Saturday, 30 January 2016

MENDELIAN INHERITANCE

MENDELIAN INHERITANCE


·         An appreciation for the concept of heredity can be traced far back in human history. Hippocrates, a famous Greek physician, was the first person to provide an explanation for hereditary traits (ca. 400 b.c.e.).

·         He suggested that “seeds” are produced by all parts of the body, which are then collected and transmitted to the offspring at the time of conception. Furthermore, he hypothesized that these seeds cause certain traits of the offspring to resemble those of the parents.

This idea, known as pangenesis, was the first attempt to explain the transmission of hereditary traits from generation to generation


·         For the next 2000 years, the ideas of Hippocrates were accepted by some and rejected by many.

·         After the invention of the microscope in the late seventeenth century, some people observed sperm and thought they could see a tiny creature inside, which they termed a homunculus (little man). This homunculus was hypothesized to be a miniature human waiting to develop within the womb of  its mother.
·         Those who held that thought, known as spermists, suggested that only the father was responsible for creating future generations and that any resemblance between mother and offspring was due to influences “within the womb.”

·         During the same time, an opposite school of thought also developed.

·         According to the ovists, the egg was solely responsible for human characteristics The only role of the sperm was to stimulate the egg onto its path of development.

·         Of course, neither of these ideas was correct. The first systematic studies of genetic crosses were carried out by Joseph Kölreuter from 1761 to 1766. In crosses between different strains of tobacco plants, he found that the offspring were usually intermediate in appearance between the two parents.

·         This led Kölreuter to conclude that both parents make equal genetic contributions to their offspring. Furthermore, his observations were consistent with blending inheritance.

·         According to this view, the factors that dictate hereditary traits can blend together from generation to generation.

·         The blended traits would then be passed to the next generation. The popular view before the 1860s, which combined the notions of pangenesis and blending inheritance, was that hereditary traits were rather malleable and could change and blend over the course of one or two generations.

·         However, the pioneering work of Gregor Mendel would prove instrumental in refuting this viewpoint.

·         In Chapter 2, we will first examine the outcome of Mendel’s crosses in pea plants. We begin our inquiry into genetics here because the inheritance patterns observed in peas are fundamentally related to inheritance patterns found in other eukaryotic species, such as humans, mice, fruit flies, and corn.

·         We will discover how Mendel’s insights into the patterns of inheritance in pea plants revealed some simple rules that govern the process of inheritance.

·         In Chapters 3 through 8 , we will explore more complex patterns of inheritance and also consider the role that chromosomes play as the carriers of the genetic material. In the second part of this chapter, we will become familiar
·         with general concepts in probability and statistics. How are statistical methods useful? First, probability calculations allow us to predict the outcomes of simple genetic crosses, as well as the outcomes of more complicated crosses described in later chapters.

·         In addition, we will learn how to use statistics to test the validity of genetic hypotheses that attempt to explain the inheritance patterns of traits.