** PLEASE PLEASE EXPLAIN, NOT JUST ANSWER ** Assume that x is a new mutant allel

Question

** PLEASE PLEASE EXPLAIN, NOT JUST ANSWER **

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

a) first cross -

X/X x x/x = progeny will be X/x.

now X/x is crossed with x/x = one X/x and one x/x means 50% X/x and 50% x/x.

so ratio of dominant to recessive phenotypes will be 1:1.

b) If the trisomic 10 pollen is viable, there would be a 5 dominant: 1 recessive ratio. The possible gametes for the trisomic 10 parent would be X/x, X, X/X, x, X, x/X.

Gametes          Progeny

X/X                 X/X/x

  x                     x/x

X/x                 X/x/x

X/x                 X/x/x

X                    X/x

X                    X/x

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