Lethal Homozygous Alleles and Their
|CA GR9-12 7.b.|
Before we start the lesson let, us review some terms used in prior lessons:
Locus - (plural= "loci") Refers to a specific location on a chromosome that affects a certain trait or characteristic. Each individual possesses two alleles at any particular locus.
Allele - One-half of a "gene pair.” Each "gene" consists of two alleles. An allele is a segment of "genetic instructions" that affect a trait or characteristic.
Trait or Characteristic - The physical manifestation of gene actions. A trait could be blue eyes, yellow coat color, and so forth.
Genotype - Refers to the exact genes and alleles (genetic code) an
Example: Huntington's is a slow degenerative disease of the nervous system the age of onset is not until later in life (30 yr-50yr).
Conditional lethal alleles may only kill the organism under
A recessive lethal allele is lethal only if two copies of the lethal allele are presented (homozygous).
Sometimes the heterozygote has a unique phenotype. The other normal allele is sufficient to compensate for the mutant one.
A dominant lethal allele is lethal even when only one copy of the allele is present.
Therefore, organisms homozygous or heterozygous for the lethal allele will die. Example: Huntington’s disease
Lethal alleles are masked by a dominant allele that permits the individual to survive and pass on the recessive to future generation.
A lethal allele may become quite common if it is closely linked to a beneficial allele of another gene or if the heterozygous conditions have some advantage as in the case of sickle cell anemia.
Example of other conditions that involve lethal alleles include the following:
Sickle cell anemia-in humans - the allele responsible for sickle cell
anemia is often lethal in the homozygous condition. The gene involves codes
for the polypeptide chain of hemoglobin, the oxygen-carrying protein found
in red blood cells and responsible for their red color. The sickle allele
results from a point mutation: a change in just one nucleotide pair, which
in this case substitutes valine for glutaimic acid. This recessive condition
is characterized by fragile red blood cells that collapse into a sickle cell
shape when the oxygen concentration is low. These cells may clog the blood
vessels and they break down more easily than normal cells. Persons with the
disease usually die at an early age.
Usually, if a gene is important, the body makes more than enough gene product to compensate for the loss of one copy of the gene. Therefore, mutations in such genes are usually recessive and are called recessive lethal alleles because they cause the death of the embryo only in the homozygous state. Often, the development of such embryos is fatal.
Each one of us is a carrier for a few lethal alleles. Fortunately, it is unlikely that we will mate with someone who is a carrier for a lethal or sub-lethal allele at the same gene locus. If we do, an average of ¼ of our children will either be severely malformed or not survive.
The inheritance of a lethal allele is difficult to follow in a family because only normal embryos will survive to term. However, the inheritance of a lethal allele can be followed through a family history if the allele also acts as a dominant allele for an unrelated phenotype.
However, individuals who carry one functional copy and one mutant copy (i.e. heterozygotes) do not develop entirely normally. They usually end up with extra fingers and/or toes. In this way, the allele acts as a dominant allele for polydactyl (P).
When polydactyl is followed through a family history, a strange phenomenon is observed. If two persons with polydactyl mate, a simple Punnet Square would predict that ¾ of the offspring would also have polydactyl. (Remember that the mutant polydactyl allele, “B” is the dominant allele and “b” is the normal allele.) We did this in former lessons.
The roles that lethal alleles play is that during species reproduction, only heterozygous offspring will survive and carry on the genes to the next generation. If those traits are heritable, they pass them to their offspring, with the result that beneficial heritable traits become more common in the next generation. The ones that do survive will thrive or have a mutation and carry a malformation and maintaining the possibility of carrying the gene on to future generations.
Experiments for Home and Classroom
There is a Teacher's Guide to this learning module at:
New Mutations (top)