dna2life_mendelism

7/29/2019 DNA2life_Mendelism

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Mendelian genetics

By Snigdha Kundu @ dna2life.com


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Mendelian inheritance was derived by Gregor Johann Mendel .Hewas an austrian monk and a son of farmer who worked with purelines of pea for eight years and hence formulated three laws ofinheritence.

Prior to Mendelian theory ,hereditary was considered as blending ofdifferent parental characteristics.


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What and why did Mendel choose for his experiments?

Mendel took pea (Pisum sativum) as his experimental topic. The reason behind hischoice:

Can be grown in small area Produces lot of offspring Self pollination gives pure breed

Artificially cross pollination can be done.


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He took seven traits of pea to conduct the experiment:

Pea shape : smooth/wrinkle Pea pod shape : inflated/constricted Pea colour : grey/white Pea pod colour : green/yellow Plant height : tall/short Pea albumin colour :yellow/green Leaf position : axial/terminal


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Terminologies :

1.Gene It can be referred as molecular2. unit of heredity of a living3. organism that is a section4. of DNA ,which encodes5. protein.

2.AlleleIt means alike; i.e two

same gene the sameposition on homologousstrand and carrying thesame trait.

3. locus

A fixed position on thestrand of DNA where a

gene or allele located.


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CharacterA biological property of an organism that can be seen ormeasured.

Homozygous It is the condition whereidentical alleles are presentat same locus.

Heterozygous

It is the condition wheredifferent alleles are present ata particular locus.

Hemizygous It is the condition where one allele is present indiploid cell. Such condition leads to sex-linkeddiseases.


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Diploid (2n) such a type of cell have two homologous copy of

chromosome,one from father and other from mother

Haploid(n) it is the number of chromosome in a gamete ofindividual.

Genotype this is the genetic natureof an individual.

Phenotype

this is the outward natureof an individual.


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Dominant Allele of a gene whose trait becomes prominent by suppressingthe others trait

RecessiveThe suppressed allele.

Gamete It is the sex cell that contains half of the genetic information for theexpression of the trait.

Zygote It is the cell formed after the fusion of two gametic cell/sex cell .

From www.mymcat.com


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Monohybrid cross

Crossing of two pea plant with different stem size: tall(T) and dwarf(t)

For homozygous tall plant = TTFor homozygous dwarf plant = tt

For P1 parent generation :

P1: TT tt

gamete: T T t t

Offsprings: Tt

Hence, genotype for all 4 offspring : TtPhenotype for all 4 offspring : tall

Offsprings are all hybrid showing tall as dominant trait and are called F1 or first filialgeneration.


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Mendel then crossed two of his offspring from F1 generation and named as F1 cross.

Tt x Tt

F1 cross : T t genotype : TT : Tt : ttT Phenotype : tall : dwarf

Genotype ratio : 1 : 2 : 1t phenotype ratio : 3 : 1

The offspring are called F2 generation.

Mendel then crossed between pure (TT) and hybrid (Tt) and named it F2 crossand F2 generation.

T TT genotype : TT : Tt

Phenotype : tallt genotype ratio : 1:1

Phenotype ratio : all tallHence, F2 generation produced all tall. Next Mendel performed dihybrid cross.

TT TT

Tt Tt


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Dihybrid cross

For P1,mendel choose; traits : seed shape and colorAllele : round(R) yellow(Y)

wrinkled(r) green(y)P1 cross :

RRYY X rryy

F1 generation : RrYy

For F2 generation : RrYy X RrYy

RY ry Ry rYRYry

RyrY

RRYY RrYy RRYy RrYY

RrYy rryy Rryy


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GENOTYPE GENOTYPE RATIOPHENOTYPE PHENOTYPE RATIO

RRYY 1

Round yellow seed

RRYy 2

RrYY 2

RrYy 4

RRyy 1

Round green seed

Rryy 2

rrYY 1

Wrinkled yellow seedrrYy 2

rryy 1

Result :

From his results, he concluded :

law of dominance Law of independent assortment Law of segregation


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Law of dominance :

It states that when different alleles for a particular triat are inherited

(heterozygous), the trait of only one (the dominant one) will be expressed. Therecessive trait's phenotype only appears in true-breeding (homozygous)individuals

Law of independent assortment :

It states that each genetic trait is produced by a pair of alleles which separate(segregate) during reproduction.

Law of seggregation :

It states that each factor (gene) is distributed (assorted) randomly andindependently of one another in the formation of gametes.


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Beyond mendel's law:

Mendel was lucky, since the characters he took were dominant. Hence phenotypes

were very clear.

But sometimes phenotypes were not not very clear ,which are later called asexceptions of Mendel's law. These are :

Incomplete dominance example : different colour of snapdragon flowers.

Co-dominance

example : human blood group

Multiple alleles example : human blood group A,B, AB, O

Polygenic trait example : different height ,hair, and eye color in human

Linked gene

Sex linked traits example : haemophilia


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!! THE END !!

dna2life_mendelism

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