Theory question 104
Mendelian Inheritance(a) Explain the following terms: (i) test cross; (ii) monohybrid cross. (b) In a mango plant, the allele for bean shaped seed is r and is recessive to round-shaped seed R. With the aid of a genetic diagram, determine the genotypes of the offspring if: (i) a homozygous bean-shaped parent is crossed with a homozygous round shaped parent; (ii) a heterozygous bean-shaped parent is crossed with a heterozygous round shaped parent.
Show the model answer
(a)(i) A test cross is a cross between an organism showing the dominant trait (unknown genotype) and a homozygous recessive organism, to find out whether the first is homozygous or heterozygous.
(ii) A monohybrid cross is a cross between parents that differ in one pair of contrasting characters (one gene), e.g. round × bean-shaped seed.
(b)(i) Parents: rr × RR; gametes: r, r and R, R; offspring: Rr, Rr, Rr, Rr. All offspring are Rr (heterozygous, round-seeded).
(ii) Bean-shaped is recessive, so a bean-shaped parent can only be rr; a heterozygous parent (Rr) is round. Treated as Rr × Rr: gametes R, r and R, r; offspring RR, Rr, Rr, rr, i.e. genotypes 1 RR : 2 Rr : 1 rr (3 round : 1 bean-shaped).
(If treated as rr × Rr: offspring 1 Rr : 1 rr.)
Each diagram must show parents, gametes, fusion (lines or a Punnett square) and offspring genotypes.
Always show parent genotypes, gametes and offspring in a genetic diagram.