……. Assignment no 1

ARRANGED BY; ; ; ZOHAIB HUSSAIN AND SAAD IQBAL

TOPIC ; ; ; ; EPISTATIC INTERACTIONS

SUBMITTED TO:::;;;;;DR SARFARZ SHAFIQ

COURSE TITLE ; ; ; ; ; ;ESSENTIAL OF GENETICS

•Epistatic Gene Interactions

Epistatic Gene Interactions

• Gene interactions occur when two or more different genes influence the outcome of a single trait

• Most morphological traits (height, weight, color) are affected by multiple genes

• Epistasis describes situation between various alleles of two genes

• Quantitative loci is a term to describe those loci controlling quantitatively measurable traits

• Pleiotropy describes situations where one gene affects multiple traits

• examine cases involving 2 loci (genes) that each have 2 alleles

• Crosses performed can be illustrated in general by– AaBb X AaBb

– Where A is dominant to a and B is dominant to b• If these two genes govern two different traits

– A 9:3:3:1 ratio is predicted among the offspring– simple Mendelian dihybrid inheritance pattern

• If these two genes do affect the same trait the 9:3:3:1 ratio may be altered– 9:3:4, or 9:7, or 9:6:1, or 8:6:2 or 12:3:1, or 13:3, or 15:1– epistatic ratios

A Cross Producing a 9:7 ratioFigure 4.18

9 C_P_ : 3 C_pp :3 ccP_ : 1 ccpp

purple white

Epistatic Gene Interaction• Complementary gene

actionEnzyme C and enzyme P

cooperate to make a product, therefore they complement one another

Epistatic Gene Interaction• Epistasis describes the situation in which

a gene masks the phenotypic effects of another gene

• Epistasis interactions arise because the two genes encode proteins that participate in sequence in a biochemical pathway

• If either loci is homozygous for a null mutation, none of that enzyme will be made and the pathway is blocked

Colorless precursor

Colorless intermediate

Purple pigment

Enzyme C Enzyme P

genotype cc

Colorless precursor

Colorless intermediate

Purple pigment

Enzyme C Enzyme P

Epistasis of Involving Sex-linked Genes

• Inheritance of the Cream-Eye allele in Drosophila– a rare fly with cream-colored eyes identified

in a true-breeding culture of flies with eosin eyes

– possible explanations• 1. Mutation of the eosin allele into a

cream allele• 2. Mutation of a 2nd gene that modifies

expression of the eosin allele

–Cream-colored eyes in fruit flies are due to the effect of a second gene that modifies the expression of the eosin allele

Testing the Hypothesis

Data obtained Cross OutcomeP cross:

Cream-eyed male X

wild-type female

F1: all red eyes

F1 cross:

F1 brother X F1 sister

F2: 104 females with red eyes

47 males with red eyes

44 males with eosin eyes

14 males with cream eyes F2 generation contains males with eosin eyes

This indicates that the cream allele is not in the same gene as the eosin allele

Interpreting the Data

Cross OutcomeP cross:

Cream-eyed male X

wild-type female

F1: all red eyes

F1 cross:

F1 brother X F1 sister

F2: 104 females with red eyes

47 males with red eyes

44 males with eosin eyes

14 males with cream eyes F2 generation contains –

151 + eye: 44 we eye: 14 ca eyea 12 : 3 : 1 ratio

Modeling the Data

Cream phenotype is recessive therefore the cream allele is recessive allele (either sex-linked or autosomal)

The mutated allele of the cream gene modifies the we allele, while the wt cream allele does not C = Normal allele

Does not modify the eosin phenotype ca = Cream allele

Modifies the eosin color to cream, does not effect wt or white allele of white gene.

Male gametes

CY

CCXw+Xw+ CCXw+Y cacaXw+Xw+ CcaXw+YCXw+

CXw+ caXw+ caY

CXw-e

caXw+

caXw-e

CCXw+Xw-e CCXw-eY CcaXw+Xw-e CcaXw-eY

CcaXw+Xw+ CcaXw+Y cacaXw+Xw+ cacaXw+Y

CcaXw+Xw-e CcaXw-eY cacaXw+Xw-e cacaXw-eY

Fem

ale

gam

etes

A Cross Involving a Two-Gene Interaction Can Still Produce a 9:3:3:1 ratio

• Inheritance of comb morphology in chicken– First example of gene interaction– William Bateson and Reginald Punnett in 1906– Four different comb morphologies

N

Figure 4.17b

The crosses of Bateson and Punnett

• F2 generation consisted of chickens with four types of combs

– 9 walnut : 3 rose : 3 pea : 1 single

• Bateson and Punnett reasoned that comb morphology is determined by two different genes– R (rose comb) is dominant to r– P (pea comb) is dominant to p – R and P are codominant (walnut comb)– rrpp produces single comb

Gene Interaction• Duplicate gene action

– Enzyme 1 and enzyme 2 are redundant

– They both make product C, therefore they duplicate each other

Duplicate Gene Action Epistasis

TV

TV

Tv

Tv

tV

tV

tv

tv

TTVV TTVv TtVV TtVv

TTVv TTvv TtVv Ttvv

TtVV TtVv ttVV ttVv

TtVv Ttvv ttVv ttvv

(b) The crosses of Shull

TTVVTriangular

ttvvOvate

TtVvAll triangular

F1 (TtVv) x F1 (TtVv)

x

F1 generation

15:1 ratio results

Bombay Phenotype

Bombay Phenotype

Bombay Phenotype

Squash Fruit Color

• Genotypes and Phenotypes:

• W-/G- white

• W-/gg white

• ww/G- green

• ww/gg yellow

Dominant Epistasis

• LETS HAVE A LOOK AT DOMINANT EPISTASIS…

• Squash fruit color is controlled by two genes.

• Gene 1 is represented by a W• Gene 2 is represented by a G

Squash Fruit Color

• Which allele is epistatic in squash color?

• How do you know?The dominant W allele is epistasis

Because every time a dominant W allele shows up in a squash genotype, the squash fruit color is white.

Wwgg x wwGg

• FOIL: Wg or wg• FOIL: wG or wg

• F1 generation genotypes:

• Phenotypes:

....THANK.

. ……YOU…..

REFRENCE BOOKS

•CELL BIOLOGY, GENETICS•EPISTASIS AND EVOLUTIONARY PROCESS BY JASON WOLF•COMBINATORIAL APPROACH TO EPISTATIS BY R LOWEN•CELL BIOLOGY AND GENETICS BY S.CHAND