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8/10/2019 Genetic Basis of Heterosis
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GENENTIC B SIS
OF
HETEROSIS
General Statements and Explanations of some
Basic Genetic Principles.
Th e crossing of unre lated individuals often
results in offspring with increased vigor. This is
called heterosis or hybrid vigor, and the genetic
background for it is not known with certainty.
There are two general modes in which het-
erosis is expressed. The firs t is an increase in
size or num ber of parts. It is the resul t of grea ter
cell activity or greater number of cells. The
second way is by an increase in biological effici-
ency such as reproductive rate or survix-a1ability.
In addition to this, there may be a reduction of
grow th and/or survival rate . that is, hybird
weakness, reversed , or negative heterosis. This
is not common, however, it does occur (18). Stern.
1948, has found one exampl e of negative hetero-
isis in Drosophilla, Hemizgotes and homozygotes
for a serie s of position alleles R+ and +3 possess
norm al venation. However, the heterozygote
R+ /+ 3 exhibits various degrees of deficient ven-
ation. Thus the genotypic combination of the
two, or the
heterozygosity at a single locus de-
ceases effectiveness (26).
Heterosis has been utilized for many plant
and animal improvements.
Farm crops: Maize, sugar beets, sorghums, forage
crops, and grasses.
Horticultural crops: Tomatoes, squashes, cucum-
bers, eggplants, onions, and annual orna-
ments.
Silkworms
Livestock: Swin e, poult ry, beef and milk ca ttle.
Vegetatively propagated plants (13).
The greatest development of heterosis has
been in
Zea
Mays (33). One hundred per cent of the
corn now grown in the corn belt is hybrid corn(l3).
Commerically, hybrid corn seed is produced by
thz double-cross as follows:
Inbred Inbred Inbred Inbred
A
B C D
(AxB)
(AxB) x (CxD)
(double-cross) (25)
A
pape r presented in a n Animal Breeding class
at Louisiana Polytechnic Institute.
The expression of each charac ter, as a rule ,
is independent of other characters. Thus. the
plan t will not be heterotic as a whole. Brieger.
1950. rep ort s that the heterotic characters affect-
ed in maize, are: height, position of e ar . size of
leaves. chlorophyll formation, root system, resist-
ance to disease, pests. and unfavorable conditions,
size an d numbe r of kernels, width and len gth of
ear, size and bran ching of tassel, and th e amount
KRUSE
of pollen shed. Earliness, lateness, row nu
plant and kernel color are not affected by
erosis (3). However, Leng reports tha t
number is affected in widely different degre
heterosis, and tha t number of kernels p er
(ear length) is the only primary yield comp
in
which large positive effects of heterosi
manifested consistently (20).
Chambers, et al, (4), reported that in
hybrid vigor is evident in number of pig
litter, and litter weights at birth but it incr
as the litter becomes less dependent upo
direct mothering ability of the dam.
Vigo
expressed to a greater extent in the incr
viability of pigs and productivity of two
cross gilts than in the increased gro wth ra
individual pigs. The ext ra number of pig
litter in most cases
was sufficient to accou
a larger percentage of th e increase in tota l
weight. Therefore, since heterosis is express
both number of pigs survived and gro wth
per pig, total weight of litter seems to be th
best over all measure of performance for com
ison of lines or crosses 4).
When a heterozygote A1 A1 resembles in
otype of the homozygotes AI AI or
A
At.
allele A1 is said to be dominant or rece
Dominance is when AI At. resembles
Dominance is absent when A A: is ex
midway between A1 AI and A: A ? . Domin
is incomplete when A1 Ar is intermediate bet
AI A, and At. A? . Overdominance is when
is more extreme-for example, larger-t
ei ther AI A1 or A. A? (25).
Mutations constantly produce delete
mutants. Natural selection quickly eliminate
dominant gene alleles but cannot eliminate re
ive gene alleles because they are covered
overpowered by good dominants. Thu s in
the deleterious alleles build up in a popula
Grea t loads of these delete rious recessive a
are present in normally crossbred popula
(25). This has been proven tr ue by Dobzh
in Drosophila pseudoobscura where a m
ity of the individua ls carry heterozygous re
ives. When these were inbred. the offspring
more highly homozygous but it resulted
in
of vigor. This vigor was restored when t h
bred lines were intercrossed (8).
