1. Law of Segregation
Question 1:
In pea plants, tallness (T) is dominant over dwarfness (t). What will be the
genotypic and phenotypic ratio in a monohybrid cross between two heterozygous
tall plants?
Answer:
Cross: Tt × Tt
Gametes: T and t
|
|
T |
t |
|
T |
TT |
Tt |
|
t |
Tt |
tt |
Genotype ratio: 1 TT : 2 Tt : 1 tt
Phenotype ratio: 3 Tall : 1 Dwarf
Explanation: The alleles separate during gamete formation (segregation) and
unite randomly at fertilization.
2. Co-dominance
Question 2:
In cattle, red coat color (RR) and white coat color (WW) show co-dominance,
producing roan (RW). If two roan cattle are crossed, what are the expected coat
color ratios?
Answer:
Cross: RW × RW
Gametes: R and W
|
|
R |
W |
|
R |
RR |
RW |
|
W |
RW |
WW |
Genotype ratio: 1 RR : 2 RW : 1 WW
Phenotype ratio: 1 Red : 2 Roan : 1 White
Explanation: Both alleles express equally in the heterozygous condition
(co-dominance).
3. Incomplete Dominance
Question 3:
In snapdragon flowers, red (RR) × white (rr) produces pink (Rr). If two pink
flowers are crossed, what will be the result?
Answer:
Cross: Rr × Rr
Gametes: R and r
|
|
R |
r |
|
R |
RR |
Rr |
|
r |
Rr |
rr |
Genotype ratio: 1 RR : 2 Rr : 1 rr
Phenotype ratio: 1 Red : 2 Pink : 1 White
Explanation: Heterozygous condition shows an intermediate phenotype (incomplete
dominance).
4. Law of Independent Assortment
Question 4:
In pea plants, round seed (R) is dominant over wrinkled (r), and yellow
cotyledon (Y) is dominant over green (y). What will be the genotypic and
phenotypic ratios when two heterozygous plants (RrYy × RrYy) are crossed?
Answer:
Cross: RrYy × RrYy
Each parent produces 4 gametes: RY, Ry, rY, ry.
|
Traits |
Ratio |
|
Round Yellow |
9 |
|
Round Green |
3 |
|
Wrinkled Yellow |
3 |
|
Wrinkled Green |
1 |
Phenotypic ratio: 9:3:3:1
Genotypic ratio: 1:2:1:2:4:2:1:2:1
Explanation: Alleles of different genes assort independently during gamete
formation.
5. Application of Mendel’s Principles
Question 5:
A plant with genotype TtRr (Tall, Round) is test-crossed with a recessive plant
ttrr. What will be the phenotypic ratio of the offspring?
Answer:
Cross: TtRr × ttrr
Gametes of F₁: TR, Tr, tR, tr
Gametes of tester: tr
|
F₁ Gamete |
Offspring Genotype |
Phenotype |
|
TR |
TtRr |
Tall, Round |
|
Tr |
Ttrr |
Tall, Wrinkled |
|
tR |
ttRr |
Dwarf, Round |
|
tr |
ttrr |
Dwarf, Wrinkled |
Phenotypic ratio: 1 Tall Round : 1 Tall
Wrinkled : 1 Dwarf Round : 1 Dwarf Wrinkled
Ratio = 1:1:1:1
Explanation: Test cross reveals the independent assortment of alleles.
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