NORMAL MENDELIAN RATIO AND ITS MODIFICATION (Includes Lethal Gene effects & EPISTASIS

DR. ASHISH PATELAssistant professor

Dept. AGB, Veterinary College, AAU, Anand

Ratios of Monohybrid cross

Phenotypic ratio 3:1 Genotypic ratio 1:2:1

Ratios of Dihybrid cross Phenotypic ratio 9:3:3:1 Genotypic ratio 1:2:2:4:1:2:1:2:1.

The law of segregation The law of purity of gametes

When a pair of alleles is brought together in a hybrid (F1) they remain together without contaminating each other and they separate or segregate from each other into a gamete during the formation of gametes.

Or

The two alleles of a gene remains separate and do not contaminate each other in F1 generation or in hybrid. At the time of gamete formation in F1, the two alleles separate out and pass into different gametes.

LAW OF INDEPENDENT ASSORTMENT

The segregation of two or more characters in the same hybrid is independent to each other. Thus any allele of one gene is equally likely to combine with any allele of the other gene and passed into same gamete.

 

No. of loci at which both parents are heterozygousn 1 2 3 4 5 10

No. of gametes / parent 2n 2 4 8 16 32 1024No. of types of genotypes in progeny 3n 3 9 27 81 243 59049No. of types of phenotypes in progeny 2n 2 4 8 16 32 1024Proportion of homozygous recessives in progeny

(¼)n ¼ 1/16 1/64 1/256 1/1024

Genotypic and Phenotypic ratio for various no. of gene pair

Genotypic ratio

Monohybrid (1:2:1)1 = 1:2:1

Di-hybrid (1:2:1)2 = 1:2:2:4:1:2:1:2:1

Poly-hybrid (1:2:1)n

Phenotypic ratio Monohybrid 3:1

Di-hybrid 9:3:3:1

Tri-hybrid 27:9:9:9:3:3:3:1

Test cross

Mono hybrid test cross – 1 : 1 Di-hybrid test cross - 1 : 1 : 1 : 1 Tri-hybrid test cross - 1 : 1 : 1 : 1 : 1 : 1 : 1 : 1

Extension of Mendelian Principles According to interaction between alleles or genes

which leads to change in the phenotypic ratio are categorized as

1. Interaction between alleles at same locus (Intra-locular interaction or intra-allelic interaction)

2. Interaction between alleles at different locus (Inter-locular interaction or inter-allelic interaction)

1. Interaction between alleles at same locus / Intra-locular interaction / intra-allelic interaction

 The traits studied by mendel were determined by the genes which acted as either dominant or as recessive.

In this type of allelic relationship, one allele completely masked the effect of other alleles and other allele masked by dominant allele. These two recessive and dominance are two extreme form of gene expression.

So, in general the dominance is an interaction at particular locus and hence this interaction is called as Intra-locular interaction / intra-allelic interaction.

The Dominance is classified in to four categories.1. Complete dominance2. Incomplete dominance (partial dominance or semi

dominance)3. Co-dominance4. Overdominance5. Lethality of genes

The phenotypic ratio of 3:1 in monohybrid crosses is obtained when there is complete dominance.

So, the extension of Mendelian ratio and principles due to

incomplete dominance, co-dominance and also due to lethal genes.

Incomplete dominance(partial dominance or semi dominance)

In this incomplete dominance the phenotype of heterozygote is intermediate to the phenotypes of the two types of homozygotes.

The genotypic and phenotypic ratios for monohybrids are 1:2:1 and 1:2:1.

E.g.Flower color in four- O’- clock plants (Mirabilis Jalapa) and Snapdragon (Antirrhinum majus), feather color in Blue Andalusian fowl

The phenotypic ratio for the monohybrid cross becomes 1:2:1 instead of 3:1.

CO-DOMINANCE The co-dominance is the type of interaction in which both

the alleles of gene are equally expressed so that the phenotype of the heterozygote exhibits a mixture of phenotypes of both homozygotes.

Or If the heterozygote exhibits a mixture of the phenotypic

characters of both homozygotes, instead of a single intermediate expression, then both alleles are called co-dominant alleles.

The genotypic and phenotypic ratios for monohybrids are 1:2:1 and 1:2:1.

E.g. MN blood group antigens in human:

Allele LM for M-type blood is codominant with allele LN for N-type blood. The heterozygotes LMLN will have both M and N antigens on the red blood cells.

