Genetics Spring 2015

Human Karyotypes and Chromosomes Behavior

Outline

The Human Karyotype - Overview

The chromosomes as seen in the cell by microscopy.

The chromosomes have been cut out of the photograph and paired with their homologs.

• Karyotypes show arranged chromosome pairs.• Standard karyotypes use the dye Giemsa generating G-bands

(gene poor regions vs R-bands).• Karyotypes are used in diagnosis.

Chromosome painting allows chromosome to display a unique color generating by a mix of many in situ hybridization probes.Chromosomes are then grouped in pairs and arranged in conventional order.

The five columns correspond to the five indicated fluorochromes and the 24 rows denote the 24 human chromosomes.

False colors representation of karyogram. Examples of arbitrary false colors:•Cy5 in Chromosome 1 is coded as white.•DEAC of Chromosome 2 coded as magenta.•TexRed of Chromosome 3 coded as light green.•FITC+SpeOra of Chromosome 12 coded as olive green and etc ….

The Karyogram shows a pathologically serious defects due to human tumor (Adenom HRT 18): the chromosomes 8, 9, 14, 17, 22 are present only single. Chromosome 19 contains parts of 10 and 3 and Chromosome 22.

The short arm of a chromosome is called “p” and the long arm “q”.

A through G refer to grouping of chromosomes by size.

The Human Karyotype – Centromere

• Centromeres are crucial for cell division as they give stability to chromosomes.

• Centromeres can be either localized or holocentric.

• When localized, centromeres can lead to metacentric, submetacentric and acrocentric chromosomes.

• A chromosome can lack centromere (acentric) or express two centromeres (dicentric).

The Human Karyotype – Dosage Compensation of X-linked Genes• In female mammals, one of

two X chromosomes is inactivated in each cell.

• Random process at about the 100-cell stage.

• Condensed X is visible in normal cell as Barr body.

• Normal females are mosaic (some cells XM, and some XP), where M = maternal chromosome, P = paternal.

Calico cat, whose pattern is the result of random X inactivation. Alleles of X give orange or black (only one active X in each female cell); unlinked autosomal gene controls white spots.

•XIST (inactive X, Xi, specific transcripts) gene expressed in the X

Inactivation Center (XIC) that generates a non-coding RNA.

•Addition of methyl group on cytosine to CpG islands(DNA methyltransferase).

•Histone code, enrichment of macroH2A.

RNA-fluorescence in situ hybridization detecting Xist RNA (red) localized on

the inactive X in a preparation of condensed chromosomes from

differentiated mouse cells

Detection of the histone variant macroH2A in interphase nuclei from females with 46,XX, 47,XXX, 48,XXXX, and 49,XXXXX karyotypes. Regions of bright fluorescence indicate presence of macroH2A associated with inactive X chromosomes.

Trisomy X, Tetrasomy X and Pentasomy X

X-inactivation escape •Distribution along the human X chromosome of the approximately 15 percent of genes that escape complete transcriptional silencing in the inactive X chromosome.•Most are at PAR region(pseudo-autosomal) where X an Y pair at meiosis.

•About 15% of genes escape X inactivation (random event, not same in all females), thus are expressed in both active and inactive X chromosomes.

•Another 10% of genes may or may not escape X inactivation (Xp).

Chromosome Abnormalities in Human Pregnancies -Ploidy refers to the number of sets of chromosomes in a cell.

-Euploidy refers to the state of a cell or organism having an integral multiple of the monoploid number, possibly excluding the sex-determining chromosomes.

-Anaploidy refers to not having euploidy.

Non-disjunctionChromosomes are not separating at meiosis (often anaphase I) and both chromosomes go to one pole, Gametes have too few or too many chromosomes. THERFORE at fertilization, the zygote gets the wrong number of chromosomes (too much or not enough).

Trisomy 21 (Down syndrome) is a viable condition, unlike most trisomies.

Percent change in number of births based on maternal and paternal age, 1997 versus 1983 (Fish et al., 2003)

Trisomy 13, Patau syndrome, is a chromosomal condition associated with severe intellectual disability and physical abnormalities in many parts of the body.Trisomy 13 occurs in about 1 in 16,000 newborns

polydactylymicrocephaly

Sex Chromosome Abnormalities

Klinefelter syndrome (47,

XXY)

and Turner syndrome (45, X) (less viable).

If genetic recombination during meiosis happens outside of the pseudoautosomal region, any ensuing offspring will be either XX male or XY female.

Metaphase chromosomes hybridized with LSI SRY probe

Undescended testes and ambiguous external genitalia were part of the diagnosis, orchidectomy of testes and vaginoplasty surgeries were performed (Gupta, 2012).

Pseudohermaphrodism case study: A 24 years old female presented with complaints of failure to menstruate.

Fertilely and Down Syndrome

Women with DS are fertile. Men with DS have traditionally been considered sterile; however, there have been two documented cases of adult men with DS fathering children.

Trisomic Segregation

In a trisomic organism, the segregation of chromosomes in meiosis is abnormal because the trisomic chromosome has two pairing partners.The trisomic chromosome can form a trivalent in some cells and one normal bivalent and one univalent in some other cells.

