sbi 3u
4. compare and contrast viruses and bacteria. (4 marks)

5. lichens do not exist except as composite organisms made of a fungus and a photosynthetic partner. (4 marks)
a) explain how lichens pose a challenge to the biological species concept.

b) do lichens pose a challenge to the phylogenetic species concept? why or why not?

6. in pigs, the allele for black coat color is dominant to the allele for white coat color. two black pigs mate, producing 5 black piglets and 1 white piglet. determine the genotypes of the parents. show your work. (4 marks)

Question

sbi 3u
4. compare and contrast viruses and bacteria. (4 marks)

5. lichens do not exist except as composite organisms made of a fungus and a photosynthetic partner. (4 marks)
 a) explain how lichens pose a challenge to the biological species concept.

b) do lichens pose a challenge to the phylogenetic species concept? why or why not?

6. in pigs, the allele for black coat color is dominant to the allele for white coat color. two black pigs mate, producing 5 black piglets and 1 white piglet. determine the genotypes of the parents. show your work. (4 marks)

Accepted Answer

### Question 4: Compare and contrast viruses and bacteria.
# Brief Explanations:
- **Structure**: Bacteria are single - celled prokaryotes with a cell wall, cell membrane, cytoplasm, and genetic material (DNA in a nucleoid). Viruses are acellular, consisting of a protein coat (capsid) and genetic material (DNA or RNA), and some have an envelope.
- **Reproduction**: Bacteria reproduce asexually by binary fission. Viruses can only reproduce by hijacking a host cell's machinery and using it to make more virus particles (lytic or lysogenic cycles).
- **Metabolism**: Bacteria can be autotrophic (make their own food) or heterotrophic (obtain food from others) and have metabolic processes like respiration or fermentation. Viruses have no metabolic processes of their own.
- **Response to Antibiotics**: Bacterial infections can often be treated with antibiotics (which target bacterial cell processes or structures). Viral infections are not treated with antibiotics (antiviral drugs work differently, often targeting viral replication).
- **Size**: Bacteria are generally larger (e.g., E. coli is about 1 - 2 μm) than viruses (e.g., influenza virus is about 80 - 120 nm).
# Answer:
- **Similarities**: Both can cause diseases in living organisms. Both contain genetic material (DNA or RNA).
- **Differences**:
    - **Structure**: Bacteria are prokaryotic cells; viruses are acellular.
    - **Reproduction**: Bacteria reproduce by binary fission; viruses replicate using host cells.
    - **Metabolism**: Bacteria have metabolic processes; viruses do not.
    - **Antibiotic Sensitivity**: Bacteria are sensitive to antibiotics; viruses are not.
    - **Size**: Bacteria are larger than viruses.

### Question 5a: Explain how lichens pose a challenge to the biological species concept.
# Brief Explanations:
The biological species concept defines a species as a group of organisms that can interbreed and produce fertile offspring. Lichens are a symbiotic association between a fungus and a photosynthetic partner (usually a cyanobacterium or a green alga). The fungus and the photosynthetic partner are from different species (and often different kingdoms). Lichens do not "interbreed" in the way the biological species concept describes. Also, a single fungus can form a lichen with different photosynthetic partners, and a single photosynthetic partner can form a lichen with different fungi. This means that the lichen "entity" does not fit the definition of a species as a group that interbreeds, since it is a composite of two (or more) different species - level organisms that are in a symbiotic relationship, not a single interbreeding group.
# Answer:
The biological species concept (BSC) defines a species as a group of organisms that can interbreed and produce fertile offspring. Lichens are a symbiosis of a fungus and a photosynthetic partner (e.g., alga/cyanobacterium). These partners are different species (often different kingdoms) and do not interbreed as a single "lichen species" (the lichen is not a single interbreeding group). Also, a fungus can form lichens with different photosynthetic partners, and a photosynthetic partner can form lichens with different fungi. Thus, lichens challenge the BSC as they don't fit the interbreeding - based species definition.

### Question 5b: Do lichens pose a challenge to the phylogenetic species concept? Why or why not?
# Brief Explanations:
The phylogenetic species concept (PSC) defines a species as the smallest monophyletic group (a group with a common ancestor and all its descendants) with a unique set of shared derived characters. Lichens are a symbiotic combination of a fungus and a photosynthetic partner. The fungus and the photosynthetic partner each have their own phylogenetic history (they belong to different lineages). A lichen does not represent a single monophyletic group because it is composed of organisms from different evolutionary lineages. So, lichens do pose a challenge to the PSC because the lichen "entity" is not a single monophyletic group, but rather a symbiotic association of two (or more) groups with their own phylogenetic histories.
# Answer:
Lichens do pose a challenge to the phylogenetic species concept (PSC). The PSC defines a species as the smallest monophyletic group (common ancestor + all descendants) with unique derived traits. Lichens are a symbiosis of a fungus and a photosynthetic partner (e.g., alga/cyanobacterium), which belong to different evolutionary lineages (not a single monophyletic group). Thus, the lichen "entity" does not fit the PSC's definition of a species as a single monophyletic group.

### Question 6: In pigs, the allele for black coat color is dominant to the allele for white coat color. Two black pigs mate, producing 5 black piglets and 1 white piglet. Determine the genotypes of the parents. Show your work.
# Explanation:
## Step 1: Define the alleles
Let the allele for black coat color be $ B $ (dominant) and the allele for white coat color be $ b $ (recessive). A white piglet must have the genotype $ bb $ (since white is recessive, it can only be expressed in the homozygous recessive state).

## Step 2: Determine the alleles from parents
Each parent must contribute one $ b $ allele to the white piglet. Since both parents are black, they must have at least one $ B $ allele (because black is dominant). So, if a parent contributes a $ b $ allele (to the white offspring) and is black (so has a $ B $ allele), the genotype of each parent must be $ Bb $.

## Step 3: Verify with Punnett square
If both parents are $ Bb $, the Punnett square is:
| | $ B $ | $ b $ |
|---|---|---|
| $ B $ | $ BB $ | $ Bb $ |
| $ b $ | $ Bb $ | $ bb $ |

The phenotypic ratio of black ($ BB $ or $ Bb $) to white ($ bb $) is $ 3:1 $. In the problem, out of 6 piglets, 5 are black and 1 is white, which is consistent with the expected ratio from a $ Bb\times Bb $ cross (since the actual numbers can vary from the expected ratio due to chance, but the genotype of the parents being $ Bb $ is the only way to get a white offspring from two black parents).
# Answer:
Both parents have the genotype $ Bb $ (heterozygous). This is because the white piglet (genotype $ bb $) must receive one $ b $ allele from each parent, and since the parents are black (dominant phenotype), they must also have a $ B $ allele, resulting in the $ Bb $ genotype. The Punnett square for a $ Bb\times Bb $ cross shows that a white ($ bb $) offspring is possible, which matches the observed offspring (1 white out of 6 piglets, consistent with the expected $ 3:1 $ black - to - white ratio for this cross).

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Question 4: Compare and contrast viruses and bacteria.

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Question 5a: Explain how lichens pose a challenge to the biological species concept.