Site hosted by Angelfire.com: Build your free website today!

MENDELIAN GENETICS

Objective:  Students will gain knowledge of Mendelian genetics and some basic inherited diseases.

Lesson

Lecture:

Mendelian Genetics
    By the middle of the 19th century the scientific world was still virtually ignorant about how organisms reproduce only that they did. The science that we now call genetics was born in an obscureAustrian monastery garden where Gregor Mendel meticulous tended his pea plants for many years, peas not being the fastest reproducing organisms. He described phenomena about peas that we now give bigger more scientific words to but his mathematical ratios have never been disputed and the characteristics he found have now been noted as genes on chromosomes.

  1. Gregor Mendel, a German monk, studied pea plants and is credited as the Father of Genetics
  2. Mendel's Rules - Mendel created four rules to explain his results.  Please note the difference between the "Rules" and the "Laws".
    1. Rule of Unit Factors - each organism has two(2) factors (called alleles) for each physical trait.  Alternative alleles explain the variations in inherited characteristics. Not all genes are as simple as tall/short or purple/white.
      • allele - different forms of the same gene (hair color comes in several varieties, all are controlled by the same genes)
    2. Rule of Dominance - some traits mask the appearance of other traits.
      • dominant - will always be seen, represented by a capital letter (ex.   "R")
      • recessive - hidden unless paired with another recessive, represented by a lowercase letter (ex.  "r")
    3. Law of Segregation - two(2) alleles separate during meiosis to form gametes
    4. Rule of Independent Assortment - each trait separates independently of each other (ex.   because you have brown hair doesn't mean that you will have green eyes)
  3. Vocabulary
    • phenotype - the physical characteristics of an organism (ex.  blue hair), it is determined by the genotype
    • genotype - the genetic makeup of an organism

    Genotype and Phenotypeclick here to see the difference between genotype and phenotype

     

    • homozygous - a pair of genes that are the same (ex.  "RR" or "rr")
    • heterozygous - a pair of genes that are different (ex. "Rr")
  4. Punnett Squares - These are the tools that we use to determine the genetic possibilities of various combinations of parents.

     

  5. Generations
    1. Test Cross - These are used to determine the missing gene in a dominant phenotype.  It crosses a dominant phenotype (R_) with a recessive phenotype (rr).  If the cross creates:
              RR x rr -> Rr 100% tall offspring, or
              Rr x rr -> Rr 50% tall offspring and rr 50% short offspring so it proves the existence of a  recessive allele in the dominant parent.
    2. P generation - represents parents
    3. F1 generation (first filial) - offspring of these (P) parents
    4. F2 generation (second filial) - offspring of these (F1) parents.

       

  6. The Exceptions to the Rule
    1. Multiple Alleles - several alleles determine a trait.  Usually found when there are more than two(2) types of trait  (ex. skin and hair color both come in more than two colors)
    2. Codominance - two (2) dominant genes exist for a single trait.  Both of the genes will be expressed equally  (ex.  When you cross a black and a white chicken you will get a "checkered" - both black and white - chicken)

       

    3. Incomplete Dominance - both the dominant and recessive genes are expressed.  This creates a new genotype.  Snapdragons are an example of flowers Mendel did not study which do not hold to the principal of dominance. The heterozygous condition does not appear as the dominant trait which one would assume to be red, but as the intermediate color - pink. (ex.  When you cross a red rose with a white rose, you will get a pink rose that is a combination of the parents.) The appearance of the F1 hybrids appear half way in between the two parents. If a red flowering plant were mated to a white flowering plant one would expect red or white to be the color of their offspring, depending on which allele is dominant. In this case all the offspring were pink in color. The reason for this is each allele for red allows 1/2 of the red pigment to be produced. RR = red or 100% of the pigment; Rr = 50% of the pigment or pink; and rr = 0% of the pigment or white. Pink is a hybrid and can never breed true. Two pinks would yield 25% red, 50% pink and 25% white offspring.

       

       

    4.  Pleiotropy- A gene can sometimes affect another characteristic. This ability of having multiple effects is called pleiotropy. Genes that control fur pigmentation in cats may have an influence on the cats eyes and brain.

    5.  Epistasis- One gene may interfere with the expression of another gene that is independently inherited. In flower color a P is required for it to exhibit purple color. PP and Pp = purple colored flowers. This can only happen if a dominant allele is present for another characteristic. PPCc =purple Ppcc = white. The C characteristic has an effect on the color of the flower.

