In a Test cross data of tomato, in which the heterozygous F1 plants were crossed with a strain that has ovate fruit, curled leaves and mottled coloration (homozygous recessive ovov cucu momo), the following progeny were observed:

Normal curled mottled 3 A
Normal curled solid 26 B
Normal flat solid 304 C
Normal flat mottled 68 D
Ovate curled mottled 310 E
Ovate curled solid 60 F
Ovate flat mottled 24 G
Ovate flat solid 5 H

Note:

Normal fruit vs. ovate fruit

Flat leaves or curled leaves

Solid color or mottled coloration

a) List the two parental genotypes of the original cross that produced the F1.

b) Designate the parental, single crossover, and double crossover classes of offspring. (Label each row of the above list: as parental, single crossover, or double crossover class of offspring. (Answer the question by giving your answers for each from A-H or copy and paste the table in the answer box).

c) Determine the order of gene loci and calculate the genetic distance of these three loci. Include calculation details. Present the data as a labeled linkage map. Drawing of the linkage map can be provided either as an uploaded document or the answers can be given for the boxes as labeled i) to v) in the following image.

Question

In a Test cross data of tomato, in which the heterozygous F1 plants were crossed with a strain that has ovate fruit, curled leaves and mottled coloration (homozygous recessive ovov cucu momo), the following progeny were observed:

Normal	curled	mottled	3	A
Normal	curled	solid	26	B
Normal	flat	solid	304	C
Normal	flat	mottled	68	D
Ovate	curled	mottled	310	E
Ovate	curled	solid	60	F
Ovate	flat	mottled	24	G
Ovate	flat	solid	5	H

Note:

Normal fruit vs. ovate fruit

Flat leaves or curled leaves

Solid color or mottled coloration

a) List the two parental genotypes of the original cross that produced the F1.

b) Designate the parental, single crossover, and double crossover classes of offspring. (Label each row of the above list: as parental, single crossover, or double crossover class of offspring. (Answer the question by giving your answers for each from A-H or copy and paste the table in the answer box).

c) Determine the order of gene loci and calculate the genetic distance of these three loci. Include calculation details. Present the data as a labeled linkage map.  Drawing of the linkage map can be provided either as an uploaded document or the answers can be given for the boxes as labeled i) to v) in the following image.

BlackTom AI · Accepted Answer

The question presents a test-cross in tomato where heterozygous F1 plants were crossed to a homozygous recessive line with ovate fruit, curled leaves, and mottled coloration. The observed progeny are grouped by three traits: fruit shape (Normal vs Ovate), leaf type (flat vs curled), and coloration (solid vs mottled). The task has three parts: (a) identify the original parental genotypes, (b) classify each progeny class as parental, single cross over (SCO), or double cross over (DCO), and (c) determine gene order and map distances.

Option a) “The original cross must have been between a homozygous dominant (Normal fruit, flat leaves, solid color: OVOV CuCu MoMo) and a homozygous recessive (ovov cucu momo). The F1 plants are all heterozygous for the dominant traits: Ov ov Cu cu Mo mo.”
- This analysis correctly recognizes that the F1 must be heterozygous for the three loci if the cross involved a dominant-phenotype homozygous parent with a recessive-phenotype homozygous parent. However, the stated genotypes mix case conventions and notation inconsistencies (OVOV CuCu MoMo vs Ov ov Cu cu Mo mo) which can be confusing. The essential idea—two homozygous parents, one with dominant phenotypes and one with recessive phenotypes, producing a heterozygous F1—is sound. A more precise statement would specify that the dominant-phenotype parent carries alleles in coupling (e.g., Fruit shape Normal (Ov? or Ov), Leaf type Flat (Cu), Color Solid (Mo)) depending on the actual coupling in the parental cross. Overall, the reasoning captures the core concept but could be tightened for clarity.

