How do know which trait is dominant rather than recessive?

Question

One of the questions from my homework states:

"In Drosophila fruit flies, a cross between a dark-bodied fly and tan-bodied fly yields 76 tan and 80 dark flies. Which trait is dominant? Explain. Based on the ratio suggested, use the letters "T" and "t" to diagram this cross."

I am not sure whether the trait with more occurence is the dominant one or not, and I also don't understand how to diagram the cross. Can someone help?

Answer 1

the capital letter is the dominant allele

from this question, we cannot tell which trait is dominant

All we can tell one fly is homozygous and another is heterozygous

one is Tt while the other is tt which gives half offspring of Tt and another half tt..which corresponds to the ratio 76:80 ~ 1:1

without additional information, we cannot decide which is the dominant trait...

Answer 2

We can construct a genetic diagram based on the information give.

Parental phenotype: Dark-bodied x Tan-bodied
Parental genotype: tt X Tt
Gametes:
- (from 1st parent) t
- (from 2nd parent) T and t

F1 generation genotype: Tt , tt
F1 generation phenotype: Tan-bodied, dark-bodied.

We can see that the tan-bodied trait is dominant. Normally a cross of a heterozygote and a homozygous recessive produces offspring in a 1:1 ratio.

There are 4 more dark flies than the tan flies does not mean dark-bodied is the dominant trait. If you observe carefully they do not deviate too much from the expected 1:1 ratio.

Answer 3

The trait for a cleft chin is recessive trait. The absence of a cleft chin is a dominant trait. I hope this explains this clearly enough but it was easy enough to double check this information on the web by doing a Google search. Since this was for your science project I just wanted to suggest this option for the future and it won't cost any of your points from answers.

Answer 4

the capital letter is always dominant. Since dark is more frequent, it must be the dominant trait. however in order to recieve the recessive tan trait the dark fly must have the recessive gene Tt. So in breeding.. Tt
t|Tt|tt|
t|Tt|tt|