- A pair of alleles govern seed size in a crop plant. ‘B’ allele responsible for bold seed is dominant over ‘b’ allele controlling small seed. An experiment was carried out to test if an identified dominant DNA marker (5kb band) is linked to alleles controlling seed size. A plant heterozygous for the marker and the alleles was crossed to a small seeded plant lacking the 5kb band. 100 progeny obtained from the cross wereanalysed for the presence and absence of the DNA marker. The result are tabulated below:
| Phenotype | Plant with bold seed | Plant with small seed |
| No. of progeny showing presence or absence of DNA marker | Prsent Absent 22 23 | Present Absent 27 28 |
Based on the above observations which one of the following conclusions is correct?
(1)The DNA marker assorts independently of the phenotype
(2) The 5kb band is linked to the allele ‘B’
(3) The 5kb band is linked to the allele ‘b’
(4) The DNA marker assorts independently with bold seed but is linked to the small seed trait
A pair of alleles governs seed size in a crop plant, where dominant B gives bold seed and recessive b gives small seed. A heterozygous plant for both the seed-size locus and a dominant 5 kb DNA marker band was test-crossed with a small-seeded plant lacking the 5 kb band (bb, marker–). One hundred progeny were scored as follows:
-
Bold seed, marker present: 22
-
Bold seed, marker absent: 23
-
Small seed, marker present: 27
-
Small seed, marker absent: 28
These four classes are almost equal (about 25 each), so the DNA marker is assorting independently of the seed-size phenotype. Therefore, the correct conclusion is:
(1) The DNA marker assorts independently of the phenotype.
Explanation of options:
-
(1) The DNA marker assorts independently of the phenotype – Correct. Equal numbers in all four phenotypic-marker combinations show independent assortment, indicating no linkage between the 5 kb DNA marker and the seed-size locus.
-
(2) The 5 kb band is linked to the allele ‘B’ – Incorrect. Linkage to B would produce an excess of bold seed with marker present and small seed without marker, not equal distribution.
-
(3) The 5 kb band is linked to the allele ‘b’ – Incorrect. Linkage to b would give more small seed with marker present and bold seed without marker, again not seen here.
-
(4) The DNA marker assorts independently with bold seed but is linked to the small seed trait – Incorrect. The data show no skewed ratio in any direction; both bold and small seeds occur about equally with and without the marker, so there is no linkage with either trait.
