Bio-210 Class Notes
Aneuploidy and Polyploidy Notes
• Aneuploidy: change in the number of individual chromosomes
o Only one chromosome is altered
o Monosomy is 2n-1
§ Trisomy is 2n+1
§ Tetrasomy is 2n+2
o Results from nondisjunction in meiosis, where homologous chromosomes or
sister chromatids do not separate
§
§ Merging with a normal gamete can create monosomic or trisomic
individuals
§
o If identical gene passed twice, it is from the parent with the gene and an issue in
meiosis II from the sister chromatids not separating.
• Polyploidy: change in the number of chromosome sets
o Homolog sets are altered for all chromosomes, not just one
o Triploidy (3x) is three copies of each homolog
§ Tetraploidy (4x) is four copies of each homolog
o More common in plants than animals
o Autopolyploidy: chromosome sets from a single species
§ Autotetraploid = 4n
§ Triploid = 3n
§ Could be defect in mitosis
, •
§ Could be defect in meiosis
•
o Allopolyploidy: chromosome sets are from two or more species
§ Allotetraploids often fertile
§
o Allotetraploid = 2n from each species
• Rearrangements: change to the structure of individual chromosomes
,Examples
• A human female with Turner syndrome (45,X) also expresses the X-linked trait
hemophilia, as did her father. Which of her parents underwent nondisjunction during
meiosis, giving rise to the gamete responsible for the syndrome?
o Because both she and her father express hemophilia, it is likely that the woman
inherited her sole X chromosome from her father and no sex chromosome from
her mother. This means that nondisjunction occurred in the mother, either
during meiosis I or meiosis II, producing an egg with no X chromosome.
• When two plants belonging to the same genus but different species are crossed, the F1
hybrid is viable and has more ornate flowers. Unfortunately, this hybrid is sterile and
can only be propagated by vegetative cuttings. Explain the sterility of the hybrid and
what would have to occur for the sterility of this hybrid to be reversed.
o The sterility of interspecific hybrids is often caused from a high proportion of
univalents in meiosis I, producing mostly inviable gametes. Viable gametes are
rare, and the likelihood of two of them “meeting” is remote. The horticulturist
might attempt to reverse the sterility by treating the hybrid with colchicine,
which, if successful, would double the chromosome number, so each
chromosome would have a homolog with which to pair during meiosis.
• What is the effect of a rare double crossover within a chromosome segment that is
heterozygous for a paracentric inversion?
o Rare double crossovers within the boundaries of a paracentric inversion
heterozygote produce only minor departures from the standard chromosomal
arrangement as long as the crossovers involve the same two chromatids. With
two-strand double crossovers, the second crossover negates the first. However,
three-strand and four-strand double crossovers have consequences that lead to
anaphase bridges as well as a high degree of genetically unbalanced gametes.
• For a species with a diploid number of 18, indicate how many chromosomes will be
present in the somatic nuclei of individuals that are haploid, triploid, tetraploid,
trisomic, and monosomic.
o haploid = 9, triploid = 27, tetraploid = 36, trisomic = 19, monosomic = 17
• Contrast the fertility of an allotetraploid with an autotriploid and an autotetraploid.
o Both allotetraploids and autotetraploids would be able to GG produce bivalents
in miosis I and would probably be equivalently fertile. Furthermore, both would
be more fertile than autotriploids, which have an odd number of chromosomes
and would frequently produce unbalanced gametes.
• Inversions are said to “suppress crossing over.” Is this terminology technically correct? If
not, restate the description accurately.
o While there is the appearance that crossing over is suppressed in inversion
heterozygotes, the phenomenon extends from the fact that the crossover
chromatids end up being abnormal in genetic content. As such, they fail to
produce viable (or competitive) gametes or lead to zygotic or embryonic death.
Therefore, rather than suppressing crossing over, the inversion actually
suppresses the recovery of crossover products.
, • Human adult hemoglobin is a tetramer containing two alpha (a) and two beta (b)
polypeptide chains. The a gene cluster on chromosome 16 and the b gene cluster on
chromosome 11 share amino acid similarities such that 61 of the amino acids of the a-
globin polypeptide (141 amino acids long) are shared in identical sequence with the b-
globin polypeptide (146 amino acids long). How might one explain the existence of two
polypeptides with partially shared function and structure on two different
chromosomes? Include in your answer a link to Ohno’s hypothesis regarding the origin
of new genes during evolution.
o Modern globin genes resulted from a duplication event in an ancestral gene
followed by a chromosomal aberration that separated the duplicated genes onto
two different chromosomes. Ohno hypothesized that essential genes would not
be free to evolve unless other genes could compensate for their functions. Gene
duplication would provide one static copy, and one copy that could be altered.