Sr. Incomplete dominance Co-dominance1. Allelic interaction in which one of the

two alleles is more expressed in the heterozygote

Allelic interaction in which one of the two alleles are equally expressed in the heterozygote

2 Phenotype of the heterozygote is intermediate to the phenotype of the two homozygote

Phenotype of the heterozygote shows the phenotype of the both homozygote

3 Alleles determining such traits are referred to as incompletely dominant allele

Alleles determining such trait referred as co-dominant alleles

4 e.g. Flower color in four- O’- clock plants (Mirabilis Jalapa

Feather color in Blue Andalusian fowl

Coat color in cattle

e.g. Coat color in cattle

MN blood groups in human

Species Trait Genotypes Phenotypes Species Trait Genotypes Phenotypes

Horse Palomino coat color

CC Chestnut (reddish)

Human

MN blood group

MM M blood group

Ccr Ccr Cremello (Whitish) NN N blood group

C Ccr Palomino (Golden yellow body with white mane and tail)

MN MN blood group

CattleCoat color in Short-horn cattle

CR CR Red

Hair structure

A1A1 Straight hair

CW CW White A2A2 Curly hair

CR CW Roan A1A2 Wavy hair

Poultry

Feather structure

FN FN or NN NormalHaemoglobin type

HbA HbA Type A

FW FW or WW Wooly HbS HbS Type S

FN FW or NW Frizzled HbA HbS Type AS

Feather color in Blue Andalusian fowl

BB or FB FB Black

WW or FW FW White

BW or FB FW Blue

Naked neck

Na Na Naked neck

na na Normal neck

Na na Naked neck with tuft of feathers

Some examples of incomplete dominance and co-dominance

OVER DOMINANCE An allelic interaction in which the phenotypic

expression of the heterozygote exceeds the phenotypic expression of either of the two homozygotes.

e.g. survival rate against Malarial parasites in sickle cell anaemia

Genotype HbA HbA HbS HbS HbA HbS

Survival Rate High Very low Highest

Lethal Gene Lethal genes are those that cause the death of the

young during pregnancy or at birth.

Genes which affect the viability of an organism are called lethal genes and the phenomenon is called lethality.

Sub lethal and Semi lethal are those genes which cause the death of the young after birth or some time later in life.

Lethal genes can be recessive, dominant, conditional, semilethal / sublethal, or synthetic, depending on the gene or genes involved.

If the lethal effect is dominant and immediate in expression, all individuals carrying the gene will die and the gene will be lost.

Dominant lethal genes are expressed in both homozygotes and heterozygotes. All individuals carrying the genes will die and the genes will be lost in populations.

Recessive lethal allele carried in the heterozygous condition has no effect but they cause death when an organism carries two copies of the lethal allele.

Recessive lethal may come to expression when mating between carriers occurs.

The phonotypic ratio is modified in to 2:1.

1. YELLOW FUR IN MICE

2 .CREEPER FOWL CONDITION

3. ACHONDROPLASIA IN CATTLE

HOW TO DETECT AND ELIMINATE LETHAL GENES?

Elimination of lethal genes from the population could be carried out by identifying the carriers (heterozygotes) and preventing them from further breeding.

Intermediate lethal genes are much easier to detect because all the individuals will exhibit some phenotypic expression of the gene.

Dominant lethals kill the individual either in homozygous or heterozygous conditions and therefore is eliminated from the population in the same generation in which it arises.

Recessive lethals kill only when in homzygous stage. They are very difficult to eliminate from the population. Heterozygous carrier parents that produce a lethal effect could be used as testers to identify others in the population.

1. Interaction between alleles at same locus / Intra-locular interaction / intra-allelic interaction

(Already discussed)Now……..2. Interaction between alleles at different locus (Non allelic interaction/ Inter-locular

interaction or inter-allelic interaction)

The phenotypic expression of alleles of one locus is modified by the alleles of the other loci is called as epistasis.

The phenomenon of two or more genes which affects single trait, in such a way that they affect the expressions of each other in various ways is known as Gene Interaction.

The gene/locus that blocks the expression of an allelic gene/locus is said to be epistatic and the gene/locus whose expression is blocked is said to be hypostatic.

When independent (non-homologous) genes located on the same or on different chromosomes interact with one another for the expression of single phenotypic trait then it is known as Inter-Allelic Interactions.

Gene interaction may involve two or more genes.

Two interacting genes produce modified dihybrid ratios.

