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genotypes
You cross two true breeding strains of zucchini, one with green fruit and the other
with yellow. The F1 plants are all green , but when these are crossed, the F2 plants
consists of 9 green and 7 yellow. How many genes are involved in determining the
zucchini color? Using your own symbols, designate which genotypes yield green and
which yield yellow zucchinis.
Solution:
Gene Interaction –complementary action:
One dominant allele of each of two genes is necessary to produce phenotype. Epistatic
interactions of the genes will give a modified 9:7 ratio.
Green zucchini is crossed with yellow Zucchini strains (True breed), so the genotype
will be
Green strain
GGYY
*
Yellow strain
ggyy
----- All are true breed
GgYy --------------- Gametic condition of progeny (Green)---F1
The cross between the F1 individuals will give 9:7 ratios in F2.
GgYy
*
GgYy ----------F1 generation
GY
Gy
gY
gy
GY
GGYY
GGYy
GgYY
GgYy
Gy
GGYy
GGyy
GgYy
Ggyy
gY
GgYY
GgYy
ggYY
ggYy
gy
GgYy
Ggyy
ggYy
ggyy
The F2 plants consists of 9 green and 7 yellow color. Complementary reaction
between the gene’s in a specific manner will produce this result.
GGYY---1
GGYy----2
GgYY----2
GgYy-----4
Total 9
The above mentioned genotypes are contained a dominant allele for each gene or
character, for example G and Y. If a dihybrid contains two Dominant gene that strain
may express green color, it’s because of interaction between two dominant genes.
GGyy------1
Ggyy-------2
Ggyy-------1
ggYY------1
ggYy-------2
Total 7
Here no one has both G and Y dominant gene, so this might be the reason to get yellow
color. The absence of G and Y dominant gene the strain couldn’t express green color,
so for green color there should be some gene interaction to express the character.
Thirteen loci controls the color of the fruit, so difference in loci for allele or gene
may involve in gene interaction and may produce variations.
There are so many gene’s interact for fruit color, but it has provided for light and
dark shades. I could not find out any proper information for 9:7 ratio as well as
green and yellow color, so please follow these information.
Thirteen loci, some of them multiple-allelic, have been identified as affecting fruit
exterior color in Cucurbita pepo L. Three major loci are D (Dark stem), l-1 (light
coloration-1), and l-2 (light coloration-2). These three loci were identified in a cross of
‘Fordhook Zuchhini’ (C. pepo subsp. Pepo Zucchini Group), which has intensecolored fruits and dark stems, with ‘Vegetable Spaghetti’ (C. pepo subsp. pepo
Vegetable Marrow Group), which has light-colored fruits and light stems. ‘Fordhook
Zucchini’ possesses genotype D/D L-1/L-1 L-2/L-2 whilst ‘Vegetable Spaghetti’
possesses genotype d/d l-1/l-1 l-2/l-2.
The complementary dominant L-1 and L-2 alleles cause the fruit to be intense, darkcolored from anthesis through maturity. When either or both is homozygous recessive,
the fruits are lightly colored. During the first week past anthesis, it is difficult to
distinguish L-1/__l-2/l-2, l-1/l-1 L-2/__, and l-1/l-1/l-2/l-2 genotypes phenotypically.
Afterwards, eachgenotype develops a characteristic pattern that allows it to be
distinguished clearly from the others. These have been designated ‘Light type 1’, ‘Light
type 2’, and ‘Plain light’, respectively.
The D gene has almost no effect on the fruits during the first week after anthesis. The
dominant D allele causes the fruits to darken during the second and third weeks after
anthesis, with recessive d/d having no effect. The darkening caused by the dominant
allele occurs independently of the allelic state of the l-1 and l-2 genes and masks their
effect, that is, D is epistatic to l-1 and l-2.
A fourth locus is pl (plain light). This gene is of relatively minor effect, determining
whether or not the ‘Light type 1’ phenotype is expressed. Plants of genotype d/d L-1/__l2/l-2 Pl/__exhibit the ‘Light type 1’ fruit phenotype whilst those of genotype d/dL-1/__l2/l-2 pl/pl exhibit the ‘Plain light’ fruit phenotype.
All F1 plants and all plants of the backcross to ‘True French’ bore intense-colored fruits.
This was true when the fruits were young as well as when they attained intermediate age.
Thus, intense coloration is dominant to light coloration. The young fruits segregated to
intense-colored and light-colored in accordance with the 9:7 ratio in the F2 and a 1:3
ratio in the backcross as expected for two complementary dominant genes conferring
intense coloration of young fruits. Therefore, ‘True French’ possesses genotype L1/L-1 L-2/L-2.
The same concept I have applied for green and yellow color, so please go through
the informations.
Hope that helps!