• Certain varieties of chrysanthemums contain 18, 36, 54, 72, and 90 chromosomes; all
are multiples of a basic set of nine chromosomes. How would you describe these
varieties genetically? What feature do the karyotypes of each variety share? A variety
with 27 chromosomes has been discovered, but it is sterile. Why?
o Given the basic haploid complement of nine unique chromosomes (see figure
below), other forms with the “n multiples” are said to be euploid, diploid (for
2n), or polyploid (for multiples above 2)—and all are autoploid.
o Karyotypes would share the same basic chromosome set, with the appropriate
number of copies of each chromosome (for example, four copies of each in an
autotetraploid).
o Individuals with 27 chromosomes are triploids (3n) and are likely to be sterile
because there are trivalents at meiosis I, which cause a relatively high number of
unbalanced gametes to be formed.
• In a cross between two varieties of corn, gl1gl1Ws3Ws3(egg parent) * Gl1Gl1ws3ws3
(pollen parent), a triploid offspring was produced with the genetic constitution
Gl1Gl1gl1Ws3ws3ws3. From which parent, egg or pollen, did the 2n gamete originate?
Is another explanation possible? Explain.
o Since two Gl1 alleles and two ws3 alleles are present in the triploid, they must
have come from the pollen parent. The wording of the problem implies that the
pollen parent contributed an unreduced (2n) gamete; however, another
explanation, dispermic fertilization, is possible. In this case, two Gl1ws3 gametes
could have fertilized the ovule.
• A woman who sought genetic counseling is found to be heterozygous for a
chromosomal rearrangement between the second and third chromosomes. Her
chromosomes, compared to those in a normal karyotype, are diagrammed. (a) What
kind of chromosomal aberration is shown? (b) Using a drawing, demonstrate how these
chromosomes would pair during meiosis. Be sure to label the different segments of the
chromosomes. (c) This woman is phenotypically normal. Does this surprise you? Why or
why not? Under what circumstances might you expect a phenotypic effect of such a
rearrangement?
Aneuploidy and Polyploidy Notes
• Aneuploidy: change in the number of individual chromosomes
o Only one chromosome is altered
o Monosomy is 2n-1
§ Trisomy is 2n+1
§ Tetrasomy is 2n+2
o Results from nondisjunction in meiosis, where homologous chromosomes or
sister chromatids do not separate
§
§ Merging with a normal gamete can create monosomic or trisomic
individuals
§
o If identical gene passed twice, it is from the parent with the gene and an issue in
meiosis II from the sister chromatids not separating.
• Polyploidy: change in the number of chromosome sets
o Homolog sets are altered for all chromosomes, not just one
o Triploidy (3x) is three copies of each homolog
§ Tetraploidy (4x) is four copies of each homolog
o More common in plants than animals
o Autopolyploidy: chromosome sets from a single species
§ Autotetraploid = 4n
§ Triploid = 3n
§ Could be defect in mitosis
, •
§ Could be defect in meiosis
•
o Allopolyploidy: chromosome sets are from two or more species
§ Allotetraploids often fertile
§
o Allotetraploid = 2n from each species
• Rearrangements: change to the structure of individual chromosomes
,Examples
• A human female with Turner syndrome (45,X) also expresses the X-linked trait
hemophilia, as did her father. Which of her parents underwent nondisjunction during
meiosis, giving rise to the gamete responsible for the syndrome?
o Because both she and her father express hemophilia, it is likely that the woman
inherited her sole X chromosome from her father and no sex chromosome from
her mother. This means that nondisjunction occurred in the mother, either
during meiosis I or meiosis II, producing an egg with no X chromosome.
• When two plants belonging to the same genus but different species are crossed, the F1
hybrid is viable and has more ornate flowers. Unfortunately, this hybrid is sterile and
can only be propagated by vegetative cuttings. Explain the sterility of the hybrid and
what would have to occur for the sterility of this hybrid to be reversed.
o The sterility of interspecific hybrids is often caused from a high proportion of
univalents in meiosis I, producing mostly inviable gametes. Viable gametes are
rare, and the likelihood of two of them “meeting” is remote. The horticulturist
might attempt to reverse the sterility by treating the hybrid with colchicine,
which, if successful, would double the chromosome number, so each
chromosome would have a homolog with which to pair during meiosis.
