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5 Human Chromosomes and Chromosome Behavior

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1 5 Human Chromosomes and Chromosome Behavior

2 Human Chromosomes Humans contain 46 chromosomes, including 22 pairs of homologous autosomes and two sex chromosomes Karyotype = stained and photographed preparation of metaphase chromosomes arranged according to their size and position of centromeres

3 Fig

4 Fig y

5 Human Chromosomes Each chromosome in karyotype is divided 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 Within each arm the regions are numbered.

6 Centromeres In metacentric it is located in middle of chromosome
• Chromosomes are classified according to the relative position of their centromeres In metacentric it is located in middle of chromosome In submetacentric—closer to one end of chromosome In acrocentric—near one end of chromosome Chromosomes with no centromere, or with two centromeres, are genetically unstable

7 Fig. 5.3

8 Human X Chromosome Females have two 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 in the nucleus of interphase cells Dosage Compensation = dosage equalization for active genes

9 Fig. 5.7

10 Human Y Chromosome Y chromosome is largely heterochromatic
Heterochromatin is 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 The pseudoautosomal region of the X and Y chromosomes has gotten progressively shorter in evolutionary time.

11 Fig. 5.10

12 Human Y Chromosome Y chromosome does not undergo recombination along most of its length, genetic markers in the Y are completely linked and remain together as the chromosome is transmitted from generation to generation The set of alleles at two or more loci present in a particular chromosome is called a haplotype The history of human populations can be traced through studies of the Y chromosome

13 Abnormal Chromosome Number
Euploid = balanced chromosome abnormality = the same relative gene dosage as in diploids (example: trisomics) Aneuploid = unbalanced set of chromosomes = relative gene dosage is upset (example: trisomy of chromosome 21) Monosomic = loss of a single chromosome copy Polysomic = extra copies of single chromosomes Chromosome abnormalities are frequent in spontaneous abortions.

14 Abnormal Chromosome Number
Monosomy or trisomy of most human autosomes unviable. There are three exceptions: trisomies of 13, 18 and 21 Down Syndrome is a genetic disorder due to trisomy 21, the most common autosomal aneuploidy in humans Frequency of Down Syndrome increases with mother’s age Amniocentesis = fetal cells are analyzed for abnormalities of chromosome number and structure Chorionic villus sampling (CVS) = cells from a zygote-derived embryonic membrane (the chorion) analyzed

15 Abnormal Chromosome Number
Trisomic chromosomes undergo abnormal segregation Trivalent = abnormal pairing of trisomic chromosomes in cell division Univalent = extra chromosome in trisomy is unpaired in cell division

16 Fig. 5.13

17 Sex Chromosome Aneuploidies
An extra X or Y chromosome usually has a relatively mild effect Trisomy-X = 47, XXX (female) Double-Y = 47, XYY (male) Klinefelter Syndrome = 47, XXY (male, sterile) Turner Syndrome = 45, X (female, sterile)

18 Chromosome Deletions Deletions = missing chromosome segment
Polytene chromosomes of Drosophila can be used to map physically the locations of deletions Any recessive allele that is uncovered by a deletion must be located inside the boundaries of the deletion = deletion mapping Large deletions are often lethal

19 Fig. 5.16

20 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 recombination Fig. 5.17

21 Red-Green Color Vision Genes
Genes for red and green pigments are close on X-chromosome Green-pigment genes may be present in multiple copies on the 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 Crossing-over between red- and green-pigment genes results in chimeric (composite) gene

22 Chromosome Inversions
Inversions = genetic rearrangements in which the order of genes in a chromosome segment is reversed Inversions do not alter the genetic content but change the linear sequence of genetic information In an inversion heterozygote, chromosomes twist into a loop in the region in which the gene order is inverted Fig. 5.22

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

24 Fig. 5.23

25 Chromosome Inversions
Pericentric inversion = includes centromere Crossing-over within a pericentric inversion loop during homologous recombination results in duplications and deletions of genetic information

26 Reciprocal Translocations
A chromosomal aberration resulting from the interchange of parts between nonhomologous chromosomes is called a translocation There is no loss of genetic information but the functions of specific genes may be altered Translocations may produce position effects = changes in gene function due to repositioning of gene Gene expression may be elevated or decreased in translocated gene

27 Reciprocal Translocation
Heterozygous translocation = one pair interchanged, one pair normal Homozygous translocation = both pairs interchanged Fig. 5.25

28 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 three genetic outcomes: Adjacent-1 segregation: homologous centromeres separate at anaphase I; gametes contain duplications and deletions

29 Reciprocal Translocation
Adjacent-2 segregation: homologous centromeres stay together at anaphase I; gametes have a segment duplication and deletion Alternate segregation: half the gametes receive both parts of the reciprocal translocation and the other half receive both normal chromosomes; all gametes are euploid, i.e have normal genetic content, but half are translocation carriers

30 Reciprocal Translocation
The duplication and deficiency of gametes produced by adjacent-1 and adjacent-2 segregation results in the semisterility of genotypes that are heterozygous for a reciprocal translocation The frequencies of each outcome is influenced by the position of the translocation breakpoints, by the number and distribution of chiasmata, and by whether the quadrivalent tends to open out into a ring-shaped structure on the metaphase plate

31 Fig. 5.26

32 Robertsonian Translocation
A special case of nonreciprocal translocation is a 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 Fig. 5.27

33 Robertsonian Translocation
Robertsonian translocations are an important risk factor to be considered in Down syndrome. When chromosome 21 is one of the acrocentrics in a Robertsonian translocation, the rearrangement leads to a familial type of Down syndrome The heterozygous carrier is phenotypically normal, but a high risk of Down syndrome results from aberrant segregation in meiosis Approximately 3 percent of children with Down syndrome are found to have one parent with such a translocation

34 Polyploidy Polyploid species have multiple complete sets of chromosomes The basic chromosome set, from which all the other genomes are formed, is called the monoploid set The haploid chromosome set is the set of chromosomes present in a gamete, irrespective of the chromosome number in the species. Fig. 529

35 Polyploidy Polyploids can arise from genome duplications occurring before or after fertilization Two mechanisms of asexual polyploidization: the increase in chromosome number takes place in meiosis through the formation of unreduced gametes that have double the normal complement of chromosomes the doubling of the chromosome number takes place in mitosis. Chromosome doubling through an abortive mitotic division is called endoreduplication

36 Polyploidy Autopolyploids have all chromosomes in the polyploid species derive from a single diploid ancestral Allopolyploids have complete sets of chromosomes from two or more different ancestral species Chromosome painting = chromosomes hybridized with fluorescent dye to show their origins Plant cells with a single set of chromosomes can be cultured

37 Fig. 5.31

38 Polyploidy The grass family illustrates the importance of polyploidy and chromo-some rearrangements in genome evolution The cereal grasses (rice, wheat, maize, millet, sugar cane, sorghum, and other cereals) are our most important crop plants Their genomes vary enormously in size: from 400 Mb found in rice to 16,000 Mb found in wheat

39 Polyploidy In spite of the large variation in chromosome number and genome size, there are a number of genetic and physical linkages between single-copy genes that are remarkably conserved in all grasses amid a background of rapidly evolving repetitive DNA Each of the conserved regions (synteny groups) can be identified in all the grasses and referred to a similar region in the rice genome.

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