Question

Phenotypically wild \(\mathrm{F}_{1}\) female Drosophila, whose mothers had light eyes \((l t)\) and fathers had straw \((s t w)\) bristles, produced the following offspring when crossed with homozygous lightstraw males: $$\begin{array}{lc} \text { Phenotype } & \text { Number } \\ \text { light-straw } & 22 \\ \text { wild } & 18 \\ \text { light } & 990 \\ \text { straw } & 970 \\ \text { Total } &{2000} \end{array}$$ Compute the map distance between the light and straw loci.

Short answer

Question: Calculate the map distance between the light (lt) and straw (stw) loci in Drosophila melanogaster based on the following data from a cross between heterozygous females and homozygous males: - light-straw: 22 offspring - wild: 18 offspring - light: 990 offspring - straw: 970 offspring Answer: The map distance between the light and straw loci is 98 centimorgans.

Step-by-step solution

Determine recombinant offspring

Recombinants are the individuals that display a different phenotype combination than the parental phenotypes, which are Light and Straw. From the given offspring numbers, we have: - light-straw: 22 - wild: 18 - light: 990 - straw: 970 The recombinant offspring are those with the light and straw phenotypes, so we have 990 light and 970 straw, totaling 1960 recombinants.

Calculate the frequency of recombinant offspring

To calculate the frequency of recombinant offspring, we divide the number of recombinant offspring (light and straw) by the total number of offspring produced. From the table, the total number of offspring produced is 2000. Frequency of recombinant offspring = (Number of recombinant offspring) / (Total number of offspring) = (990 + 970) / 2000 = 1960 / 2000 = 0.98

Calculate the map distance between the light and straw loci

To calculate the map distance, we multiply the frequency of recombinant offspring by 100, as map distances are expressed in centimorgans (cM). Map distance = (Frequency of recombinant offspring) * 100 = (0.98) * 100 = 98 cM The map distance between the light and straw loci is 98 centimorgans.

Key concepts

Recombinant Offspring

Recombinant offspring refer to the progeny derived from sexual reproduction that have a different combination of traits than either of their parents. This phenomenon is a direct result of the process of recombination, which can occur during meiosis, the type of cell division involved in sexual reproduction. When two genes are located on the same chromosome, they are said to exhibit genetic linkage, which influences the likelihood that they will be inherited together; however, recombination can break this linkage, leading to recombinant offspring.

In our Drosophila genetics example, recombinant offspring were identified based on their unique light or straw phenotypes, distinct from their wild-type parents. This was crucial for calculating the genetic map distance, a concept further explained in subsequent sections. Recombinant offspring, in larger genetic studies, provide powerful insights into the arrangement and distances between genes on a chromosome.

Genetic Linkage

Genetic linkage is a key concept in genetics which points to the tendency of certain genes to be inherited together because they reside closely on the same chromosome. This proximity restricts the independent assortment of these genes during the formation of gametes in meiosis. As a result, genes that are closely linked do not assort independently according to Mendel's second law, but rather are inherited together unless separated by recombination.

Importance of Genetic Linkage

Understanding genetic linkage is critical for constructing genetic maps which depict the relative positions of genes on a chromosome. The presence of linkage provides insights into the physical arrangement of genes and helps predict the outcome of genetic crosses. In the context of the exercise, understanding that the light and straw genes are linked was pivotal for determining the frequency of the recombinant offspring and thus, the distance between these genes.

Centimorgan

A centimorgan (cM) is a unit of measure used in genetic mapping. Named after the pioneering geneticist Thomas Hunt Morgan, it represents the distance between chromosome positions for which one product of meiosis in 100 is recombinant. A distance of 1 cM suggests that a pair of genetic markers are separated by 1% chance of crossing over occurring between them per generation.

Mathematically, if the frequency of recombinant offspring in a genetic cross is 0.98, as in our Drosophila exercise, this means that the two genes exhibit 98 recombinations for every 100 offspring, or a 98% chance of recombination. Consequently, the distance between them would be 98 cM. This metric facilitates the creation of genetic maps which are critical tools for geneticists, especially in the fields of genetic research and breeding.

Drosophila Genetics

Drosophila melanogaster, more commonly known as the fruit fly, is a premier model organism in the field of genetics. Its popularity in genetic research is due to its short life cycle, high fecundity, and relatively simple genome. Researchers have used Drosophila extensively to study fundamental genetic concepts such as inheritance patterns, genetic linkage, and mutations.

Application in Genetic Mapping

The ease of breeding and the clear phenotypic variations make Drosophila a prime choice for genetic mapping exercises like the one in our example. By analyzing the different phenotypes of the offspring—wild-type, light, and straw—one can infer how genes are linked and calculate the genetic map distances between the loci. The results of such Drosophila studies have had widespread implications, advancing our understanding of genetics in more complex organisms, including humans.