Understanding Genetic Ratios and Sex Determination

The question involves understanding how sex is determined in different organisms and how genetic factors like the X:A ratio and Barr bodies are related to it. We are given individuals with two haploid sets of autosomes (effectively diploid for autosomes) and no Y chromosome.

The X:A ratio is calculated as the number of X chromosomes divided by the number of sets of autosomes. In these individuals, there are 2 sets of autosomes (A = 2). The Y chromosome is absent.

Let's analyze each case based on the given X:A ratio and organism:

Case i: X:A ratio = 0.5

• The X:A ratio is given as $\frac{\text{Number of X}}{\text{Number of sets of autosomes}} = \frac{\text{Number of X}}{2} = 0.5$.
• This implies the Number of X chromosomes = $0.5 \times 2 = 1$.
• Since there is no Y chromosome, the genotype is XO.

• Organism A: Human

  • In humans, individuals with an XO genotype (Turner syndrome) are female. So, the primary sex is Female (II).
  • Barr bodies are inactivated X chromosomes found in mammalian somatic cells. The number of Barr bodies is typically calculated as the number of X chromosomes minus one (N-1 rule).
  • With 1 X chromosome, the number of Barr bodies = $1 - 1 = 0$. So, the number of Barr bodies is zero (a).

• Organism B: Drosophila

  • In Drosophila, sex is determined by the X:A ratio. An X:A ratio of 0.5 (as in XO or XY individuals) results in a male phenotype. So, the primary sex is Male (I).
  • Barr bodies are not typically observed or used for sex determination in Drosophila in the same way as in mammals. Dosage compensation occurs differently. For the purpose of this question and matching options, let's assume 0 Barr bodies are expected in Drosophila lacking a Y chromosome, similar to the mammalian XO case or simply because the mammalian Barr body concept doesn't apply directly. The option 'a' (zero) is available.

Case ii: X:A ratio = 2

• The X:A ratio is given as $\frac{\text{Number of X}}{\text{Number of sets of autosomes}} = \frac{\text{Number of X}}{2} = 2$.
• This implies the Number of X chromosomes = $2 \times 2 = 4$.
• Since there is no Y chromosome, the genotype is XXXX.

• Organism A: Human

  • In humans, individuals with four X chromosomes (XXXX) are female. So, the primary sex is Female (II).
  • Using the N-1 rule for Barr bodies, with 4 X chromosomes, the number of Barr bodies = $4 - 1 = 3$. So, the number of Barr bodies is three (d).

• Organism B: Drosophila

  • In Drosophila, an X:A ratio greater than 1 (like 2) results in a metafemale (also called superfemale) phenotype. So, the primary sex is Metafemale (III).
  • As discussed earlier, Barr bodies are generally considered 0 in Drosophila in this context, or at least not following the mammalian N-1 rule with observable structures. The option 'a' (zero) is available.

Summarizing the findings:

Now, let's match these results with the options provided:

• Case i, Organism A, Primary Sex II, Barr bodies a (i - A - II - a)
• Case ii, Organism A, Primary Sex II, Barr bodies d (ii - A - II - d)
• Case i, Organism B, Primary Sex I, Barr bodies a (i - B - I - a)
• Case ii, Organism B, Primary Sex III, Barr bodies a (ii - B - III - a)

Comparing this sequence (i - A - II - a; ii - A - II - d; i - B - I - a; ii - B - III - a) with the given options, we find the correct match.