RECAP• Manx cat

M m

M MM Mm

m Mm mm

Manx

Death

Normal

PLEIOTROPY

• Is multiple effects of a single gene on the phenotype

• Occurs for several reasons– suppression of single gene can affect cell function in

more than one way– A gene may be expressed in different cell types in a

multicellular organism– A gene may be expressed at different stages of

development– Eg cystic fibrosis, inability to recycle salt causing extra

salty sweat

5- GENE INTERACTIONS

Compiled bySiti Sarah JumaliLevel 3 Room 14

Ext 2123

Polygene interactions

• Mendel studied one gene for height but he did not realize that there are other genes that also affect height

• i.e. – Tall strain KK LL MM NN OO PP QQ RR SS TT– Dwarf strain KK LL MM NN OO PP QQ RR SS tt

EPISTASIS

• An inheritance pattern in which the alleles of one gene mask the phenotypic effects of the alleles of a different gene

EPISTASIS• Occurs when: gene at one locus affects the expression of

an allele at a different locus b.• eg: fur color in mice • 1) B is allele for black fur

2) b is allele for brown fur 3) BUT, must have allele C at a different locus to have colored fur 4) if different locus has cc, then fur is colorless even if B or b is present 5) see what happens when BbCc mouse is crossed with BbCc mouse(a) expect a (9:3:3:1 ratio) (b) get a 9:3:4

Chicken comb (recessive epistasis)

WT Rose R_ppPea rrP_Walnut R_P_Single rrpp

R is dominant to rP is dominant to pRP is codominant (walnut)rrpp single comb

Epistasis on Chicken Comb

• Alleles of one gene mask the phenotype of alleles on another gene. In this case, rrpp masks a walnut comb

Epistasis on Flower

• 9:7 ratio is a modification of phenotypic but not genotypic Mendelian ratios.

• The C and P genes independently assort, the presence of a recessive genotype at one locus (i.e., cc or pp) masks the effects of the alleles at the other locus.

• There are 9 combinations of alleles in the F1 generation that feature at least 1 dominant C and 1 dominant P allele, which would yield a purple flower phenotype

• Conversely, there are seven combinations that result in either a cc or a pp, which would yield the white flower phenotype-hence, the 9:7 ratio of purple to white flowers.

9:7 phenotypic ratio

CP Cp cP cp

CP CCPP CCPp CcPP CcPp

Cp CCPp CCpp CcPp Ccpp

cP CcPP CcPp ccPP ccPp

cp CcPp Ccpp ccPp ccpp

• In this case, white masks purple petal

9:3:4 ratio

• Agouti x Agouti– BbCc x BbCc– Brown agouti &

albino mask agouti– c epistatic to B;

b epistatic to C

Ratio Description Name(s) of Relationship (Used by Some Authors)

9:3:3:1 Complete dominance at both gene pairs; new phenotypes result from interaction between dominant alleles, as well as from interaction between both homozygous recessives

Not named because the ratio looks likeindependent assortment

9:4:3 Complete dominance at both gene pairs; however, when 1 gene is homozygous recessive, it hides the phenotype of the other gene

Recessive epistasis

9:7 Complete dominance at both gene pairs; however, when either gene is homozygous recessive, it hides the effect of the other gene

Duplicate recessive epistasis

12:3:1 Complete dominance at both gene pairs; however, when one gene is dominant, it hides the phenotype of the other gene

Dominant epistasis

15:1 Complete dominance at both gene pairs; however, when either gene is dominant, it hides the effects of the other gene

Duplicate dominant epistasis

13:3 Complete dominance at both gene pairs; however, when eithergene is dominant, it hides the effects of the other gene

Dominant and recessive epistasis

9:6:1 Complete dominance at both gene pairs; however, when eithergene is dominant, it hides the effects of the other gene

Duplicate interaction

7:6:3 Complete dominance at one gene pair and partial dominance at the other; when homozygous recessive, the first gene is epistatic to the second gene

No name

3:6:3:4 Complete dominance at one gene pair and partial dominance at the other; when homozygous recessive, either gene hides the effects of the other gene; when both genes are homozygousrecessive, the second gene hides the effects of the first

No name

11:5 Complete dominance for both gene pairs only if both kinds ofdominant alleles are present; otherwise, the recessivephenotype appears

No name

• All of these are also called as gene modifier effect by other geneticists

Question• Manx cats are heterozygous for a dominant mutation that results

in no tails (or very short tails), large hind legs, and a distinctive gait. The mating of two Manx cats yields two Manx kittens for each normal, long-tailed kitten, rather than three-to-one as would be predicted from Mendelian genetics. Therefore, the mutation causing the Manx cat phenotype is likely a(n) __________ allele.

• A. pleiotropic • B. codominant • C. epistatic • D. lethal • E. sex-linked