Chapter 4: Modification of Mendelian RatiosAllele

*Wild-type allele*Mutant allele

Conventional symbols for alleles:

recessive allele - initial letter of the name of the recessive trait, lowercased and italicized

dominant allele - same letter in uppercase

F1 Generation

RR rr

Gametes

P Generation

F2 Generation Sperm

Gametes

Red White

R r

RrPink

R r

R rR

r

RR Rr

rrRr

Eggs12

12

12

12

12

12

In incomplete dominance, F1 hybrids have an appearance in between the phenotypes of the two parents.

Homozygousfor ability to make

LDL receptors

Severe diseaseMild diseaseCell

Normal

LDLreceptor

LDL

Homozygousfor inability to make

LDL receptors

HeterozygousHH Hh hh

GEN

OTY

PEPH

ENO

TYPE

Hypercholesterolemia•Dangerously high levels of cholesterol in the blood.•Is a human trait that is incompletely dominant.•Heterozygotes have blood cholesterol levels about 2X normal.•Homozygotes have blood cholesterol levels about 5X normal.

Codominance:

Condition in which the phenotypic effects of a genes alleles are fully and simultaneously expressed in the heterozygote.

Example:

MN Blood group - red blood cells contain a transmembrane glycoprotein (glycophorin); two different forms of this protein exist, M and N

BloodGroup

(Phenotype)Genotypes Red Blood Cells

O

A

B

AB

ii

IAIB

IBIB

orIBi

IAIA

or IAi

Carbohydrate A

Carbohydrate B

Multiple Alleles

Examples:

*Table 4.1: over 100 alleles at a given locus in Drosophila

*ABO Blood group in humans

*Characterized by the presence of glycoprotein antigens on the surface of red blood cells

*Distinct from the M and N antigens

*Also exhibits codomiance

Lethal Alleles: Example: Coat color in mice*A = agouti = wild-type allele*AY = yellow = mutant allele

EpistasisExample: In Drosophila, gene:eyeless

*Black (B) is dominant to brown (b)

*Second gene responsible for allowing pigment to be deposited in hair,

C = presencec = absence (colorless)

*Gene product interactions

– Pleiotropy is the impact of a single gene on more than one character.

Pleiotropy Multiple traits(e.g., sickle-celldisease)

Singlegene

Multiple genes

Polygenicinheritance Single trait

(e.g., skin color)

– Polygenic inheritance is the additive effects of two or more genes on a single phenotype.

F1 Generation

P Generation

F2 Generation Sperm

AABBCC(very dark)

Eggs

aabbcc(very light)

AaBbCcAaBbCc

18

164 64

66415

6420

6415

646 1

64

18181818181818

18

18

18

18

18

18

18

18