• What is the effect of a rare double crossover within a chromosome segment that is
heterozygous for a paracentric inversion?
o Rare double crossovers within the boundaries of a paracentric inversion
heterozygote produce only minor departures from the standard chromosomal
arrangement as long as the crossovers involve the same two chromatids. With
two-strand double crossovers, the second crossover negates the first. However,
three-strand and four-strand double crossovers have consequences that lead to
anaphase bridges as well as a high degree of genetically unbalanced gametes.
• For a species with a diploid number of 18, indicate how many chromosomes will be
present in the somatic nuclei of individuals that are haploid, triploid, tetraploid,
trisomic, and monosomic.
o haploid = 9, triploid = 27, tetraploid = 36, trisomic = 19, monosomic = 17
• Contrast the fertility of an allotetraploid with an autotriploid and an autotetraploid.
o Both allotetraploids and autotetraploids would be able to GG produce bivalents
in miosis I and would probably be equivalently fertile. Furthermore, both would
be more fertile than autotriploids, which have an odd number of chromosomes
and would frequently produce unbalanced gametes.
• Inversions are said to “suppress crossing over.” Is this terminology technically correct? If
not, restate the description accurately.
o While there is the appearance that crossing over is suppressed in inversion
heterozygotes, the phenomenon extends from the fact that the crossover
chromatids end up being abnormal in genetic content. As such, they fail to
produce viable (or competitive) gametes or lead to zygotic or embryonic death.
Therefore, rather than suppressing crossing over, the inversion actually
suppresses the recovery of crossover products.
, • Human adult hemoglobin is a tetramer containing two alpha (a) and two beta (b)
polypeptide chains. The a gene cluster on chromosome 16 and the b gene cluster on
chromosome 11 share amino acid similarities such that 61 of the amino acids of the a-
globin polypeptide (141 amino acids long) are shared in identical sequence with the b-
globin polypeptide (146 amino acids long). How might one explain the existence of two
polypeptides with partially shared function and structure on two different
chromosomes? Include in your answer a link to Ohno’s hypothesis regarding the origin
of new genes during evolution.
o Modern globin genes resulted from a duplication event in an ancestral gene
followed by a chromosomal aberration that separated the duplicated genes onto
two different chromosomes. Ohno hypothesized that essential genes would not
be free to evolve unless other genes could compensate for their functions. Gene
duplication would provide one static copy, and one copy that could be altered.
• Certain varieties of chrysanthemums contain 18, 36, 54, 72, and 90 chromosomes; all
are multiples of a basic set of nine chromosomes. How would you describe these
varieties genetically? What feature do the karyotypes of each variety share? A variety
with 27 chromosomes has been discovered, but it is sterile. Why?
o Given the basic haploid complement of nine unique chromosomes (see figure
below), other forms with the “n multiples” are said to be euploid, diploid (for
2n), or polyploid (for multiples above 2)—and all are autoploid.
o Karyotypes would share the same basic chromosome set, with the appropriate
number of copies of each chromosome (for example, four copies of each in an
autotetraploid).
o Individuals with 27 chromosomes are triploids (3n) and are likely to be sterile
because there are trivalents at meiosis I, which cause a relatively high number of
unbalanced gametes to be formed.
• In a cross between two varieties of corn, gl1gl1Ws3Ws3(egg parent) * Gl1Gl1ws3ws3
(pollen parent), a triploid offspring was produced with the genetic constitution
Gl1Gl1gl1Ws3ws3ws3. From which parent, egg or pollen, did the 2n gamete originate?
Is another explanation possible? Explain.
o Since two Gl1 alleles and two ws3 alleles are present in the triploid, they must
have come from the pollen parent. The wording of the problem implies that the
pollen parent contributed an unreduced (2n) gamete; however, another
explanation, dispermic fertilization, is possible. In this case, two Gl1ws3 gametes
could have fertilized the ovule.
• A woman who sought genetic counseling is found to be heterozygous for a
chromosomal rearrangement between the second and third chromosomes. Her
chromosomes, compared to those in a normal karyotype, are diagrammed. (a) What
kind of chromosomal aberration is shown? (b) Using a drawing, demonstrate how these
chromosomes would pair during meiosis. Be sure to label the different segments of the
chromosomes. (c) This woman is phenotypically normal. Does this surprise you? Why or
why not? Under what circumstances might you expect a phenotypic effect of such a
rearrangement?
