles (TT or tt) for a trait are said to be HOMOZYGOUS
Mendel called these: true-breeding
Organisms with 2 different alleles for a particular trait are said to be HETEROZYGOUS
Mendel called these hybrids

20. Punnett Squares
Punnett squares predict the possible offspring of a cross between two known genotypes.
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Mendelian Genetics

21. 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.

22. Punnett square—monohybrid cross
Punnett Squares
Punnett square—monohybrid cross
The number of squares is determined by the number of different types of alleles produced by each parent.
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Mendelian Genetics

23. Probability
Definition: the likelihood that a particular event will occur
Ex: flip a coin: ½ or 50% chance it will come up heads
If you flip a coin 3 times in a row, what is the probability it will come up heads 3 times in a row?
Each event is an independent therefore it is:
½ x ½ x ½ = 1/8
Past outcomes do not influence future outcomes
The principles of probability can be used to predict the outcomes of genetic crosses

24. Probabilities Predict Averages
Probabilities predict the outcomes of a LARGE number of events
Probabilities cannot predict precise outcomes of an event
Ex: flip a coin twice you may get 100% heads
You need to flip the coin many, many times to get close to 50%

25. 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.

26. 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.

27. What is the expected ratio for this cross? Tt x tt

28. Punnett Square Rules
1. Choose a letter to represent the dominant allele and capitalize it (choose a letter that is easy to distinguish between upper-case and lower-case).
2. Use the same letter but use lower case to represent the recessive allele.
3. Put the male on the left of the square and the female on the top.
4. Write "genotype" and "phenotype" below the square.
5. Use "x" to indicate a cross.
6. Write the dominant allele first if you are writing out a hybrid.

29. Punnett square—dihybrid cross
Punnett Squares
Punnett square—dihybrid cross
Four types of alleles from the male gametes and four types of alleles from the female gametes can be produced.
The resulting phenotypic ratio is 9:3:3:1.
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Mendelian Genetics

30. Probability
Mendel’s results were not exactly a 9:3:3:1 ratio, but the larger the number of offspring involved, the more likely it will match the results predicted by Punnett squares.
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Mendelian Genetics