Describe the experiment conducted by T.H. Morgan on Drosophila melanogaster involving eye colour and body colour. How did the results deviate from Mendelian inheritance pattern ? (c) Explain the two genetic terms used by Morgan for his observations.
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Remember this rule for genetic distance: Recombination frequency is directly proportional to the physical distance between genes. - Close together = Tight Linkage = Low Recombination ($1.3\%$ for body/eye color).
Far apart = Loose Linkage = High Recombination ($37.2\%$ for eye color/wing size).
Step 1: Understanding the Question:
The question asks for a description of Thomas Hunt Morgan's dihybrid crosses in fruit flies (Drosophila melanogaster) involving eye color and body color traits. It also asks how his data deviated from standard Mendelian ratios and requires an explanation of the terms linkage and recombination. Step 2: Key Formula or Approach:
Mendel's Law of Independent Assortment predicts a $9:3:3:1$ phenotypic ratio in the $\text{F}_2$ generation of a dihybrid cross, or a $1:1:1:1$ ratio in a dihybrid test cross. When genes are located close together on the same chromosome, they alter these expected proportions due to genetic linkage. Step 3: Detailed Explanation: (a) Morgan's Experiment: Morgan performed a dihybrid cross using fruit flies to study two X-linked genes: body color and eye color. He crossed yellow-bodied, white-eyed females (carrying recessive mutant alleles, $yw$) with wild-type brown-bodied, red-eyed males (carrying dominant alleles, $y^+w^+$). He then intercrossed the resulting $\text{F}_1$ generation offspring to analyze the trait combinations in the $\text{F}_2$ generation. (b) Deviation from Mendel: According to Mendel's Law of Independent Assortment, the two genes should separate independently, producing an $\text{F}_2$ generation with a $9:3:3:1$ ratio, where $37.5\%$ of the offspring display new, non-parental trait combinations (recombinant phenotypes). However, Morgan observed a significant deviation: $98.7\%$ of the $\text{F_2$ offspring retained the exact parental combinations}, while only $1.3\%$ showed recombinant traits. This proved that the genes did not sort independently. (c) Morgan's Genetic Terms: To explain these findings, Morgan introduced two key concepts:
1. Linkage: This describes the physical association of two or more genes located on the same chromosome. Because they are physically linked on the same DNA strand, they tend to be inherited together as a unit during meiosis, keeping parental trait combinations intact.
2. Recombination: This describes the generation of non-parental gene combinations in offspring. It occurs during prophase I of meiosis, when homologous chromosomes cross over and exchange genetic material, swapping alleles between chromatids. Morgan noted that the rate of recombination depends directly on the distance between genes: genes located very close together show tight linkage and low recombination rates (like eye and body color at $1.3\%$), while genes farther apart show weaker linkage and higher recombination rates. Step 4: Final Answer:
(a) Morgan crossed yellow-bodied, white-eyed female flies with brown-bodied, red-eyed male flies, and then intercrossed their $\text{F}_1$ offspring to study how the traits were passed down.
(b) The results deviated from the expected Mendelian dihybrid ratios because parental phenotypes appeared at a very high rate ($98.7\%$), while recombinants appeared at a very low rate ($1.3\%$), showing that independent assortment did not occur.
(c) Linkage refers to the physical association of genes on the same chromosome, which keeps them together. Recombination refers to the formation of new, non-parental gene combinations due to crossing over during meiosis.
