Possible response:

Gene pairs and chromosome inheritance

Introduction: Genes are segments of DNA that determine traits and are located in specific positions, called loci, on chromosomes. Chromosomes are structures made of DNA and proteins that carry genetic information from parents to offspring. Inheritance patterns of genes depend on various factors, such as their location, linkage, and segregation during meiosis.

Gene pairs: In the given chromosome, there are five genes arranged in a linear order: p-q-r-s-t. Each gene occupies a specific locus on the chromosome, and can exist in different forms, called alleles, that may affect the phenotype (observable trait) of an organism. Gene pairs are formed when homologous chromosomes (one from each parent) pair up during meiosis, and each allele has a chance to segregate into a different gamete (reproductive cell).

Probability of inheritance: The probability of two genes being inherited together depends on their location relative to each other, and whether they are linked or unlinked. Linked genes are located close to each other on the same chromosome and tend to be inherited together, unless crossing-over occurs during meiosis. Unlinked genes are located far apart on different chromosomes and tend to assort independently, unless they are on the same sex chromosome or affected by epistasis (interaction of genes).

Least probability of inheritance: The gene pairs that have the least probability of being inherited together are those that are farthest apart on the chromosome, because they are more likely to be separated by crossing-over during meiosis. Therefore, the correct answer to the given question is (d) p and s, because they are located at opposite ends of the chromosome and have the maximum physical distance between them. The other gene pairs have varying degrees of linkage and proximity, and may have higher chances of being inherited together. For example, (a) p and q are adjacent and may be linked, (b) r and s are adjacent and may be linked, and (c) s and t are adjacent and may be linked. However, these possibilities depend on the specific alleles involved and the frequency of crossing-over in the region.