Human Genetics

The study of human genetics requires alternative methods Studies of pedigrees are problematic because human families are small

Karyotype Karyotyping is the analysis of chromosomes Cytogenetics is the study of chromosomes and inheritance Cytogenetics is based on studies of humans as well as Drosophila and other organisms

Preparing a karyotype Harvested cells are first cltured The cells are then treated with colchicine which arrests the cells in metaphase, and then treated and stained to observe the chromosomes Chromosomes can be photographed or visualized using a computer, and then analyzed Chromosomes are identified by size, position of the centromere, and banding and staining regions

Why Analyse Chromosomes and Genes Why Analyse Chromosomes and Genes? Genetic errors arise from deletions or insertions of genetic material, abnormal numbers of whole chromosomes or genes, and even from misplacement of a single base in the DNA sequence. Genetic abnormalities can range from relatively harmless to severe: from vitamin deficiencies and food allergies to cancer, birth defects and infant mortality.

Chromosomal abnormalities Autosome chromosomal abnormalities Sex chromosomal abnormalities Both could be affected in the structure or the number of the chromosomes

Abnormalities in chromosome number cause certain human disorders Polyploidy Aneuploidy

Polyploidy Polyploidy is the presence of additional sets of chromosomes Polyploidy is common in plants Polyploidy is typically lethal in animals Polyploidy is due to nondisjunction or multiple fertilizations of an egg

Aneuploidy B. Aneuploidy is a condition of an extra chromosome Extra chromosome: trisomic for a particular chromosome Or a missing chromosome: haploid for that chromosome Aneuploidies result from nondisjunction during meiosis or mitosis

Down syndrome Persons with Down syndrome are usually trisomic for chromosome 21 Down syndrome is a relatively common chromosomal abnormality Morphological characteristics include differences in the face, tongue, and hands, and individuals are mentally retarded Down syndrome is typically caused by a trisomy of chromosome 21, usually caused by nondisjunction during oogenesis

4. Down syndrome is much more common in babies with older mothers, perhaps because an older woman is less likely to spontaneously abort a Down fetus 5. Affected individuals may vary in the severity of their symptoms 6. Other trisomies are typically inviable, as are monosomies

Sex chromosomal abnormalties Most sex chromosome aneuploidies are less severe than autosome abnormalities

Klinefelter syndrome Individuals with Klinefelter syndrome have an XXY karyotype Klinefelter males have small testes and are often sterile Klinefelter males are often tall and have female-like breast development; about half are mentally impaired

Turner syndrome Persons with Turner syndrome have only one X chromosome and no Y chromosome Turner syndrome females are designated as XO, and have undeveloped reproductive structures and are sterile No Barr bodies are apparent in persons with Turner syndrome

XYY Disorders Some essentially normal males have an XYY karyotype Persons with an extra Y chromosome are fertile, phenotypically male, and often are very tall and have acne

Aneuploidies usually result in prenatal death Chromosome abnormalities are relatively common at conception but usually result in a miscarriage Approximately 20% of all pregnancies may be spontaneously aborted, often due to major chromosomal abnormalities

Abnormalities in chromosome structure cause certain disorders Translocation Translocation is attachment of part of a chromosome to a nonhomologous chromosome This is the cause of approximately 4% of Down syndrome

Deletion A deletion is loss of part of a chromosome Most large deletions are nonviable An example of a deletion is the cri-du-chat syndrome

Fragile sites Fragile sites are weak points at specific sites in chromatids Fragile sites appear to be held together by thin strands of chromatin Fragile X syndrome is a relatively common cause of mental retardation, particularly in males