Saturday, 30 January 2016

MENDEL’S LAWS OF INHERITANCE


MENDEL’S LAWS OF INHERITANCE

 


Gregor Johann Mendel, born in 1822, is now remembered as the father of genetics  .


He grew up on a small farm in Hyncice (formerly Heinzendorf) in northern Moravia, which
was then a part of Austria and is now a part of the Czech Republic.

As a young boy, he worked with his father grafting trees to improve the family orchard. Undoubtedly, his success at grafting taught him that precision and attention to detail are important elements of success.

These qualities would later be important in his experiments as an adult scientist. Instead of farming, however,
Mendel was accepted into the Augustinian monastery of St. Thomas, completed his studies for the priesthood, and was ordained in 1847.

 Soon after becoming a priest, Mendel worked for a short time as a substitute teacher. To continue that role, he
needed to obtain a teaching license from the government. Surprisingly, he failed the licensing exam due to poor answers in the areas of physics and natural history.

Therefore, Mendel then enrolled at the University of Vienna to expand his knowledge in these two areas.

 Mendel’s training in physics and mathematics taught him to perceive the world as an orderly place, governed
by natural laws. In his studies, Mendel learned that these natural laws could be stated as simple mathematical relationships.

In 1856, Mendel began his historic studies on pea plants. For 8 years, he grew and crossed thousands of pea plants on a small 115- by 23-foot plot.

 He kept meticulously accurate records that included quantitative data concerning the outcome of his
crosses.

He published his work, entitled “Experiments on Plant Hybrids,” in 1866. This paper was largely ignored by scientists at that time, possibly because of its title.

 Another reason his work went unrecognized could be tied to a lack of understanding of chromosomes and their transmission,   Nevertheless, Mendel’s ground-breaking work allowed  him to propose the natural laws that now provide a framework for our understanding of genetics.

Prior to his death in 1884, Mendel reflected, “My scientific work has brought me a great deal of satisfaction and I am convinced that it will be appreciated before long by the whole world.”

Sixteen years later, in 1900, the work of Mendel was independently rediscovered by three biologists with an interest in plant genetics: Hugo de Vries of Holland, Carl Correns of Germany, and Erich von Tschermak of Austria. Within a few years, the influence of Mendel’s studies was felt around the world.

In this section, we will examine Mendel’s experiments and consider their monumental significance in the field of genetics.

Mendel Chose Pea Plants as His Experimental Organism


Mendel’s study of genetics grew out of his interest in ornamental flowers.

Prior to his work with pea plants, many plant breeders had conducted experiments aimed at obtaining flowers with new varieties of colors.

When two distinct individuals with different characteristics are mated, or crossed, to each other, this is called
a hybridization experiment, and the offspring are referred to as hybrids. For example, a hybridization experiment could involve a cross between a purple-flowered plant and a white-flowered plant. Mendel was particularly intrigued, in such experiments, by the consistency with which offspring of subsequent generations showed characteristics of one or the other parent. His intellectual foundation in physics and the natural sciences led him to



Structure of a pea flower



Pollination and fertilization in angiosperms


Pollination and fertilization in angiosperms




Flower structure and pollination in pea plants.  The pea flower can produce both pollen and egg cells. The pollen grains are produced within the anthers, and the egg cells are produced within the ovules that are contained within the ovary. A modified petal called a keel encloses the anthers and ovaries. (b) Photograph of a flowering pea plant.

 A pollen grain must first land on the stigma. After this occurs, the pollen sends out a long tube through which two sperm cells travel toward an ovule to reach an egg cell.

The fusion between a sperm and an egg cell results in fertilization and creates a zygote. A second sperm fuses with a central cell containing two polar nuclei to create the endosperm. The endosperm provides a nutritive material for the developing embryo.

consider that this regularity might be rooted in natural laws that could be expressed mathematically. To uncover these laws, he realized that he would need to carry out quantitative experiments in which the numbers of offspring carrying certain traits were carefully recorded and analyzed. Mendel chose the garden pea, Pisum sativum, to investigate the natural laws that govern plant hybrids. The morphological features of this plant are shown in Figure 

Several properties of this species were particularly advantageous for studying plant hybridization. First, the species was available in several varieties that had decisively different physical characteristics.

