mendelian inheritance definition

mendelian inheritance definition In organismswhich reproduce by the union of male and
female reproductive cells (or, gametes), the expression
of physical characteristics in offspring inherited from
their parents through transmitted genetic information
are governed by the principle of segregation and the
rules of gene expression. The principle of segregation
states that, in general, each copy of a gene present in
the parental genomes has an equal probability of being
transmitted to offspring to its homologue – a parent who
possesses the variants A and a of one gene is equally
likely to transmit either A or a to their offspring. However,
the characteristics expressed by the offspring once
its genome is set are governed by Mendelian modes of
trait inheritance. Some gene variants may be preferentially
expressed to others, and these variants are called
‘dominant’ while the less preferred variants are labelled
‘recessive’. For example, at a fictional gene GZ1, there
are two possible genetic variants, or ‘alleles’, B and b.
A family has a father with a GZ1 genotype, the identity
of both homologues at the site, or Bb; the mother
also has genotype Bb, and their three children 1, 2 and
3 each have genotypes of BB, Bb and bb respectively.
Accordingly, different manifestations of the trait influence
by GZ1 are observed depending on the mode of
inheritance:
1) If the allele B is dominant to the recessive allele
b, and we are interested in the trait governed by B,
we say that the trait follows a dominant Mendelian
mode of inheritance. That is, the trait expressed by
B will be expressed by any individual carrying even
one copy of B. In the family described, the mother,
father and children 1 and 2 all express the trait
associated with B.
2) If the allele B is dominant to the recessive allele b,
and we are interested in the trait governed by b, we
say that the trait follows a recessive mode of inheritance.
The trait expressed by b will be expressed in
individuals carrying no copies of B. In the family
described, only child 3 expresses the trait associated
with b.
3) If alleles B and b do not compete, but are equally
and/or differently expressed, we say that we observe
a codominant mode of inheritance. The expression
of the trait varies by genotype. In the family
described, the mother, father and child 2 will
express similar traits; child 1 and child 3 will express
traits different from each other and their other family
members.
4) If the allele B is dominant to the recessive allele b,
and the variation in the level of trait is based on the
number of copies of B which they possess, then we
say thatwe observedmode of inheritance is additive.
In the family described, child 1 would have the highest
level of the additively-inherited trait; the mother,
father and child 2 would have a similar level of the
trait; and child 3 would have the lowest level of the
trait.
Mendelian patterns of inheritance are similarly applied
to sex chromosomes as they are to autosomes. In
females, the above Mendelian patterns of inheritance
are applicable to genes present on the two X chromosomes,
while in males, whatever gene variant is present
at a particular site on the X chromosome is automatically
the variant which is expressed, as there is only
one homologue of X in males. All variants at all genes
on the Y chromosome are likewise expressed in males.