68. The following picture represents a gel profile of a pair of DNA markers observed P1 and P2, their F1 progeny and F2 progeny. Four different profiles were observed in case of F2 progeny showing a given profile is indicated in brackets
Based on the above observation, which one of the following statements is correct?
(1) Co-dominant DNA markers were used for this study.
(2) The polymorphic DNA bands represents two independent genes.
(3) If the P1 parent was crossed to the Ft individual, the progeny will show all the four profiles as observed in the case of F2 progeny.
(4) If an F2 progeny which does not show either of the DNA markers (last lane of the above gel) is crossed to a P1individual the obtained progeny will have two types of individual, one which shows a band and the other where no band is observed
The correct option is (4). In this gel‑based DNA marker question, the marker behaves as a dominant “presence/absence” marker for a single locus, and only statement 4 gives the correct genetic outcome for the described cross.
Question recap and basic interpretation
The figure shows gel profiles for a pair of DNA markers in two parents (P1 and P2), their F1, and four F2 band patterns whose frequencies are 93, 28, 33 and 11, respectively. P1 has only the upper band, P2 has only the lower band, F1 has both bands, and F2 individuals show four patterns: only upper band, only lower band, both bands, or no band. These are segregating alleles of a dominant molecular marker where “band present” is dominant to “band absent.”
Why option (1) is incorrect
Statement (1): Co-dominant DNA markers were used for this study.
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Co‑dominant DNA markers allow clear distinction among both homozygotes and the heterozygote at a locus because each allele produces its own scorable band, typically giving a 1:2:1 genotype ratio in F2 (P1 type : F1 type : P2 type) without a “null” class.
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In the given gel, P1 and P2 each show only one band, F1 shows two bands, and F2 shows four different profiles because of presence or absence of bands, including a lane with no band at all; this presence/absence behavior is characteristic of a dominant marker, not a strictly co‑dominant one, so statement (1) is false.
Why option (2) is correct
Statement (2): The polymorphic DNA bands represent two independent genes.
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The four F2 profiles and their frequencies (93, 28, 33, 11) are consistent with segregation of two independent dominant loci, each scored as band present or absent; combining two such loci naturally yields 22=4 phenotypic classes in F2.
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Small deviations from an exact 9:3:3:1 count are expected due to sample size, and the presence of all four band combinations (both bands, only upper, only lower, no band) indicates that the two bands segregate independently as separate polymorphic loci, so this statement is true.
Why option (3) is incorrect
Statement (3): If the P1 parent was crossed to the F1 individual, the progeny will show all the four profiles as observed in the case of F2 progeny.
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Crossing P1 (only upper band) with F1 (both bands) for each dominant locus is effectively a backcross, which yields only two genotypes per locus (heterozygote vs P1‑type homozygote), not four; when combining two loci, the maximum number of phenotypic banding classes in such a backcross is three, not four.
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Therefore a P1 × F1 cross cannot recover the “no‑band” double‑recessive class seen in F2, so all four F2 profiles will not appear in this backcross progeny and statement (3) is false.
Why option (4) is correct
Statement (4): If an F2 progeny which does not show either of the DNA markers (last lane of the gel) is crossed to a P1 individual, the obtained progeny will have two types of individuals, one which shows a band and the other where no band is observed.
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The F2 individual with no bands is homozygous recessive (null) at both marker loci; P1 carries the dominant allele (band present) for at least one locus, so the cross is between a double‑recessive and a dominant‑allele‑containing genotype.
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For each locus, this cross gives 1:1 segregation for band present vs absent; across two loci, progeny are either all‑band‑absent (if they inherit recessive alleles) or band‑present (if they inherit dominant alleles), producing exactly two phenotypic classes—“band” and “no band”—as described, so statement (4) is correct and is the best answer.
Summary of options
| Option | Statement essence | Correct? | Brief reason |
|---|---|---|---|
| (1) | Marker is co‑dominant | No | Pattern shows dominant presence/absence plus null class, not pure co‑dominance. |
| (2) | Two independent polymorphic genes | Yes | Four F2 profiles with approximate 9:3:3:1‑type segregation imply two loci. |
| (3) | P1 × F1 gives all four F2 profiles | No | Backcross cannot generate the double‑null class; at most three phenotypes. |
| (4) | Null F2 × P1 gives only “band” and “no band” | Yes | Cross between double‑recessive and dominant genotype yields two phenotypic classes; this matches the statement and is the correct answer. |
