WORKED PROBLEMS

Problem 1

A chromosome has the following segments, where · represents the centromere.

What types of chromosome mutations are required to change this chromosome into each of the following chromosomes? (In some cases, more than one chromosome mutation may be required.)

• a. A B E · F G
• b. A E D C B · F G
• c. A B A B C D E · F G
• d. A F · E D C B G
• e. A B C D E E D C · F G

Solution Strategy

What information is required in your answer to the problem?

Types of chromosome mutations that would lead to the chromosome shown.

What information is provided to solve the problem?

• The original gene segments found on the chromosome.
• The altered gene segments that occur after the mutations.

For help with this problem, review:

Section 8.2.

Solution Steps

• a. The mutated chromosome (A B E · F G) is missing segment C D; so this mutation is a deletion.
• b. The mutated chromosome (A E D C B · F G) has one and only one copy of all the gene segments, but segment B C D E has been inverted 180 degrees. Because the centromere has not changed location and is not in the inverted region, this chromosome mutation is a paracentric inversion.
• c. The mutated chromosome (A B A B C D E · F G) is longer than normal, and we see that segment A B has been duplicated. This mutation is a tandem duplication.
• d. The mutated chromosome (A F · E D C B G) is normal length, but the gene order and the location of the centromere have changed; this mutation is therefore a pericentric inversion of region (B C D E · F).
• e. The mutated chromosome (A B C D E E D C · F G) contains a duplication (C D E) that is also inverted; so this chromosome has undergone a duplication and a paracentric inversion.

Problem 2

Species I is diploid (2n = 4) with chromosomes AABB; related species II is diploid (2n = 6) with chromosomes MMNNOO. Give the chromosomes that would be found in individuals with the following chromosome mutations.

• a. Autotriploidy in species I
• b. Allotetraploidy including species I and II
• c. Monosomy in species I
• d. Trisomy in species II for chromosome M
• e. Tetrasomy in species I for chromosome A
• f. Allotriploidy including species I and II
• g. Nullisomy in species II for chromosome N

Solution Strategy

What information is required in your answer to the problem?

The letter designations of chromosomes that will be found in individuals with each type of mutation.

What information is provided to solve the problem?

• Species I is diploid with 2n = 4.
• Species I has chromosomes AABB.
• Species II is diploid with 2n = 6.
• Species II has MMNNOO.

For help with this problem, review:

Sections 8.3 and 8.4.

Solution Steps

• a. An autotriploid is 3n, with all the chromosomes coming from a single species; so an autotriploid of species I would have chromosomes AAABBB (3n = 6).

  Hint: First determine the haploid genome complement for each species. For species I, n = 2 with chromosomes AB and, for species II, n = 3 with chromosomes MNO.

• b. An allotetraploid is 4n, with the chromosomes coming from more than one species. An allotetraploid could consist of 2n from species I and 2n from species II, giving the allotetraploid (4n = 2 + 2 + 3 + 3 = 10) chromosomes AABBMMNNOO. An allotetraploid could also possess 3n from species I and 1n from species II (4n = 2 + 2 + 2 + 3 = 9; AAABBBMNO) or 1 n from species I and 3n from species II (4n = 2 + 3 + 3 + 3 = 11; ABMMMNNNOOO).
• c. A monosomic is missing a single chromosome; so a monosomic for species I would be 2n − 1 = 4 − 1 = 3. The monosomy might include either of the two chromosome pairs, with chromosomes ABB or AAB.
• d. Trisomy requires an extra chromosome; so a trisomic of species II for chromosome M would be 2n + 1 = 6 + 1 = 7 (MMMNNOO).
• e. A tetrasomic has two extra homologous chromosomes; so a tetrasomic of species I for chromosome A would be 2n + 2 = 4 + 2 = 6 (AAAABB).
• f. An allotriploid is 3n with the chromosomes coming two from different species; so an all otriploid could be 3n = 2 + 2 + 3 = 7 (AABBMNO) or 3n = 2 + 3 + 3 = 8 (ABMMNNOO).
• g. A nullisomic is missing both chromosomes of a homologous pair; so a nullisomic of species II for chromosome N would be 2n − 2 = 6 − 2 = 4 (MMOO).