Y-: yellow S-: star yy: black ss: starless
All the sneetches want their children to have stars on their bellies. An embryo with either YYss or yySS genotype is lethal. Y and S are two independently assorting autosomal genes. A true breeding yellow star bellied sneetch mated with a true breeding black starless sneetch and produced 50 F1 sneetches. The F1 then mated with each other and produced 840 F2 progeny.
What proportion of black star F2 would be true breeding?
Among 840 F2 sneetches, how many of them are black star bellied sneetches?
Among 840 F2 sneetches, how many of them are black starless bellied sneetches?

Explanation:

Available data:

1st cross: A true-breeding yellow star bellied sneetch with a true-breeding black starless sneetch

Parentals)  YYSS   x   yyss

F1) 100% YySs

    N = 50

2nd Cross:

Parentals) YySs    x     YySs

Gametes) YS, Ys, yS, ys

                 YS, Ys, yS, ys

Punnett square)     YS        Ys        yS         ys

                     YS    YYSS   YYSs   YySS    YySs

                     Ys    YYSs    YYss    YySs    Yyss

                     yS    YySS     YySs    yySS   yySs

                     ys     YySs     Yyss    yySs    yyss

F1) N = 840

14 100% of the progeny 840 individuals

2 yySs X = 14.28% X = 120 individuals yySs

1 yyss X = 7.14% X = 59.97 individuals yyss

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