Biology · Variation and Genetics
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A cross in which three fourths of the offspring appear dominant is:
- A
Tt x TT
- B
TT x tt
- C
Tt x tt
- D
Tt xTt
- E
TT x TT
Option D is the correct answer.
Results are TT, Tt, Tt, tt.3/4 is dominant, 1/4 is recessive (tt).
If one parent is heterozygous (Tt) and the other is homozygous dominant (TT)
All offspring are dominant in appearance.
Phenotypic ratio = 100% dominant (4:0)
Conclusion: Incorrect — not 3/4 dominant.
Gametes from TT are all T, and from tt are all t. So all offspring are Tt (heterozygous).
All show the dominant trait.
Phenotypic ratio = 100% dominant (4:0) Conclusion: Incorrect — not 3/4 dominant.
Gametes from Tt are T and t, from tt are t only.Phenotypic ratio = 1:1 (50% dominant, 50% recessive)
Conclusion: Incorrect — only half are dominant.
Option D is the correct answer.
Results are TT, Tt, Tt, tt.3/4 is dominant, 1/4 is recessive (tt).
Both parents are homozygous dominant, so all offspring will be TT (dominant).
Phenotypic ratio = 100% dominant (4:0)
Conclusion: Incorrect — all dominant, not 3/4.
Tagged under Biology · Variation and Genetics · 2015