Numerical And Structural Chromosome Abnormalities

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Euploid

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Typical complement of chromosomes, an exact multiple of haploid

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Aneuploid

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"Not a good set," something wrong with chromosome complement

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Nondisjunction

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Failure of homologous pair of chromosomes to disjoin during meiosis 1, or failure for chromatids to separate in M2 or mitosis

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Monosomy

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One one member of chromosomal pair is present

Only viable monosomy in humans is Turner Syndrome (monosomy X)

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Mosaicism

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2 or more distinct cell lines from a single zygote differing because of mutation or nondisjunction

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Frequency of chromosomal abnormalities

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All chromosomal abnormalities:

1:154 live births

Most common:Trisomy 21 about 1:830
Balanced structural rearrangements

Sex chromosome aneuploidies about 1:1000(47,XXX; 47, XXY; 47,XYY)

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Phenotypic findings in Trisomy 21

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Mental retardation; Low nasal bridge; loose muscle tone; Congenital heart disease; Small, low set ears; Protruding tongue; intestinal problems; 15-fold increase in leukemia; increased incidence of Alzheimers

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Critical region on chromosome 21 that appears responsible for most Phenotypic findings in Trisomy 21

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Region 22 on the long arm of chromosome 21

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What are the other viable trisomies? What are their phenotypic findings?

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<b> Trisomy 18 </b> (Edward syndrome). Phenotypic features: Congenital heart disease; failure to thrive; several mental retardation; hypertonia (stiff); prominent occiput; recession of jaws; low-set malformed ears; intestinal abnormalities; rocker bottom feet

<b> Trisomy 13 </b>: Patau syndrome. Lethal in nearly all cases by age of 6 months. Findings include congenital heart disease; mental retardation; problems with muscle tone; cleft lip/palate; polydactyly, syndactyly; rocker bottom feet; polycystic kidneys

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Triploidy

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69 chromosomes

Can occur when two sperm fertilize single egg, or failure in a meiotic division

two paternal complements being associated with proliferation of the placenta and well-grown fetus

Two maternal complements demonstrate severe intrauterine growth retardation and macrocephaly.

Incompatible with life.

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Errors in meiosis I

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Failure of reduction division such that homologs segregate together

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Errors in meiosis II

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Failure of chromatids to separate

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Robertsonian translocation

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Translocation formed between two acrocentric chromosomes by fusion at or near the centromere, with loss of the short arms.

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Pairing of Robertsonian translocation at meiosis.

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Pairing occurs between homologous sequences, but given the translocation, there are three chromosomes sharing homologous regions, resulting in a trivalent.

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What type of gametes can a Robertsonian translocation produce when combined with a normal chromosomal haploid complement?

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1) Normal gamete + Normal spouse = Normal child

2) Balanced gamete + Normal spouse = Balanced carrier child

3) Unbalanced gamete + Normal spouse = Unbalanced child (Down syndrome:compatible with life)

4-6) Unbalanced gamete with either Trisomy 14, Monosomy 14, or Monosomy 21, all incompatible with life.

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Reciprocal translocation

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Chromosomal rearrangement involving exchange of segments between nonhomologous chromosomes.

Formed when single break occurs in two chromosomes, and they switch. Ideally no material is gained or lost, but sometimes partial monosomy can happen

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Balanced reciprocal translocation pairing at meiosis

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Homologous sequences pair up during meiosis, even though they're on different chromosomes. End up with a tetravalent (instead of bivalent) structure.

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What type of gametes can a reciprocal translocation produce when combined with a normal chromosomal haploid complement?

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1) Normal gamete + Normal spouse = Normal child

2) Balanced gamete + Normal spouse = Balanced carrier child

3-6) Unbalanced gamete + Normal spouse = Unbalanced child with partial trisomy and monosomy.

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Chromosomal deletions: terminal

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Believed to be caused when a single break happens in a chromosome, and the material distal to that break is lost. Results in partial monosomy.

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Cri-du-chat syndrome

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chromosomal condition that results when a piece of chromosome 5 is missing. (Written as 5p-). Infants with this condition often have a high-pitched cry that sounds like that of a cat. The disorder is characterized by intellectual disability and delayed development, small head size (microcephaly), low birth weight, and weak muscle tone (hypotonia) in infancy. Affected individuals also have distinctive facial features, including widely set eyes (hypertelorism), low-set ears, a small jaw, and a rounded face. Some children with cri-du-chat syndrome are born with a heart defect.

1:20-50k births

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Interstitial deletion

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Results from two breaks in a single chromosome, and the chromosome material between the two breaks is lost, resulting in unbalanced complement.

Increases in severity the more material is lost.

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Prader-Willi syndrome

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Results from <b> paternal </b> interstitial deletion of chromosome 15

del(15)(q11q13)

Enormous appetite, obesity, small hands and feet, hypogonadism in males, short stature, developmental delay

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Angelman syndrome

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Results from <b> maternal </b> interstitial deletion of chromosome 15

del(15)(q11q13)

Puppetlike walking, episodes of inappropriate laughter, growth deficiencies, developmental delay, speech problems

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Imprinting

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Modification of genetic material take place depending on whether genetic info is maternal or paternal.

Usually from differences in methylation

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Inversions

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chromosomal rearrangement in which a segment of a chromosome is reversed end to end. If centromere is included in inversion, the inversion is <b> pericentric </b>, if not, it is <b> paracentric </b>.

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Relative incidence of structural abnormalities

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Translocations > Inversion > other Abnormalities