Mendel and the Gene Idea_1_ by hcj


									Mendel and the Gene Idea
Mendel and the Gene Idea
4 Mendel’s law of independent assortment:
  – Each allele pair segregates independently of
    other gene pairs during gamete formation.
Mendel and the Gene Idea
4 Rule of Multiplication:
  – The probability that an independent events will
    occur simultaneously is the product of their
    individual probabilities!
Mendel and the Gene Idea
4 What is the probability that a trihybrid cross
    between two organisms with genotypes
    AaBbCc and AaBbCc will produce an
    offspring with a genotype aabbcc?
4   Aa x Aa : probability of aa     =     1/4
4   Bb x Bb : probability of bb     =     1/4
4   Cc x Cc : probability of cc     =     1/4
4   Therefore      1/4 x 1/4 x 1/4 = 1/64
Mendel and the Gene Idea
4 Rule of Addition:
  – The probability of an event that can occur in
    two or more independent ways is the sum of the
    separate probabilities of the different ways
Mendel and the Gene Idea
4 What fraction of the offspring from the
  following cross of garden peas, would show
  recessive phenotypes for at least two of the
  three traits? PpYyRr x Ppyyrr
  –   ppyyRr    1/4 x 1/2 x 1/2 = 1/16
  –   ppYyrr    1/4 x 1/2 x 1/2 = 1/16
  –   Ppyyrr    1/2 x 1/2 x 1/2 = 2/16 (1/8)
  –   PPyyrr    1/4 x 1/2 x 1/2 = 1/16
  –   ppyyrr    1/4 x 1/2 x 1/2 = 1/16
  –                               6/16 or 3/8
Mendel and the Gene Idea
4 Test Cross: The breeding of an organism
  with an unknown genotype with a
  homozygous recessive.
Mendel and the Gene Idea
Mendel and the Gene Idea
4 Incomplete dominance:
   – Pattern of inheritance in which the dominate
     phenotype is not fully expressed in the
     heterozygote, resulting in a phenotype
     intermediate between the homozygote dominate
     and recessive characteristics.
                        – Blends
Mendel and the Gene Idea
4 Codominance:
  – Inheritance characterized by full expression of
    both alleles in the heterozygote.
                      – Blood type
                          – AA
                          – BB
                          – AB
Mendel and the Gene Idea
4 Multiple Alleles
  – some genes may have multiple alleles; that is,
    more than just two alternative forms of a gene.

   – These multiple alleles occupy a single locus
             – Blood typing A, B, and O
                     – IA IB and i
Multiple Alleles Continued…
     Possible Genotype                 Phenotype
           OO                              O
         AA or AO                          A
         BB or BO                          B
           AB                             AB

What would be the phenotype and probability for the cross
between AO x BO?
Mendel and the Gene Idea
4 Human disorders
  – Recessive alleles that cause human disorders
    are usually defective versions of normal alleles.
  – Recessively inherited disorders range from
    nonlethal traits (albinism) to lethal diseases
    (cystic fibrosis)
 Mendel and the Gene Idea
4 Cystic fibrosis: most common lethal genetic
  disease in the US among Caucasians
                  – Frequency 1 in 2500
  – 4% of the Caucasians are carriers
  – The dominate allele codes for a membrane
    protein that controls chloride traffic across the
    cell membrane. -absent in homozygous recessive
  – Disease symptoms result from the accumulation
    of thickened mucus in the lungs and pancreas.
Mendel and the Gene Idea
4 Tay-Sachs disease: Incidence is about 100x
  higher in central European Jews than among
  Mediterranean Jews
                 – Frequency 1 in 3600
  – Brain cells of babies are unable to metabolize
    gangliosides - lipid
  – As lipids increase in the brain, the infant suffers
    seizures, blindness, and degeneration of motor
    skills and mental performance.
  – Child dies after a few years
Mendel and the Gene Idea
4 Pleiotropy:
   – The ability of a single gene to have multiple
     phenotypic effects.
   – There are many hereditary diseases in which a
     single defective gene causes complex sets of
     symptoms       Sickle-cell Anemia
Mendel and the Gene Idea
4 Sickle-cell disease: most common in
  African Americans 1 in 400.
  – Single amino acid substitution in hemoglobin
  – Abnormal hemoglobin molecules tend to link
    together and crystallize when blood O2 is lower
    than normal
  – Causes the erythrocyte to form a sickle shape
  – Sickle cells clog small vessels causing pain,
    discomfort and fever.
Mendel and the Gene Idea
4 Sickle Cell
   – 1 in 10 African Americans are carriers
   – Codominance - carriers may experience
     discomfort during periods of low blood Ox
     levels - other wise function normally
   – Believed to be result of malaria infestation in
     tropical regions
Mendel and the Gene Idea
4 Sickle cell:
   – The probability of inheriting the same rare
     harmful allele from both parents, is greater if
     the parents are closely related.
Mendel and the Gene Idea
4 Huntington’s disease
  – Lethal dominate (late acting)
  – a degenerative disease of the nervous system
  – does not show until age 35-40
  – irreversible to death
  – found at the tip of chromosome #4
 Mendel and the Gene Idea
4 PKU (phenylketonuria):
   – recessive      1 in 15,000
   – patient cannot break down the amino acid
   – build up to toxic levels causes mental retardation
   – Controlled with a special diet to avoid toxic

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