A fully dominant allele will be able to express itself only
when one copy is present ?as in heterozygous condition, whereas the alternative
allele will be fully recessive.
When, in case of full dominance, the homozygous dominant
cannot be differentiated from heterozygous dominant; that is at the phenotypic
For example: The disease the Phenylketonuria is case of recessive
mutations. PKU is caused by a defective allele coding for the enzyme Phenylalanine
hydroxylase(PAH). In the absence of PAH, the phenylalanine in the food is not
broken down in the body and thus accumulates. As a result of which phenylalanine
is converted into phenyl pyruvic acid which reaches he blood through the
bloodstream and there impedes normal development, leading to mental retardation.
Why is defective allele recessive?
One “dose” of the wild-type allele produces enough PAH to
breakdown the phenylalanine entering the body. The PSH gene is also said to be
Hence, both P/p and P/P doses are enough to result in normal
Assume that 16 units of a gene’s product are needed for
normal chemistry and that each wild type allele can produce 10 units. Then two
wild type alleles will produce 2 units. But what will happen if one of the
mutation is a null mutation which produces non-functional protein. Then a null
mutation with a single type of wild allele will produce 10+0= 10 units.
This is known as haplo insufficiency; the same responsible
for DiGeorge syndrome in humans, a condition with cardiovascular and craniofacial
Polypeptides with this type of mutation act as ‘spoilers’ or
‘rogues’. In some cases, the gene product is a unit of homodimeric protein—composed
of two units of the same type. In the heterozygote (+/M), the spoiler polypeptide
binds to the wild polypeptide and distorts it or interferes with its function.
Example: in case of genes coding for collagen protein.
Some mutations in this give rise to osteogenesis imperfecta-brittle
bone disease. (Collagen protein is a trimer).
When a pure-breeding wild-type four-o’clock plant line
having red petals is crossed with a pure line having white petals, the F1
has pink petals. If the F2 is produced by selfing the F1,the
¼ of the plants have red petals
½ of the plant have pink petals
¼ of the plant have white petals
The occurrence of the intermediate character is a result of
A classic example is the ABO blood grouping, where there is
co dominance of antigen alleles. These three alleles interact in several ways
to produce the four blood types of blood groups of the ABO system. The combinations
result in six different genotypes:
IA/IA , IA/I A
IB/IB, IB/I B
The human disease sickle cell anaemia, the gene concerned
encodes molecule haemoglobin which is responsible for transport of oxygen in
blood vessels. The alleles HbA and HbS produce three
possible genotypes as follows:
: normal; red blood cells
: severe. Fatal anaemia, abnormal
haemoglobin causes red blood cells to become sickle cell
HBA/HbS : no anaemia; red blood cells only sickle shaped
under low oxygen concentrations.
RECESSIVE LETHAL ALLELES
An allele that is
capable of causing death of an organism is called a lethal all??ele.
The diagnostic test for lethality- the test is well illustrated
by one of the prototypic examples of a lethal allele, a coat-colour allele in
mice. Normal wild-type mice have coats with a dark pigmentation overall. A mutation
called yellow (coat colour) shows an interesting pattern. If any yellow mouse
is mated with a homozygous wild-type mouse, a 1:1 ratio of yellow to wild type
mice is always observed in the progeny.
This suggests that a yellow mouse is always heterozygous for
the yellow allele and that the yellow allele is dominant over the wild type. However,
if any two yellow mice are crossed with each other, the result is,
Yellow x Yellow ? 2/3 yellow, 1/3 wild type