Question

which of the following would most likely result in an abnormal chromosome number?
failure of homologous chromosomes to separate during meiosis.
dna exposed to radiation after the birth of the organism.
mutations occurring during the fetal stages.
the deletion of dna during translation.

Explanation:

• Failure of homologous chromosomes to separate during meiosis: This is called non - disjunction. During meiosis, homologous chromosomes (in meiosis I) or sister chromatids (in meiosis II) should separate. If they fail to do so, the resulting gametes will have an abnormal number of chromosomes. For example, in humans, if non - disjunction occurs during meiosis I of gamete formation, gametes can have 22 or 24 chromosomes instead of the normal 23. When these gametes fuse with a normal gamete (with 23 chromosomes) during fertilization, the zygote will have an abnormal chromosome number (45 or 47).
• DNA exposed to radiation after the birth of the organism: Radiation can cause mutations in DNA (such as base - pair substitutions, deletions, insertions). But these mutations affect the nucleotide sequence of genes within chromosomes, not the number of chromosomes.
• Mutations occurring during the fetal stages: Similar to the above, these mutations change the genetic code within genes (e.g., a point mutation like sickle - cell anemia where a single base - pair change occurs in the β - globin gene). They do not change the chromosome number.
• The deletion of DNA during translation: Translation is the process of protein synthesis from mRNA. DNA is not deleted during translation. Even if there were an error in DNA replication (which is a different process), a deletion in DNA would affect the gene sequence, not the chromosome number.

Answer:

Failure of homologous chromosomes to separate during meiosis.