Inbreeding increases the proportion o
mozygotes in the population.
For exampl
F.L f a monohybrid cross AA x aa is 25AA
25aa. 50 percent are homozygous; 50 percen
heterozygous. Suppose all plants ar e s
The en tir e progeny of homozygous individua
main homozygous, but only half of the hete r
ous, individuals remain heterozygous, the
half become homozygous. This is illus trat
Table I.
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HUNTSVILLE ha
HUNTSVILLE Historical Huntsville
preparing a welcome-of-tomorrow for the annu
meeting of two agricu ltura l associations set f
April
5 6 7.
While not in official sessions. the Nation
Association of Colleges and Teachers of Agricu
ture and Delta Tau Alpha Honor Fraternity w
have an opportunitv for tours in an area that
fast becoming the leading tourist attraction in t
great Southwest.
Meeiings will be held
n
Sam Houston Sta
Tecchers College's glamorous new 1.8 millio
Lowman Student Center.
Conve~itiondelegates will enjoy a tour o
the historic campus.
The Austin College Buil
ing. built in
1851.
is the oldest building west
the Mississippi still in use for educational pu
poses. Thc newest ol the college's ma ny faci
ties is the Farrington Science building. Stu den
study here under ideal conditions
in
well-equi
ped laboratories. Jus t one of the many adva
tages for study that the science students have
Lhe planatnrium.
The Graphic Arts Building houses the on
complete graphic arts department in the worl
Along with studies in journalism, photograph
and photo-engraving. the complete Southwe
School of P rin ting is housed in this building .
The modern new agriculture laboratory w
be of interes t to convention delegates. The 89
acre Country Campus offers agricultural studen
splendid opportunities to demonstrate and stud
in agronomy. horticulture. dairy. livestock. an
poultry farming. Country Campus is also th
home of Sam Houston' s excellent 9-hole go
course in the rolling hills of Walker County.
Historic Huntsville is the home of th e St a
Prison. The well-kept buildings of both th
main unit and the farm units. are a clear refle
tion of Texas' advanced prison system. In tun
with their striving for rehabilitation. the priso
ers are entertained each October by well-know
performer s at the annu al Prison Rodeo. Th
rodeo staged for the prisoners is visited by hund
reds of rodeo lovers from the Lone Star sta
as well as other states.
Across from the prison, on the shortest high
\vay in the state. is the grave of G enera l Sam
Houston, first president of the Republic ol' Texa
Nestled in the tall pines of east Texas
Sam Houston Park . The serene park grounds fea
tur e a museum in Sam Houston's honor. Adja
cent to the museum is the small home of th
farnous Texan, a vivid exa mple of how the gr ea
man lived. small log cabin houses th e law
books he used in his practice. Behind t he whi t
frame house is the newly built War and Peac
House. A tri but e to Texas' own soldiers is show
here in World War I and I1 relics.
en N
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of ommerce
Overlooking the park is the i~ostalgic team-
boclt House the two-story home fashioned after
a
lMississippi steamboat in which the Texas Gen-
eral spent his last days.
Only nine miles from the Sam Houston Park
is Huntsv ille State Park. Thousands of Texans
flock annually to the 21.000-acre recreational
area. Pleasant holidays are spent fishing camp-
ing boating and skiing.
Driving back into scenic Huntsville. NACTA
and DTA delegates trave l through par t of the
155 000 acre National Forest. Driving through
this beautiful wooded area. they end up back on
campus under th e tall spires of Old Main.
News Bureau. Sam Houston State
Huntsville. Texas
M RCH
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GENETIC BASIS . . .
(Continued From Page 9)
the interaction can be described in ter ms of the
ability to synthesize specific metabolites. Emerson
decribed this Neurospora heterocaryon (physio-
logical apprcximation to a heterozygote in a
diploid organism) with a one-gene difference be-
tween the component nuclei showing superior
growth to either homocaryotic strain.
