30. In Drosophila, balancer chromosomes are used to keep all the alleles on one chromosome together. A balancer contains multiple inversions; so that when it recombines with the corresponding wild type chromosome, no viable cross over products are formed. Balancers also carry an allele for a dominant phenotype.
A Drosophila male with sepia eye color is crossed to a female carrying a third chromosome balancer (TM6B). The allele for sepia phenotype (se) is located on chromosome 3 and is recessive to the wild type eye color. The dominant marker for TM6B is a tubbyphenotype. Further, an individual homozygous for TM6B balancer does not survive. F1 progeny with tubby phenotype is sib-mated. The F2 progeny is expected to have:
(1) only sepia eye color
(2) sepia. tubby and wild type flies in a ratio of 1:2:1
(3) sepia and tubby flies in a ratio of 1:2
(4) sepia and wild type flies in ratio of 3:1
Introduction:
Balancer chromosomes like TM6B in Drosophila melanogaster are essential tools used to maintain linked alleles and suppress recombination. The TM6B balancer carries multiple inversions with a dominant tubby marker and causes lethality when homozygous. The sepia eye color mutation (se) is recessive and located on chromosome 3. Understanding the expected F2 progeny phenotype ratios when crossing flies with sepia and TM6B balancer sheds light on balancer genetics and Mendelian inheritance patterns.
Explanation of Options:
(1) Only sepia eye color:
This option is incorrect because the TM6B balancer carries a dominant tubby phenotype marker, which will segregate along with the sepia mutation in heterozygous flies, so tubby flies will appear in the progeny.
(2) Sepia, tubby, and wild type flies in a ratio of 1:2:1:
This is the correct expected ratio. Since TM6B is dominant for tubby and homozygous lethal, the F1 tubby flies are heterozygous (Tm6B/se) and when sib-mated, the F2 will show three phenotypes:
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tubby flies heterozygous for TM6B dominant marker (2/4)
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sepia flies homozygous recessive for se without TM6B (1/4)
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wild type flies homozygous without TM6B or se (1/4)
Homozygous TM6B flies do not survive, explaining why the ratio is 1:2:1 for sepia:tubby:wild type.
(3) Sepia and tubby flies in a ratio of 1:2:
This is not correct because the wild type flies without tubby or sepia phenotype are also expected to appear in the F2, making three phenotypes total.
(4) Sepia and wild type flies in ratio 3:1:
This ratio would be expected if the balancer and tubby phenotype were not involved, focusing just on sepia as a Mendelian recessive trait. This ignores the presence of TM6B and tubby dominant marker.
Summary of Genetic Cross:
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Parental cross: Sepia (recessive) male × TM6B (tubby, dominant) balancer female
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F1: Tubby heterozygous (balancer/sepi) phenotype flies sib-mated
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F2 expected phenotypes with approximate ratios: sepia (1), tubby (2), wild type (1); TM6B homozygotes die
This classic example illustrates the use of balancer chromosomes in Drosophila genetics to maintain mutations and control recombination, while dominant visible markers (tubby) aid in identifying genotypes.
References:
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TM6B balancer characteristics and tubby dominant phenotype, homozygous lethal nature
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Sepia eye color as a recessive mutation on chromosome 3
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Expected Mendelian segregation patterns involving balancer chromosomes and linked markers
