Question

In this chapter, we focused on the Mendelian postulates, probability, and pedigree analysis. We also considered some of the methods and reasoning by which these ideas, concepts, and techniques were developed. On the basis of these discussions, what answers would you propose to the following questions: (a) How was Mendel able to derive postulates concerning the behavior of "unit factors" during gamete formation, when he could not directly observe them? (b) How do we know whether an organism expressing a dominant trait is homozygous or heterozygous? (c) In analyzing genetic data, how do we know whether deviation from the expected ratio is due to chance rather than to another, independent factor? (d) since experimental crosses are not performed in humans, how do we know how traits are inherited?

Short answer

Answer: To determine whether an organism expressing a dominant trait is homozygous or heterozygous for that trait, we perform a test cross. This involves crossing the organism with an individual that is homozygous recessive for the trait. If the offspring all express the dominant trait, then the original organism is most likely homozygous dominant. However, if there are offspring that express both the dominant and the recessive traits, then the original organism must be heterozygous.

Step-by-step solution

Mendel derived his postulates concerning the behavior of "unit factors" (now known as genes) during gamete formation through a series of well-designed experiments on pea plants. He observed traits in the pea plants, such as seed shape and flower color, and cross-bred the plants to track how these traits were inherited across generations. By analyzing the results of these crosses, he formulated laws of inheritance that described the behavior of these unit factors even though he could not directly observe them. #b) Determining Homozygous or Heterozygous#

In order to determine whether an organism expressing a dominant trait is homozygous (AA) or heterozygous (Aa) for that trait, we perform a test cross. This involves crossing the organism with an individual that is homozygous recessive (aa) for the trait. If the offspring all express the dominant trait, then the original organism is most likely homozygous dominant. However, if there are offspring that express both the dominant and the recessive traits, then the original organism must be heterozygous. #c) Deviation from Expected Ratio#

In analyzing genetic data, we can use the chi-squared test to determine whether the deviation from the expected ratio is due to chance or another independent factor. The chi-squared test compares the observed frequency of each phenotype in the offspring to the expected frequency based on Mendelian ratios. If the calculated chi-squared value is small, it means that the deviation from the expected ratio is likely due to chance or random sampling error. If the chi-squared value is large, it suggests that there may be another independent factor affecting the inheritance of the traits. #d) Inheritance in Humans#

Since experimental crosses cannot be performed in humans for ethical reasons, we study inheritance patterns through pedigree analysis. A pedigree is a family tree that indicates the presence or absence of a trait in each family member across generations. By examining the pattern of inheritance of a trait in a pedigree, we can make inferences about the possible genotypes of individuals and how traits are inherited. Additionally, geneticists can use the principles of Mendelian inheritance, probability, and statistical analysis to predict the likelihood of certain traits being passed on to offspring.