Biochemi-
cally, it appear that one homocaryotic strain
synthesizes too little vitamin para-amino benzoic
acid,
wheress the other produces an excess that
promotes a reaction deleterious to growth. The
heterocaryon achieves a proper balance that is
enough for growth and not to the extent that it
is deleterious (1).
Bonnier, 2). concluded that the single
assumption of overdominance will make many
accessory assumptions unnecessary. As long as
there is not biochemical proof to the contrary,
the assumption of overdominance seems to be the
simplest explanation for heterosis
2).
Other Facis and Ideas on Heierosis.
The simplest kind of t ru e heterosis--euheter-
osis-is the
r sult
of a superior dominant allele
sheltering a deleterious recessive mutant
9).
Thus heterosis proper or euheterosis is a product
of muta tion and selection pressures. Luxur iance
of heterozygotes is an evolutionary accident (8).
Luxuriance is observed in some hybrids between
normally self-ferterlized species, races. or strains.
This luxur iance is not due to sheltering of dele ter-
ous genes nor due to overdominance. Luxur iance
is from the evolutionary standpoint, an accidental
condition brought about by complementary action
of genes found in the parental form crossed. These
cases do not seem to be bet ter adaptable and also
ar e more frequently found in domesticated species.
Luxuriance is, really. pseudoheterosis 9).
It is interesting to note in the roport of
Wellhausen that Chalqueno corn. a hybrid of
Conicc and Tuxpeno: two distinct races of corn
will
not outdo either parent in their respective
location as measured by yield. yet in a new
envi ronment i t has an excess of vigor.
Certainly
the genotype is no different. The difference in
this case must be due to different interactions be-
tween over-all gene action and environment (31).
Jn Drosophila paeudoobscura both of th e two
kinds of heterosis ar e reasonably well known and
understood. Firs t, there is the presence of deleter-
ious recessive mutant gene being sheltered by
their normal dominant alleles in a population.
Accumulation of these deleterious genes is a by-
product of the muta tion process.
Secondly, there
are
complexes of linked polygenes which give spe-
cific heterotic interaction effects in heterozy-
gotes (overdominance). This kind of heterosis is en-
gendered by natura l selection and a form of
ada pta tio n of species to its environment
(8).
Hayman said that heterosis is a
composite
phenomena: possible causes are epistasis. over-
dominance, and accumulation of favorable domin-
ants in the heterozygotes (14).
Many factors may influence hybrid vigor.
Genes that have no dominance may be the ones
that have a major effect on heterosis.
Then
chromosomal deletions may occur which is
complete elimination of a normal locus.
T
are also dominant unfavorable genes pre
In the homozygote, they are not completely le
but seldom produce seed or pollen.
In t he h
ozygote there is a marked reduction in
growth, and reproduction as compared wi th e
parent (18).
In choromosomal resrrangements, suc
inversions and translocations. genes without
ation are placed in different spatial relations
other genes. In altered position they have d
ent effects 18). Dobzhansky and Rhodes in
suggested a method of locating genes by para
tric inversions. In plant s heterozygous for
inversions. the crossovers that occur within
inverted segments are only rarely recovere
viable gametes. Thus. the inve rted segme
inherited as a urlit and all genes contained w
the segment are completely linked with very
exceptions. In plants without inversions. w
crossing over breaks up favorable combina
of minor genes. the existance of them is diff
to demonstrate. Thus. if the same characteri
show up in all followinq generations. then i
be assumed that these characteristics were loc
in the inverted section. Sprague, 1941. used
method and found that plants with heterozy
inverted segments were superior to homozy
inverted segment in yield and kernel weigh
the difference as significant. Chao, study in
bred lines of corn by this method, found tha t g
increasing ear height in most of these inbred
were present in chromosome 6 of the ln3 a s
and in the long arm of chromosome 3. These g
did not show overdominance (5).
