Question

question 16
you want to produce a brood of dragons with small tail spikes. your lab partner says that both dragon parents should have small spikes. is this the best cross to produce small - spiked dragons? if not, which cross is better?
he is correct. tt x tt was the best cross.
he is wrong. tt x tt is a better cross.
he is wrong. tt x tt is a better cross.
he is wrong. tt x tt is a better cross.
he is wrong. tt x tt is a better cross.
he is wrong. tt x tt is a better cross.
clear selection

Explanation:

Step1: Define trait genetics

Assume small tail spikes are a recessive trait, coded by allele $t$; dominant allele $T$ gives non-small (large) spikes. So small-spiked dragons have genotype $tt$.

Step2: Analyze partner's cross (Tt x Tt)

Offspring genotypes:
$$\frac{1}{4} TT, \frac{1}{2} Tt, \frac{1}{4} tt$$
Only 25% will have small spikes.

Step3: Analyze tt x tt cross

Offspring genotypes:
$$100\% tt$$
All offspring have small spikes.

Step4: Compare all crosses

Other crosses (TT x TT, TT x Tt, TT x tt, Tt x tt) produce 0%, 50%, 0%, 50% small-spiked offspring respectively, all less than 100%.

Answer:

He is wrong. tt x tt is a better cross.