Example for trait

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Extension to Mendel’s Law

1. Alleles’ interaction

2. Pleiotropism

3. Epistasis interaction

4. Linkage and crossing over

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a) Incomplete dominance

b) Lethal allele

c) Co dominant allele

d) Multiple allele

e) Penetrance and expressivity

Alleles’ interaction

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Lack of dominant and recessive relationship

Allele - same degree of expression in heterozygous condition

a) Incomplete dominance

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Q1) The 4O’ clock plant has one variety with red flowers and other

variety with white flowers. From a pure crossing of these two

varieties all the F1 has pink flowers. The F2 offspring produce 142

pink, 68 white and 73 red flowers. How these flowers are inheriting?

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Q2) Give the genotypes and phenotypes of progeny from the

following crosses in the 4O’ clock plant.

i. White X White

ii. White X Pink

iii. Red X Pink

iv. Red X Red

v. Pink X Pink

vi. Red X White

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b) Lethal allele

Some genes - cause death during early stage of development

(before sexual maturation)

A fully dominant lethal allele - arise by mutation from a wild

type allele

Dominant lethal genes - lost from the heterozygous

organism called as gene erosion

Most of all lethal genes are recessive (death only

homozygous)

There are two types of lethal alleles

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1. One that has no obvious phenotypic effect in heterozygous

a. Mice coat colour

Dominant homozygous causes death during embryonic stage

b. Aurea gene in Antirrhinum majus

It produces yellow leaf due to lack of chlorophyll in heterozygous

c. Dexter gene in cattle

d. Achondroplastic dwarfness in man

Coat colour in mice

Dexter gene in cattle

Antirrhinum majus

Achondroplastic dwarfness

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2. One that exhibit a distinctive phenotype when heterozygous

The amount of chlorophyll in snapdragons is controlled by a pair of

alleles C1 and C2

One of which C2 exhibits lethal effect when homozygous and a

distinctive colour phenotype when heterozygous

Thus with regard to colour these alleles are incomplete dominant

However, with regard to viability the C2 allele is fully recessive that

the C2 allele only causes death when C1 is absent

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Snapdragon plant

Alleles that lack of dominant and recessive relationship and are both

observed phenotypically to some degree are co-dominant

This means that the phenotypic effect of each allele is observable in

heterozygous condition

Hence the heterozygous genotypes give rise to a phenotype

distinctly different from either of the homozygous genotype but

possess characteristics of each other

For co-dominant alleles all upper case base symbol with different

superscripts are used.

c. Co - dominant allele

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Qu) Coat colours of short horned breed of cattle represent a classical

example of co-dominant allele. Red is governed by the genotype

CRCR, roan (mixture of red and white) by CRCW and white is CWCW.

a. When roan short horns are crossed among themselves what

genotypic and phenotypic ratios are expected among their progeny?

b. If red short horns are crossed with roans and the F1 progeny are

crossed among themselves to produce the F2. What percentage of F2

will probably be roan?

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More than two alleles - at a gene locus in a population

The series called multiple allele series

These alleles may have arisen as a result of mutation of the

dominant (wild) allele in a gene pair

d. Multiple allele

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Common characters of multiple allele

1. They occupy same locus within the homologous chromosome. In a diploid cell

any two alleles of such allele series are present in a pair of homologous

chromosome. The gamete of an organism contains only one allele of such

series.

2. Multiple allele controls the particular character but with varying degree of

efficiency.

3. The normal gene of the series act as dominant, overall such series alleles may

also behave as dominant, recessive and co-dominant among them.

4. The capital letter is commonly used to designate allele that is dominant to all

others in the series. The corresponding lower case letter designates the allele

that is recessive to all others in the series.

5. Other alleles, intermediate in their degree of dominance between these two

extremes are usually assigned the upper case letter with some suitable

superscript.

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Examples:

i. Eye colour of Drosophila

ii. Coat colour in rabbit

iii. Blood group in man

iv. Self sterility genes in plants. Eg: Red clove and Tobacco

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Eye colour of Drosophila

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Differences in environmental condition/ in genetic backgrounds

may cause individuals that are genetically identical at a particular

locus to exhibit different phenotypes

The percentage of individuals with a particular gene combination

that exhibit the corresponding character to any degree represents

the penetrance of the trait.

e. Penetrance and expressivity

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The polydactyl (extra fingers and or toes) in human can be produced

by a dominant gene P. The wild type condition with five digits on

each limb is produced by the recessive gene p. However, some

heterozygous individuals (Pp) are not polydactylous. If 20% of Pp

individuals do not show polydactyl (that are wild type) the gene has

a penetrance of 80%. A trait although penetrant may be quite

variable in its level of expression. The degree of effect produce by a

penetrant genotype is term expressivity.

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A gene that has multiple phenotypic effects

It is a special interest because it helps in understanding the

relationships between different organisms of the same individuals

Examples:

I. Drosophila bar eyed individuals may be significantly altered by

the wing nature

II. Seed coat colour gene of sweet pea controls flower colour and

also red spot in leaf axils

III. Sickle cell anemia in human

2. Pleiotropism

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