determining_gene_order_by_three_point

Question

Assuming no recombination, how many different types of gametes can be produced by the wild-type female parent heterozygous for all three mutations?

Correct.In the absence of recombination the wild-type female parent can only pass on one of her two homologous chromosomes to each gamete. Since she is heterozygous for all three mutations, her homologous chromosomes are not identical. Therefore she can produce two different types of gametes.

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Incorrect. In the absence of recombination the wild-type female parent can only pass on one of her two homologous chromosomes to each gamete. Since she is heterozygous for all three mutations, her homologous chromosomes are not identical. Therefore she can produce two different types of gametes.

Question

How many different types of recombinant gametes could be produced by the wild-type female parent heterozygous for all three mutations?

Correct. In the wild-type, triple-heterozygous female parent, there are eight different types of gametes possible. Two of these are non-recombinant (no crossing over) and the other six are recombinant (produced by cross over between scarlet and spineless, cross over between spineless and ebony, and double cross over).

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Incorrect. In the wild-type, triple-heterozygous female parent, there are eight different types of gametes possible. Two of these are non-recombinant (no crossing over) and the other six are recombinant (produced by cross over between scarlet and spineless, cross over between spineless and ebony, and double cross over).

Question

How many different types of gametes could be produced by the triple-mutant male parent?

Correct. The triple-mutant male parent is homozygous for all three mutations. Since scarlet, spineless and ebony mutations are all on chromosome 3 and both homologous chromosomes contain all three mutations, only one type of gamete can be formed.

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Incorrect. The triple-mutant male parent is homozygous for all three mutations. Since scarlet, spineless and ebony mutations are all on chromosome 3 and both homologous chromosomes contain all three mutations, only one type of gamete can be formed.

Question

Assuming that gene order is as determined in the original animation, what would be the most common three-point test cross progeny types if, in the triple-heterozygous female parent, the scarlet eye and spineless bristle mutations were coupled and the ebony body mutation was in repulsion?

wild-type progeny and triple-mutant progeny

ebony progeny and scarlet, spineless progeny

ebony, scarlet progeny and spineless progeny

ebony, spineless progeny and scarlet progeny

Correct.The most common progeny in a three-point test cross represent those arising from non-recombinant gametes in the triple-heterozygote parent. If the scarlet and spineless mutations are coupled, these two mutations will segregate together on a non-recombinant gamete and the ebony mutation will segregate separately on the other non-recombinant gamete since ebony is in repulsion.

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Incorrect. The most common progeny in a three-point test cross represent those arising from non-recombinant gametes in the triple-heterozygote parent. If the scarlet and spineless mutations are coupled, these two mutations will segregate together on a non-recombinant gamete and the ebony mutation will segregate separately on the other non-recombinant gamete since ebony is in repulsion.

Question

Assuming that gene order is as determined in the original animation, what would be the least common three-point test cross progeny types if, in the triple heterozygous female parent, the scarlet eye and spineless bristle mutations were coupled and the ebony body mutation was in repulsion?

ebony, scarlet progeny and spineless progeny

wild-type progeny and triple-mutant progeny

ebony, spineless progeny and scarlet progeny

ebony progeny and scarlet, spineless progeny

Correct. The least common progeny in a three-point test cross represent those arising from the double cross over gametes in the triple-heterozygote parent. In the original animation, spineless locus is between the scarlet and ebony locus. Therefore, if the scarlet and spineless mutations are coupled and the ebony mutation is repulsed, the double cross over gametes are those containing only the ebony and spineless mutations and those containing only the scarlet mutation.

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Incorrect. The least common progeny in a three-point test cross represent those arising from the double cross over gametes in the triple-heterozygote parent. In the original animation, spineless locus is between the scarlet and ebony locus. Therefore, if the scarlet and spineless mutations are coupled and the ebony mutation is repulsed, the double cross over gametes are those containing only the ebony and spineless mutations and those containing only the scarlet mutation.

Question

Could gene order be determined if rather than eight phenotypes of progeny, as seen in the animation, there were only six phenotypes of progeny observed (representing three reciprocal progeny groups)?

Yes, the double recombinants will be the two types of progeny that will be missing and can be assumed as they are the least frequent type of recombination, in this case zero.

No, in a three-point test cross all eight phenotypic classes of progeny must be observed to determine gene order.

Yes, the pair of least frequently observed progeny will still represent the double recombinants.

Correct. In a three-point test cross, the triple-heterozygote can always theoretically produce eight different kinds of gametes (2 non-recombinant or parental types and six recombinant types) which should result in eight phenotypic classes. The double cross over recombinants are always the least frequent type of gamete produced. Therefore, if only six types of progeny are represented in the offspring, the missing two phenotypes can be assumed to be those that would have resulted from double cross over gametes.

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Incorrect. In a three-point test cross, the triple-heterozygote can always theoretically produce eight different kinds of gametes (2 non-recombinant or parental types and six recombinant types) which should result in eight phenotypic classes. The double cross over recombinants are always the least frequent type of gamete produced. Therefore, if only six types of progeny are represented in the offspring, the missing two phenotypes can be assumed to be those that would have resulted from double cross over gametes.

Question

In the animation, the original triple-heterozygous female fly had all of the mutations in a coupled arrangement. If, instead, the original wild-type, triple-heterozygous female were the result of two true breeding parents, one being scarlet and ebony and the other being spineless, which of the following statements would be TRUE?

One could no longer do a three-point test cross because the mutations would not be in coupled arrangement in the wild-type female.

Nothing would change because the female is still heterozygous for all three linked mutations.

Wild-type and triple-mutant phenotypes would be the least frequent progeny in the three point test cross.

Scarlet, ebony progeny and spineless progeny would be the least frequent progeny types in the three-point test cross.

Correct. If the triple-heterozygous, wild-type female were the result of a mating between two true breeding parental stocks, with one parent being scarlet and ebony and the other parent being spineless, scarlet and ebony would be coupled and spineless in repulsion in the wild-type female. This means that the double cross over gametes would be wild-type gametes and triple-mutant gametes. Since the double cross over gametes are the least frequent type of gametes in a three-point test cross, the wild-type and triple-mutant phenotypes will be the least frequent types of progeny.

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Incorrect. If the triple-heterozygous, wild-type female were the result of a mating between two true breeding parental stocks, with one parent being scarlet and ebony and the other parent being spineless, scarlet and ebony would be coupled and spineless in repulsion in the wild-type female. This means that the double cross over gametes would be wild-type gametes and triple-mutant gametes. Since the double cross over gametes are the least frequent type of gametes in a three-point test cross, the wild-type and triple-mutant phenotypes will be the least frequent types of progeny.

Chapter 7. Animation Activity: Determining Gene Order by Three-Point Cross

Animation Assessment

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