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2.5: Karyotypes Describe Chromosome Number and Structure

2.5.1 Karyograms are images of real chromosomes

Each eukaryotic species has its nuclear genome divided among a number of chromosomes that is characteristic of that species.  For example, a haploid human nucleus (i.e. sperm or egg) normally has 23 chromosomes (n=23), and a diploid human nucleus has 23 pairs of chromosomes (2n=46). A karyotype is the complete set of chromosomes of an individual.  The cell was in metaphase so each of the 46 structures is a replicated chromosome even though it is hard to see the two sister chromatids for each chromosome at this resolution. As expected there are 46 chromosomes. Note that the chromosomes have different lengths. In fact, human chromosomes were named based upon this feature.  Our largest chromosome is called 1, our next longest is 2, and so on. By convention the chromosomes are arranged into the pattern shown in Figure 2.15 and the resulting image is called a karyogram. A karyogram allows a geneticist to determine a person's karyotype - a written description of their chromosomes including anything out of the ordinary.

Figure 2.15:
Karyogram of a normal human male karytype.

Various stains and fluorescent dyes are used to produce characteristic banding patterns to distinguish all 23 chromosomes. The number of chromosomes varies between species, but there appears to be very little correlation between chromosome number and either the complexity of an organism or its total amount genomic DNA. 

2.5.2  Autosomes and Sex Chromosomes

In the figure above note that most of the chromosomes are paired (same length, centromere location, and banding pattern). These chromosomes are called autosomes. However note that two of the chromosomes, the X and the Y do not look alike. These are sex chromosomes. In humans males have one of each while females have two X chromosomes. Autosomes are those chromosomes present in the same number in males and females while sex chromosomes are those that are not. When sex chromosomes were first discovered their function was unknown and the name X was used to indicate this mystery. The next ones were named Y, then Z, and then W.

The combination of sex chromosomes within a species is associated with either male or female individuals. In mammals, fruit flies, and some flowering plants embryos, those with two X chromosomes develop into females while those with an X and a Y become males. In birds, moths, and butterflies males are ZZ and females are ZW. Because sex chromosomes have arisen multiple times during evolution the molecular mechanism(s) through which they determine sex differs among those organisms. For example, although humans and Drosophila both have X and Y sex chromosomes, they have different mechanisms for determining sex .

In mammals, the sex chromosomes evolved just after the divergence of the monotreme lineage from the lineage that led to placental and marsupial mammals. Thus nearly every mammal species uses the same sex determination system. During embryogenesis the gonads will develop into either ovaries or testes. A gene present only on the Y chromosome called TDF encodes a protein that makes the gonads mature into testes. XX embryos do not have this gene and their gonads mature into ovaries instead (default). Once formed the testes produce sex hormones that direct the rest of the developing embryo to become male, while the ovaries make different sex hormones that promote female development. The testes and ovaries are also the organs where gametes (sperm or eggs) are produced.

How do the sex chromosome behave during meiosis? Well, in those individuals with two of the same chromosome (i.e. homogametic sexes: XX females and ZZ males) the chromosomes pair and segregate during meiosis I the same as autosomes do. During meiosis in XY males or ZW females (heterogametic sexes) the sex chromosomes pair with each other (Figure 2.16). In mammals the consequence of this is that all egg cells will carry an X chromosome while the sperm cells will carry either an X or a Y chromosome. Half of the offspring will receive two X chromosomes and become female while half will receive an X and a Y and become male.

Figure 2.16:  Meiosis in an XY mammal. The stages shown are anaphase I, anaphase II, and mature sperm. Note how half of the sperm contain Y chromosomes and half contain X chromosomes.     

2.5.2 Aneuploidy - Changes in Chromosome Number

Analysis of karyotypes can identify chromosomal abnormalities, including aneuploidy, which is the addition or subtraction of a chromosome from a pair of homologs.  More specifically, the absence of one member of a pair of homologous chromosomes is called monosomy (only one remains).  On the other hand, in a trisomy, there are three, rather than two (disomy), homologs of a particular chromosome.  Different types of aneuploidy are sometimes represented symbolically; if 2n symbolizes the normal number of chromosomes in a cell, then 2n-1 indicates monosomy and 2n+1 represents trisomy.  The addition or loss of a whole chromosome is a mutation, a change in the genotype of a cell or organism.

The most familiar human aneuploidy is trisomy-21 (i.e. three copies of chromosome 21), which is one cause of Down syndrome.  Most (but not all) other human aneuploidies are lethal at an early stage of embryonic development.  Note that aneuploidy usually affects only one set of homologs within a karyotype, and is therefore distinct from polyploidy, in which the entire chromosome set is duplicated (see below).  Aneuploidy is almost always deleterious, whereas polyploidy appears to be beneficial in some organisms, particularly many species of food plants.

Aneuploidy can arise due to a non-disjunction event, which is the failure of at least one pair of chromosomes or chromatids to segregate during mitosis or meiosis. Non-disjunction will generate gametes with extra and missing chromosomes.

2.5.3 Chromosomal abnormalities

Structural defects in chromosomes are another type of abnormality that can be detected in karyotypes (Fig 2.17).  These defects include deletions, duplications, and inversions, which all involve changes in a segment of a single chromosome. Insertions and translocations involve two non-homologous chromosomes.  In an insertion, DNA from one chromosome is moved to a non-homologous chromosome in a unidirectional manner. In a translocation, the transfer of chromosomal segments is bidirectional and reciprocal – a reciprocal translocation.  

Figure 2.17:
Structural abberations in chromosomes.

Structural defects affect only part of a chromosome (a subset of genes), and so tend to be less harmful than aneuploidy. In fact, there are many examples of ancient chromosomal rearrangements in the genomes of species including our own.  Duplications of some small chromosomal segments, in particular, may have some evolutionary advantage by providing extra copies of some genes, which can then evolve in new, potentially beneficial, ways.

Chromosomal abnormalities arise in many different ways, some of which can be traced to rare errors in natural cellular processes such as DNA replication. Chromosome breakage also occurs infrequently as the result of physical damage (such as ionizing radiation), movement of some types of transposons, and other factors.  During the repair of a broken chromosome, deletions, insertions, translocations and even inversions can be introduced.

2.5.4 Structural defects and pairing of chromosomes in meiosis.

The loss or gain of chromosome segments interferes with the pairing of homologous chromosomes in prophase I of meiosis. With deletions and duplications there is a region of unpaired chromosome that forms a loop out from the rest of the paired chromosome. These loops can be seen cytologically and used to determine the location of the aberration along the chromosome. In inversions, the pairing also forms a loop and crossovers within the loop region led to the formation of unbalanced gamete products that, when combined with a normal gamete at fertilization, produce lethal zygotes (see Gene Balance below).  Translocations also can produce unbalance gametes because the proper segregation of chromosomes is compromised. There are three possible ways that two translocated chromosomes can segregate (alternate, adjacent-I, adjacent-2) and only one leads to balanced gametes.