    6. Polygenic Traits- Quantitative traits or having 3 sets of alleles for a characteristic. Skin color is polygenic. There are 6 genes responsible for this characteristic. BBBBBB= Very dark pigmentation where as bbbbbb = the opposite very light pigmentation. All the other genotypes are intermediates of these combinations.

    7. Autosomal traits - those traits NOT determined by the sex-chromosomes (chromosomes #1 - 22)
    8. Sex-Linked Traits - traits determined by the sex chromosomes. (chromosome #23)  Humans have 22 pairs of autosomes and one pair of sex chromosomes. Some traits and diseases are carried on the sex chromosome pair, most often on the "Y" chromosome contributed by the father. This makes tracing some inherited diseases through families rather easy. Most of these are also recessive genes; so they do not show up in a family unless the mother also has a family history of the disease or trait (is heterozygous). (i.e.  colorblindness, male pattern baldness, hemophilia, etc.)
      1. "X" chromosome - carries genetic information for the body;  is recessive for gender (XX = female)
      2. "Y" chromosome - determines "maleness" ONLY.  There is no other genetic information on the "Y" chromosome; is dominant for gender (XY = male)

  7. Human Inherited Genetic Disorders

    1. Autosomal - Recessive Diseases - These diseases require that the afflicted have two recessive genes for the trait.  There are "carriers" which have both a dominant and recessive gene for the trait.  These "carriers" don't have the disease but can pass it to their offspring if mated with another "carrier"

      1. Cystic fibrosis - Is most frequent in the Caucasian population. The location of the gene is known.  It produces a failure of the cell membrane pumps so the chloride ion is transported improperly. Symptoms include excessive mucus which clogs lungs, liver and pancreas. Inevitable complications lead to premature death, usually childhood or teenage years. Pre-natal testing and some preliminary gene therapy is available, but there is not a true cure

      2. Sickle-cell anemia - Is most frequent in African Americans. The location of gene is known.  It causes an abnormal hemoglobin formation from single incorrect amino acid.  The abnormal hemoglobin leads to poor circulation and can lead to strokes and heart attacks certainly pain. Pre-natal testing is available, but there is no cure.

      3. Tay-Sachs - Is most frequent among the Jewish population. The location of gene is known.  It causes a defective enzyme which deteriorates the central nervous system in infancy and causes death. Pre-natal testing is available, but there is no cure.

      4. Phenylketonuria - There are no true population boundaries for this disease. The location of gene is known.  It causes a lack of one enzyme - phenylalanine hydroxylase - which prevents people from metabolizing phenylalanine. Pre-natal testing is available after birth and if it is caught,  the person just cannot consume any phenylalanine (which is easier said than done). The consequences of not metabolizing phenylalanine, especially in infancy, is a failure of the brain cells to develop properly. There is no cure, and the enzyme cannot be added.  Adults "grow out of" this disease.

    2. Autosomal - Dominant Diseases - These diseases require that the afflicted have only a single dominant allele for the trait.  There are NO CARRIERS and afflicted parents have a 50% chance of having afflicted offspring.

      1. Polydactylism - It causes the body to develop 6 digits instead of the normal 5. There is no real test available because this is not a life threatening situation. Cure has always been simple surgical removal after birth. Is much more common that most people realize 1 out of 400 babies born.

      2. Huntington's Disease - causes the brain tissue to gradually deteriorate causing loss of muscle tone first and then memory loss before death. Onset of the disease is usually in the early 30's. The gene location is known but no test is available and there is no cure.

    3. Sex - Linked Diseases -  These diseases are usually carried on the X chromosome which means that men are more likely to be affected by these diseases than women.

      1. Duchenne's Muscular Dystrophy - caused by sex-linked recessive gene, degrades the myelin coating of nerves which stimulate muscles causing them to gradually waste away. Gene is known and a test to identify is available but no cure or treatment available.

      2. Hemophilia - caused by a sex-linked recessive gene, generally Caucasian males but females can be affected. Defective blood clotting, factor VII missing, this immunoglobin protein can be administered but so much is needed, not just when a person gets cut but when a child is growing that it still makes a "normal" life difficult and reduces life expectancy. With the new problems that AIDS has added to blood supplies and blood banks it has multiplied problems of hemophiliacs. The gene location is known, but no cure, only a treatment, is available.

 

 

 
../images/homework.gif (7988 bytes)

 

 

 

  Return to CCS Web Academy    Return to Cumberland County Schools
Email Instructor    Desktop