Loci
are
known that have different effec
the different parts of an organism. These g
may be so close together that they never
crossing over or they may be compound g
with multiple effects. That a chromosome
linear arran gement of loci. each of which is a
of a single gene with a one-effect-function.
over-simplification. There are also single g
with multiple effects. This has been proven
especially tru e in growth production illustrate
chlorophyll formation (18). However the dev
ment of cholorophyll is affected by man y g
all, in this case. involved in the developme
.this heitable characte r. Each recesive a
when homozygous allows formation of par ti
no pigmentation resulting in an albino.
generally believed that the majority. if no
genes for albinism, a f f ~ c t different step
chlorophyll production. Thus if one step fai
go to completion. albinism results (16).
It has also been suggested that heterosis
be the result of t he interaction between genes
cytoplasm. Within species, differences in rec
cal crosses are rare; however, in two diff
flowering types of tobacco, crosses show a m
nal effect (18).
A
I?
Shull preferred
explanation that heterosia was due to a stim
resulting from a changed nucleus on a relat
unaltered cytoplasm.
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Whaley reported that heterosis is associated
with the ability of the hybrid to synthesize or
to utilize one or several specific substances involv-
ed in fundam ental growth processes of organ-
isms. Much evidence indicates that primary
heterosis effect is concerned with growt h sub-
stances whose predominant activity is registered
in the earl y part of the development cycle. Many
hybrid plants gain their advantage a few hours
after germination. The primary gro wth activities
at thi s stage involves the unfolding of t he
enzymatic pattern. Here the hybrid has its ad-
vantage (32). Gartner,et
al
(1953), found that the
degree of heterosis in the experimental plants of
A n i i r r h i n u m majus L. (snapdragon) was greatly
influenced by the amount of solar radiation.
Heterotic ability of the F1 to retain the indol-
acetic acid has been demonstrated. The greater
abili ty of the hybrids to retain and utilize growth
substance under high light conditions permits
greate r expansion of plant tissue and t hus gives
the additional growth increment that can cumu-
latively result in heterosis
(1
1).
Cases are reported where a deleterious re-
cessive gene caused the heterozygote to be no
bette r than the homozygous normal. These genes
are lethal or nearly lethal. They are all unfavor-
able
(17).
Sternei al, 26), tested the viability
of Drosphila melanogasfer for 75 sex linked recess-
ive lethals. It was found tha t on the average the
viabi lity of the heterozygote for lethals was .965.
a decrease in viability of approximately
4
percent
(27).
By way of a summary, it can be said tha t
heterosis can involve only recombinations of al-
ready existing alleles, unless by rare chance
mutation occurs. We are thus concerned with an
interpreta tion limited to diffe rent types of re-
combinations. and to different kinds of gene action
resul ting from these recombinations (32). The
basic principle of all explanations is tha t different
gene combinations from the same restricted pool
of genes will give many different phenotypes (30).
In conclusion, heterosis is apparently not due to
any single genetic cause.
1
Beadle, G. W.. 1953, Heterosis, Journa l of Here -
dity, 44:88
2. Bonnier, G., 1961 Esperi ments on Hybr id Super-
iori ty in Drosophila melanogaster: Egg Layi ng
Capacity and Larval Survival, Genetics. 46:9-24
3.
Brieger, F. G., 1950, Genetic Basic of Hete rosis in
blaize, Genetics, 35: 420-45.
4.
Cham bers , D. and Whatley, J. A., 1951, Heterosis in
Crosses of Inbred Lines of Duroc Swine,
Jou rna l of Animal Science, 10:505-15.
5.
Chao, C. Y., 1959, Heterotic Effects of a Chromoso-
mal Segment in Maize, Genetics. 44:657-77.
6. Crow, J. F., 1948, Alternative Hypothesis of Hy-
brid Vigor. Genetics. 33:477-87.
7. Crow, Jam es F., Dominance and Overdominance,
edited by John W Gowen, 1952 Heterosis. Iowa
Sta te College Press, Arnes. Iowa.
8. Dobzhansky, T.. 1950, Genetics of National Pop-
ulations: Orgin of Heterosis Through Natural
Selection in Population of Drosophila pseudoob-
scura, Genetics, 35:288-302.
9. Dobzhansky,
T.
