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Mendelian Genetics
Gregor Mendel’s research
• Mendel proposed the first theory about the
units of inheritance (what we now call
genes) and described two fundamental rules
governing how traits are inherited.
• Mendel’s work is recognized today as one
of the greatest breakthroughs in the history
of science.
• In one experiment he made a cross between
two true-breeding strains, one with wrinkled
seeds and another with round seeds.
• This is called a monohybrid cross because
the two plants involved in the cross differ
only in one character (in this case, seed
shape).
• The plants involved in the original cross are
called the parental or P generation. In this
experiment, Mendel found that all the
progeny plants, the first filial or F1
generation, produced only round seeds.
The wrinkle trait seemed to have been
masked, or “dominated,” by the round trait
in the F1 seeds.
• Mendel called the round trait dominant and the
wrinkled trait recessive. All seven monohybrid
crosses behaved in a similar manner; they
produced F1 progeny processing the trait of one of
the parents.
• Mendel then planted the F1 seeds for each cross,
raised the plants, and allowed them to self
pollinate to produce the second filial or F2
generation.
• He found that both dominant and recessive types
appeared in the F2 generation in a numerical ratio
of 3 dominant to 1 recessive.
• Mendel proposed that genes exist in
different forms and can consequently give
different traits.
• Today we call different forms of one gene
an allele.
Autosomal Dominant
• If the trait were dominant, we would use
the following designations:
A = the trait (a genetic disease or
abnormality, dominant)
a = normal (recessive)
Autosomal Recessive
• If the trait were recessive, we would use the
following designations:
A = normal (dominant)
a = the trait (a genetic disease or
abnormality, recessive)
X-linked Recessive
• We use the following symbols for
X-linked recessive:
XA = normal
Xa = the trait (a genetic
disease or abnormality)
Y = Y chromosome (males
only)
Rr
rr
Rr
rr
XA XA
XA Xa
XA Y
Xa Y*
* = trait is expressed
Assignment
• Pg. 178, Section 1 Review 1-7
• Due on Wednesday (1/15)