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• Mendelian inheritance describes
inheritance patterns that obey two laws
– Law of segregation
– Law of independent assortment
• Simple Mendelian inheritance involves
– A single gene with two different alleles
– Alleles display a simple dominant/recessive
• In this chapter we will examine traits that
deviate from the simple dominant/recessive
• The inheritance patterns of these traits still
obey Mendelian laws
– However, they are more complex and
interesting than Mendel had realized
• There are many ways in which two alleles
of a single gene may govern the outcome of
a trait
• Mendelian inheritance describes several
patterns that involve single gene. What are
they? (submit 19/1/2011)
– These patterns are examined with two goals in mind
• 1. Understanding the relationship between the
molecular expression of a gene and the trait itself
• 2. The outcome of crosses
• Prevalent alleles in a population are termed wildtype alleles
– These typically encode proteins that
• Function normally
• Are made in the right amounts
• Alleles that have been altered by mutation are
termed mutant alleles
– These tend to be rare in natural populations
– They are likely to cause a reduction in the amount or
function of the encoded protein
– Such mutant alleles are often inherited in a recessive
• Consider, for example, the traits that Mendel studied
Wild-type (dominant) allele Mutant (recessive) allele
Purple flowers
White flowers
Axial flowers
Terminal flowers
Yellow seeds
Green seeds
Round seeds
Wrinkled seeds
Smooth pods
Constricted pods
Green pods
Yellow pods
Tall plants
Another example is from Drosophila
Wild-type (dominant) allele Mutant (recessive) allele
Red eyes
White eyes
Normal wings
Miniature wings
• Genetic diseases are caused by mutant alleles
• In many human genetic diseases , the recessive allele contains
a mutation. What kind of genetic diseases that you know of
containing recessive alleles? (Submit 19/1/2011)
– Preventing the allele from producing a fully functional
• In a simple dominant/recessive relationship, the
recessive allele does not affect the phenotype of the
– So how can the wild-type phenotype of the
heterozygote be explained?
Lethal Alleles
• Essential genes are those that are absolutely
required for survival
– The absence of their protein product leads to a lethal
• Nonessential genes are those not absolutely
required for survival
• A lethal allele is one that has the potential to
cause the death of an organism
– Resulting mutations in essential genes
– Inherited in a recessive manner
• Many lethal alleles prevent cell division
– These will kill an organism at an early age
• Some lethal alleles exert their effect later in life
– Huntington disease
• Characterized by progressive degeneration of the nervous
system, dementia and early death
• The age of onset of the disease is usually between 30 to 50
• Conditional lethal alleles may kill an organism only
when certain environmental conditions prevail
– Temperature-sensitive (ts) lethals
• A developing Drosophila larva may be killed at 30 C
• But it will survive if grown at 22 C
• Semilethal alleles
– Kill some individuals in a population, not all of them
– Environmental factors and other genes may help
prevent the detrimental effects of semilethal genes
• A lethal allele may produce ratios that seemingly
deviate from Mendelian ratios
• An example is the “creeper” allele in chicken
– Creepers have shortened legs and must creep along
– Such birds also have shortened wings
– Creeper chicken are heterozygous
Creeper X Normal
Creeper X Creeper
1 creeper : 1 normal
1 normal : 2 creeper
Creeper is a dominant allele
Creeper is lethal in the
homozygous state
Incomplete Dominance
• In incomplete dominance the heterozygote
exhibits a phenotype that is intermediate
between the corresponding homozygotes
• Example:
– Four o’clock (Mirabilis jalapa) flower color
– Two alleles
• CR = wild-type allele for red flower color
• CW = allele for white flower color
1:2:1 phenotypic
ratio NOT the 3:1
ratio observed in
simple Mendelian
In this case, 50% of
the CR protein is not
sufficient to produce
the red phenotype
Figure 2
Incomplete Dominance
• Whether a trait is dominant or incompletely
dominant may depend on how closely the trait is
• Take, for example, the characteristic of pea shape
– Mendel visually concluded that
• RR and Rr genotypes produced round peas
• rr genotypes produced wrinkled peas
– However, a microscopic examination of round peas
reveals that not all round peas are “created equal”
Multiple Alleles
• Many genes have multiple alleles
– Three or more different alleles
• An interesting example is coat color in rabbits
– Four different alleles
• C (full coat color)
• cch (chinchilla pattern of coat color)
– Partial defect in pigmentation
• ch (himalayan pattern of coat color)
– Pigmentation in only certain parts of the body
• c (albino)
– Lack of pigmentation
– The dominance hierarchy is as follows:
• C > cch > ch > c
– Figure 4 illustrates the relationship between
phenotype and genotype
• Agouti (wild type)
• Chinchilla (mutant)
• Himalayan(mutant)
• Light grey
• Albino (mutant)
c+c+, c+cch, c+ch, c+c
cchch, cchc
• Caused by tyrosinase; producing melanin
• Two types of melanin: eumelanin (black
pigment) and phaeomelanin (orange/yellow
• The himalayan pattern of coat color is an example
of a temperature-sensitive conditional allele
– The enzyme encoded by this gene is functional only at
low temperatures
• Therefore, dark fur will only occur in cooler areas of the body
• This is also the case in the Siamese pattern of coat color in cats
• The ABO blood group provides another example of
multiple alleles
• It is determined by the type of antigen present on
the surface of red blood cells
– WHAT ARE ANTIGENS? (Submit 19/1/2011)
• As shown in Table 1, there are three different types
of antigens found on red blood
– Antigen A, which is controlled by allele IA
– Antigen B, which is controlled by allele IB
– Antigen O, which is controlled by allele i
• Allele i is recessive to both IA and IB
• Alleles IA and IB are codominant
– They are both expressed in a heterozygous individual
• The carbohydrate tree on the surface of RBCs is
composed of three sugars
• A fourth can be added by the enzyme glycosyl
– The i allele encodes a defective enzyme
• The carbohydrate tree is unchanged
– IA encodes a form of the enzyme that can add the sugar
N-acetylgalactosamine to the carbohydrate tree
– IB encodes a form of the enzyme that can add the sugar
galactose to the carbohydrate tree
• Thus, the A and B antigens are different enough to
be recognized by different antibodies
• For safe blood transfusions to occur, the donor’s
blood must be an appropriate match with the
recipient’s blood
• For example, if a type O individual received blood
from a type A, type B or type AB blood
– Antibodies in the recipient blood will react with antigens in
the donated blood cells
– This causes the donated blood to agglutinate
– A life-threatening situation may result because of clogging
of blood vessels