High School

For questions 12-14 consider the following scenario. Timothy has two closely related frogs, species H and species S. These two species have diverged enough that their chromosomes cannot pair up during meiosis; however, their gene content is relatively the same. As a result of this divergence, species H has n=13, while for species S, n=9. When crossing two haploid gametes of these diploid species, Timothy can form an F1, but it tends to be sterile. However, very rarely, Timothy can isolate F2s, which are fertile. What concept is demonstrated in this F2?

a. Autopolyploidy
b. Allopolyploidy
c. Monoploidy
d. Monosomy
e. Trisomy

Answer :

The scenario described is an example related to plant and animal breeding, specifically in genetics concerning the formation of F1 and F2 offspring from two different species.

  1. Understanding the Terms:

    • Species H and Species S are two related species whose chromosomes have diverged in such a way that they cannot pair up during meiosis. Here, 'n' represents the number of chromosomes in the gametes; species H has [tex]n=13[/tex] and species S has [tex]n=9[/tex].
  2. F1 and F2 Generations:

    • The F1 generation is the first generation of offspring resulting from a cross between two different species or genetic lines. These F1 individuals are typically sterile because their chromosomes cannot form homologous pairs during meiosis.
    • Occasionally, however, viable and fertile F2 generations (second filial generation) can arise.
  3. Concept of Allopolyploidy:

    • The rare fertile F2 offspring mentioned in the scenario demonstrate a concept known as allopolyploidy. This occurs when two different species with distinct sets of chromosomes produce a hybrid, which then undergoes chromosome doubling, resulting in a new, stable species with multiple sets of chromosomes from more than one species.
    • Allopolyploid species can be fertile because the doubled chromosome sets allow pairing during meiosis, overcoming the initial sterility issue seen in the F1 generation.
  4. Why This is Not Other Options:

    • Autopolyploidy refers to chromosome doubling within a single species, not between species as in this scenario.
    • Monoploidy (having a single set of chromosomes) and monosomy (having one chromosome from a pair missing) are not relevant to the formation of fertile hybrids.
    • Trisomy involves the presence of an extra chromosome, which is not related to the fertility in this specific context.

The correct multiple-choice option is b. Allopolyploidy.

This scenario illustrates the importance of chromosome behavior in hybrid species and contributes valuable insights into the mechanisms of speciation and hybrid vigor.