Question

which of the following statements describes the chromosome theory of inheritance as expressed in the early twentieth century?
individuals inherit particular chromosomes attached to genes.
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.
no more than a single pair of chromosomes can be found in a healthy normal cell.

Explanation:

The chromosome theory of inheritance, proposed in the early 20th century, builds on Mendel's work. It states that genes (Mendelian genes) are located at specific positions (loci) on chromosomes. During meiosis (the process of gamete formation), chromosomes segregate, and thus the genes on them also segregate.

• The first option is incorrect because genes are on chromosomes, not the other way around.
• The third option is wrong as homologous chromosomes carry genes (not give rise to genes in the way described). Crossover involves the exchange of genetic material between homologous chromosomes but not in the sense of "giving rise to genes" as stated.
• The fourth option is incorrect as most healthy normal cells (except gametes) have multiple pairs of chromosomes (e.g., human somatic cells have 23 pairs).

Answer:

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