3. Inheritance and Variation

-CREATED BY-

PROF. DESHMUKH A. B.

ASST. TEACHER

AGASTI ARTS, COMMERCE & DADASAHEB RUPWATE SCIENCE JUNIOR COLLEGE, AKOLE , TAL. – AKOLE, DIST. - AHMEDNAGAR

3.5 Deviation from Mendel’s findings

3.5 Deviations from Mendel’s findings :

• Few generalizations were arrived at by Mendel, on the basis of his experiments of garden pea plant- such as,
• i. Single trait Single gene Two alleles.

e.g., Height of Stem (T and t)

• ii. Two alleles show interaction in which one is completely dominant.

e.g., In a Cross between Tall (TT) X Dwarf (tt) F₁

Hybrids – all Tall (Tt)

• iii. Factors (genes) for different traits present on different chromosomes assort independently.

Height of Stem

Seed Shape

Seed Cotyledon Colour

Pod Shape

Flower Colour

Flower Position

Pod Colour

• With the passage of time,
• number of deviations were observed/ identified in the post-Mendelian era,
• that deviations gave additional information on the patterns of inheritance.
• These deviations are then described as Neo-Mendelism.

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• It was observed that the phenotypic expression of a gene can be modified or influenced by the other gene.
• These gene interactions are of two types.
• i. Intragenic interactions :
• Occur between the alleles of same gene

e.g., incomplete dominance and co-dominance.

• It also occurs between the multiple allele series of a gene.

• ii. Intergenic (non-allelic) interactions:
• Occur between the alleles of different genes present on the same or different chromosomes.
• e.g.,
• pleiotropy,
• polygenes,
• epistasis,
• supplementary and complementary genes, etc.

a. Incomplete dominance:

• Here, both the alleles (genes) of an allelomorphic pair express themselves partially.
• One allele (gene) cannot suppress the expression of the other allele (gene) completely.
• In such case, there is an intermediate expression in the F₁ hybrid.
• A well-known example is the flower colour of Mirabilis jalapa.
• If a red-flowered (RR) plant is crossed with a white-flowered (rr) plant, then F₁ offsprings have pink (Rr) flowers.

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Result :

Genotypic ratio –

1RR : 2Rr : 1rr

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Phenotypic ratio –

1Red : 2 Pink :1 White

b. Co-dominance :

• In co-dominance, both the alleles (genes) of an allelomorphic pair express themselves equally in F₁ hybrids.
• Such alleles which are able to express themselves equally independently in hybrids, are called co-dominant alleles.
• Thus, in co-dominance both alleles are expressed.
• Classic example of co-dominance is coat colour in cattle.

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• There are two types
• one with red coat (with red colour hair) and
• other with white coat (with white hair).
• When red cattles (RR) are crossed with white cattles (WW), F₁ hybrids (RW) are roan.
• Roans have the mixture of red and white colour hair.
• Thus, both the traits are expressed equally.
• In F₂ generation red (RR), roans (RW) and white (WW) are produced in the ratio 1:2:1.
• Thus, in Co-dominance, the genotypic and phenotypic ratios are identical.

Parents ( P ):

Phenotype:

Genotype:

Gametes:

First Filial Generation

( F₁ ):

Red (♀)

White (♂)

X

RR

WW

(R)

(W)

RW

Roans

F₁ Hybrids:

Phenotype:

Genotype:

Gametes:

Second Filial Generation

( F₂ ):

Roan (♀)

X

RW

(R)

(W)

Roan (♂)

RW

(R)

(W)

Red

Roan

White

RR (♀)

RW (♀)

WW (♂)

RW (♂)

Phenotypic Ratio:

Phenotype:

Genotype:

Red : Roan : White

1 : 2 : 1

Genotypic Ratio:

RR : RW : WW

1 : 2 : 1

c. Multiple alleles :

• More than two alternative forms (alleles) of a gene in a population occupying the same locus on a chromosome or its homologue, are known as multiple alleles.
• Multiple alleles arise by mutations of the wild type of gene.
• A gene can mutate several times producing a series of alternative expression.
• Different alleles in a series show
• dominant- recessive relation or
• may show co-dominance or
• incomplete dominance among themselves.
• Wild type is dominant over all other mutant alleles.

• In Drosophila, large number of multiple alleles are known.
• e.g., The size of wings from normal wings to vestigial (no) wings, i.e., just stumps, is due to one allele (vg) in homozygous condition.
• The normal wing is wild type while vestigial wing is recessive type.

Phosphate

Pentose Sugar

Nitrogenous Base

A / G / C / T

Nucleotide

Gene Mutations

Chromosomal Mutations

• Another good example of multiple alleles is A, B, O blood groups in human beings.
• The gene I controls the ABO blood groups.
• It has three alleles – I^A, I^B and i
• The allele I^A & I^B produce a slightly different form of the antigen.
• Allele i does not produce any antigen.
• Since humans are diploid organisms, each person possess any two of the three I gene alleles.
• I^A and I^B are co-dominants and are completely dominant over i.
• There are six different genotypes and four different phenotypes.

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For information only

d. Pleiotropy :

• When a single gene controls two (or more) different traits, it is called pleiotropic gene, and the phenomenon is called pleiotropy or pleiotropism.
• The phenotypic ratio is 1:2 instead of 3:1 because of the death of recessive homozygote.
• The disease, sickle-cell anaemia, is caused by a gene Hb^s.
• Normal or healthy gene Hb^A is dominant.
• The carriers (heterozygotes Hb^A/ Hb^s) show signs of mild anaemia as their RBCs become sickle-shaped i.e., half- moon- shaped only under abnormally low O₂ concentration.

• The homozygotes with recessive gene Hb^s however, die of total anaemia.
• Thus, the gene for sickle-cell anaemia is lethal in homozygous condition and produces sickle cell trait in heterozygous carrier.
• Two different expressions are produced by a single gene.
• A marriage between two carriers will produce normal, carriers and sickle-cell anaemic children in 1:2:1 ratio.
• Sickle cell anaemics die leaving carriers and normals in the ratio 1:2.
• The heterozygotes or carriers can be identified by microscopic examination of blood.