Table of contents

1. Introduction to Biology2h 42m
2. Chemistry3h 37m
3. Water1h 26m
4. Biomolecules2h 23m
5. Cell Components2h 26m
6. The Membrane2h 31m
7. Energy and Metabolism2h 0m
8. Respiration2h 40m
9. Photosynthesis2h 49m
10. Cell Signaling59m
11. Cell Division2h 47m
12. Meiosis2h 0m
13. Mendelian Genetics4h 44m
14. DNA Synthesis2h 27m
15. Gene Expression3h 6m
16. Regulation of Expression3h 31m
17. Viruses37m
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22. Evolution of Populations3h 53m
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24. History of Life on Earth2h 6m
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26. Prokaryotes4h 59m
27. Protists1h 12m
28. Plants1h 22m
29. Fungi36m
- Fungi
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30. Overview of Animals34m
- Overview of Animals
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31. Invertebrates1h 2m
32. Vertebrates50m
33. Plant Anatomy1h 3m
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35. Soil37m
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  20m
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  16m
36. Plant Reproduction47m
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  33m
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  14m
37. Plant Sensation and Response1h 9m
38. Animal Form and Function1h 19m
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- Digestion
  1h 3m
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  7m
40. Circulatory System1h 49m
41. Immune System1h 12m
42. Osmoregulation and Excretion50m
- Osmoregulation and Excretion
  50m
43. Endocrine System1h 4m
- Endocrine System
  1h 4m
44. Animal Reproduction1h 2m
- Animal Reproduction
  1h 2m
45. Nervous System1h 55m
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  1h 8m
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  46m
46. Sensory Systems46m
- Sensory System
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47. Muscle Systems23m
- Musculoskeletal System
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48. Ecology3h 11m
49. Animal Behavior28m
- Animal Behavior
  28m
50. Population Ecology3h 41m
51. Community Ecology2h 46m
52. Ecosystems2h 36m
53. Conservation Biology24m
- Conservation Biology
  24m

13. Mendelian Genetics

Punnett Square Probability

General Biology

13. Mendelian Genetics

Punnett Square Probability

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2:03 minutes

Problem 17

Textbook Question

The breakthrough that led Bateson and Punnett to recognize the existence of linked genes was the appearance of unexpected results after they crossed double heterozygous pea plants (PpLl) with each other. Imagine that you have a group of Labrador retrievers that are all heterozygous for both coat color and blindness (BbNn). If you used this group of dogs to produce 160 puppies, how many puppies of each phenotype do you expect to get if the genes are not linked? How would the results differ if the genes are in fact linked?

Verified step by step guidance

Step 1: Understand the problem. The problem involves two genes (B and N) in Labrador retrievers, each with two alleles. B (dominant) determines coat color, and N (dominant) determines blindness. The dogs are heterozygous for both traits (BbNn). You are tasked with predicting the phenotypic ratios of 160 puppies under two scenarios: (1) the genes are not linked, and (2) the genes are linked.

Step 2: For the unlinked genes scenario, apply Mendel's law of independent assortment. This means the alleles for coat color (B and b) and blindness (N and n) segregate independently during gamete formation. Use a dihybrid Punnett square to determine the phenotypic ratios. The phenotypes are: (1) Black coat, not blind (B_N_), (2) Black coat, blind (B_nn), (3) Brown coat, not blind (bbN_), and (4) Brown coat, blind (bbnn).

Step 3: Calculate the expected phenotypic ratios for unlinked genes. From the Punnett square, the phenotypic ratio for a dihybrid cross (BbNn x BbNn) is 9:3:3:1. This means 9/16 of the offspring will have the first phenotype, 3/16 the second, 3/16 the third, and 1/16 the fourth. Multiply these fractions by the total number of puppies (160) to determine the expected number of puppies for each phenotype.

Step 4: For the linked genes scenario, consider that the genes are located close together on the same chromosome and are inherited together unless crossing over occurs. This would alter the phenotypic ratios. If no crossing over occurs, only the parental phenotypes (Black coat, not blind and Brown coat, blind) will be observed. If crossing over occurs, recombinant phenotypes (Black coat, blind and Brown coat, not blind) will also appear, but in lower frequencies. The exact ratios depend on the recombination frequency, which is not provided in the problem.

Step 5: Summarize the differences. For unlinked genes, the phenotypic ratios follow the 9:3:3:1 pattern. For linked genes, the ratios deviate from this pattern, with more parental phenotypes and fewer recombinant phenotypes. To calculate exact numbers for the linked scenario, additional information about the recombination frequency would be needed.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Mendelian Genetics

Mendelian genetics is the study of how traits are inherited through generations based on the principles established by Gregor Mendel. It involves understanding dominant and recessive alleles, as well as the segregation and independent assortment of genes during gamete formation. This framework allows us to predict the phenotypic ratios of offspring from parental genotypes, particularly in dihybrid crosses.

Phenotypic Ratios

Phenotypic ratios represent the relative frequencies of different phenotypes in the offspring resulting from a genetic cross. In a dihybrid cross of two heterozygous individuals (BbNn), the expected phenotypic ratio for unlinked genes is typically 9:3:3:1. This ratio helps in predicting the distribution of traits, such as coat color and blindness in the puppies, based on the combinations of alleles inherited from the parents.

Linked Genes

Linked genes are genes that are located close to each other on the same chromosome and tend to be inherited together during meiosis. When genes are linked, the expected phenotypic ratios deviate from those predicted by Mendelian genetics, as they do not assort independently. This can lead to fewer recombinant phenotypes in the offspring, affecting the overall distribution of traits, such as coat color and blindness in the Labrador puppies.

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Textbook Question

Both Tim and Jan have a widow's peak (see Module 9.8), but Mike has a straight hairline.

What are their genotypes?

What is the probability that Tim and Jan's next child will have freckles and a straight hairline?

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Textbook Question

What is the probability that each of the following pairs of parents will produce the indicated offspring? (Assume independent assortment of all gene pairs.)

a. AABBCC×aabbcc→AaBbCc

b. AABbCc×AaBbCc→AAbbCC

c. AaBbCc×AaBbCc→AaBbCc

d. aaBbCC×AABbcc→AaBbCc

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Textbook Question

In rabbits, black hair depends on a dominant allele, B, and brown hair on a recessive allele, b. Short hair is due to a dominant allele, S, and long hair to a recessive allele, s. If a true-breeding black short-haired male is mated with a brown long-haired female, describe their offspring. What will be the genotypes of the offspring? If two of these F1 rabbits are mated, what phenotypes would you expect among their offspring? In what proportions?

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Textbook Question

In fruit flies, the genes for wing shape and body stripes are linked. In a fly whose genotype is WwSs, W is linked to S, and w is linked to s. Show how this fly can produce gametes containing four different combinations of alleles.

Which are parental-type gametes?

Which are recombinant gametes? How are the recombinants produced?

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Multiple Choice

In a Punnett square where both parents are heterozygous for attached earlobes (Ee), what is the probability that their offspring will have unattached earlobes?

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Multiple Choice

Why can the principles of probability be used to predict the outcomes of genetic crosses?

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