Mendel and the Gene Idea
Mendel and the Gene Idea
 Mendel’s law of independent assortment:
– Each allele pair segregates independently of
other gene pairs during gamete formation.
Mendel and the Gene Idea
Dominance
Incomplete Dominance
Codominance
Mendel and the Gene Idea
 Codominance:
– Inheritance characterized by full expression of
both alleles in the heterozygote.
– Blood type
– AA
– BB
– AB
Mendel and the Gene Idea
 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
 Test Cross: The breeding of an organism
with an unknown genotype with a
homozygous recessive.
Mendel and the Gene Idea
 Rule of Multiplication:
– The probability that an independent events will
occur simultaneously is the product of their
individual probabilities!
Mendel and the Gene Idea
 What is the probability that a trihybrid cross
between two organisms with genotypes
AaBbCc and AaBbCc will produce an
offspring with a genotype aabbcc?
 Aa x Aa : probability of aa
=
1/4
 Bb x Bb : probability of bb
=
1/4
 Cc x Cc : probability of cc
=
1/4
 Therefore
1/4 x 1/4 x 1/4 = 1/64
Mendel and the Gene Idea
 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
 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
ppYyrr
Ppyyrr
PPyyrr
ppyyrr
1/4 x 1/2 x 1/2 = 1/16
1/4 x 1/2 x 1/2 = 1/16
1/2 x 1/2 x 1/2 = 2/16 (1/8)
1/4 x 1/2 x 1/2 = 1/16
1/4 x 1/2 x 1/2 = 1/16
6/16 or 3/8
Mendel and the Gene Idea
 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
Mendel and the Gene Idea
 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
 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
 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
 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
 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
 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
 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
 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
 PKU (phenylketonuria):
– recessive
1 in 15,000
– patient cannot break down the amino acid
phenylalanine
– build up to toxic levels causes mental retardation
– tx with a special diet to avoid toxic levels
Chi Square
X2 = Σ (O-E)2

E
 The alternate hypothesis in this experiment
is to validate the use of the prediction!
 The null hypothesis in this experiment is to
discredit the use of the prediction!
Chi Square
 Degree of freedom: number of possible
phenotypic categories minus one.
Chi Square
 Evaluate the calculated value with the chart!
 If your calculated value is less than the
chart value -- then you fail to reject the null!
Chi Square
 Last, evaluate your p= value
 If you fail to reject your null, then this value
identifies the percent chance that any
difference in your results is due to random
chance!