The sking of the orange South American toad secretes a lethal neurotoxin. Assume

Question

The sking of the orange South American toad secretes a lethal neurotoxin. Assume that the toxin is produced by an enzyme encoded by a gene that has two alleles: T (dominant, wild type enzyme) and t (recessive, no enzyme). The neurotoxin is secreted by means of a protein S produced by the wild type gene S. The recessive allele s does not produce protein S. A toad that cannot manufacture the toxin and is also Ss is mated with a toad that cannot secrete the toxin and is heterozygous at T.

What fraction of the progeny will be poisonous to the touch aka able to both manufacture and to secrete the toxin?

Explanation / Answer

A toad that cannot manufacture the toxin and is also Ss is mated with a toad that cannot secrete the toxin and is heterozygous at T.

That is: ttSs x Ttss

The fraction of the progeny will be poisonous to the touch aka able to both manufacture and to secrete the toxin is = 4/16 or 1/4

tS tS ts ts Ts TtSs (poisonous) TtSs (poisonous) Ttss (non poisonous) Ttss (non poisonous) Ts TtSs (poisonous) TtSs (poisonous) Ttss (non poisonous) Ttss (non poisonous) ts ttSs (non poisonous) ttSs (non poisonous) ttss (non poisonous) ttss (non poisonous) ts ttSs (non poisonous) ttSs (non poisonous) ttss (non poisonous) ttss (non poisonous)

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