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General Biology II: 12.4 Monohybrid Cross and the Punnett Square

General Biology II
12.4 Monohybrid Cross and the Punnett Square
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table of contents
  1. Cover
  2. Title Page
  3. Copyright
  4. Table Of Contents
  5. Reference Information
  6. The Process of Science
  7. 3. Biological Molecules
  8. 4. Structure of DNA
  9. 5. DNA Replication
  10. 6. Protein Synthesis
    1. 6.1 What are proteins and what do they do?
    2. 6.2 What is a gene?
    3. 6.3 How do genes direct the production of proteins?
    4. 6.4 Transcription: from DNA to mRNA
    5. 6.5 Eukaryotic RNA Processing
    6. 6.6 Translation
    7. 6.7 The Genetic Code
    8. Optional Section - Micropigs
  11. 7. Mutations
    1. How Gene Mutations Occur
    2. Intro to Genetic Disorders
    3. Do all gene affect health and development?
    4. Types of Mutations
    5. Changes in Numbers of Genes
    6. Changes in Chromosome Number
    7. Complex Multifactorial Disorders
    8. Genetic Predispositions
    9. Genetics and Statistics
  12. Gene Regulation
    1. 8.1 Prokaryotic versus Eukaryotic Gene Expression
    2. 8.2 What is the epigenome?
    3. 8.3 Alternative RNA splicing
  13. 9. Biotechnology
    1. 9.1 Manipulating Genetic Material
    2. 9.2 Cloning
    3. 9.3 Genetic Engineering
    4. 9.4 Biotechnology in Medicine and Agriculture
    5. 9.5 Genomics and Proteomics
    6. 9.6 Applying Genomics
    7. 9.7 Proteomics
  14. 10. Cell Division - Binary Fission and Mitosis
    1. 10.1 Prokaryotic Cell Division
    2. 10.2 Eukaryotic Cell Division
    3. 10.3 Control of the Cell Cycle
    4. 10.4 Cancer and the Cell Cycle
  15. 11. Meiosis
    1. 11.1 Sexual Reproduction
    2. 11.2 Overview of Meiosis
    3. 11.3 Interphase
    4. 11.4 Meiosis I
    5. 11.5 Meiosis II
    6. 11.6 Comparing Meiosis and Mitosis
    7. 11.7 Errors in Meiosis
  16. 12. Patterns of Inheritance
    1. 12.1 Mendelian Genetics
    2. 12.2 Garden Pea Characteristics Revealed the Basics of Heredity
    3. 12.3 Phenotypes and Genotypes
    4. 12.4 Monohybrid Cross and the Punnett Square
    5. 12.5 Laws of Inheritance
    6. 12.6 Extensions of the Laws of Inheritance
    7. 12.7 Multiple Alleles
    8. 12.8 Sex-Linked Traits
    9. 12.9 Linked Genes Violate the Law of Independent Assortment
    10. 12.10 Epistasis
  17. Genetics: Dog Coat Color
    1. Introduction to Genetics
    2. Pedigrees and Punnett Squares
    3. Black fur color: a dominant trait
    4. Yellow fur color: a recessive trait
    5. Epistasis: the relationship between black, brown, and yellow fur
    6. Brindle color: partial dominance and epistasis
    7. Incomplete dominance: when traits blend
    8. White spotting: When there's more than two alleles
    9. Hemophilia: a sex-linked disorder
    10. Overall phenotypes: putting it all together
    11. Additional complexity
    12. It's not all in the genes

12.4 Monohybrid Cross and the Punnett Square

When fertilization occurs between two true-breeding parents that differ by only the characteristic being studied, the process is called a monohybrid cross, and the resulting offspring are called monohybrids. Mendel performed seven types of monohybrid crosses, each involving contrasting traits for different characteristics. Out of these crosses, all of the F1 offspring had the phenotype of one parent, and the F2 offspring had a 3:1 phenotypic ratio. On the basis of these results, Mendel postulated that each parent in the monohybrid cross contributed one of two paired unit factors to each offspring, and every possible combination of unit factors was equally likely.

The results of Mendel’s research can be explained in terms of probabilities, which are mathematical measures of likelihood. The probability of an event is calculated by the number of times the event occurs divided by the total number of opportunities for the event to occur. A probability of one (100 percent) for some event indicates that it is guaranteed to occur, whereas a probability of zero (0 percent) indicates that it is guaranteed to not occur, and a probability of 0.5 (50 percent) means it has an equal chance of occurring or not occurring.

To demonstrate this with a monohybrid cross, consider the case of true-breeding pea plants with yellow versus green seeds. The dominant seed color is yellow; therefore, the parental genotypes were YY for the plants with yellow seeds and yy for the plants with green seeds. A Punnett square, devised by the British geneticist Reginald Punnett, is useful for determining probabilities because it is drawn to predict all possible outcomes of all possible random fertilization events and their expected frequencies. Figure 7 shows a Punnett square for a cross between a plant with yellow peas and one with green peas. To prepare a Punnett square, all possible combinations of the parental alleles (the genotypes of the gametes) are listed along the top (for one parent) and side (for the other parent) of a grid. The combinations of egg and sperm gametes are then made in the boxes in the table on the basis of which alleles are combining. Each box then represents the diploid genotype of a zygote, or fertilized egg. Because each possibility is equally likely, genotypic ratios can be determined from a Punnett square. If the pattern of inheritance (dominant and recessive) is known, the phenotypic ratios can be inferred as well. For a monohybrid cross of two true-breeding parents, each parent contributes one type of allele. In this case, only one genotype is possible in the F1 offspring. All offspring are Yy and have yellow seeds.

10i.monohybrid
Figure 7: This Punnett square shows the cross between plants with yellow seeds and green seeds. The cross between the true-breeding P plants produces F1 heterozygotes that can be self-fertilized. The self-cross of the F1 generation can be analyzed with a Punnett square to predict the genotypes of the F2 generation. Given an inheritance pattern of dominant-recessive, the genotypic and phenotypic ratios can then be determined.

When the F1 offspring are crossed with each other, each has an equal probability of contributing either a Y or a y to the F2 offspring. The result is a 1 in 4 (25 percent) probability of both parents contributing a Y, resulting in an offspring with a yellow phenotype; a 25 percent probability of parent A contributing a Y and parent B a y, resulting in offspring with a yellow phenotype; a 25 percent probability of parent A contributing a y and parent B a Y, also resulting in a yellow phenotype; and a (25 percent) probability of both parents contributing a y, resulting in a green phenotype. When counting all four possible outcomes, there is a 3 in 4 probability of offspring having the yellow phenotype and a 1 in 4 probability of offspring having the green phenotype. This explains why the results of Mendel’s F2 generation occurred in a 3:1 phenotypic ratio. Using large numbers of crosses, Mendel was able to calculate probabilities, found that they fit the model of inheritance, and use these to predict the outcomes of other crosses.

References

Unless otherwise noted, images on this page are licensed under CC-BY 4.0 by OpenStax.

OpenStax, Biology. OpenStax CNX. May 27, 2016 http://cnx.org/contents/GFy_h8cu@10.57:4qg08nt-@8/Characteristics-and-Traits

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Copyright © 2016 by Lisa Bartee and Christine Anderson. Mt Hood Community College Biology 102 by Lisa Bartee and Christine Anderson is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.
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