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Introduction
to Genetics
The Work of Gregor Mendel
Genetics
• Scientific study of heredity
• Gregor Mendel
–Born in 1822 in Czech Republic
–Worked in monastery garden while
teaching H.S. (math & science)
–Worked w/ true-breeding pea plants
• If allowed to self-pollinate, produce
offspring identical to themselves
Genes and Dominance
• Mendel studied 7 traits
–Specific characteristic that varies
from one individual to another–
color, height
–Each trait had two contrasting
characters – greed seeds vs. yellow
–Crossed true-breeding plants of
each type to opposing character
Genes and Dominance
• True-breeding yellow seed X Truebreeding green seed
• Original pair of plants = P
(parental) generation
• Offspring = F1 (first filial)
• Offspring of crosses between
parents w/ different traits = hybrids
• What did the F1 hybrids
look like? Did they look like
a blend of both parents?
No!
• All F1 offspring only
showed the character of
one parent.
Mendel’s Two Conclusions
• 1. Biological inheritance is
determined by factors that
are passed on from one
generation to the next
–Genes – chemical factors
that determine traits
• Each trait that Mendel studied
was controlled by one gene
that occurred in two
contrasting forms
–Ex: Gene for Plant Height =
One form for tall one for short
• Alleles – different forms of a
gene
• 2. Principle of Dominance –
some alleles are dominant
and others are recessive
–If there is a dominant
present, that form is shown
–If there are two recessive
present, the recessive is
shown
Segregation
• Did the recessive alleles
disappear, or were they still
present ?
–To find the answer, he allowed F1
generation plants to self-pollinate
to create an F2 generation
• What happened?
–Traits reappeared. ¼ of the F2
Segregation, cont’d
• Law of Segregation
–Two alleles separate from
each other so that each
gamete carries only a single
copy of each gene
–Ex: Fig. 11-5
Introduction
to Genetics
Genetics and
Probability
Genetics and Probability
• Probability
–The likelihood that a particular event
will occur
• Ex: tossing a quarter
–Heads = 1 in 2
–Tails = 1 in 2
–3 tails in a row ½ x ½ x ½ = 1/8
–Principles can be used to predict
outcome of genetic crosses
Genetic Vocabulary
• Character
– Detectable
inheritable feature
of an organism
• Trait
– Variant of
inheritable
character
= allele / alleles
• Dominant
– Capital letter (SS)
• Recessive
– Lower case (ss)
• Two alleles for
each character
– Mom and dad
Genetic Vocabulary
• Genotype
• Phenotype
–The genetic
–The
makeup of
physical
an organism
traits of an
–Written in
organism
letter format
–PP, pp, Pp
Genetic Vocabulary
• Homozygous
• Heterozygous
–Having two
identical
alleles for a
given trait
–Having two
different
alleles for a
given trait
• PP
• ss
• Pp
• Ss
Basic Crosses
Recessive/Dominant
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