Transcript 3. LETHAL ALLELES

3. LETHAL ALLELES
Compiled by
Siti Sarah Jumali
Level 3 Room 14
Ext 2123
LETHAL ALLELES
• Alleles that cause an organism to die only when present in
homozygous condition, where the gene involved is an
essential gene.
• Instead of mutations alter the appearance of a living
organism, a mutant allele could cause death.
• When an essential gene is mutated, it can result in a
lethal phenotype.
• If the mutation is caused by a dominant lethal allele,
the heterozygote for the allele will show the lethal
phenotype, the homozygote dominant is impossible.
• If the mutation is caused by a recessive lethal allele, the
homozygote for the allele will have the lethal phenotype.
Most lethal genes are recessive.
What is it?
• Lethality refers to Genome changes of an organism which produce
effects severe enough to cause death. Lethality is a condition in which
death of a certain genotype occurs prematurely.
• Fully dominant lethal allele kills the carrier in both homozygous and
heterozygous conditions.
• Occasionally arises by mutation from a normal allele.
• Completely lethal genes usually cause death of the zygote, later in the
embryonic development or even after birth or hatching.
• Complete lethality, thus is the case where no individual of a certain
genotype attain the age of reproduction. However, in many cases
lethal genes become operative at the time the individuals become
sexually mature. Such lethal genes which handicap but do not destroy
their possessor are called subvital, sublethal or semilethal genes. The
lethal alleles modify the 3: 1 phenotypic ratio into 2 : 1.
• Many lethal alleles prevent cell division
– These will kill an organism at an early age
• Some lethal alleles exert their effect later in life
– Huntington disease
• Characterized by progressive degeneration of the nervous
system, dementia and early death
• The age of onset of the disease is usually between 30 to 50
• Conditional lethal alleles may kill an organism only
when certain environmental conditions prevail
– Temperature-sensitive (ts) lethals
• A developing Drosophila larva may be killed at 30 C
• But it will survive if grown at 22 C
• Semilethal alleles
– Kill some individuals in a population, not all of them
– Environmental factors and other genes may help
prevent the detrimental effects of semilethal genes
• A lethal allele may produce ratios that seemingly
deviate from Mendelian ratios
• An example is the “creeper” allele in chicken
– Creepers have shortened legs and must creep along
– Such birds also have shortened wings
– Creeper chicken are heterozygous
DOMINANT LETHAL ALLELES
• Modifies 3:1 ratio to 2:1
• Organism die before they can produce the
progeny.
• So, the mutant dominant lethal allele is
removed from the population in the same
generation it arose
• Fully dominant lethal allele kills the carrier in
both homozygous and heterozygous
conditions.
DOMINANT LETHAL ALLELES
CONT’D
Lethal allele segregation
A 2:1 ratio among the progeny of a cross results from the segregation of a lethal allele.
© 2005 W. H. Freeman Pierce, Benjamin. Genetics: A Conceptual Approach, 2nd ed. (New York:
W. H. Freeman and Company), 105. Used with permission. All rights reserved
Example
• Huntington's disease
- although it is not described as
lethal, it is invariably lethal in
that the victim experiences
gradual neural degeneration and
mental deterioration for some
years before death occurs.
The Pedigree
RECESSIVE LETHAL ALLELE
• The recessive lethal allele kills the carrier
individual only in homozygous condition. They
maybe of two kinds
(i) one which has no obvious phenotypic effect in
heterozygotes and
(ii) one which exhibits a distinctive phenotype
when in heterozygous condition.
• Occasionally arise from normal allele mutation
• In many cases, lethal genes become operative at
the onset of sexual maturity
examples
• Duchenne muscular dystrophy, cystic
fibrosis, or Tay-Sachs disease, sickle-cell
anemia etc
• Cystic fibrosis-due to a poorly functioning
Cl- ion transport protein in the lungs
• Tay-Sachs disease- due to an enzyme
which is unable to breakdown specific
membrane lipids
• many other lethal alleles are so lethal that
the offspring die as embryos
Other examples
• One coat color of ranch foxes
- caused by recessive lethal gene.
-This gene causes a death if both recessive
alleles are possessed by the same individual.
RECESSIVE LETHAL ALLELE