Problem 13 - Campbell Biology Concepts & Connections

Question

In fruit flies, the genes for wing shape and body stripes are linked. In a fly whose genotype is \(W w S s, W\) is linked to \(S\), and \(w\) is linked to \(s\). Show how this fly can produce gametes containing four different combinations of alleles. Which are parental-type gametes? Which are recombinant gametes? How are the recombinants produced?

Step-by-Step Solution

Answer

The four gametes are \(WS, ws, Ws, wS\). Parental types: \(WS, ws\). Recombinants: \(Ws, wS\). Recombinants are produced by recombination during meiosis.

Step 1 – Identify the genotype and linkage

The fly's genotype is given as \(WwSs\). The alleles are linked such that \(W\) is linked to \(S\), and \(w\) is linked to \(s\). This means the chromosomes are arranged as \(WS\) and \(ws\).

Step 2 – Possible gametes without recombination

Without recombination, the fly can produce gametes that contain the alleles present on one chromosome. Therefore, the possible gametes are: \(WS\) and \(ws\). These are the parental-type gametes because they are directly inherited from the parent.

Step 3 – Understand recombination

Recombination occurs during meiosis when homologous chromosomes exchange genetic material. This process can produce new allele combinations that are not found in the parent.

Step 4 – Possible gametes with recombination

With recombination, the fly can produce new allele combinations. The resulting recombinant gametes will be \(Ws\) and \(wS\). These gametes contain a mix of alleles from both chromosomes.

Step 5 – Categorize the gametes

The complete set of possible gametes are: \(WS, ws, Ws, wS\). \(WS\) and \(ws\) are the parental-type gametes. \(Ws\) and \(wS\) are the recombinant gametes.

Key Concepts

linked genes

Linked genes are genes that are located close to each other on the same chromosome. Because of their proximity, they tend to be inherited together during the process of meiosis. For instance, in fruit flies, genes for wing shape (W) and body stripes (S) are linked. This linkage means they are typically inherited as a pair, leading to gametes that contain either the combination of the linked genes (e.g., WS or ws). This result is known as the parental-type gametes.
However, genetic recombination can introduce variability by mixing these linked genes, resulting in recombinant gametes (e.g., Ws or wS).
Understanding how linked genes work helps predict the possible outcomes of genetic cross experiments and knowing why certain traits are inherited together.

gamete formation

Gamete formation is a crucial process in sexual reproduction where reproductive cells, called gametes, are created. In organisms like fruit flies, gametes carry only one set of chromosomes, half the number found in body cells. During meiosis, diploid cells divide to form haploid gametes.
In the case of fruit flies with the genotype WwSs, four types of gametes can be produced:

• WS (parental type)
• ws (parental type)
• Ws (recombinant type)
• wS (recombinant type)

Without recombination, only WS and ws would be formed, as these are the original linked combinations.
However, recombination introduces new allele combinations, producing Ws and wS.
This process adds genetic diversity, which is important for the evolution and adaptability of species.

meiosis

Meiosis is a type of cell division that reduces the number of chromosomes by half, resulting in the production of haploid gametes. It's a multi-step process:

• Prophase I: Homologous chromosomes pair up and exchange segments (crossing over).
• Metaphase I: Chromosome pairs align in the cell's center.
• Anaphase I: Homologous chromosomes are pulled apart to opposite poles.
• Telophase I and Cytokinesis: The cell divides into two haploid cells.
• Prophase II, Metaphase II, Anaphase II, Telophase II, and Cytokinesis: Similar to the first division but separates sister chromatids, resulting in four haploid cells.

During Prophase I, genetic recombination occurs when homologous chromosomes exchange segments. This shuffling of genetic material leads to the creation of recombinant gametes.
In our example with fruit flies, recombination can produce new allele combinations (e.g., Ws and wS) that differ from the parent's chromosomes.
This recombination during meiosis is a key source of genetic diversity in sexually reproducing organisms.