|
Home
|
Recessive Information Resources
The term recessive gene refers
to an allele that causes a detectable or visible characteristic that is only
seen in an organism with two copies of the same allele. Since each person
has two copies of every gene (one inherited from the mother and one from the
father), a recessive deficiency is only expressed if the offspring inherits
two recessive alleles (25% probability when both parents carry the recessive
trait).
Therefore, a recessive genetic disorder will only manifest in
patients who have received two copies of mutated genes.
Dominant phenes will produce characteristic traits that mask the recessive
traits from being expressed in the phenotype of an individual. A dominant
trait creates a heterozygous (Aa) genotype, with the dominant allele
effectively blocking instructions from the other recessive allele. For
example, the allele for blood type ‘A’ is dominant over the allele for blood
type ‘O’. For offspring to be produced with type ‘O’ blood, they must
inherit two of the recessive alleles from their parents.
The term ‘homozygous dominant’ (homo – same) refers to an individual carrying two
dominant alleles for a particular trait. Heterozygous (heteros - different )
refers to when the individual has one dominant and one recessive allele for
a particular trait.
The most straightforward type of dominance is ‘simple-’ or ‘complete
dominance’. This is the type described by Gregor Mendel in his pea
experiments, where the dominant allele is purple (P) and the recessive
allele is white (p).
In any individual plant, the alleles in a chromosome
pair can be either (PP) both purple, (pp) both alleles white or (Pp) one
purple and one white allele. Where the alleles are homozygous, the trait
they represent will be expressed. Individuals carrying heterozygous alleles
will only display the characteristic of the dominant allele – so (Pp) would
be a purple flower since the recessive allele is repressed.
Incomplete dominance (or partial dominance) is a heterozygous genotype that
produces a phenotype with intermediate characteristics. An example of this
type of partial dominance is the allele for red pigment in carnation flowers.
In this example (R) is the allele for red pigment, (R’) is the allele for no
pigment.
Offspring with an (RR) phenotype will produce a lot of red pigment,
resulting in red flowers. Similarly, (R’R’) offspring make no red pigment
and will produce white flowers. Offspring with either (R’R) or (RR’)
phenotypes will produce a moderate amount of red pigment, combining to
produce pink flowers. Before Mendel’s research was understood, it was widely
believed the pattern of inheritance for all organisms was this type of
mixture of characteristics, e.g. that a tall father and short mother would
produce a medium-height offspring.
Codominance means that neither phenotype is completely dominant. Instead, a
heterozygous individual will display the characteristics of both phenotypes.
For example, when two individuals with AB type blood have offspring, they
can have either A, B or AB type blood, giving a 1A: 2AB: 1B ratio. This
ratio is in comparison to the 3:1 ratio found when one allele is dominant
over the other for a particular phenotype.
Many genetic traits are not controlled by a single set of alleles, but
rather a complex assortment of different alleles, each with their own
dominance relationships. This means that establishing dominance patterns
between alleles may not be straightforward. |
