This document provides an introduction to gene interactions, also known as neo-Mendelism or post-Mendelian genetics. It discusses how multiple gene pairs can interact to influence a single trait, modifying Mendel's expected inheritance ratios. Specifically, it describes different types of gene interactions including dominance, epistasis, inhibitory genes, masking genes, complementary/duplicate recessive genes, and duplicate dominant genes. These gene interactions result in non-Mendelian inheritance ratios such as 9:3:4, 13:3, 12:3:1, 9:7, and 15:1. The document uses examples from coat color in mice, feather color in fowl, and flower color in sweet peas to illustrate these concepts

1. Welcome

2. SAVITRIBAI PHULE PUNE
UNIVERSITY
DEPARTMENT OF EDUCATION & EXTENSION
Presented By -
Apeksha Shrikant Kurane
(TY. BSc. Bed)

3. Title -
Neo-Mendelism
(Gene
Interaction)

4. Introduction
■ According to Mendel (1822-1884), each trait in an organism is determined by a pair
of genes (two genes).
■ After the discovery of Mendel's laws of inheritance, the later studies have revealed
that in many instances two, three or even more pairs of different (non-allelic)
genes control (influence / affect) one character.
■ e.g. 20 different genes by their interaction express the eye colour in fruit-fly
(Drosophila melanogaster) and more than 20 different genes interact and express
the colour of seed coat in maize.
■ a trait or phenotypic character is controlled by the combined action of two or more
than two pairs of different (non-allelic) genes, it is known as Interaction of genes.
■ The basic Mendel ratios viz. 3:1 (monohybrid ratio in F{2} ) and 9: 3: 3: 1 (dihybrid
ratio in F{2} are modified.
■ A different pattern of inheritance was discovered called Gene interactions. This
study is known as Post-mendelian genetics or Neo-mendelism.

5. Genetic Interaction
■ While studying Mendel's work, we have observed that, determination of single
phenotypic trait of an organism occurs with various interactions of two alleles or
allelomorphs of a single gene.
■ It is such that, a single gene might show either simple (complete) dominance over
other or both the alleles can exhibit partial or incomplete dominance or show
codominant relationship.
■ These kinds of genetic interactions occur in between the two alleles of a single type
of gene and are mentioned as intra-allelic or allelic genetic interactions. These
kinds of genetic interactions give the classical ratios of 3: 1 and 9:3:3:1
■ In interallelic genetic interactions, the independent (non-homologous) genes
located on the same or on different chromosomes interact with one another for the
expression of single phenotypic trait of an organism.
■ The discovery of the inter-allelic genetic interactions has been made after Mendel
and they can be best understood by considering the way in which a phenotypic trait
is controlled by a gene.

6. Difference Between Dominance
Epistasis -
 Dominance
■ The occurrence of
dominance involves intra-
allelic gene suppression,
or the overpowering effect
of one allele on the
expression of another
allele at the same locus.
 Epistasis
■ The phenomenon of
epistasis involves inter-
allelic gene suppression or
the masking effect which
one gene locus has upon
the expression of another.

7. Recessive Epistasis (9:3:4)
■ If the recessive genotype at one locus (example aa) suppresses
the expression of alleles at the locus B, a locus is said to exhibit
recessive epistasis over the B locus.
■ Only if the dominant allele is present at the A locus, the alleles
of the hypostatic B locus can be expressed. The genotypes A-B
and A-bb produce additional phenotypes. The 9:3:3:1 ratio
becomes 9:3:4 ratio.
■ The example of recessive epistasis is coat colours in common
mice. It occurs in a number of coat colours, i.e., agouti, black
and albino. The agouti colour pattern is commonly found (wild
type) and is characterized by colour banded hairs in which the
part nearest the skin is gray, then a yellow band and finally the
distal part is either black or brown. The albino mouse lacks
totally in pigments and has white hairs and pink eyes.

8. ■ When a homozygous black (CCaa) is crossed with a homozygous
albino (ccAA) in F1; all agouti (CcAa) offspring's appear. When,
the F1; agouti are crossed among themselves in F2 agouti, black
and albino offspring's appear in the ratio of 9:3:4 as in the table.
P1 : Black Albino
CCaa ccAA
P₁ gametes : (Ca) (cA)
F1 Agouti
CcAa

9. Recessive Epistasis – 9:3:4

10. Inhibitory Genes (13:3)
■ Sometimes, the dominant alleles of one gene locus (A) in
homozygous (AA) and heterozygous (Aa) condition and the
homozygous recessive alleles (bb) of another gene locus (B)
produce the same phenotype, the F₂ phenotypic ratio becomes
13: 3 instead of 9:3:3:1.
■ In such case, the genotype AABB, AABb, AaBB, AaBb, AAbb and
Aabb produce same phenotype and the genotype aaBB, aaBb
and aabb produce another but same phenotype.

11. ■ Example: Complete dominance at both gene pairs, however
one gene when dominant epistatic to the other, and the
second gene when homozygous recessive, epistatic to the
first. Feather Colour of Fowl:
■ Gene pair 'A': Colour inhibition is dominant to colour
appearance.
■ Gene pair 'B': Colour is dominant to white.
■ Interaction: Dominant colour inhibition prevents colour
expression even when colour is present, and colour gene,
when homozygous recessive, prevents colour even when,
dominant inhibitor is absent.

13. Masking Genes (12:3:1)/ Dominant
Epistasis
■ When a dominant allele at one locus, for example, the A,
produces a certain phenotype irrespective of the allelic condition
of the other locus, then the A locus is said to be epistatic to the B
locus, since the dominant allele A is able to express itself in the
presence of either b or B, this is the case of a dominant epistasis.
■ Only when the genotype of the individual is homozygous
recessive at epistatic locus (aa), alleles of hypostatic locus (B or
b) can be expressed.
■ Thus, the genotypes A-B and A-bb produce same phenotype,
whereas aaB and aabb produces two additional phenotypes. The
original 9:3:3:1 ratio becomes modified into 12:3:1 ratio.

15. Complementary/ Duplicate
Recessive Genes (9:7)
■ If both the gene loci have homozygous recessive alleles and both of
them produce identical phenotype, the F₂ ratio would be 9: 7
instead of 9:3:3: 1.
■ The genotype aaBB, aaBb, AAbb, Aabb and aabb produce same
phenotype. Both dominant alleles when are present together only
then they can complement each other. This is known as
complementary gene.
■ Example: Complete dominance at both gene pairs, but either
recessive homozygote is epistatic to the effect of the other gene.
In sweet pea flower colour is expressed by --
■ Gene pair 'C' - Purple dominant over white.

16. CP = 9 Purple
Cp = 3 White
cP = 3 White
cp = 1 White
9 : 7
Purple White

17. Duplicate Dominant Gene/ Duplicate
Interaction without Cumulative Effect
(15:1)
■ If a dominant allele of both gene loci produces the same
phenotype without cumulative effect, i.e. independently, the
ratio will be 15:1.
■ Example : Complete dominance at both gene pairs, but either
gene when dominant, epistatic to the other.
■ e.g. Seed capsule of shepherd's purse (Capsella bursa-
pastoris).
■ Gene pair 'A': Triangular shape dominant over ovoid.
■ Gene pair 'B': Triangular shape dominant over ovoid (double
recessive).

18. AB = 9 Triangular
Ab = 3 Triangular
aB = 3 Triangular
ab = 1 Ovoid (Top shaped)
15 : 1
Triangular Ovoid

19. Thank You