An aquatic arthropod called a Cyclops has antennae that are either smooth or barbed. The allele for barbs is dominant. In the same organism, resistance to pesticides is a recessive trait. Make a “key” to show all the possible genotypes (and phenotypes) of this organism. Use the rabbit key to help you if you’re lost.
Key:
The correct answer and explanation is :
Sure! To create a key for the Cyclops organism with two traits, we need to break down the genetic possibilities based on the dominant and recessive alleles. The two traits in question are:
- Antennae type (smooth or barbed)
- Barbed antennae is the dominant trait (B).
- Smooth antennae is the recessive trait (b).
- Resistance to pesticides (resistant or susceptible)
- Resistance to pesticides is recessive (r), so an organism must have two copies of the recessive allele (rr) to be resistant.
- Susceptibility to pesticides is the dominant trait (R), so the organism can either have one dominant allele (Rr) or two dominant alleles (RR) to be susceptible.
Key:
- B = Barbed antennae (dominant)
- b = Smooth antennae (recessive)
- R = Susceptible to pesticides (dominant)
- r = Resistant to pesticides (recessive)
Step 1: List the possible combinations of alleles for each trait.
- For antennae:
- BB (Barbed)
- Bb (Barbed)
- bb (Smooth)
- For pesticide resistance:
- RR (Susceptible)
- Rr (Susceptible)
- rr (Resistant)
Step 2: Combine the alleles for both traits.
The Cyclops organism has two traits, so we will combine the possible alleles for antennae and pesticide resistance:
- BBRR – Barbed antennae, Susceptible to pesticides
- BBRr – Barbed antennae, Susceptible to pesticides
- BbRR – Barbed antennae, Susceptible to pesticides
- BbRr – Barbed antennae, Susceptible to pesticides
- bbRR – Smooth antennae, Susceptible to pesticides
- bbRr – Smooth antennae, Susceptible to pesticides
- BBrr – Barbed antennae, Resistant to pesticides
- Bbr – Barbed antennae, Resistant to pesticides
- bbrr – Smooth antennae, Resistant to pesticides
Step 3: Write the phenotypes based on the alleles.
- Barbed antennae, Susceptible to pesticides: Any genotype with at least one B and either R or Rr (BBRR, BBRr, BbRR, BbRr).
- Smooth antennae, Susceptible to pesticides: bb and either RR or Rr (bbRR, bbRr).
- Barbed antennae, Resistant to pesticides: Any genotype with at least one B and rr (BBrr, Bbr).
- Smooth antennae, Resistant to pesticides: bb and rr (bbrr).
Explanation:
- Dominant vs. Recessive: The barbed antennae trait is dominant (B), meaning the organism only needs one copy of the dominant allele to have barbed antennae. For resistance to pesticides, however, the trait is recessive (r), so the organism must have two copies of the recessive allele (rr) to be resistant. If it has at least one dominant allele (R), it will be susceptible to pesticides.
- Genotype vs. Phenotype: The genotype refers to the specific alleles an organism has (e.g., BBRR), while the phenotype describes the observable traits (e.g., barbed antennae and susceptible to pesticides).