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Particulate Inheritance Patterns
Blended Inheritance
•  Articulated by Aristotle
•  Offspring result from the combination of parental
“essences”. •  Phenotypes of offspring should be intermediate to that of
parents
Particulate Inheritance
•  Theory proposed by Mendel •  First articulated in 1865
•  Result of work done with Pisum sativum
–  Established “pure strains”
–  Considered each character (trait) individually
–  Applied statistics to the data
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Definitions
•  Dominant allele - an allele that expresses
its phenotypic effect even when
heterozygous with a recessive allele
•  Recessive allele - an allele whose
phenotypic effect is not expressed in a
heterozygote. Definitions
•  Parental generation (P1) - the original
organisms under study
•  First filial generation - the offspring that
results from a cross of the parental
generation
Reciprocal Cross
•  Organisms that have different sexes can be
crossed in two different ways Phenotype A male X Phenotype B female Phenotype B male X Phenotype A female •  Mendel concluded that reciprocal crosses
are equal 2
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Mendel’s Crosses
Significance of 3:1 ratio
Mendel’s Conclusions
•  Existence of particles of inheritance •  These particles of inheritance are inherited
in pairs
•  Members of of the pair segregate into
gametes
•  Gametes carry only one set of chromosomes
•  Gametes combine without regard to the
members of gene pair that are carried. 3
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Mendel’s 5 points
Mendel’s First Law
The Law of Segregation
The two members of a gene pair segregate
from each other into the gametes; so half the
gametes carry one member of the pair and
the other half of the gametes carry the other
member of the pair Definitions
•  Monohybrid - a heterozygote resulting from
the cross of two pure parental strains. •  Selfing- a cross in which individuals are
allowed to self-fertilize (plants) or two
phenotypically identical individuals from
the same generation are allowed to mate.
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Definitions
•  Dihybrid - a individual heterozygous at two
gene loci. Results from the mating of two
pure parental strains. •  Dihybrid cross - the selfing of dihybrid
individuals. 3/4 of these round seeds
will be yellow
3/4 of the F2 will be round
1/4 of these round seeds
will be green
3/4 of these wrinkled
seeds will be yellow
1/4 of the F2 will be wrinkled
1/4 of these wrinkled
seeds will be green
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Mendel’s Second Law The Law of Independent
Assortment
During gamete formation, the segregation of
the alleles of one gene is independent of the
segregation of the alleles of another gene
For genes whose loci are located on different
chromosomes. Calculating Genetic Ratios
•  Punnett square
•  Forked line
•  Probabilities Punnett Square
•  For every loci that is
heterozygous (n)
•  the number of gametes
is 2n
•  the number of
genotypes is 3n
•  the number of possible
combinations is 4n
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Forked Line
1/4CC
1/4BB
1/4 AA
1/2Bb
1/2Cc
1/4cc
1/4 bb
Product Rule
The probability of independent events
occurring together is the product of the
probabilities of the individual events
Sum Rule
The probability of either of two mutually
exclusive events occurring together is the
sum of their individual probabilities
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Now Solve:
For the cross:
AaBbCcDdEe X AaBbCcDdEe
What is the probability of getting an offspring
with the genotype of aaBbCCDdee?
(1/4)(1/2)(1/4)(1/2)(1/4) = 1/256
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