26. In an experiment involving mapping of 3 genes (a, b andc) in Drosophila, a three point test cross is carried out.The parental cross was AAbbCCXaaBBcc. The genotypesof the double crossovers are: Aabbcc and aaBbCc. Basedon this, determine the order of the genes.
(1) acb (2) cab
(3) abc (4) bac
Introduction to Three-Point Test Cross in Drosophila
A three-point test cross is an essential genetic technique in Drosophila used for mapping the order and distance between three linked genes on a chromosome. This method involves crossing a heterozygous organism for three gene loci with a homozygous recessive tester, allowing for analysis of various recombinant and parental types to deduce gene order and recombination frequencies.
Understanding the Parental and Double Crossover Genotypes
Given a cross between AAbbCC and aaBBcc in Drosophila, the parental chromosomes are:
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Parental chromosome 1: A b C
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Parental chromosome 2: a B c
The genotypes of the double crossover offspring are Aabbcc and aaBbCc. Double crossovers differ from parental genotypes by switching the middle gene, indicating the gene order places the gene that changes in double crossovers in the middle position.
Determining Gene Order From Options
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Option (1) acb: This order suggests gene b is last, but double crossovers show changing alleles for b, which conflicts with this order.
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Option (2) cab: Indicates gene a is in the middle, inconsistent with the observed changes in b in double crossovers.
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Option (3) abc: This order fits the data since double crossovers involve changes in gene b, which is the middle gene, while genes a and c remain consistent with parental types.
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Option (4) bac: This would mean gene a is in the middle with changes expected there, but the data points to changes in gene b instead.
Thus, option (3) abc correctly represents the gene order.
Explanation of Double Crossover Role in Mapping Gene Order
Double crossovers are crucial because they cause changes only in the middle gene when compared to parental chromosomes. By identifying which gene switches in the double crossover progeny, one can pinpoint the gene order. In this example, switches in b alleles in Aabbcc and aaBbCc show gene b is in the middle, making the gene order abc.
This explanation helps you confidently determine gene order from three-point test cross data in Drosophila, a foundational skill in genetics and molecular biology research and education.
