ADVERTISEMENTS:
The following points highlights the top fifteen cases on monohybrid cross.
Case # 1. What will be the appearance of (a) F1 and (b) F2 progenies when a pure (homozygous) tall pea plant is crossed with a pure (homozygous) dwarf pea plant?
Tallness (T) gene is dominant over dwarfness (t) gene.
ADVERTISEMENTS:
Solution:
Pure (homozygous) tall pea plant = TT
Pure (homozygous) dwarf pea plant = tt
(a) Parents:
Thus, the off-springs of F1 generation will be heterozygous tall.
(b) Here the F1 hybrids, i.e., heterozygous tall (Tt) are self-pollinated which may result into following possibilities:
Therefore, 3 plants will be tall and one plant will be dwarf in F2 generation showing a ratio of 3: 1.
Results:
(a) The result of F1 would be the production of heterozygous tall (Tt).
(b) The result of F2 would be the production of tall and dwarf in a ratio of 3: 1. Out of 3 tall plants one would be pure (homozygous) tall (TT) and two would be heterozygous tall (Tt). The dwarf would be a pure homozygous dwarf (tt).
Case # 2. When a plant homozygous for tall is crossed with a plant homozygous for dwarf, what will be the appearance of the off-springs of a cross of F1 with its tall parent?
Solution:
ADVERTISEMENTS:
When a plant homozygous for tall (TT) is crossed with a plant homozygous for dwarf (tt) the off-springs in the F1 will be the production of heterozygous tall (Tt) as shown above in Exercise No. 1.
Now such a heterozygous tall (Tt), when back- crossed with its tall parent (TT), will show the following results:
Results:
ADVERTISEMENTS:
In such a case all the plants will be tall.
Case # 3. When a plant homozygous for tall is crossed with a plant homozygous for dwarf, what will be the appearance of the off-springs of a cross of F1 with its dwarf parent?
Solution:
When a plant homozygous for tall (TT) is crossed with a plant homozygous for dwarf (tt), the off-springs in the F, will be heterozygous tall (Tt).
ADVERTISEMENTS:
When such heterozygous tall (Tt) is back-crossed with dwarf parent (tt) the following will be the result:
Result:
In such cases the result would be the production of tall and dwarf in a ratio of 1: 1.
ADVERTISEMENTS:
Case # 4. Work out for the genotypes of the parents of the cross between a tall and a dwarf pea plant which result into about one half of the tall and one half of dwarf off-springs.
Solution:
Because the ratio is 1: 1, so it is a test cross between a pure dwarf (tt) and a heterozygous tall (Tt).
The genotypes may also be shown as follows:
Heterozygous tall (Tt) will form the gemetes T and t.
Pure dwarf (tt) will form the gemetes t and t.
ADVERTISEMENTS:
Result:
Heterozygous tall = 50%
Pure dwarf = 50%
A ratio of 1: 1 will be resulted.
ADVERTISEMENTS:
Case # 5. Work out the probabilities of the off-springs in a case when heterozygous tall hybrids of F1generation are self-fertilized.
Solution:
Heterozygous tall hybrids are represented as Tt.
Result:
A ratio of 3 (tall): 1 (dwarf) will be resulted. So tall off-springs will be more in number.
Case # 6. What will be the result in F] generation when a homozygous white male rabbit is crossed with a homozygous black female rabbit?
Also work out the back cross.
(a) Back Cross With Male Parent:
Case # 7. How can we produce such seeds which, when sown, yield pink-coloured Four O’ clock plants?
Solution:
Pink coloured Four O’ clock plants are generally the hybrids and can be produced by crossing the homozygous red flowered plants with homozygous white flowered plants as under:
Case # 8. What type of the gametes and flower colours of the off-springs would be produced in the following crosses?
(a) RR x rr and (b) Rr x Rr.
Case # 9. What type of gametes and the flower colours of the off-springs would be produced in the following cross?
Results:
50% pink and 50% white flowers would be produced.
Case # 10. What type of gametes and flower colours of the off-springs would be produced in the following cross:
Result:
50% red and 50% pink flowers would be produced.
Dihybrid Cross Exercises:
Case # 11. What will be the result of selfing the F1 generation in a cross when round and yellow seeded pea plants (YYRR) are crossed with green and wrinkled seeded pea plant (yyrr)?
Solution:
It is an exercise of dihybrid cross.
The result will be yellow round (9), yellow wrinkled (3), green round (3) and green wrinkled (1). So it shows the ratio of 9:3:3:1.
Case # 12. When round and yellow seeded pea plants (YYR R) are crossed with green wrinkled (yyrr) seeded pea plants the F t are yellow and round seeded plants (YyRr).
What will be the result when this F1 is crossed with round and yellow seeded parents?
Solution:
Following results may be obtained when F (YyRr) are crossed with pure yellow and round seeded parent (YYRR).
Case # 13. When round and yellow seeded pea plants (YYRR) are crossed with green and wrinkled (yyrr) seeded pea plants the F} are yellow and round seeded plants (YyRr).
What will be the results when this Ft is crossed with green and wrinkled seeded (yyrr) parents?
Solution:
Following results will be obtained when F (YyRr) are crossed with green and wrinkled seeded (yyrr) parents:
Result:
Four different kinds of plants (yellow round, yellow wrinkled, green round and green wrinkled) in 1: 1:1:1 ratio would be formed.
Case # 14. Find out the phenotypic appearance of the off-springs of the following cross, in which the genotypes of the parents are given:
yyRr x Yyrr
Solution:
It is an exercise of dihybrid ratio. It is based on the facts that yellow colour is dominant over green character, and the roundness is dominant over wrinkledness.
Results:
A ratio of 1 (yellow round): 1 (yellow wrinkled): 1 (green round): 1 (green wrinkled) will be produced.
Case # 15. Find out the phenotypic appearance of the off-springs of the following cross, in which the genotypes of the parent are given.
YyRr x Yyrr
Solution:
It is an example of dihybrid ratio where yellow character is dominant over green, and round is dominant over wrinkled.
Result:
A phenotypic ratio of 3 (yellow round): 3 (yellow wrinkled): 1 (green round): 1 (green wrinkled) will be produced.
Q. 16. Find out the phenotypic value of the offspring of the following cross, in which the genotypes of the parents are given.
YYRr x YyRr
Solution:
In this exercise of dihybrid cross following phenotypic off-springs will be resulted:
Result:
A ratio of 6 (yellow round): 2 (yellow wrinkled) would be resulted.
