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 level, R/R=R/r.
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 PAH gene is also said to be
Hence, both P/p and P/P doses are enough to result in
normal cellular chemistry.
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 abnormalities.
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 result is-
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 incomplete dominance.
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 biconcave shape
HbS/HbS : 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 allele.
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
Yellow x Yellow ? 2/3 yellow, 1/3 wild type
The tailless Manx phenotype in cats, is produced by an
allele that is lethal in the homozygous condition. A single dose of the Manx
allele ML, interferes with the normal spinal development in such a
way, that the cat born are tailless in case of ML/M heterozygote. Whereas
the ML/ML homozygotes do not survive the embryonic stage.
Whether an allele is lethal or not, depends on the
environment in which the organism has to survive. Human hereditary diseases, for
example, Sickle cell anaemia and Cystic fibrosis are deadly without treatment.
TEMPERATURE SENSITIVE ALLELES
There are certain alleles that are members of a
general class of temperature-sensitive
mutations. Their phenotype is wild type at the permissive temperature (often room temperature) but mutant at some higher
Temperature sensitive alleles are thought to be caused
by twisting/ bending of protein prone shape to an inactive conformation at the