H., Natu re and Orgi n of Heteros
edited by John W. Gowen, 1952, Heterosis, Io
Statc College Press Ames Iowa.
10
England. D. C. and
L M.
Winters , 1953, Effects
Genetic Diversity and Performance of Inb
Lines per se on Hybrid Vigor in Swin
Journal of Animal Science, 12: 836-47.
11. Gartner,
J. B.,
and others , 1953, Effect of Indo
acetic Acid and Amount of Solar Radiation
Heterosis in the Snapdragon, Science. 117:5
5.
12. Go\r.en, Jo hn W., Hybr id Vigor in Drosophil
edited by John
W.
Gowen, 1952, Hetero
Iowa State College Press, Ames. Iowa.
13. Hayes. H. K., Development of th e Heterosis C
cept, edited by Jo hn W. Gowen, 1952, Hetero
Iowa Sta te College Press, Arne.. Iowa .
14. Hayman,
R.
L., 1957 Interaction, Hetcrosis, A
Diallel Crosses. Genetics. 42:336-55
15. Henderson,
M.
T., 1949. Consideration of
Genetic Explanations of Heterosis, Agrono
Journa l, 41 :123-6.
1G. Irwin.
M.
R. Specificity of Gene Effects, edi
by Joh n W. Gowen, 1952. Heterosis, Iowa S t
College Press, Ames, Iowa.
17. Jones, D. F.. 1945, Heterosis Resu lting fr
Degenera tive Changes. Genetics., 30527-42
18. Jones, Donald
F.,
Plasmagenes and Chromoge
in Heterosis, edited by Jo hn W. Gowen, 19
Heterosis, Iowa State College Press, Ames, Iow
19. Jones,
D. F.,
1957, Gene Action i n Heteros
Genetics, 42:92-103.
20. Leng, E.
R.
1954, Effects of Heterosis on th e Ma
Components of Grain Yield in Corn, Agrono
Journal, 46:502-506.
21. Rickey, F. D.. 1945, Bruce's Explanation of Hyb
Vigor, Journa l of Hered ity , 36:243-4.
22. Shu ll, G.
H.,
1948, Wha t is Heterosis? Genet
33:439-46.
23. Shul l, George Harrison, Beginning of th e Hetero
Concept, edited by John W. Gowe n, 1
Heterosis. Iowa Sta te College Press, Ames, Io
24.
Sierlr, C.
F.
and L. &I. Winters . 1951, Study
Heterosis in Swine, Journ al of Ani mal Scien
10:104-11.
25. Sinno tt, Edmund W.. Dunn, L. C.. and Dobzhans
Theodosius, 1958. Principles of Genetics, M
Graw
-
Hill Book Company, Inc., New Yo
Chapter 19, Pages 254-268.
26. Stern, C. 1948, Negative Heterosis and Decrea
Effectiveness of Alleles in Heterozygote
Genetics, 33:215-19.
27. Ste rn, C., an d other s. 1952, Viability of He te
zygotes for Lethals. Genetics, 37:413-49.
78 Strauss,
F.
S., and J. W. Gowen, 1943, Hetero
Its Mechanism in Terms of Chromosmes Un
in Egg Production of Drosophila melanogasr
Abstract. Genetics, 28:93.
9. Tantaway, A 0 1957, Heterosis and Gene
Variance in Hybrids Between
Inbred Lines
Drosophila melanogaster. in Relation to the Level
Homozygosity, Genetics, 42535-43.
30. Wagner, Robert P., and Mitchell, Herschell. 1
Genetics and Metabolism. Joh n Wiley So
Inc., New york.
31.
Wellhausen,
E.
J., Heterosis in a New Populatio
edited by John W. Gowen, 1952, Heterosis, Io
State College Press, Ames, Iowa.
32. Whaley, W. Gordon,
Physiology of Gen e Act
in Hybrids, e&ted by John W. Gowen, 1
Heterosis. Iowa S tat e College Press , Ames, Io
33. Zirkle, Conway, Early Ideas on Inbreeding
Crossbreeding, edited by John W. Gowen, 1
Heierozis, Iowa State College Press. Ames, Io