Gregor Mendel’s Laws

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Gregor Mendels Laws

Mendelian inheritance (orMendelian

genetics orMendelism) is a set of

primary tenets relating to the transmission

of hereditary characteristics from parent

organisms to their children.

The laws of inheritance were derived by

Gregor Mendel, an Austrian monk.


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History

Between 1856 and 1863, he cultivated andtested some 28,000 pea plants.

His experiments brought forth two

generalizations which later became knownas Mendel's Laws of HeredityorMendelian inheritance.

Although his results were published in1865, it went largely un noticed, but it wasrediscovered by three scientists in 1900.


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Mendels experiments.

Mendel discovered that by crossing white flower

and purple flower plants, the offspring was

purple flowered, rather than being a mix of the

two. He then conceived the idea of heredity units,

which he called "factors", one which is a

recessive characteristic and the other dominant.

Mendel said that factors, later called genes,normally occur in pairs in ordinary body cells, yet

segregate during the formation of sex cells.


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Terminology

Allele: One of two or more forms a gene may take.

Dominant: An allele whose expression overpowers theeffect of a second form of the same gene.

Gamete: A reproductive cell.

Heterozygous: A condition in which two alleles for agiven gene are different from each other.

Homozygous: A condition in which two alleles for agiven gene are the same.

Recessive: An allele whose effects are concealed inoffspring by the dominant allele in the pair.


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Mendel's Law of Segregation

Mendel's law of segregation, also known

as Mendel's First Law, essentially has

four parts.

1. Alleles exist- They are alternate forms

of the same gene- for eg. Gene which

codes for colour of the pea pod or flower-

gene is same but can be either green oryellow.


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2. For each characteristic feature, an organisminherits two alleles, one from each parent, i.e.one from the father and one from the mother. Intrue breeding organisms- both alleles will be

same-TT/tt and in hybrid, it will be Tt. 3. If the two alleles differ, then one, the

dominant allele, is fully expressed in theorganism's appearance; the other, therecessive allele, has no noticeable effect onthe organism's appearance. In other words,the dominant allele is expressed in thephenotype of the organism;


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However this does not always hold true,today.

There are several examples we know that

disprove this "law", e.g. Mirabilis jalapa,the "Japanese wonder flower.This iscalled incomplete dominance.

There is also codominance e.g. Humanblood types where A and B arecodominant and O is recessive.


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4. The two alleles for each

characteristic segregate during gamete

production.

The two alleles of the organism are

separated into different gametes, ensuring

variation.


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The Law of Segregation states that the

members of each pair of alleles separate

when gametes are formed.

A gamete will receive one allele or the

other.

A


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Genetic diagram

A genetic diagram is done in order to find out thepossibilities of a cross between two knownparents.

Mendels always started the crosses with pure

bred parents for one character, calledmonohybrid.

The diagram is also known as the Punnettsquare.

In the following diagram, parents are one puregreen (GG) and the other pure yellow (gg). Theresulting offspring are Gg the F1 generation.


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Second law of inheritance

An individual's physical appearance or phenotype isdetermined by its alleles. An individual possesses twoalleles for each trait; one allele is given by the femaleparent and the other by the male parent. They are

passed on when an individual matures and producesgametes, egg and sperm. When gametes form thepaired alleles separate randomly so that each gametereceives a copy of one of the two alleles. The presenceof an allele doesn't promise that the trait will be

expressed in the individual that possesses it. Inheterozygous individuals the only allele that in expressedis the dominant. The recessive allele is present but itsexpression is hidden.


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Mendels Second law

The Law of Independent assortment states

that two or more pairs of alleles segregate

independently of one another during

gamete formation.


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Mendels Second law

The genetic diagram again begins with two

pure bred parents, this time with two

characters or dihybrid.

They are pure green and round seeds

(GGYY) and pure yellow and wrinkled

seeds ( ggyy). The F1 generation is a

hybrid of GgYy, which is crossed again togive the following results.


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Gregor Mendel’s Laws

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