BASIC GENETIC TERMS-IV

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BASIC GENETIC TERMS-IV

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History and development of Genetics

• Us humans are curious creatures.

• From really old times, this curiosity provided realization of basic premise of heredity.

• Children look like their father or mother in animals,

• Plants resemble their ancestor

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Early view of inheritance

• Moist Vapour Theory:

• Pythagoras (580-500 B.C.) To explain why children looks like their father he believed that each organ of the body of male produced moist vapours during

coitus which formed the body parts of the embryo.

• And children looks like their mother cause they

develop in mothers body.

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• Fluid Theory:

• Empedocles (504-433 B.C.), the pro-pounder of four

humour theory, proposed that each body part produced a fluid. The fluid of different body parts of the two parents mixes up and is used in the formation of embryo.

• Any defect in the descent and mixing up of the fluids

results in missing of characters of one parent or both the parents.

• According to the ratio of the fluids child phenotype changes.

• Siblings don’t look like each other…

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• Reproductive Blood Theory:

• Aristotle (384-322) thought that the males produce highly purified reproductive blood containing the nutrients from all body parts.

• Females also produce reproductive blood but this is impure.

And it has to excreted from the body. (menstural cycle)

• The two reproductive bloods coagulate in the body of the female and form the embryo.

• Due to purity of reproductive blood, the contri bution of

characters by the male is more than the female.

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• Due to purity of reproductive blood, the contri

bution of characters by the male is more than the female.

• This belied lasted very long. Even now horse breeders believe this to be true.

• Also blood relatives can’t get married.

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Blossoming modern genetics

• Preformation hypothesis:

miniature humans inside sperm and egg cell.

A tiny person inside a sperm, as drawn by Nicolaas Hartsoeker in 1695

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• William Harvey (1620) cut of the uterus of recently mated dears and looked for tiny dears to prove

prrformation hypothesis. But he couldn’t find it.

• He saw embryo development after few days of mating. So he disproved the hypothesis.

• After development of myroscope spem and egg cell visualised.

Blossoming modern genetics

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• Pangenesis is a hypothetical mechanism for heredity was first described by Hippocrates (BC500-400) than developed and published by Darwin.

Darwin proposed this hypothesis to fill a major gap in his evolutionary theory.

“each part of the body continually emitted its own type of

small organic particles called gemmules that aggregated in the gonads, contributing heritable information to the gametes”

Pangenesis means originating from everywhere

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McComas, W. F. (2012). Darwin's invention: inheritance & the" mad dream" of pangenesis. The american biology Teacher, 74(2), 86-91.

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Pangenesis solved several problems

• assuming that “gemmules” could lie dormant for generations,

“reversion” (the sudden reappearance of old traits) is easily explained.

• Regeneration of tissues provided by “gemmules” previously generated by that tissue.

• If an organ developed a new ability new gemmules can eat up old ones



• Source of new variation through use and disuse.

Let’s go back to

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How did Pangenesis hypothesis prove to be wrong?

• The most serious challenge to pangenesis came from Francis Galton (Darwin’s half-cousin).

• he tested the gemmules (which were freely circulating.

• Took blood from mongrel rabbits and transfused their blood with that of the pure-bred silver-gray variety.

• gemmules circulating in the blood of one variety would cross over to the other and change the inheritance pattern in future

generations.

• It did not, and the silver-gray rabbits continued to breed without any inference.

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Blending hypothesis

• Progeny inherits any characteristic as the average of the parents' values of that characteristic.

• crossing of a red flower variety with a white variety of the same species would yield pink-flowered

offspring.

• The pangenesis hypothesis implied as blending

inheritance.

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• Genetic material in oocytes and sperm blends in upon fertilization.

• Trouble: you never get an

«average» individual

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Blending hypothesis

• Objections to Blending Inheritance:

• Many individuals show ancestral characters not found in immediate parents. The phenomenon is called atavism (L.

atavus- great grandfather, grandfather or forefather),

reversion or throw-back. Some persons are able to move pinna or external ear.

• Thus both smooth and hairiness occurred on the leaves of one generation only to separate in the subsequent

generation. This proved that the traits have particulate nature

and remain discrete.

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Two fundamental arguments

• Organism inherit developmental information from their parents.

1- arguments based on observations

regarding patterns of inheritance like the resemblance of Jackie and Selma.

2- arguments concerned with the

localization of hereditary factors inside cells

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Two fundamental arguments lead humans to assume

 Traits have alternate forms.

 Traits can stay dormant. They are represented in the individuals by discrete particulate entities which do not get blended or modified.

 One alternate of a trait may be exhibited more often than the other. Other alternate form of a trait may remain hidden for one or more generations and then reappear in the unchanged state.

 Particular forms of two or more traits may occur together in one generation and separate in subsequent generations.

 Out of the two alternate traits present together in an individual, only one is expressed.

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• Medel proposed particulate inheritance theory

– Characters are defined by units and the units are carried over for generations

• This theory explained most of the phenotypes

• In 1860s he defined biological factors which play an important role inheritance, and named them as

“units”.

Mendelian Inheritance

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Peak into epigenetics history:

• Epigenetics term was proposed by Conrad Hal Waddington

• the background of the emergence of this term;

– Non-Mendelian inheritance, – Zygote development into

different types of cells,

https://www.nature.com/articles/nrg933 Developmental biologist, paleontologist, geneticist, embryologist and philosopher

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• Non-Mendelian inhertiance  same genetic material different phenotypes;

Penetrance, Expressivity

• Zygote development  from one cell to an organism

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Zygote development

from one cell to an organism

Zygote

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What is the answer?

• How do we have different cells?

• Why identical twins are not really identical?

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Epigenetics

Greek prefix epi over above

Study of inherited phenotype alterations or gene expressions that do not rely on the changes within DNA sequences which includes;

DNA methylation,

histone modification,

some noncoding RNAs Epigenetics

genetics