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Amniocentesis Amniocentesis: A type of possibility for reducing human suffering. Screening of newborn and fetuses for serious inherited diseases, tests for identifying carriers, and genetic counseling – all these tools of modern medicine rely on the Mendelian model of inheritance. To this day, scientists use Mendel's principles to explain the most basic phenomena of inheritance.

Learning Objectives

After completing the topic, the student will be able to:

  • Discuss the scientific approach of Mendel to identify laws of inheritance.
  • Explore the reasons for Mendel's success. Define and discuss the terms: test cross, back cross, dominant and recessive genes.
  • Define and discuss law of dominance, law of independent assortment and law of segregation.
  • Predict the chances of producing gametes and offsprings by using Punnett square and laws of probability.
  • Define and describe other forms of inheritance : polygenic inheritance, epistasis, co and incomplete dominance, pleiotropy.
  • Demonstrate how a pedigree may be used to determine the mode of inheritance of a genetic trait.
  • Evaluate how genetic testing helps to determine genetic disorders.

Mendel's Peas Mendel's peas Mendel conducted experiments by way of breeding garden peas which helped him to discover the principles of heredity. The modern science of genetics traces its roots to the observations made by Gregor Johann Mendel, a German–Czech Augustinian monk and scientist who made detailed studies of the nature of inheritance in plants.
Mendelian Genetics

The modern science of genetics traces its roots to the observations made by Gregor Johann Mendel, a German–Czech Augustinian monk and scientist who made detailed studies of the nature of inheritance in plants. The word "genetics" (derived from the Greek word "genno" – to give birth) itself was coined in 1905 by William Bateson, a significant proponent of Mendel's work, to describe the study of inheritance.

Mendel conducted experiments by way of breeding garden peas which helped him to discover the principles of heredity. In the decades following the rediscovery and popularization of Mendel's work, numerous experiments sought to elucidate the molecular basis of DNA.

In 1910 Thomas Hunt Morgan argued that genes reside on chromosomes, based on observations of a sex–linked white eye mutation in fruit flies. In 1913 his student Alfred Sturtevant used the phenomenon of genetic linkage and the associated recombination rates to demonstrate and map the linear arrangement of genes on the chromosome.

Although the significance of Mendel's work was not recognized until the turn of the 20th century, the independent rediscovery of these laws formed the foundation of the modern science of genetics.

Cross pollination Cross pollination between pea plants. Cross between purple colored flowers and white colored flowers in a pea plant.
Mendel's experiments

Mendel tried cross pollination between pea plants with different traits such as varied colors of flowers. Mendel observed that the flowers of each pea plant were either purple or white – and never an intermediate between the two colors. These different, discrete versions of the same gene are called "alleles".

Mendel conducted experiments on true–breeding plants only, with traits that are discrete (either–or type) which enabled him to discover the particulate nature of inheritance. For example, Mendel cross–pollinated two contrasting, true–breeding pea varieties – for example, purple–flowered plants and white–flowered plants - which is called hybridization.

The parents are referred to as the P generation (parental generation), and their hybrid offspring are the F1 generation (first filial generation). Self pollination of F1 hybrids produce an F2 generation (second filial generation). Mendel could discover the basic patterns of inheritance by analyzing F2 generation plants.

The F1 hybrids from a cross between purple–flowered and white–flowered pea plants had flowers just as purple as the purple–flowered parents. But, when F1 generation plants which have purple flowers self–pollinate, one fourth of the F2 generation plants had white flowers. Mendel called the purple color as a dominant trait and white flower as a recessive trait. Mendel observed the same pattern of inheritance in six other characters, each represented by two different traits. Mendel developed the laws of inheritance to explain the 3:1 inheritance pattern that he consistently observed among the F2 offspring in his pea experiments.

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