In the snail Cepaea nemoralis, an autosomal allele causing a banded shell (BB) i
ID: 61578 • Letter: I
Question
In the snail Cepaea nemoralis, an autosomal allele causing a banded shell (BB) is recessive to the allele for unbanded shell (BO). Genes at a different locus determine the background color of the shell; here, yellow (CY) is recessive to brown (CBw). A banded, yellow snail is crossed with a homozygous brown, unbanded snail. The F1are then crossed with banded, yellow snails (a testcross). a. What will be the results of the testcross if the loci that control banding and color are linked with no crossing over? b. What will be the results of the testcross if the loci assort independently? c. What will be the results of the testcross if the loci are linked and 20 map units apart? In the snail Cepaea nemoralis, an autosomal allele causing a banded shell (BB) is recessive to the allele for unbanded shell (BO). Genes at a different locus determine the background color of the shell; here, yellow (CY) is recessive to brown (CBw). A banded, yellow snail is crossed with a homozygous brown, unbanded snail. The F1are then crossed with banded, yellow snails (a testcross). a. What will be the results of the testcross if the loci that control banding and color are linked with no crossing over? b. What will be the results of the testcross if the loci assort independently? c. What will be the results of the testcross if the loci are linked and 20 map units apart? In the snail Cepaea nemoralis, an autosomal allele causing a banded shell (BB) is recessive to the allele for unbanded shell (BO). Genes at a different locus determine the background color of the shell; here, yellow (CY) is recessive to brown (CBw). A banded, yellow snail is crossed with a homozygous brown, unbanded snail. The F1are then crossed with banded, yellow snails (a testcross). a. What will be the results of the testcross if the loci that control banding and color are linked with no crossing over? b. What will be the results of the testcross if the loci assort independently? c. What will be the results of the testcross if the loci are linked and 20 map units apart?Explanation / Answer
(a) P generation - BB -----> CYxBO -------> CBW ---------> BB CYBO CBW
F1 generation BB ------------> CY ------------> BO CBWF
or F2 generation: BB ------------> CYxBB -------> CY --------> BO CBWBB
CYBB -------->CYBB ---------> CY --------------> BB CYBO CBW which has 50% banded yellow and 50% unbanded brown.
(b)
P generation is BBBBCYCYx BOBOCBWCBW
F1 generation BOBBCBWCYF
or F2 generation BOBBCBWCY x BBBBCYCYBO
CBWBOCYBBC BWBBCYBBCY BOBBCBWCY Unbanded brown - BOBBCYCY Unbanded yellow - BBBBCBWCY Banded brown - BBBBCYCY .
Banded yellow = 25% unbanded brown = 25% unbanded yellow = 25% banded brown = 25% banded yellow
(c)
P generation - BB ------> CYxBO --------> CBW ------------> BB CYBO CBW
F1 generation - BB --------> CY ----------> BO CBWF
or F2 generation: BB ----------> CYxBB --------> CY ------------> BO CBWBB CYGenes are linked and are 20 mu apart – the F1 double heterozygote should form recombinant gametes 20% of the time (10% BBCBW and 10%BOCY), and nonrecombinant gametes 80% of the time (40% BBCY and 40% BOCBW).BB --------> CYBO -------->CBWBB -------> CBWBO ---------> CY --------> BB CYBB CYBB CYBB CY with 40% banded yellow , 40% unbanded brown , 10% banded brown and 10% banded yellow