# Mendel and the Gene Idea_1_ by hcj

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```									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
– 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
phenylalanine
– build up to toxic levels causes mental retardation
– Controlled with a special diet to avoid toxic
levels

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