Punnett Squares
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Name: _________________________ Date: ______ Period: ___
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Big Question:
A Punnett square is a chart that shows all the possible combinations of alleles that can result from a genetic cross (mating). Probability is the likelihood that a particular event will occur. In a genetic cross, the likelihood that an allele from each parent is passed on to its offspring is based on the laws of probability . We can use a Punnett square to calculate the probability that offspring with a certain combination of alleles will result. An organism’s genotype is its allele (gene) combination. An organism’s phenotype is its physical appearance or its visible traits. In co-dominance or blending , the alleles are neither dominant nor recessive -- both alleles are expressed in the offspring.
I) The Laws of Probability?
A) The laws of probability determine what is LIKELY to occur,
NOT what actually DOES occur.
B) The laws of probability apply to the allele that each parent will pass to its offspring.
C) Using the laws of probability we can determine the
CHANCES of offspring with certain traits to be born.
II) Punnett Square
A) A Punnett square is a table that shows ALL the possible combinations of alleles in offspring of parents that reproduce sexually. Using Punnett squares, geneticists can predict the probability of occurrence of a particular trait in the offspring.
B) Punnett squares Do NOT show what the next offspring will be.
They show the chances that the offspring will acquire certain traits from their parents.
III) Genotypes and Phenotypes
A) Genotype: Indicates the alleles that the organism has inherited.
Example : Bb, TT, ss.
B) Phenotype: Indicates the physical traits of the organism.
Example: Curly hair, tall, blue eyes
IV) Co-dominance or Blending
In co-dominance or blending, the alleles are neither dominant, nor recessive. Neither allele is masked by the other and both alleles are expressed in the offspring. Examples in humans: Blood type AB, sickle cell anemia.
Rev: 4/16/2020

Punnett Squares
1.
A number that describes how likely it is that a certain event will occur is called
______________________.
2.
A chart that shows all the possible allele combinations that can result from a genetic cross is called a(n) ______________________________.
3.
______________ indicates the alleles the organism has inherited from its parent(s).
4.
True or False? The laws of probability determine what will occur in a genetic cross.
5.
S s
S s
7.
True or False: Both parents in question 5 are homozygous.
6.
Bb Bb h bb bb
8.
If (S) is for smooth coat and (s) is for rough coat in guinea pigs in question 5, the probability that the offspring will have smooth coat is ______ percent.
9.
The probability of offspring in question 6 being heterozygous is __________________ percent.
10.
In question 6, if (B) is for Black coat in guinea pigs and (b) is for white coat, the chance of the offspring having black coat is ________ percent.
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Mini-Essay (10 pts) Read, Understand, and Answer: In pea plants, the allele for smooth pods (S) is dominant over the allele for pinched pods (s). Construct a Punnett square that shows a cross between a SS plant and a Ss plant. Predict what percent of the offspring are likely to have smooth pods.
Explain your answer.
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Self: 10 9 8 7 6 5 4 3 2 1 0
Honest and fair self-evaluation helps you improve.
Other: 10 9 8 7 6 5 4 3 2 1 0
You are evaluating the work of the person
NOT the person.
If your handwriting is hard to read, maximum score is 5. You can type your essay and paste in the space.
Rev: 4/16/2020