Problems of gene interaction

2. Content # 5:
Gene Interaction & Lethality
Aleeha Muzaffar
BSc.(Hons) Biotech
2nd Proff
Akhuwat F.I.R.S.T

3. Gene Interaction:
“Two alleles or allelomorphs of a single
gene(intra-allelic) or two alleles of different
genes located on same chromosome or on
different chromosome (inter-allelic) interacts
with one another is called gene interaction.”
Interaction is of different types;
 One allele dominant over other recessive
allele (Complete dominance)

4.  Both allele express themselves partially (Incomplete
dominance)
 Both alleles have equal expression (Codominance)
Discovery:
In 1906,Bateson & Punnett modifies the classical
mendelian inheritance and gives the concept of gene
interaction that is;
“Genes are not nearly separate elements producing
distinct individual effect but they could interact with
each other to give completely novel(different)
phenotype.”

5. Gene is a chemical determiner. Whereas a
phenotypic trait results from the combined
action of many genes and their products
constantly interacting with the environment.
 Allele A gives product A
 Allele B gives product B
Allele A + Allele B=C product
Product C is different from A & B due to the
interaction of both genes.

6. Example:
Comb shape in poultry
 RR=Rose comb dominant, rr=single comb recessive
 PP=Pea comb dominant, pp=single comb recessive
Rose comb × Pea comb
RRpp rrPP
Rp rP
RrPp
(All Walnut)

7. Walnut × Walnut
RrPp RrPp
RP,Rp,rP,rp RP,Rp,rP,rp
Rp Rp rP rp
RP RRPp RRPp RrPP RrPp
Rp RRpp RRpp RrPp Rrpp
rP RrPp RrPp rrPP rrPp
rp Rrpp Rrpp rrPp rrpp

8. Example:
Petal color in blue-eyed Mary plants
mm=magenta color
ww=white color
WWMM,WwMM,WWMm,WwMm=blue color
Cross b/w WwMm × WwMm.What is F2
generation and their ratio?

9. Types of gene interaction:
 Epistasis
 Lethality

10. Epistasis:
“Masking” or “Hiding”
The kind of interaction in which gene at one
locus masks the expression of gene at second
locus is called epistasis.
 Term epistasis used by Bateson in 1906.
 A gene or locus that suppress the action of
another gene or locus is called Epistatic
gene or suppressor gene.

11.  The gene or locus that is suppressed by a
epistatic gene is called Hypostatic gene.
Types of Epistasis:
Four types of epistasis
 Dominant epistasis
 Recessive epistasis
 Dominant and recessive(Alternate epistasis)
 Duplicate dominant genes

12. Dominant epistasis:
Dominant allele at one gene pair hides the effect
of another gene pair at other locus.
Example:
Coat color of Dog
AA,Aa=color inhibition(white) dominant over
color
BB,Bb=black color
aa,bb=brown color
Epistatic alleles=AA,Aa
Hypostatic allele=BB,Bb,bb

13. Cross b/w AABB × aabb. What is the
phenotype of parents and offsprings?
Cross b/w AaBb × AaBb.What is the F2
generation and their phenotypic ratio?
Cross b/w AABB × Aabb. What is the progeny
and phenotype of parents?
Cross b/w aaBb × aabb. What is the result?

14. Recessive Epistasis:
Homozygous recessive gene hides the effect of
another gene.
Example:
Coat color in mouse
cc=albino dominant over black
aa=black color
Cc,CC=Agouti color
Aa,AA=Color expression

15. Cross b/w AAcc × aaCC. Find the F1 and
F2 generation with ratio?
Cross b/w Ccaa × ccAA. Find the
progeny and their phenotypic ratio?

16. Duplicate gene action:
“Two identical genes showing the same
phenotypic action but localized in different
regions of a chromosomes or on different
chromosome.”
Example:
Seed capsules of shepherd’s purse
A,B=triangular shaped
a,b=Top-shaped

17. Both A & B are required for triangular shape.
When BB or Bb present with aa then it is also
triangular shape.
Cross b/w AABB × aabb. Find F1 and F2
generation and ratio of F2?

18. Alternate Epistasis:
“One gene A hides the effect of other gene B
or vice versa. When each pair mutually
epistatic to each other is called alternate
epistasis.”
Example:
Primula petal color
B=blue color due to malvidin substance
b=white color recessive
D=suppresses the effect of B

19. Cross b/w DDBB × ddbb.
F1=?, F2=?, Ratio of F2 with phenotype?

20. Lethality:
A condition in which certain genes have
serious effects that the organisms is unable
to live. Genes that cause the death of an
organism are known as lethal gene.
 Lethal genes are of two types
 Dominant lethal genes
 Reccesive lethal genes
 Lethal genes have high degree of
penetrance and expressivity

21.  The lethal genes which gives the survival
chance to the affected genotype after the
maturity that gene called the Semilethal
genes.
 The lethal genes which have no or little
survival chance among affected genotype at
infant stage are called Completely lethal
genes.
 The lethal genes which are unable to show
phenotypic expression and give the
individual chance of survival is called Non-
lethal genes.

22. Dominant lethal gene:
“This gene kills both in homozygous and
heterozygous states but the homozygous
recessive individual survive.”
 Individuals with a dominant lethal gene die
before they can leave progeny.
 It can be caused easily by x-rays treatment
of the reproductive organs of the parents.

23.  Single dose of this gene causes the abnormal
skin growth ,severe mental defects and
multiple tumors.
 Example:
 Epiloia
 Retinoblastoma

24. Recessive lethal genes:
This kills the individual only when genes in them
are in homozygous state
 Phenotypic effect of genes either obvious
or a distinct when in heterozygotes.
 Lethal genes or alleles modified the
phenotypic ratio from 3:1 to 2:1.

25. Example:
Manx Cat
 M=tailless, lethal in Homozygous
 M=tail
Allele interferes with normal spinal
development in heterozygous cats this results
in lack of tail.
Cross b/w Mm × Mm.What is the progeny?

26. Mexican hairless dogs
 hh=hairy normal trait
 Hh=hairless one mutation present
 HH=lethal
Cross b/w Hh × Hh.What is the progeny?

27. Amputated calves
Condition where the head, legs and lower
jaws are missing
aa=lethal gene
a+=normal gene
Cross b/w a+ × a+.Find the progeny?

28. Lethal Genes in Humans:
Congenital ichthyosis:
 It occurs only when homozygous occurs for its
recessive lethal gene.
 In this children at birth affected by crusted
leathery skin with deep fissures down to the
subcutaneous tissues, the fissures leads to the
bleeding, infection and death.

29. Infantile amaurotic idiocy:
 It is also due to recessive genes.
 Bearers of this genotype begin to lose their eye
sight b/w the age of four to seven years
 The complete blindness is followed by mental
degeneration and finally death before
adolescence.

30. Cooley’s anaemia(Thalassemia):
 Haemoglobin disease somewhat similar to sickle
cell anaemia.
 Occurs mostly in children and is nearly 100% fatal.
 It occurs due to homozygous recessive gene(cc)
and cause death of patient.
 If it occurs due to heterozygous condition(Cc),
results in a mild form of this disease called
microcythemia or thalassemia minor.