Many strains of the garden pea were available that varied in the appearance of their height, flowers, seeds, and pods. A second important issue is the ease of making crosses. In flowering plants, reproduction occurs by a pollination event

 Male gametes (sperm) are produced within pollen grains formed in the anthers, and the female gametes (eggs) are contained within ovules that form in the ovaries. For fertilization to occur, a pollen grain lands on the stigma, which stimulates the growth of a pollen tube.

This enables sperm cells to enter the stigma and migrate toward an ovule. Fertilization occurs when a sperm enters the micropyle, an opening in the ovule wall, and fuses with an egg cell. The term gamete is used to describe haploid reproductive cells that can unite to form a zygote.

 It should be emphasized, however, that the process that produces gametes in animals is quite different from the way that gametes are produced in plants and fungi.

In some experiments, Mendel wanted to carry out self fertilization, which means that the pollen and egg are derived from the same plant. In peas, a modified petal known as the keel covers the reproductive structures of the plant.

Because of this covering, pea plants naturally reproduce by self-fertilization. Usually, pollination occurs even before the flower opens. In other experiments, however, Mendel wanted to make crosses between different plants.

 How did he accomplish this goal? Fortunately, pea plants contain relatively large flowers that are easy to manipulate, making it possible to make crosses between two particular plants and study their outcomes.

 This process, known as cross-fertilization, requires that the pollen from one plant be placed on the stigma of another plant.

 This procedure is shown in Figure  Mendel was able to pry open immature flowers and remove the anthers before they produced pollen. Therefore, these flowers could not self- fertilize.

He would then obtain pollen from another plant by gently touching its mature anthers with a paintbrush. Mendel applied this pollen to the stigma of the flower that already had its anthers removed.

In this way, he was able to cross-fertilize his pea plants and thereby obtain any type of hybrid he wanted.


cross-fertilized two different pea plants



How Mendel cross-fertilized two different pea plants.

 This illustration depicts a cross between a plant with purple flowers and another plant with white flowers. The offspring from this cross are the result of pollination of the purple flower using pollen from a white flower.

Mendel Studied Seven Characteristics That Bred True


When he initiated his studies, Mendel obtained several varieties of peas that were considered to be distinct. These plants were different with regard to many morphological characteristics.

 The general characteristics of an organism are called characters. The terms trait and variant are typically used to describe the specific properties of a character.

 For example, eye color is a character of humans and blue eyes is a trait (or variant) found in some people.
Over the course of 2 years, Mendel tested his pea strains to determine if their characteristics bred true.

 This means that a trait did not vary in appearance from generation to generation. For example,
if the seeds from a pea plant were yellow, the next generation would also produce yellow seeds. Likewise, if these offspring were allowed to self-fertilize, all of their offspring would also produce yellow seeds, and so on.

 A variety that continues to produce the same trait after several generations of self-fertilization is called a
true-breeding line, or strain.

Mendel next concentrated his efforts on the analysis of characteristics that were clearly distinguishable between different true-breeding lines. Figure  illustrates the seven character that Mendel eventually chose to follow in his breeding experiments.

All seven were found in two variants. A variant (or trait) may be found in two or more versions within a single species. For example, one character he followed was height, which was found in two variants: tall and dwarf plants. Mendel studied this character by crossing the variants to each other.

A cross in which an experimenter is observing only one character is called a monohybrid
cross, also called a single-factor cross.

 When the two parents are different variants for a given character, this type of cross produces single-character hybrids, also known as monohybrids.




An illustration of the seven characters that Mendel studied. Each character was found as two variants that were decisively different from each other.





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.