Question 1 (6 points) Assume that x is a new mutant allele in corn. An x/x plant

Question

Question 1 (6 points)
Assume that x is a new mutant allele in corn. An x/x plant is crossed with a triple-10 individual (trisomic for chromosome 10 only) carrying only wild-type dominant alleles at the X locus (Cross 1). Trisomic progeny for chromosome 10 are recovered and crossed back to the x/x plant (Cross 2).

(a) What ratio of dominant to recessive phenotypes is expected if the X locus is not in chromosome 10? (Hint: Write out the parental and progeny genotypes for the X locus for both Cross 1 and Cross 2 keeping in mind that it is not on chromosome 10).

(b) What ratio of dominant to recessive phenotypes is expected if the X locus is in chromosome 10? (Hint: Write out the parental and progeny genotypes for the X locus for both Cross 1 and Cross 2 keeping in mind that it now is on chromosome 10)

Explanation / Answer

Cross 1 – xx (plant carrying mutant allele) is crossed with XX (plant carrying wild type dominant allele)

Expected genotype- Xx, Xx, Xx, Xx (all the plants will exhibit wild type phenotype). Hence ratio of dominant: recessive phenotype is 4:0.

Cross 2- Xx (plants from cross 1) is crossed with xx (plant carrying mutant allele)

Expected genotypes- Xx, Xx, xx, xx. Hence, the ratio of dominant to recessive phenotype is 1:1.

Cross 1 – xx (plant carrying mutant allele) is crossed with XXX (plant carrying wild type dominant allele with trisomy of chromosome 10)

Expected genotype- Xx, XXx, Xx, XXx (all the plants will exhibit wild type phenotype). Hence ratio of dominant: recessive phenotype is 4:0.

Plants from cross 1- XXx and Xx. But the trisomy from chromosome 10 is recovered, hence the genotype of plants from cross 1 is Xx.

Cross 2- Xx (plants from cross 1) is crossed with xx (plant carrying mutant allele)

Expected genotypes- Xx, Xx, xx, xx. Hence, the ratio of dominant to recessive phenotype is 1:1.

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