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2g. Know how to predict possible combinations of alleles in a zygote from the genetic makeup of the parents. 1

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**Punnett’s Squares Reginald Punnett: (1905) English ___________**

Devised a short- hand way of finding the expected proportions of possible ___________ in the offspring of a cross Good for showing all the possible combinations of ______________ and the likelihood that each will occur Punnett’s Squares 2

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**11-2 Probability and Punnett Squares**

photo credit: W. Perry Conway/CORBIS 3

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**Genetics and Probability**

How do geneticists use the principles of probability? Genetics and Probability The likelihood that a particular event will occur is called _______________________. The principles of probability can be used to predict the outcomes of ____________ _________________. 4

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**Punnett Squares How do geneticists use Punnett squares?**

The gene combinations that might result from a genetic cross can be determined by drawing a diagram known as a _______________________. Punnett squares can be used to predict and compare the genetic _______________ that will result from a cross. 5

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Punnett Squares A capital letter represents the __________ allele for tall. A lowercase letter represents the ______________ allele for short. In this example, T = _____ t = ______ 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. 6

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**BABY STEPS: 1. determine the genotypes of the parent organisms 2**

BABY STEPS: 1. determine the genotypes of the parent organisms 2. write down your "cross" (mating) 3. draw a p-square 4. "split" the letters of the genotype for each parent & put them "outside" the p-square 5. determine the possible genotypes of the offspring by filling in the p-square 6. summarize results (genotypes & phenotypes of offspring) 7. bask in the glow of your accomplishment !

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Punnett Squares _______________ produced by each F1 parent are shown along the top and left side. 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. 8

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Punnett Squares 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. 9

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Punnett Squares Organisms that have two identical alleles for a particular trait are said to be _________________. (TT, tt) Organisms that have two different alleles for the same trait are ____________________. (Tt) ________________ organisms are true- breeding for a particular trait. ________________ organisms are hybrid for a particular trait. 10

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Punnett Squares All of the tall plants have the same _________________, or physical characteristics. The tall plants do not have the same ____________________, or genetic makeup. One third of the tall plants are TT, while two thirds of the tall plants are Tt. 11

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Punnett Squares The plants have different _______________ (TT and Tt), but they have the same ______________(tall). Although these plants have different genotypes (TT and Tt), they have the same phenotype (tall). TT Homozygous Tt Heterozygous 12

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**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. 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. 13

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**Probability and Segregation**

Because the allele for tallness (T) is _______________ 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 ___:___. The predicted ratio showed up in Mendel’s experiments indicating that _______________ did occur. 14

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**Probabilities Predict Averages**

_____________________ predict the average outcome of a large number of events. Probability _______________ 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. 15

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