MENDELISM

ANA 807 PRESENTATION

BY

ADESEJI WASIU ADEBAYODepartment of Anatomy,

University of Ilorin.

08/46KA006January, 2015

OUTLINEINTRODUCTIONGENETICS BEFORE MENDELMENDEL EXPERIMENTSMENDEL’S PRINCIPLES OF INHERITANCEAPPLICATION OF MENEDL’S PRINCIPLES

TO HUMAN GENETICSMEDELIAN PRINCIPLES IN HUMAN

GENETICSCONCLUSIONREFERENCES

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INTRODUCTIONMendelism simply put, refers to the laws of

inheritance postulated by Gregor Mendel.Gregor Mendel was an Austrian Monk who

lived between 1822-84.He experimented on various plants species

and animalHe was the first to state the universal laws

governing inheritance of traits. Gregor Mendel is known as father of modern

genetics

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GENETICS BEFORE MENDELBefore Mendel Heredity was a mystery“The laws governing inheritance are

quite unknown; no one can say why the same peculiarity … is sometimes inherited and sometimes not so; why the child often reverts in certain characters to its grandfather or grandmother or other much more remote ancestor…”

- Charles Darwin, Origin of Species 1872

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GENETICS BEFORE MENDELThe concept of inheritance of physical ‘units’

(later called genes) was accepted, and scientists had reported on many hybridization experiments in both animals and plants.

Yet no one had set forth principles of inheritance which could be used as a universal theory to explain how traits in offspring can be predicted from traits in the parents.

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MENDEL EXPERIMENTSGregor experimented with

pea plants, by crossing various strains and observing the characteristics of their offspring.

He also experiment on drosophila, but much success was recorded on pea plants

Between 1856 and 1863, Mendel cultivated and tested some 28,000 pea plants.

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MENDEL EXPERIMENTSThe choice of Garden pea (Pisum sativum) was due to the following reasons:

Can be grown in a small area Produce lots of offspring Produce pure plants when allowed to self-pollinate several generations Can be artificially cross-pollinated

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MENDEL EXPERIMENTSOne peculiarity of pea reproduction is that the petals of

the flower close down tightly, preventing pollen grains from entering or leaving. This enforces a system of self-fertilization, in which the male and female gametes from the same flower unite with each other to produce seeds.

As a result, individual pea strains are highly inbred, displaying little if any genetic variation from one generation to the next.

Because of this uniformity, we say that such strains are true-breeding

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MENDEL EXPERIMENTSThe true breed traits of the pea plant are:Seed shape --- Round (R) or Wrinkled (r)Seed Color ---- Yellow (Y) or  Green (y)Pod Shape --- Smooth (S) or wrinkled (s)Pod Color ---  Green (G) or Yellow (g)Seed Coat Color ---Gray (G) or White (g)Flower position---Axial (A) or Terminal (a)Plant Height --- Tall (T) or Short (t)Flower color --- Purple (P) or white (p)

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Mendel focused on the study of inheritance of one trait at a time.

In an experiment, Mendel crossed tall and dwarf pea plants to investigate how height was inherited.

This type of crossing between parents differing in only one trait or in which only one trait is being considered is termed monohybrid cross.

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MENDEL EXPERIMENTSAll the offspring from the cross (First

filialgeneration- F1) were all tall regardless of the way he performed the cross (tall male with dwarf female or dwarf male with tall female).

Mendel noted that the dwarf characteristic seemed to have disappeared in the progeny of the cross.

MENDEL EXPERIMENTSMendel decided to cross the F1 generation with each

other to see if the dwarf trait would reappear in the next generation.

When he examined the progeny (F2 generation), he found that they consisted of both tall and dwarf plants in a ratio of approximately 3:1.

Mendel inferred that these hybrids carried a latent genetic factor for dwarfness, one that was masked by the expression of another factor for tallness.

He said that the latent factor was recessive and that the expressed factor was dominant.

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MENDEL EXPERIMENTSMendel repeated the experiment examining

the transmission of other traits such as flower colour, pod colour.

The same pattern and results was observed

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Dihybrid Crosses: The Principle of Independent Assortment

Mendel also performed experiments with plants that differed in two traits.

The purpose of the experiments was to see if the two seed traits, color and texture, were inherited independently.

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He crossed plants that produced yellow, round seeds with plants that produced green, wrinkled seeds.

F1 seeds were all yellow and round, the alleles for these two characteristics were dominant.

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MENDEL’S PRINCIPLES OF INHERITANCEThe result of Mendel’s experiments led to the

principles of inheritance.

It was published in 1865 through a single research paper, ‘Experiments in Plant Hybridization’

Unfortunately, this paper languished in obscurity until 1900, when it was rediscovered by three botanists—Hugo de Vries in Holland, Carl Correns in Germany, and Eric von Tschermak-Seysenegg in Austria.

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Genes In PairsGenes In Pairs: Genetic characters are

controlled by factors (gene) that exist in pairs of alleles in individual organisms and are passed from parents to their offspring. When two organisms produce offspring, each parent gives the offspring one of the alleles from each pair

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Dominance and RecessivenessDominance and Recessiveness: When two

unlike alleles responsible for a single character are present in a single individual, one allele can mask the expression of another allele. That is, one allele is dominant to the other. The latter is said to be recessive.

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The Law of SegregationThe Law of Segregation: During the

formation of gametes, the paired alleles separate (segregate) randomly so that each gamete receives one allele or the other.

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The Law of Independent Assortment The Law of Independent Assortment: During

gamete formation, segregating pairs of alleles assort independently of each other. Example: genes on different chromosomes will segregate independently. Linked genes (close together on one chromosome) do not follow this law.

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MEDELIAN PRINCIPLES IN HUMAN GENETICSMendel’s principles can be applied to

study the inheritance of traits in humans.The genetic analysis of human heredity

depends on family records, which are often incomplete.

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Pedigrees diagram the relationships among family members where

squares designate males and circles designate females,

a horizontal line represents mating and

vertical lines beneath reflect offspring.

Birth order is left to right,

afflicted individuals are shaded or filled in

Pedigrees : are used to identify dominant and recessive traits in human families

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Mendelian inheritance in humans

d) Earlobe attachment

a) Mid-digital hair

b) Tongue rolling

c) Widow's peak

e) Hitchhiker's

thumb

f) Relative finger length

All these traits (and more) are transmitted in Mendelian fashion in humans

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CONCLUSIONMENDEL MADE US UNDERSRAND

GENETICS