Option b) “Annotated Progeny Table: A: Normal, curled, mottled, 3 – Double crossover (DCO); B: Normal, curled, solid, 26 – Single crossover (SCO); C: Normal, flat, solid, 304 – Parental (P); D: Normal, flat, mottled, 68 – Single crossover (SCO); E: Ovate, curled, mottled, 310 – Parental (P); F: Ovate, curled, solid, 60 – Single crossover (SCO); G: Ovate, flat, mottled, 24 – Single crossover (SCO); H: Ovate, flat, solid, 5 – Double crossover (DCO). c) Step 1: Parental classes are the most frequent: C (304) and E (310)....
- The classification hinges on identifying parental classes as the most frequent phenotypes and DCO as the least frequent. Here, C and E are indeed the two most frequent classes observed, which supports treating them as parental. The proposal that A (3) and H (5) are the DCOs, while plausible given their very small counts, must be checked against a full understanding of how SCO and DCO manifest across three loci with the particular coupling phase in F1. The author then uses those identifications to infer the gene order as Fruit shape (Ov/ov), Color (Mo/mo), Leaf (Cu/cu). This inference is challenged by potential mislabeling of the F1 phase and by inconsistencies in which traits are considered middle genes. In short, the approach follows general mapping logic but the conclusions (especially the middle gene and the map calculation steps) require careful re-checking against the actual SCO/DCO patterns.
- Notably, the step-by-step distance calculations appear inconsistent and contain arithmetic errors: total progeny 800 is assumed, but summing the listed counts yields 800, which is correct, yet the distances are then reported as 12.5% and 11.75% for ov–mo and mo–cu, respectively, after mixing sums in a nonstandard way. The calculation of SCO vs DCO contributions should be consistent and explicit for each interval, and the mapping should account for the phase arrangement of the F1 (which determines which crossovers produce the observed phenotypes). The presented final distances (12.5% and 11.75%) do not align cleanly with the SCO totals (68+24+26+60 = 178 for one interval, etc.), indicating miscalculation or misinterpretation of which progeny types count toward which interval. Therefore, while the intent is reasonable, the numeric conclusions and the suggested gene order require re-evaluation with a formal, interval-by-interval calculation.

Option c) “Determine the order of gene loci and calculate the genetic distance of these three loci. Include calculation details. Present the data as a labeled linkage map. Drawing … or answers can be given for i) to v) in the image.”
- This option asks for the core objective of linkage analysis: deducing gene order and computing map distances, then presenting a map. The provided answer attempts to derive an order (Fruit shape, Color, Leaf) and offers distances between adjacent loci (ov–mo and mo–cu) along with a proposed map layout. However, as noted, there are inconsistencies and potential errors in the underlying computations within the proposed steps (e.g., which phenotypes are considered SCO vs DCO, how the middle gene is assigned, and how distances are calculated from SCO counts). A robust solution would: (1) clearly identify F1 coupling phase, (2) classify each progeny class with explicit reasoning for parental vs SCO vs DCO using the two-point or three-point test cross framework, (3) compute each interval distance as the sum of SCO progeny for that interval divided by total progeny, times 100, (4) check consistency with the expected three-point cross patterns, and (5) present a clean map with ordered loci and three pairwise distances that sum to the total map distance (approximately the sum of the two adjacent interval distances in a three-point cross, acknowledging interference). As written, option c provides a structure and intent but falls short in delivering rigorous, accurate computations and a consistently justified gene order. It would benefit from a recalculation using the standard three-point cross method and a clearer articulation of the F1 phase and recombination patterns.

Summary of evaluation:
- Option a correctly identifies the basic cross logic (two homozygous parents producing a heterozygous F1), but could improve precision and notation consistency for clarity.
- Option b follows the general framework of classifying progeny into parental, SCO, and DCO categories and attempts to deduce gene order, yet contains inconsistencies in phase assignment and a questionable arithmetic treatment of recombination distances, suggesting a need for a careful re-check of counts and their interval associations.
- Option c captures the intended goal of determining gene order and map distances and emphasizes presenting a linkage map, but the accompanying calculations and the drawn conclusions require rigorous rework to ensure the gene order and distance values are correct and reproducible from the data.

To fully validate any of these options, one would re-run the three-point test-cross analysis with explicit tabulation of recombination events, determine phase relationships from the F1, categorize each progeny class with unambiguous justification, and recompute the distances with standard formulas to yield a coherent map. The key pitfalls to watch for are misidentifying parental classes, mislabeling SCO vs DCO categories, and arithmetic errors in distance calculations.

AGRI30048_2025_SM2 Practice Questions for the Mid Semester Test - UNMARKED REVISION QUESTIONS

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