1. Human Karyotypes and Chromosome Behavior5
Human Karyotypes and Chromosome Behavior

2. Human Chromosomes
Humans contain 46 chromosomes, including 22 pairs of homologous chromosomes and two sex chromosomes
Karyotype = stained and photographed preparation of metaphase chromosomes arranged in homologous pairs in descending size order

4. Human Chromosomes
Chromosome maps are prepared by dividing the chromosome into two regions (arms) separated by the centromere
p = short arm (petit); q = long arm
p and q arms are divided into numbered bands and interband regions based on pattern of staining

5. Centromeres
Centromeres may be located in different regions of a chromosome:
Metacentric = located in middle of chromosome
Submetacentric = located closer to one end of chromosome
Acrocentric = located near one end of chromosome

7. Human X Chromosome Females=2 copies of X chromosome
One copy of X is randomly inactivated in all somatic cells
Females are genetic mosaics for genes on the X chromosome; only one X allele is active in each cell
Barr body = inactive X chromosome
Dosage Compensation= dosage equalization for active genes

9. Human Y Chromosome
Y chromosome is largely heterochromatic (condensed inactive chromatin)
Important regions of Y chromosome:
-pseudoautosomal region=region of shared X-Y homology
-SRY=master sex controller gene which encodes testis determining factor (TDF) for male development

10. Abnormal Chromosome Number
Euploid=balanced chromosome abnormality=equal number of copies
aneuploid=unbalanced set of chromosomes=unequal copy number
monosomic=loss of a single chromosome copy

11. Abnormal Chromosome Number
Trisomy of chromosome 21 = most common autosomal (non-sex chromosome) aneuploidy
Down Syndrome = genetic disorder due to trisomy 21 = aneuploidy
Amniocentesis=fetal cells are analyzed for abnormalties of chromosome number and structure

12. Extra or Missing Chromosomes
Polysomy = extra copies of single chromosomes in a cell
Trisomy = extra copy of chromosome
Trivalent = abnormal pairing of trisomic chromosomes in cell division
Univalent = extra chromosome in trisomy is unpaired in cell division

14. Sex Chromosome Aneuploidies
trisomy-X=47, XXX (female)
double-Y=47, XYY (male)
Klinefelter Syndrome=47, XXY (male, sterile)
Turner Syndrome=45, X (female, sterile)
Fragile-X syndrome= X chromosome instability resulting from high copy number of trinucleotide repeat

15. Chromosome Deletions Deletions = missing chromosome segment
Polytene chromosomes of Drosophila can be used to map physically the locations of deletions
Testcross mapping shows which wild-type alleles are lost by deletion
Large deletions are often lethal

17. Gene Duplications
Duplication = chromosome segment present in multiple copies
Tandem duplications = repeated segments are adjacent
Tandem duplications often result from unequal crossing-over due to mispairing of homologous chromosomes during meiotic recombi- nation

19. Red-Green Color Vision Genes
Green-pigment genes may be present in multiple copies on the X-chromosome due to mispairing and unequal crossing-over
Unequal crossing-over between these genes during meiotic recombination can also result in gene deletion and color-blindness
Results in chimeric (composite) gene

20. Chromosome Inversions
Inversions = genetic rearrangements in which the order of genes is reversed in a chromosome segment
Inversions do not alter the genetic content but change the linear sequence of genetic information
At synapsis = homologous pairing, inversion loops form

22. Chromosome Inversions
Paracentric inversion = does not include centromere; Pericentric inversion = includes centromere
Crossing-over within a paracentric inversion loop during recombination produces one acentric (no centromere) and one dicentric (two centromeres) chromosome

24. Chromosome Inversions
Crossing-over within a pericentric inversion loop during homologous recombination results in duplications and deletions of genetic information
Deletions and duplications occur because inversion loop causes misalignment of chromosomes during homologous pairing = synapsis

26. Reciprocal Translocations
Reciprocal translocations = exchange of genetic segments between non-homologous (unrelated) chromosomes
There is no loss of genetic information but the functions of specific genes may be altered
Reciprocal translocation may affect one or both pairs of chromosomes

27. Reciprocal Translocation
Heterozygous translocation = only one pair of non-homologous chromosomes is affected
Homozygous translocation = both pairs of non-homologous chromosomes are affected

29. Reciprocal Translocations
Synapsis involving heterozygous reciprocal translocation results in pairing of four pairs of sister chromatids = quadrivalent
Chromosome pairs may segregate in several ways during meiosis, with varying genetic outcomes in gametes

30. Reciprocal Translocation
3 outcomes of meiotic segregation:
Adjacent-1 segregation: homologous centromeres separate at anaphase I; gametes contain duplications and deletions
Adjacent-2 segregation: homologous centromeres stay together at anaphase I; gametes have a segment duplication and deletion

31. Reciprocal Translocation
Alternate Segregation: half the gametes receive both parts of the reciprocal translocation and the other half receive both normal chromosomes; all gametes are euploid = normal genetic content, but half are translocation carriers

32. Robertsonian Translocation
Robertsonian translocation = fusion of two acrocentric chromosomes in the centromere region
Translocation results in apparent loss of one chromosome in karyotype analysis
Genetic information is lost in the tips of the translocated acrocentric chromosomes

34. Robertsonian Translocation
Meiotic segregation (14:21):
Adjacent-1 segregation: 1/2 gametes have extra copy of most of chromosome 21 (fertilization will produce Down Syndrome = trisomy 21); 1/2 gametes have no copy of 21
Adjacent-2 segregation: 1/2 gametes contain duplications; 1/2 lack chromosome 14

35. Robertsonian Translocation
Alternate Segregation: all gametes are euploid; 1/2 are carriers of the Robertsonian translocation
Translocations may produce position effects = changes in gene function due to repositioning of gene
Gene expression = elevated or decreased in translocated gene

36. Polyploidy
Polyploidy = genome composed of multiple complete sets of chromosomes; occurs in plants
Haploid = set of unpaired chromosomes found in gametes
Diploid = set of paired homologous chromosomes found in most cells; total number is fixed for a species

38. Chromosome Number
monoploid = basic set of chromosomes; number fixed for species
diploid = 2 copies of monoploid set
triploid = 3 copies of monoploid set
tetraploid = 4 copies of monoploid set
hexaploid = 6 copies of monoploid set

39. Endoreduplication
Endoreduplication= doubling of the chromosome complement
Tetraploidy=chromosomes fail to segregate during meiosis or mitosis
autopolyploidy=chromosomes derived from single diploid
Chromosome painting = chromosomes hybridized with fluorescent dye to show origins