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Introduction to Genetics
Chapter 11
What is genetics?
 Genetics is the scientific study of heredity
Mendel’s Work
 In the mid-1800s, an Austrian monk
named Gregor Mendel studied how traits
were passed from one generation to the
next using pea plants
 His ideas form the foundation of the
principles of heredity, known as Mendelian
genetics
Mendel’s Work
 Pea plants can produce
purebreds (genetically
identical offspring to the
parent plant through selfpollination)
 Mendel did crosspollination to interbreed
the pea plants
Mendel’s Work
 Mendel studied 7 traits (specific
characteristics), such as seed color or
plant height
 The parent generation is called the P
generation
 The offspring from those parents are called
the F1 generation (first filial)
Mendel’s F1 crosses
Mendel’s Conclusions
1. Biological inheritance is determined by
factors (genes) that are passed from one
generation to the next
Each trait is controlled by one gene
occurring in two contrasting forms – the
different forms of each gene are called
alleles
for example, the gene for plant height has
alleles for both tall and short plants
Mendel’s Conclusions
2. Some alleles are dominant and others
are recessive
if the dominant allele is present, that form
of the trait will always be present
the recessive trait will only show if there
is no dominant allele
Mendel’s F2 Cross
 Mendel allowed the F1 generation to selfpollinate
Mendel’s F2 Cross
 Alleles are segregated from each other so that
each gamete (egg, sperm) carries only a single
copy of each gene
Probability
 The likelihood that an event will occur is
called probability
 For example, coin flipping:
- the probability of flipping heads is ½
(50%)
- the probability of flipping three heads in
a row is ½ x ½ x ½ = 1/8 (1 in 8 chance)
 Past outcomes do not affect future
ones!
Punnett Squares
 A Punnett square is a
diagram to identify
possible combinations
of alleles in offspring
when two parents are
“crossed”
Two types of alleles
 DOMINANT alleles are
represented by
CAPITAL letters
ex: T is for tall
 recessive alleles are
represented by
lowercase letters
ex: t is for short
Dominant or recessive?
 Which of the following alleles are
DOMINANT?
A
t
q
R
s
 Which of the following alleles are
recessive?
A
t
q
R
s
Two types of alleles
 Each person has 2 alleles – one from mom, one
from dad
 homozygous means you have 2 of the same
allele (either both dominant, TT, or both
recessive, tt)
also known as purebred
 heterozygous means you have 2 different
alleles (one dominant, one recessive: Tt)
also known as hybrid
Homozygous or heterozygous?
 Label the following as homozygous (homoz)
or heterozygous (heteroz):
Hh
heteroz
BB
homoz
rr
homoz
Ss
heteroz
Tt
heteroz
qq
homoz
HH
homoz
Genotype and phenotype
 A phenotype is the
physical characteristic
(what it looks like)
ex: tall plants
 A genotype is the genetic
make-up (the actual
alleles)
ex: TT or Tt
Monohybrid Crosses
 single gene
cross (each
parent has
one gene,
each with two
alleles)
Monohybrid Crosses
 parents go
along the top
and side
 offspring
shown in the
middle 4
squares
Monohybrid Crosses
 cross a
homozygous
tall plant
with a
homozygous
short plant