IB Biology HL IA – 7

Measuring the Effect of the Distance of Two Genes in Chromosome number 3 of Drosophila melanogaster on Recombination Frequencies

1. Research Question

To what extent does the distance of two genes (1, 12, 13, 32, 33, 45 cM) in chromosome number 3 of Drosophila melanogaster affect recombination frequency measured in percentage (%)?

2. Background Information

Genetic linkage is where two genes sit close together on the same chromosome and tend to be inherited together as linkage groups. During meiosis, the alleles of two different genes are likely to pass as a whole unit to gametes and thereby expressing parental configuration of alleles and phenotypes. In other words, the linkage reduces the amount of genetic variation in the offspring (Khan, 2016).

In 1911, Thomas Hunt Morgan, provided a major breakthrough to the deviation from Mendel’s second law of inheritance and discovered non-Mendelian ratios in Drosophila melanogaster (which now will be shortened as D. melanogaster). Morgan studied two autosomal genes on the same chromosome: one of genes affecting eye color and another one affecting wing length ( In fruit fly genetics, the normal fly is often referred to a “wild type” and its allele is denoted by a superscript + over the mutant allele. Any fly expressing a certain phenotypic mutation is called a “mutant” and specific code is designated for its allele such as pr and vg. Morgan employed a ‘heterozygous test cross’ between a double heterozygous fly (pr vg/pr+ vg+) and homozygous recessive (pr vg/pr vg) of parents to observe phenotypic classes of F1 offspring. As a result, four classes of offspring were observed in unequal numbers. Two of them containing parental chromosome configurations largely detected and the remaining two classes having new chromosome configurations, so-called recombinants, under-represented. The number of recombinants will be always appeared a way less than the number of parental configurations of alleles because crossing-over does not happen all the time. From here, Morgan not only found genes are positioned on the same chromosome but also established the idea of crossing-over and recombinants. (Griffiths, 2000).

The heterozygous test cross is a great medium to figure out whether two genes are linked or not by observing phenotypic ratio of the offspring. The heterozygous test cross of linked genes produces 1:1:0.1:0.1 phenotypic ratio of the offspring. However, if two genes are unlinked, the results of this heterozygous test cross comply with the law of independent assortment, giving an equal ratio of four possible phenotypes – 1:1:1:1 ratio. This all relates back to the relationship between the frequency of crossing-over and the distance between two genes. If two genes are close together, crossing-over happens rarely and thereby resulting in few recombinants. Whereas, two genes that are relatively further apart have more recombination events and thus having a greater number of recombinants. This explains why the cross involving two genes that are far apart produces the Mendelian ratio in the phenotype of their offspring: 1:1:1:1 ratio – which also accounts for the phenotypic ratio of a dihybrid cross involving two genes on the separate chromosome. In other words, two genes found on different chromosome, or far apart on the same chromosome, are both said to be unlinked (BBC, 2014)…