Two genes influencing the same character Example: Comb pattern in poultry Normally, certain specific breeds have a specific comb

pattern. • Wyandotte – Rose comb • Brahmas – Pea comb • Leghorns – Single comb

Crosses between Rose combed and single combed variety showed that Rose was dominant over single and a 3:1 ratio appeared in the F2.

Crosses between Pea combed and single combed variety showed that Pea was dominant over single and a 3:1 ratio appeared in the F2.

Rose Pea

Single Walnut

When Rose was crossed with Pea, all the offspring showed a new comb form known as “Walnut”

When the F1 Walnut combed birds were inbred, in the F2 generation Walnut, Rose, Pea and Single combed ones also appeared.

The phenotypic ratio in F2 is 9:3:3:1 ratio. In this ratio, out of 16 progenies, nine Walnut comb, three Rose comb, three Pea comb, one Single comb.

Differences from normal dihybrid inheritance are • The F1 resembles neither the parent (Walnut

comb) • Apparently novel characters appear in F2 (Single

comb) • Walnut character results from an interaction

between two independently inherited dominant Rose and Pea genes.

Single comb results from interaction of their two recessive alleles.

Recessive Epistasis also called as “Supplementary gene action”When one gene is homozygous recessive, it hides the phenotype of the other gene. (cc epistatic to R and r )

When recessive alleles at one locus mask the expression of both (dominant and recessive) alleles at another locus it is known as recessive epistasis.

Dominant Epistasis also called as “Masking gene action”When one gene is dominant, it hides the phenotype of the other gene. (I epistatic to B and b ) When a dominant allele at one locus can mask the expression of both alleles (dominant and recessive) at another locus, it is known as dominant epistasis.

Dominant-Recessive Epistasis also called as “Inhibitory gene action”When either gene is dominant, it hides the effects of the other gene. (I epistatic to C and c, cc epistatic to I and i, So, I and cc produce identical phenotypes)

A dominant allele at one locus can mask the expression of both (dominant and recessive) alleles at second locus.

Duplicate Recessive Epistasis also called as “Complementary gene action”When either gene is homozygous recessive, it hides the effect of the other gene. (cc epistatic to P & p,pp epistatic to C and c )

When recessive alleles at either of the two loci can mask the expression of dominant alleles at the two loci, it is called duplicate recessive epistasis.

Duplicate dominance Epistasis also called as “Duplicate gene action”When either gene is dominant, it hides the effects of the other gene.(F epistatic to ss,S epistatic to ff)

When a dominant allele at either of two loci can mask the expression of recessive alleles at the two loci, it is known as duplicate dominant epistasis.

Duplicate gene with Interaction also called as “Polymeric gene action”•When both genes are dominant, it hides the effects of recessive allele.(R and S interact)•When only either gene is dominant, it alone can not hides the effects of the other gene. (only R can not hide the effects of rr and ss) (only S can not hide the effects of rr and ss)•Two dominant alleles have similar effect when they are separate, but produce enhanced effect when they come together. Such gene interaction is known as polymeric gene interaction.

GENETIC EXPLANATION F2 PHENOTYPIC RATIOAABB AABb AaBB AaBb AAbb Aabb aaBB aaBb Aabb

Classical Dihybrid Ratio 9 3 3 1Recessive Epistasis When one gene is homozygous recessive, it hides the phenotype of the other gene. (aa epistatic to B and b )

9 3 4

Dominant Epistasis When one gene is dominant, it hides the phenotype of the other gene. (A epistatic to B and b )

12 3 1

Dominant and Recessive Epistasis When either gene is dominant, it hides the effects of the other gene. (A epistatic to B and b, bb epistatic to A and a, A and bb produce identical phenotypes)

13 3

Duplicate Recessive Epistasis When either gene is homozygous recessive, it hides the effect of the other gene. (aa epistatic to Bb, bb epistatic to A and a )

9 7

Duplicate Dominant Epistasis When either gene is dominant, it hides the effects of the other gene.(A epistatic to B and b, B eptistatic to A and a )

15 1

Duplicate Interaction When either gene is dominant, it hides the effects of the other gene. (A and B interact)

9 6 1

Complete dominance at one locus and Incomplete dominance at another locus (co-dominance) 3 6 1 2 3 1

Complete dominance lacking at either locus (co-dominance at both locus) 1 2 2 4 1 2 1 2 1

Homozygous recessive lethal at either locus 1 2 2 4 0