Medical and Molecular Genetics
Lecture 4 Human Cytogenetics
1) Diagram the parts of a metaphase chromosome.
2) Describe the phases of mitosis and meiosis. Mitosis has five phases: Interphase where the chromosomal material replicates itself, Prophase where chromosomes condense, Metaphase where the nuclear membrane disappears, spindles form, and the chromosomes align themselves on the equatorial plane, Anaphase where sister chromatids separate and are drawn to the opposite poles of the cell, Telophase the chromosomes become less distinct, nuclear membranes reform around the new nuclei, and the cell pinches at the equator to form two new daughter cells. Meiosis has two divisions with four phases each. Meiosis I reduces the chromosome number from diploid to haploid through the following stages: Prophase I where the chromosomes condense and allow synapsis and crossing over, Metaphase I where the nuclear membrane disappears, spindles form, and paired chromosomes align themselves on the equatorial plane, Anaphase I the two members of the bivalent disjoin and their respective centromeres draw the pairs of sister chromatids to the opposite poles,
Telophase I the two haploid sets of chromosomes have grouped at opposite poles of the cell. Meiois II is the disjunction of sister chromatids to form four haploid gametes:
Prophase II is the same, Metaphase II is the same but sister chromatids line up alone with no homologous pair, Anaphase II separates sister chromatids leaving only one copy of each chromosome for each daughter cell, and Telophase II is the same.
3) Differentiate among metacentric, submetacentric, and acrocentric chromosomes.
Metacentric chromosomes have a central centromere, Submetacentric chromosomes have an off-center centromere, and Acrocentric chromosomes are recognizable by the centromere being near one end.
4) Describe the standard techniques of chromosome analysis. The chromosomes are visualized by a pictorial representation, called a karyotype, where they are arranged from largest to smallest. The cells most commonly used are T lymphocytes because they are capable of rapid growth and division in culture. Analysis is performed during metaphase or prometaphase. Lymphocytes are cultured for two mitotic cycles, poisoned with colchicine (preventing chromatid separation), swollen with hypotonic salt (dispersing chromosomes), and finally fixed on a glass slide and stained. The

metaphase cells are photographed under the microscope, enlarged, cut out and arranged. Each chromosome pair also has a unique banding pattern used in differentiation. G-banding is the most common technique using a Giemsa stain following treatment with trypsin (to digest some of the binding proteins). Dark bands are AT-rich, condensed and non-transcribed, light staining bands are GC-rich, elongated and transcribed. Non-staining gaps, known as fragile sites, may be normal variants, but one site at the end of Xq has been identified in X-linked mental retardation.
5) Define a Barr body and calculate the predicted number of Barr bodies based on a variety of sex chromosome complements. A Barr body is a type of heterochromatin which is present in the interphase nucleus of cells containing more than one X chromosome. The number of Barr bodies present reflects the X chromosome compliment (number of X chromosomes = number of Barr bodies + one active non-condensed X chromosome).
6) List the genetic consequences of chromosome analysis in clinical medicine.
Numerical abnormalities may be euploid with a multiple of 23 (46,XX or 69,XXX) or aneuploid (47,XX,+21 or 47,XY,+18) resulting from nondisjunction in meiosis I or
II. Structural abnormalities consist of deletions, duplications, ring chromosomes, inversions and translocations.
7) Define euploidy and aneuploidy. Euploidy – an exact multiple of the haploid chromosome. Aneuploidy – not an exact multiple of the haploid chromosome number.
8) Diagram the structure of the following chromosomal rearrangements: deletions, duplications, inversions, reciprocal translocations, Robertsonian translocations and ring chromosomes.

9) Describe the products of meiotic segregation in a reciprocal translocation during
alternate, adjacent-1, and adjacent-2 segregations. Alternate segregation results in normal or balanced karyotypes associated with a normal phenotype. Adjacent segregation results in abnormal, unbalanced karyotype with both excess and deficiency of chromosomal material. Adjacent-1 is when homologous centromeres go to different gametes. Adjacent-2 is when homologous centromeres go to the same gamete.
10) Differentiate among the clinical features of trisomy 21, trisomy 18, trisomy 13,
Turner syndrome, and Klinefelter syndrome.
Syndrome Clinical features
Trisomy 21 Down syndrome, most common (1 in 700 births), patients have: short stature, characteristic facial appearance, heart defects (45%) and gastrointestinal defects (5%), developmental disabilities, limp muscle tone in infancy, and menal retardation.
Trisomy 18 Low birthweight, small mouth, prominent occiput, short sternum, cardiac defects (>80%), and decreased viability. Life expectancy ~ l week.
Trisomy 13 Holoprosencephaly, cleft lip and palate, extra digits, and decreased viability (~ 1 week).
Turner syndrome
Monosomy X (45,X): female phenotype (mostly normal), short stature, absent secondary sexual characteristics, excess nuchal skin, broad chest, increased carrying angle of the arms, and peripheral edema. Typically have primary amenorrhea.
Klinefelter syndrome
Trisomy (47, XXY): male phenotype, tall stature, developmental disabilities, gynecomastia (breast development), small testes and infertility.