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Chapter 16 · Part 10 of 10
  1. Chapter overview
  2. 1Chromosome sets and homologous pairs
  3. 2The cell cycle and its checkpoints
  4. 3Sister chromatids and the spindle
  5. 4The phases of mitosis
  6. 5Meiosis I separates homologs
  7. 6Meiosis II and how meiosis differs from mitosis
  8. 7Asexual reproduction and the cost of sex
  9. 8Genetic diversity and sexual life cycles
  10. 9Chromosome shapes and changes in structure
  11. 10Changes in chromosome number

Changes in chromosome number

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Besides changing shape, chromosomes can change in number. The change can involve whole sets of chromosomes or just one chromosome within a set, and the difference between those two matters a great deal.

Whole sets vs. single chromosomes

The suffix -ploid refers to complete sets of chromosomes. An organism is euploid when its chromosomes come in complete sets, whether that’s one set, two, or more. A normal human, with two full sets, is euploid. The examples below are for a species with n = 4.

Term Chromosomes present Count
Diploid (normal) Two complete sets (2n) 8
Polyploid Three or more complete sets: triploid (3n), tetraploid (4n), and so on 12 if triploid
Trisomy One chromosome present three times (2n + 1) 9
Monosomy One chromosome present only once (2n − 1) 7

Diploids and polyploids are both euploid, because every set is complete. Trisomy and monosomy are kinds of aneuploidy: a change in the number of one particular chromosome, so the total is no longer an exact multiple of a set. A person with three copies of chromosome 21 has trisomy 21; one missing a copy of a chromosome has a monosomy.

Check yourselfA plant species has n = 7. One plant has 21 chromosomes and another has 15. Classify each.

The plant with 21 has three complete sets (21 ÷ 7 = 3), so it’s a triploid polyploid. The plant with 15 is 2n + 1, an aneuploid with a trisomy.

Nondisjunction

Many changes in number start with a sorting mistake. Nondisjunction is a failure of chromosomes to separate properly during cell division. It can happen in meiosis I, when a pair of homologs moves to the same pole, or in meiosis II, when a pair of sister chromatids does.

  1. In anaphase I, one pair of homologs fails to separate, and both move into the same cell.
  2. Meiosis II proceeds normally in both cells.
  3. Two gametes carry one chromosome too many (n + 1), and two carry one too few (n − 1).
  4. If one of these gametes fuses with a normal gamete, the zygote is aneuploid, and so is every cell of the organism that grows from it.
A cell with three pairs of homologous chromosomes goes through meiosis I, but one pair, shown in green, does not separate: both green homologs move into the left cell. After meiosis II, the two cells on the left each have four chromosomes, including two green ones, and the two cells on the right have only two chromosomes and no green one.
Figure 16.9 One pair that fails to separate in meiosis I makes every gamete abnormal. Half carry an extra chromosome and half are missing one.
Check yourselfA sperm carrying 22 chromosomes fertilizes a normal egg. How many chromosomes does the zygote have, and what is this called?
  1. The egg adds 23, so the zygote is 2n − 1, a monosomy.

Aneuploidy in humans

Errors in chromosome number arise surprisingly often when human gametes form. Most embryos with the wrong number don’t develop, and these errors cause a large share of early miscarriages. A few aneuploid conditions survive to birth:

Chromosomes Syndrome Main features
Trisomy 21 Down Cognitive disability, flattened face, short stature
Trisomy 18 Edwards Severe disabilities, early death
Trisomy 13 Patau Severe disabilities, organ defects, early death
XXY Klinefelter Male, no sperm, sexually immature
XYY Jacobs Male, tall
XXX Triple X Female, tall and thin
X0 (one X only) Turner Female, short stature, sexually undeveloped

The autosomal trisomies that survive involve small chromosomes, which carry fewer genes. Trisomies of the other autosomes, and most monosomies, are thought to be lethal. The risk of having a child with an abnormal chromosome number rises with parents’ age, probably because nondisjunction is more likely in older cells. For Down syndrome, the risk climbs sharply in a mother’s later reproductive years.

Why aneuploidy is harmful

For many genes, a cell makes gene product in proportion to how many copies of the gene it has.

An aneuploid organism has wrong amounts of the products of many genes at once, all on one chromosome, while the rest of its genes stay normal. That imbalance is why aneuploidy is usually harmful in any eukaryote.

Polyploidy in animals and plants

Animals tolerate polyploidy poorly; in mammals it’s generally lethal. A few amphibians and reptiles are polyploid, and male honeybees, which hatch from unfertilized eggs, are haploid.

Plants are different. Polyploidy is common in plants, and polyploid plants tend to be larger and hardier than their diploid relatives and to tolerate harsher conditions. Many crops and garden flowers are polyploid. Bread wheat, for example, has six sets of chromosomes, inherited from three different wild grasses.

Practice questions

  1. 1 of 4

    A girl has Turner syndrome, with a single X and no second sex chromosome. Which pair of gametes most likely formed her?

  2. 2 of 4

    An organism whose chromosomes all come in complete sets, whether two sets or more, is described as

  3. 3 of 4

    Which factor raises the chance of having a child with an abnormal chromosome number?

  4. 4 of 4

    One pair of homologs fails to separate in meiosis I, and meiosis II proceeds normally. What are the four resulting gametes like?