GENETICS

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Human gametes have 23 chromosomes (-?- halploid chromosome number n=23), while most somatic cells have 46 chromosomes (diploid chromosome number 2n=46).

Genes occupy sites on chromosomes (loci) and occur in alternative forms -?- Alleles.

The human haploid genome contains sufficient DNA (3x109 nucleotides) to encode more than 100,000 genes of average size, 40,000 nucleotides, or 40 kilobases. The usual human karyotype displays 500-1,000 bands on the 22 autosomes, plus the X or Y sex chromosomes, predicting that each chromosomal band contains more than -?- 100 genes.

Mendelian diseases exhibit autosomal dominant, autosomal recessive, or X-linked inheritance, while multifactorial diseases (e.g., cleft palate, diabetes mellitus, schizophrenia, hypertension) are determined by multiple genes plus the -?- Environment.

Characteristics of autosomal dominant disease include a vertical pedigree pattern, affliction of both males and females, variable expressivity (variable severity among affected individuals), frequent new -?- mutations, and a 50-percent recurrence risk for offspring of affected individuals.
COROLLARY: Germline mosaicism may produce affected siblings with autosomal dominant disease when -?- neitherparent is affected.

Characteristics of autosomal recessive disease include a -?- horizontal pedigree pattern, afflictionof males and females, frequent consanguinity (inbreeding), frequent carriers (heterozygotes without manifestations of disease), and a -?- 25 percent recurrence risk for carrier parents.
COROLLARY: Normal siblings of individuals with autosomal recessive disease has a -?- 2/3 chance of being a carrier.

Characteristics of X-linked recessive diseases include an -?- Oblique pedigree pattern, affliction of males only, frequent female carriers and a 25-percent recurrence risk for carrier females.
COROLLARY: Haldane's law predicts a 2/3 chance that the mother of an affected male with X-linked recessive disease will be a carrier (and a -?- 1/3 chance the affected male represents a new mutation).

Ethnic correlations with mendelian disorders include higher frequencies of -?- Cystic fibrosis in whites, sickle cell anemia in blacks, B-thalassemia in Italians and Greeks, a-thalassemia in Asians, and Tay-Sach's disease in jews.

Advanced maternal age is associated with higher risks of chromosomal disorders (e.g., Down syndrome, trisomy 13), while advanced paternal age is associated with higher risks for new mutations (e.g., those producing achondroplasia or -?- Marfan syndrome).

The Hardy-Weinberg law predicts allele frequencies in an idealized population according to the formula p2 + 2pq + q2 =1. Applied to cystic fibrosis, the law predicts that homozygotes (q) have a frequency of 1 in -?- 1600, predicting that carriers (2pq) have a frequency of 1 in 20.

A karyotype is an ordered arrangement of chromosomes that is described by cytogenetic notation. A karyotype can be obtained from dividing cells (blood leukocytes, bone marrow, fibroblasts, amniocytes) but not from -?- Frozen or formalin-fixed cells.

Cytogenetic notatin includes the chromosome number (usually 46), description of the sex chromosomes (usually XX or XY), and an indication of missing, extra, or -?- Rearranged chromosomes.

DNA diagnosis examines specific regions of genes for altered nucleotide sequences or deletions that affect gene expression and function; techniques include Southern blotting, gene amplification with the polymerase chain reaction (PCR), and mutant allele detection by -?- Hybridization with allele-specific oligonucleotides (ASO). Chromosome microdeletions encompass several genes and are detected by fluorescent in situ hybridization (FISH).

Nonmendelian inheritance mechanisms include mitochondrial inheritance (exhibiting maternal transmission), expansion of -?- Triplet repeats 9exhibiting anticipation in pedigrees as in the Fragile X syndrome), and genomic imprinting (exhibiting different phenotypes according to maternal or paternal origin of the aberrant genes).

Prenatal diagnosis can include fetal ultrasound, maternal -?- Serum studies, or sampling of cells from the fetoplacental unit by chorionic villus sampling (CVS at 8-10 weeks, amniocentesis at 12-18 weeks, or percutaneous umbilical sampling (PUBS) from 16 weeks to term).

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