2. When a cross is made between dominant long and full endosperm (LLSS) and recessive short and shrunken endosperm (Ilss). The progeny obtained 98.3 % were long and full endosperm and short and shrunken, rest 1.7 % were long shrunken endosperm and short full endosperm. Such a ratio was observed due to
(1) Epitasis              (2) Linkage
(3) Co-dominance   (4) Over dominance

Introduction:
In genetic studies on maize, crossing dominant long and full endosperm (LLSS) with recessive short and shrunken endosperm (llss) often yields varying progeny phenotypes. This article explores why the observed progeny ratio of predominantly long and full or short and shrunken endosperm, with a small percentage of recombinant phenotypes, occurs. We analyze the possible causes including epistasis, linkage, co-dominance, and overdominance, and explain the genetic implications of each term.

Detailed Explanation of the Cross and Genetic Concepts:

• The cross is between a dominant phenotype (LLSS) and a recessive phenotype (llss) resulting in progeny that display 98.3% parental-type phenotypes (long-full and short-shrunken) and 1.7% recombinant phenotypes (long-shrunken and short-full). This ratio strongly suggests genes may not assort independently.

• Epistasis: This is the interaction between two or more genes where the expression of one gene masks or modifies the effect of another gene. It does not usually produce recombinant phenotypes but rather alters phenotypic ratios through gene interaction where one gene’s effect dominates another.​

• Linkage: Genes located close together on the same chromosome tend to be inherited together and do not assort independently; this leads to a higher proportion of parental-type combinations and fewer recombinant types. The small percentage (1.7%) of recombinants supports the idea of linkage, as crossover frequency is low but present, indicating genes controlling endosperm traits are linked.​

• Co-dominance: This refers to both alleles being expressed equally in the phenotype of heterozygotes (e.g., blood groups), but it does not explain the observed phenotypic proportions in this maize cross, as progeny types involve complete dominance rather than shared expression.

• Overdominance: This occurs when heterozygotes have a phenotype superior or different from either homozygote. It typically produces a distinct phenotype rather than the parental or recombinant types seen here.

Conclusion:
The ratio of progeny obtained in the maize cross (dominant long/full endosperm crossed with recessive short/shrunken) is mainly due to linkage between the genes controlling the traits, as indicated by the high percentage of parental phenotypes and the presence of a small fraction of recombinant types. Epistasis, co-dominance, and overdominance do not adequately explain this phenotype distribution in this case.

Hence, the correct answer is (2) Linkage.

References:

• Epistasis definition and impact on phenotypes​

• Gene linkage and recombinant frequency in genetics