LECTURE 4

2.2 Crosses that deviate from the Mendelian inheritance. 2.2.1 Codominant allele 2.2.2 Incomplete dominant allele 2.2.3 Multiple alleles

2.2.4 Polygenes/polygenic inheritance

OBJECTIVES

At the end of the lesson, students should be able to :

• Explain codominant alleles.• Calculate the genotypic and phenotypic

ratio (1:2:1).• Explain incomplete dominant alleles.• Calculate the genotypic and phenotypic

ratio (1:2:1).• Explain multiple alleles• Explain polygenes/polygenic inheritance

Codominant allele

• both alleles of a pair are fully expressed in a heterozygous form

• Eg : The existence of three different human blood groups called the M,N and MN blood groups.

• Another example is ABO human blood group.

• Genotypic ratio 1:2:1

ABO Blood Group

IOIO / iiO

IAIBAB

IBIB , IBIOB

IAIA , IAIOA

GenotypePhenotype

(Blood group)

e.g: Cross between LMLM and LN LN

P : LMLM X LNLN

G : LM LM LN LN

F1 : LM LN

F1 X F1

P : LM LN X LM LN

G : LM LN LM LN

F2 : LM LM LM LN LMLN LNLN

Phenotypic ratio:- 1 producing M antigen: 2 producing both antigens: 1 producing N antigen

Genotypic ratio:- 1 LM LM :2 LM LN :1LNLN

• Different from Mendel’s law because the phenotypic ratio is 1:2:1 in F2

generation instead of 3:1.

Incomplete Dominant Allele• One allele is not fully dominant over its

partner, so in the heterozygous condition, the total product is intermediate between that of the dominant and recessive alleles.

• Eg :color of snapdragon flower Antirrhinum.• Heterozygotes for color alleles have pink

colors in contrast to red ( dominant homozygotes) and white (recessive heterozygotes).

• The phenotypic ratio for the monohybrid cross then becomes 1:2:1 instead of 3:1. (for F2)

Incomplete dominance in snapdragons

Summary of dominance relationships

Multiple Alleles

• When more than two different forms of alleles, they are referred to as multiple alleles.

• Eg : ABO blood type alleles in humans.

• There are 4 blood types : A, B, AB and O.

• The ABO locus has three common alleles : IA , IB , IO.

Multiple alleles control the ABO blood groups.

Poligenes

• In many characters such as human skin color and height, an either-or classification is impossible, because the characters vary in the population gradually.

• These are called quantitative characters. • Quantitative variation usually indicates

polygenic inheritance, an additive effect of two or more genes on a single phenotypic character.

• For example, skin pigmentation in humans is controlled by at least three (probably more) separately inherited genes.

• Let us consider three genes, with a dark-skin allele for each gene (A,B,C) contributing one ‘unit’ of darkness to the phenotype and being incompletely dominant to the other alleles (a,b,c).

• An AABBCC person would be very dark, while an aabbcc individual would be very light.

• An AaBbCc person would have skin of an intermediate shade.

• Because the alleles have a cumulative effect, the genotypes AaBbCc and AABbcc would make the same genetic contribution (three units) to skin darkness.

• This polygenic inheritance could result in a bell-shaped curve, called a normal distribution, for skin darkness among the members of a hypothetical population.

• Environmental factors, such as exposure to the sun, also affect the skin-color phenotype.

Individuals based on degrees of skin darkness.

PPHHVery darkXXXX

PPHh; PpHhDarkXXX

PPhh; ppHH; PpHh

Quite darkXX

ppHh ; PphhFairX

pphhVery fair-

GenotypePhenotype0 darkness

Differences between multiple alleles and polygenes

- Phenotypes that controlled by polygenes can be influence by environmental factors while multiple alleles is not influence by environmental factors

- For multiple alleles, phenotypes control involved genes that have multiple alleles (exist more than two allelic forms) while in polygenes the phenotype is controlled by more than one gene

• Polygenic traits involved continuous variation/ quantitative variation while multiple alleles traits are expressed as discontinuous variation.