Download Human karyotype

Biology 212 General Genetics
Spring 2007
Lectures 20 & 21: Human Genetics I & II: Chromosome Defects
Reading: Text Chap. 5 to p. 168, pp. 174-189
Lecture outline:
1. Human karyotypes
2. Sex chromosomes
3. Variations in chromosome #
4. Variations in chromosome structure
5. Polyploidy
Lecture:
1. Human karyotypes
Karyotype: An array of all the chromosomes in a metaphase cell of an organism.
Human karyotype
46, XX female
46, XY male
consists of 22 pairs of autosomes, 1 pair of sex chromosomes
Preparation of a karyotype:
(see Fig. 5.2)
1) Treat blood cells with colchicine to arrest dividing cells in metaphase
2) Spread on a glass slide
3) Stain with Giemsa
4) Photograph
5) Use computer to cut and paste chromosome images to create karyotypes
6) Chromosomes arranged in pairs from largest (#1) to smallest (#22), followed by
sex chromosomes XX or XY
Chromosome painting
Uses fluorescently tagged DNA markers to identify each chromosome (see Fig. 5.1) by
hybridization
Anatomy of a chromosome
telomere
shorter arm = p (petit)
centromere
longer arm = q (queue: a long line)
telomere
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Role of centromere
 Stabilize chromosome movement during cell division
 Site where chromosome attaches to spindle
Acentric chromosome: Chromosome that lacks a centromere; segregates abnormally in
mitosis or meiosis.
Dicentric chromosome: Chromosome with two centromeres; unstable, usually breaks
during cell division.
Centromere position in normal chromosomes can vary:
Metacentric: centromere in middle of chromosome
Submetacentric: centromere closer to one end
Acrocentric: centromere near the end of the chromosome
2. Sex chromosomes
female 46, XX
male 46, XY
X chromosome
 female has twice the genetic material than males for genes on the X chromosome
 dosage compensation: adjust for levels of gene expression
 in humans and many organisms dosage compensation occurs by X-inactivation
 the X-chromosome inactivated in each cell is randomly chosen
 can observe inactive X chromosome as a Barr body in the cell nucleus
Y chromosome
 Few genes are represented
 Encodes SRY, master sex-determining gene
 SRY encodes a transcription factor that is the testis determining factor
 Transcription factors help determine the level of transcription of a gene
 The pseudoautosomal region on the Y chromosome pairs with a region on the X
 The Y chromosome is progressively shortening in mammalian evolution
3. Variations in chromosome number
Chromosome abnormalities occur in about 7.5% of all miscarriages.
Types of common abnormalities (Table 5.1)
Trisomy: one extra chromosome
Monosomy: missing one chromosome
45, X (Turner syndrome) only viable human monosomy
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Translocation: a chromosome rearrangement
Polyploid: extra set of chromosomes
Aneuploidy
 Monosomy and trisomy are examples
 Contains extra or missing chromosomes
 Have an unbalanced chromosome set
 Chromosomes behave unstably in cell division
A common trisomy: Down syndrome = Trisomy 21
 Affected individuals have 3 copies of chromosome 21
 Affects 1/750 live births
Symptoms:
 Mental retardation
 Short stature
 Similar face shape
 Shortened life span (<50 years)
 Often have heart defects
 Happy, responsive
Major cause of Down syndrome:
 Non-disjunction = improper chromosome segregation in meiosis I or meiosis II
 Occurs more commonly in oogenesis
 Likelihood of conceiving affected child mainly depends on age of mother (Fig.
5.12)
Minor cause: hereditary form
 Chromosome translocation
Sex chromosome abnormalities
Due to non-disjunction in meiosis
47, XXX female, mostly normal
47, XYY male, tall, phenotypically mostly normal
47, XXY male, Klinefelter syndrome, tall sterile, often mentally impaired
45, XO female, Turner syndrome, short, sterile—doesn’t reach sexual maturity
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4. Variations in chromosome structure
A. Deletion or deficiency
A B C D E -->


ABE
Missing a segment of the chromosome
Large mutations are usually lethal
B. Duplication
A B C D E --> A B B C D E


Region of the chromosome is present twice
Produced by unequal crossing over
C. Inversion
A B C D E --> A D C B E



A segment of the chromosome with the genes in reverse order
Inverted segment may include the centromere = pericentric inversion
OR inverted segment may be outside the region with the centromere = paracentric
inversion
D. Translocation
A B C D E --> A B Y Z
XYZ



XCDE
Exchange of parts between non-homologous chromosomes
Reciprocal translocation (balanced translocation) is swapping the ends of the
chromosomes
Translocation may not be reciprocal
Individuals with chromosome defects have high risk of errors in meiosis
Refer to practice problems that look at pairing of chromosomes in heterozygotes
5. Polyploidy = extra full set of chromosomes


lethal in humans
common in plants to have extra chromosome sets
haploid = chromosome set in a gamete
diploid = chromosome set in most eukaryotic cells = 2 monoploid sets
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triploid = 3 monoploid sets
tetraploid = 4 monoploid sets
 In evolution of plants it is common for hybridization of gametes to occur from
different but related species to produce a new polyploid species

In agriculture, polyploids often produce larger flowers and or larger or more
desirable fruit.

A few other organisms are also polyploid: trout are tetraploids
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