Question

which of the following statements describes the chromosome theory of inheritance as expressed in the early twentieth century?
mendelian genes are at specific loci on the chromosome and in turn segregate during meiosis.
homologous chromosomes give rise to some genes and crossover chromosomes to other genes.
individuals inherit particular chromosomes attached to genes.
no more than a single pair of chromosomes can be found in a healthy normal cell.

Explanation:

The chromosome theory of inheritance, developed in the early 20th century, states that Mendelian genes are located at specific loci on chromosomes and that chromosomes segregate during meiosis. This is a key tenet of the theory. The other options are incorrect: A healthy normal cell has multiple pairs of chromosomes (so the first option is wrong). It's genes that are on chromosomes, not the other way around (so the second option is wrong). While homologous chromosomes can undergo crossover, the core statement of the chromosome theory as asked here is about gene - locus and segregation (so the third option is less relevant compared to the fourth one).

Answer:

Mendelian genes are at specific loci on the chromosome and in turn segregate during meiosis.