A] CYTOGENETICS

• Chromosomal theory of inheritance - genes are a part of the chromosomes
• Prenatal testing
  • Chorionic villus sampling involves the collection of a small amount of fetal tissue from the placenta and constructing a karotype from the dividing embryonic cells.  
    • This test can be done in the 8^th – 10^th weeks of pregnancy.  
  • Amniocentesis involves the collection of floating embryonic cells (that have been sloughed off by the developing fetus) by extracting a small amount of amniotic fluid (the fluid which surrounds the developing fetus). 
    • This procedure is normally accomplished during the 14^th to 16^th weeks of pregnancy, with results coming a couple of weeks later, after culturing of the cells has been accomplished.  
  • Both of these tests will provide information on the structure of the chromosomes, but not on genetic problems that may be present within the DNA
• Chromosomes
• composed of DNA and protein
• located in the nucleus of cells
• only visible under the light microscope when a cell is actively dividing
• all cells of the body (with the exception of the reproductive cells - eggs & sperm) contain the exact same number of chromosomes
• all members of the same species have the same number of chromosomes in their cells
• each chromosome has a compliment - another chromosome of the same size and shape that carries information on the same characteristics of the organism - these pairs of chromosomes are called homologous pairs of chromosomes
• Structure of chromosomes
• during one phase of mitosis, called Metaphase, each chromosome appears as two strands connected together at one spot
• the strands are called chromatids
• the point of attachment is called the centromere
• if a cell is smashed during this phase of mitosis, the individual chromosomes are randomly spread out to enable viewing of individual chromosomes
• a photograph can be taken of the chromosomes and enlarged to show even more detail
• these enlarged chromosomes can be cut out and arranged into their homologous pairs by matching up those that are the same size, shape, and have the centromere located in the same spot along the length of the chromosome
• the result is a karotype - a precise ordering and pairing of the chromosomes of a cell

B] HUMAN CHROMOSOMES

• Normal humans have a total of 46 chromosomes
• 23 homologous pairs of chromosomes
• 22 of these pairs are found in both males and females - the autosomes
• 1 pair is different in males vs. females - the sex chromosomes
• females have 2 chromosomes that look alike and are designated the X chromosomes
• males have one of the X chromosomes and a unique chromosome found only in males and smaller than the X, it is called the Y chromosome
• therefore, males can be designated XY and females XX
• Chromosomal abnormalities
• normally, if a fertilized egg has one too many or one too few chromosomes, a human will not develop
• in a few cases, a human will develop even if the fertilized egg has a wrong number of chromosomes
• Down syndrome - caused by having one extra autosome,
• caused by an extra copy of chromosome #21 (one of the smallest human chromosomes)
• distinct facial features and different degrees of mental retardation are associated with this syndrome
• Kleinfelter syndrome - caused by having an extra sex chromosome
• sex chromosomes = XXY
• has a penis, but may develop secondary sexual characteristics of females (e.g. breast development)
• usually does not affect intelligence

C] PATTERNS OF INHERITANCE

• Autosomal dominant inheritance
• these traits follow general Mendelian patterns of inheritance
• the trait is located on an autosome
• if you show the given trait you possess at least one copy of the dominant allele
• examples: having mid-digital hair; being able to taste the chemical PTC; having blood type A or B, etc.
• other information concerning blood types
• alleles are written a special way:
• I^A = blood type A allele
• I^B = blood type B allele
• i = blood type O allele
• blood type O is the most common (recessive does not mean uncommon)
• blood type AB (I^AI^B) is an example of codominance - a situation where both alleles show their effect at the same time without mixing
• Ausosomal recessive inheritance
• these traits follow general Mendelian patterns of inheritance
• the trait is located on an autosome
• if you show the given trait you possess two copies of the recessive allele
• examples: “hitchhiker’s thumb”, blond hair, blue eyes
• Sex influenced inheritance
• in these cases, the dominance or recessiveness of an allele depends on the sex of the person possessing the allele
• no difference between the sexes of the phenotype of a person who is homozygous for a given allele
• difference in phenotype comes into play when a person is heterozygous - in male one of the alleles is dominant and shows its affect while in females the other allele is dominant and shows its effect
• examples: short index finger and male pattern baldness
• Sex linked inheritance
• deals with traits actually located on the sex chromosomes
• these traits can be on the X or Y, but most cases involve traits located on the X chromosome
• some information on the large X chromosome is not on the smaller Y chromosome
• example of sex linked inheritance associated with the X chromosome is colorblindness - information on this trait is only found on the X chromosome, not on the Y chromosome
• Dermatoglyphics - study of patterns of the creasing of the skin (especially the hands)
• example of person specific genetic information due to mutations during development (different even between identical twins)
• three basic types of fingerprints - arch, loop, & whorl
• other examples: Simian crease and palmer angle