This document discusses balanced lethal systems in organisms. It provides examples of balanced lethal systems in Drosophila involving the curly and plum genes, and in Oenothera plants. In Oenothera, there are two types of balanced lethal mechanisms - one involving gametic lethality and the other involving zygotic lethality. The balanced lethal systems ensure that only heterozygotes survive by eliminating homozygotes for lethal alleles.

1. Balanced
Lethal
System
Submitted By :
Akshita Awasthi
(A-2018-30-038)
Submitted To:
Dr R.K.Mittal &Dr Swaranlata

2. LETHAL GENES
 Some genes in an organism are known to cause the death of the
organism. These genes can be either dominant or recessive, but
the organism must be homozygous for them.
 Hence, alleles of a lethal gene show a deviation from the
normal Mendelian inheritance. For instance, the Mendelian
inheritance shows a phenotypic ratio of 3:1, whereas in the case
of expression of lethal alleles there is a deviation from this
ratio.
 Lethal alleles are produced when mutation in a usual allele
distorts the function of an essential gene. This in turn results in
a phenotype, which when expressed, is fatal to the organism
carrying them.

4.  When the allele is fully dominant it can cause the death of the
organism in both heterozygous and homozygous conditions. But
when the allele is recessive in nature the organism dies only when
the lethal allele is in the homozygous condition.
 Types of lethal alleles
 Lethal alleles can be further classified into the following:
 recessive lethals
 dominant lethals
 conditional lethals
 balanced lethals
 gametic lethals.

5. Balanced lethals
 Two different lethal systems can operate in an organism, and
balance each other's effect.
 Both genes are tightly linked in the repulsion phase of linkage
where a dominant allele of one gene is linked to a recessive
allele of another gene.
 In such cases, one parent contributes the dominant allele of
gene one and recessive allele of gene two, while the other parent
contributes the recessive allele of gene one and dominant allele
of gene two.
 This arrangement of lethal alleles maintains a heterozygous
combination, and ensures that homozygotes for lethal
chromosomes do not occur.

6. Balanced lethal system
 Balanced lethal system was first reported by Muller (1917) in
Drosophila where:
 if dominant alleles of two traits, each associated with a recessive
lethal effect, are present in heterozygous repulsion phase,
homozygotes will not survive and permanent hybridity will be
maintained.
 A similar balanced lethal system operates in Euoenotheras, so that
the heterozygous of different races is maintained in progeny over
generations.
 In Oenothera lamarckiana, the lethality is zygotic .

7. Balanced lethal systems of Drosophila would therefore,
lead to 50% ovule abortion or seed set.
In most species of Oenothera, two gametic complexes
are found and are called alpha(α) and beta (β) the former
being present in all functional egg cells and the latter in
all functional pollen grains. While beta is eliminated in
eggs, alpha is eliminated in all functional pollen, due to
failure of development of gametes containing these
respective gametic complexes.
This leads to 50% pollen abortion but a full seed set.
The full ovule fertility or seed set is due to the fact that,

8.  the two types of gametes will function irrespective of its
position in linear tetrad,only that megaspore functions
which carries alpha complex.
 Since alpha complex in the egg always unites with beta
complex in pollen, plants breed true for heterozygous
condition.
 Thus there are two balanced lethal mechanisms,one
having zygotic lethality and the other involving gametic
lethality. The gametic and zygotic lethality leads to
survival of only heterozygotes. It may be noticed that in
gametic lethality,only one of the male side, the other type
being functional

9.  on the female side, thus giving rise to only one type of progeny,
which will be heterozygous.
 In zygotic lethality on the other hand, both type of gametes will
function on male as well as on female side, but the
homozygotes progeny due to recessive lethal genes will not
survive.

10. Balanced Lethal system involving
curly(Cy), and plum (Pm) genes
Drosophila
Two principles involved in evolving
balanced lethal system in Oenothera.
(A) gametic lethality, (B) zygotic lethality.

11. Interchange heterozygosity in Oenothera
 Subgenus Euoenothera of genus Oenothera has been studied
during 1920-1930 & cytogenetic structure leading to evolution
in this group was examined. This group has 2n=14 & all 7
chromosomes of a haploid complement have median
centromeres. Different species in the subgenus Euoenothera,
can be classified in three groups:

12. i) first group is represented by species showing bivalents or small
rings at meiosis. e.g. O.hookeri, O.grandiflora
ii) Second group is represented by species forming rings of various
sizes at meiosis indicating the presence of interchanges. These
rings are not permanent but are maintained due to their
superiority in adaptive value. e.g. O.irrigua
iii) the third group is represented by those having permanent
translocation heterozygosity involving all chromosome so that a
ring of 14 chromosome is regularly formed .

13.  In O.lamarckiana, produces one bivalent and a ring of only
12 instead of a ring of 14 chromosomes is observed.
 In O.lamarckiana alternate segregation in the ring of 12
gave two complexes: 3.4, 12.11, 7.6, 5.8, 14.13, 10.9, and
4.12, 11.7, 6.5, 8.14, 13.10, 9.3, each also bearing the 1.2
chromosome of the segregating bivalent.
 Thus each complex of six chromosomes in this case would
be considered as a single linkage group.
 According to terminology invented by Renner, the two
complexes above were named velans and gaudens,
respectively.

15. The interesting and surprising, however, was that
O.lamarckiana bred true to these complexes and did not
produce either velans/velans or gaudens/gaudens, although
both homozygous are chromosomally balanced.
The answer to this puzzle came from Muller’s observation of
balanced lethal genes in Drosophila.
Apparently, recessive lethals were maintained in both the
velans and gaudens complexes, so that homozygous
combinations were lethal. In lamarckiana this lethality affects
the zygotes,

16.  so that half the seeds did not germinate.In other Oenothera
complexes, gametophytes were affected so that only the
male gametophytes of one complex were viable in the
female gametophytes of the other, i.e., only translocation
heterozygous combinations could be produced .

17. REFERENCES
 Cytology Genetics and Evolution :P.K.Gupta
 Genetics : M.W.Strickberger