11.2 Applying Mendel’s Principles
Probability and Punnett Squares
1. What is probability? Probability is the likelihood that a certain event will occur
2. In a parent pea plant with the allele pair Gg, what is the probability that one gamete will contain the G allele? ½ or 50%
3. Complete the graphic organizer to define the characteristics of homozygous and heterozygous genotypes and phenotypes.
Both alleles are the same
The alleles are different
The phenotype depends on whether both alleles are recessive or dominant The phenotype will be that produced by the dominant allele
4. The dominant allele for smooth pod shape in peas is S. The recessive allele for constricted pod shape is s. In the Punnett square, show the result of crossing two heterozygous parents (Ss). Write the genotype and the phenotype of each type of offspring in the space provided.
For Questions 5–9, refer to the Punnett square above.
5. What is the probability of a heterozygous offspring? Explain your answer. Probability is 1/2
6. What is the probability of a homozygous offspring? Explain. 1/2
7. What is the probability of a homozygous recessive offspring? 1/4
8. What is the probability of a smooth phenotype?
9. What is the probability of a homozygous recessive individual (ss) producing a gamete with a dominant allele (S)? Explain. ZERO because the individual did not inherit a dominant allele
10. State the principle of independent assortment below.
Genes for different traits segregate independently during the formation of gametes
11. Using the principle of independent assortment, complete the Punnett square to show the results of an F1 cross between two individuals heterozygous for both pod color (C = green and c = yellow) and pod shape (S = smooth and s + constricted). The gametes and some of the genotypes of the F2 offspring are given.
For Questions 12–15, refer to the Punnett square above.
12. Which genotype belongs to an offspring that is homozygous recessive for both traits? What is the probability of that genotype? The genotype is ccss and the probability is 1/16
13. What is the phenotype of an individual heterozygous for both traits? The phenotype is green pod color and smooth pod shape
14. What is the probability of an F2 offspring having the green pod color and smooth pod shape? Explain. (Note: Remember that more than one genotype can produce this phenotype.) Th)e probability is 9/16 (the combined probability of CCSS,CcSs, CCSs and CcSs
15. The Punnett square predicts a 9:3:3:1 ratio for phenotypes. Explain what that ratio means. It means that the probability of an offspring that exhibits both dominant traits (green pod and smooth pod shape) is 9/16. The probability of one dominant and one recessive trait (either greed pod color and constricted shape OR yellow pod and smooth shape) is 3/16. The probability of both recessive traits showing in the phenotype (yellow color and constricted shape) is 1/16.
Summary of Mendel’s Principles
For Questions 16–20, complete each statement by writing the correct word or words 16. The units that determine the inheritance of biological characteristics are genes. 17. A form of a gene is a(n)allele.
18. If two or more forms of a gene exist, some may be dominant and others may be recessive. 19. The offspring of most sexually reproducing organisms have two copies of each gene. One came from each parent. 20. Alleles from different genes usually segregateindependently from each other when gametes form.
For Questions 21–25, match the term with its description.
E 21. Determine traitsA. parents
B 22. Can be two of these in one geneB. alleles
C 23. Allele that is expressedC. dominant
A 24. Where genes come fromD. segregate
D 25. What genes do during gamete formationE. genes
26.Explain the importance of Thomas Hunt Morgan’s experiments with fruit flies. Why was his work an important addition to Mendel’s research? He experimented with fruit flies instead of plants. He was the first to discover that Mendel’s principles apply to organisms other than plants
27.Four sisters begin attending your school. One has brown hair and brown eyes. Another has brown hair and blue eyes. The third also has blue eyes, but blond hair. The fourth has blond hair, too, but she has brown eyes. Explain how the principle of independent segregation accounts for these sisters having four different phenotypes for two traits. If their parents are heterozygous for both traits, the genes and alleles segregate independently, resulting in all possible phenotypes combinations: dominant-dominant, dominant-recessive, recessive-dominant, and recessive-recessive