Drosophila females of wild-type appearance but heterozygous for three autosomal genes are mated with malesshowing the three corresponding autosomal recessivetraits: glassy eyes, coal-colored bodies, and striped thoraxes. One thousand (1000) progeny of this cross aredistributed in the following phenotypic classes:Wild type 27Striped thorax 11Coal body 484Glassy eyes, coal body 8Glassy eyes, striped thorax 441Glassy eyes, coal body, striped thorax 29a. Draw a genetic map based on these data.b. Show the arrangement of alleles on the two homologous chromosomes in the parent females.c. Normal-appearing males containing the samechromosomes as the parent females in the precedingcross are mated with females showing glassy eyes,coal-colored bodies, and striped thoraxes. Of 1000progeny produced, indicate the numbers of thevarious phenotypic classes you would expect.

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Asked Mar 10, 2020
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Drosophila females of wild-type appearance but heterozygous for three autosomal genes are mated with males
showing the three corresponding autosomal recessive
traits: glassy eyes, coal-colored bodies, and striped thoraxes. One thousand (1000) progeny of this cross are
distributed in the following phenotypic classes:
Wild type 27
Striped thorax 11
Coal body 484
Glassy eyes, coal body 8
Glassy eyes, striped thorax 441
Glassy eyes, coal body, striped thorax 29
a. Draw a genetic map based on these data.
b. Show the arrangement of alleles on the two homologous chromosomes in the parent females.
c. Normal-appearing males containing the same
chromosomes as the parent females in the preceding
cross are mated with females showing glassy eyes,
coal-colored bodies, and striped thoraxes. Of 1000
progeny produced, indicate the numbers of the
various phenotypic classes you would expect.

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Expert Answer

Step 1
  • Let us represent the alleles responsible for wildtype eyes, wildtype bodies, and wildtype thoraxes as E, B, and T. The recessive forms of these alleles responsible for glassy eyes, coat-colored body, and striped thorax can be represented as e, b, and t.
  • The genotype of the heterozygous female drosophila having the wildtype traits is EeBbTt. The genotype of the male drosophila expressing the recessive trait is eebbtt.
Step 2
  • The cross between EeBbTt and eebbtt drosophila would produce both recombinant and non-recombinant progenies. The recombinant progenies will form as a result of crossing over between genes of the female parent. This is because crossing over in the male parent would not generate recombinant gametes as both the chromosomes carry the same recessive alleles.
  • The different progenies produced as a result of this cross is given. In these cases, the highest number of progenies have a parental or non-recombinant phenotype. The lowest number of progenies are those that are produced as a result of a double cross over between the genes. This is because the formation of chiasma in nearby regions is a very infrequent event. The progenies with intermediate numbers between non-recombinant and double crossover are considered single cross over progeny. The table below assigns the progenies with various phenotypes as non-recombinant, single cross over and double crossover.

 

Phenotype

Number

Type

Coal body

484

Non-recombinant (parental)

Glassy eyes, striped thorax

441

Non-recombinant (parental)

Glassy eyes, coal body, striped thorax

29

Single crossover

Wild type

27

Single crossover

Striped thorax

11

Double crossover

Glassy eyes, coal body

8

Double crossover

Step 3
  • From the given non-recombinant progenies, the phenotypes of parents can be determined. The male parent contributes only recessive genes. In the coal body progeny, the genotype is EbT/ebt. The glassy eyes and striped thorax progeny have the genotype eBt/ebt. Therefore, the genotype of the female parent is EbT/eBt and the male parent is ebt/ebt. The schematic representation is shown below.

Biology homework question answer, step 3, image 1

  • By looking at the genotype of a double crossover progeny the gene order can be determined. This is because during double crossover event, only the middle gene is moved from one chromosome to its homologous counterpart. It can be seen that the glassy eye allele is moved as one of the double crossover progenies h...

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