This document discusses balanced and unbalanced lethal genes. It provides the example of the yellow mouse, which is lethal when homozygous for the yellow gene due to failure of yellow sperm to penetrate yellow eggs. It then describes balanced lethal systems, where two different recessive lethal genes balance each other's effects to allow survival of heterozygotes. Muller's discovery of the balanced lethal genes beaded and Le in Drosophila is described. The document also discusses chromosome complexes in Oenothera lamarckiana, where the alpha and beta gametic complexes lead to gametic lethality, while complexes gaudens and velans show zygotic lethality.

1. ACHARYA N.G. RANGAAGRICULTURAL UNIVERSITY
S.V. AGRICULTURAL COLLEGE , TIRUPATI
Department of Genetics and Plant Breeding
Balanced Lethals and Chromosome Complexes
Submitted by
M. Venkata Rama Sai
TAM/2022-24

2. Lethality
 Perhaps the most serious effect a gene can have on an
organism is lethality
 In 1905, a French geneticist reported the inheritance of a
mouse body colour gene ‘Yellow’ that did not appear to fit
into normal mendalian segregation
 In this case, Cuenot couldn’t find homozygous yellow
body coated mice, when back crossed yellow to agouti
and he assumed that it’s because the yellow sperm
couldn’t penetrate yellow egg
 But later, Castle and Little offered explanation that the
yellow homozygous progeny did formed but it was died
in utero

3.  According to them yellow had a dominant effect on coat colour trait but
recessive effect on lethality, so that homozygous yellow are inviable
 A cross of Yellow x Yellow always produced 2:1 (Heterozygous yellow :
Agouti) instead of 1:2:1
 Lethal mutations are produced when mutation in a usual allele distorts the
function of an essential gene . This in turn results in a phenotype when
expressed, lethal to the concerned organism
 Types of lethal alleles:
1) Dominant lethals
2) Recessive lethals
3) Conditional lethals
4) Gametic lethals
5) Balanced lethals

4. Balanced lethals
 Two different lethal systems operate in an organism and balance
each other’s effects – Balanced lethals
 It was first reported by ‘Muller’ in Drosophila
 Muller observed a peculiar stock of drosophila flies which are
always heterozygous for a particular lethal gene ‘beaded’.

5.  This beaded has dominant effect on wing shape as well
as it is lethal in homozygous state. So all beaded winged
ones are heterozygous ( Similar to heterozygous yellow
mouse) and the recessive ones are normal winged ones.
 A cross of Beaded x Beaded should result in 2:1 Beaded
heterozygotes and recessive normal winged homozygotes.
 But Muller didn’t even observed normal winged ones in
this unusual stock. Since the beaded ones are
heterozygous and normal ones are homozygous the
question arised was what has happened to homozygotes?

6.  Then Muller found that the homozygous chromosome carrying
normal allele for beaded also carried a gene that was lethal in
homozygous condition i.e., ‘Le’
 Since beaded and Le are recessive lethals and are not allelic,
only heterozygous beaded are survived when two beaded
heterozygotes are crossed.
 The beaded heterozygotes are therefore self perpetuating as long
as the homologous chromosome also contains a different
recessive lethal factors.
 This balancing effect between two different lethals in a self
perpetuating stock was named as Balanced lethals

7.  Here both genes are tightly linked in a repulsion phase, where
a dominant allele of one gene is linked to a recessive allele of
other 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 the survival of the Heterozygotes and
maintains permanent hybridity. At the same time it causes the
lethality of both homozygotes.

8. B le
b Le
B le
b Le
B le b Le
B le
b Le
B le
b Le
B le
b Le
X
B le
B le
b Le
b Le
DIES
DIES
PARENTS
PROGENY

9. Balanced lethal Mechanisms:
 There are two balanced lethal mechanisms : 1) Zygotic lethality
2) Gametic lethality
 Both the Gametic and Zygotic lethality leads to production of
heterozygotes
 Zygotic lethality: In this, both types of gametes will be functional
on male as well as on female side. But the homozygote progeny do
not survive due to recessive lethals. Ex: Oenothera lamarckiana
 Gametic lethality : In both male and female gametes, certain type
of non-functional gametes are produced and only 50 percent of
gametes are viable. Ex: biennis, Muricata etc
Oenothera lamarckina

10. CHROMOSOME COMPLEXES
 De Vries experimented on a plant called Oenothera lamarckiana.
When Oenothera was self pollinated or crossed with other races most of
its progeny are similar to the parents, but a few were different plants
 It was later revealed that most of the variants isolated are due to its
anamolous chromosome behaviour given by Otto Renner
 The chromosome forms multiple ring at meiotic metaphase and
telomeres only involve in recombination and plant shows complex
heterozygosity i.e, maternal and paternal chromosomes are arranged in
alternate disjunction.
 In Oenothera sps. each functional gamete normally contains seven
chromosomes which together maybe described as gametic complex.
Each species has two such gametic complexes and are also called as
Renner complexes

11. Gametic lethality in Oenothera lamarckiana
Thus, in Oenothera the two gametic complexes are found
and are called alpha α and beta β.
The alpha is present in functional egg cells and beta in
functional pollen grains . Similarly the egg cells containing
beta are non functional and pollen containing alpha are
non functional.
This leads to 50 percent gamete abortion but a full seed
set. This is due to the fact that in female gametophyte the
megaspore which is carrying alpha complex only forms
embryo.
Thus alpha complex always unites with beta complex in
pollen, plants breed true for heterozygous condition.
α β
α
β
αβ
Gametic lethality

12.  Although only two complexes are present in each race , these gene complexes
differ in different races so that a large number of complexes occur in Oenothera.
 Each complex is given a specific name and are collectively called as Renner
complexes.
 Ex: 1) biennis – albicans and rubens = only Rubens type pollen and albicans egg
2) Muricata – rigens and curvans = only curvans pollen and rigens egg
3) lamarckiana – gaudens and flavens = gaudens and flavens pollen
Zygotic lethality :
 When lamarckiana is self fertilized half of seed did not germinate. But when it is
crossed with other races all seeds are viable
 Then Renner assumed that half of seed or embryo must be undergoing
homozygotic lethality and the plant must be heterozygous for two gene
complexes

13.  For instance , in Oenothera lamarckiana, complexes are gaudens
and velans which is indicated by G and V .
 These complexes are lethal when present in homozygous condition
and the lethality occurs in zygote condition.
 Here the gametes are fully functional in both males and females and
both the homozygous zygotes undergoes zygotic lethality.
 Thus the self perpetuating heterozygotes are produced here since
because of the presence of two lethal systems called Balanced lethals
GG GV
GV VV
G V
G
V

15. References:
 Cytogenetics – Genetics and evolution : P. K. Gupta
 Genetics : Strickberger
 Renner’s studies in Oenothera lamarckiana research paper