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
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.