Chromosome Abnormalities in Human PregnanciesEnvironmental Effects on NondisjunctionRisk factors include radiation, alcohol, smoking, pesticides and other chemical such as bisphenol A.

Abnormalities of Chromosome Structure

A, Terminal and interstitial deletions, each generating an acentric fragment.B, Unequal crossing over between segments of homologous chromosomes or between sister chromatids (duplicated or deleted segment indicated by the brackets).C, Ring chromosome with two acentric fragments.D, Generation of an isochromosome for the long arm of a chromosome.E, Robertsonian translocation between two acrocentric chromosomes.F, Insertion of a segment of one chromosome into a nonhomologous chromosome.

Breakpoint can be mapped through testcrosses, thus

generating genetic maps.

Conventional symbols of chromosome abnormalities allows for ease of reference.

Isochromosome 21 leads to Down Syndrome

Chromosomal Deletion & Duplication - DeletionsDeletions are a type of aberrations in chromosome structure and specifically refer to a chromosome having a missing segment of DNA.The severity of the phenotype depends on the missing length and the genotype of the individual.Deletions can arise from simple breakage and reunion or through ectopic recombinations.

Azoospermia Factor (AZF)

Chromosomal Deletion & Duplication – Deletion Mapping

Chromosomal Deletion & Duplication – DuplicationDuplications are another type of aberrations in chromosome structure and specifically refer to a chromosome having a duplicated segments of DNA.Tandem duplication can have their own phenotypes (bar phenotype)Tandem duplication, through unequal crossing over, can produce more repeated sequences.

•Trichromatism refers to normal vision, dichromatism or monochromatism refers to impaired vision.•Red-green color blindness is and X-linked recessive disorder.•Unequal crossing over can occur in eye pigment genes (red-green) leading to color blindness.

Organization of red-pigment and

green-pigment genes in three

wildtype X chromosomes.

Origin of multiple green-pigment

genes by unequal crossing over, lack

of green-pigment gene in other chromosome.

•Depending on the type of crossing over that occurs, several different types of chromosomal aberrations will result.•Lack of green pigment induces deuteranopia , lack of red pigment induces protomatopia and lack of blue pigment induces tritanopia.

Ishihara color blindness test

Genetics of Chromosomal Inversions - Inversions Inversions are another type of aberrations in chromosome structure and specifically refer to a chromosome having a reversed order of genes in a particular DNA sequence.Inversions can occur through a two-break event or through ectopic recombination between inverted repeat sequences.During meiosis, heterozygous inversion leads to an inversion loop where the genes align at the prophase I synapsis complex.

If there is no crossing over within the inversion, then the homologous chromosomes separate normally during anaphase I.

Genetics of Chromosomal Inversions – Paracentric and Pericentric Inversions

Genetics of Chromosomal Inversions – Paracentric inversions

If there is crossing over within the inversion and the inversion is paracentric, then the homologous chromosomes do not separate normally and dicentric and acentric chromosomes are generated.

Genetics of Chromosomal Inversions – Pericentric inversions If there is crossing over within the inversion and the inversion is pericentric, then the homologous chromosomes separate normally and monocentric chromosomes are generated, even if chromatids carry deletion and duplications of selected genes.

Chromosomal Translocations - Translocations

In a reciprocal translocation two broken off chromosome pieces of non-homologous chromosomes are exchanged. This is a relatively frequent anomaly. One finds it with an incidence of 1:500 newborns.Reciprocal translocations are frequently balanced because the entire genetic material is present. Problems occur, though, in gamete formation.

Another frequently observed anomaly (1:1'000 newborns) is the robertsonian translocation, which occurs between two acrocentric chromosomes of groups G and D. It is also referred to as the centric fusion of two acrocentric chromosomes.It is a special kind of translocation in that on the acrocentric chromosomes (most often chromosomes 14 and 21 or 22) the very short, satellite-bearing arm is lost and a centric fusion t(14q21q or 14q22q) of the two remainder chromosomes, i.e., the long arms of the two pieces, results.

Chromosomal Translocations - Reciprocal TranslocationsMeiosis in a translocation heterozygote has genetic complications called semisterility, as half the gametes are euploid (normal) and half of the gametes are aneuploid (abnormal) after the formation of the quadrivalent in the meiotic synapsis.

Segregation of homologous chromosomes can occurs in three ways:

1. Adjacent-1 or disjunctional (unbalanced gametes)

2. Adjacent-2 or nondisjunctional (unbalanced gametes)

3. Alternate (balanced gametes)

1. Adjacent-1 or disjunctional (unbalanced gametes)

2. Adjacent-2 or nondisjunctional (unbalanced gametes)

3. Alternate (balanced gametes)

Chromosomal Translocations – Robertsonian TranslocationsA Robertsonian translocation can join chromosome 21 to any other acrocentric (ex. 14) chromosome; also causes Down syndrome.Heterozygous carriers of a Robertsonian translocation are phenotypically normal but a high risk of Down syndrome results from aberrant segregation in meiosis.