Introduction to Genetics
Genetics is the study of heredity
The inheritance of traits
A trait is a characteristic passed from parent to
offspring. Some human traits include:
In the 1800’s Gregor Mendel used pea
plants to study the inheritance of traits
A trait is a specific characteristic that varies
between one individual in a population to
another
When fertilization takes place, it
produces a new cell
Pea plants self fertilize so they inherit
their own characteristics
These are true breeding and
produce purebreed offspring
Mendel removed the male parts of the pea flowers
and made controlled crosses between plants with
different traits
This is called cross pollination
Cross pollination produces seeds that have two
different parents
Mendel studied seven traits in the pea plants
He chose these traits because they show only one of the two
contrasting characteristics
 The different forms of a gene that produce
traits are called Alleles
 The first cross breeding of different parent
plants creates a hybrid filial
 Also called F1 generation
 Crosses between parents with different traits
are called hybrids
 When Mendel crossed true breeding tall pea
plants with true breeding short plants he
created a hybrid
Mendel crossed tall and short pea plants to create a hybrid
All of the offspring in the F1 generation were tall because the allele
for the tall trait was dominant over the allele for the short trait
Did the allele for the recessive trait disappear?
When he let the F1 generation self pollinate the recessive trait re-appeared
At some point, the recessive allele segregated from the dominant allele
When Mendel repeated other crosses
he obtained similar results!
Which traits for the pea color and parakeet color are recessive?
How do you know?
Mendel realized that the principal of probability can be used
to predict the outcome of genetic crosses
Mendel concluded that traits are
inherited through the passing of
factors from parents to offspring
Today these chemical factors
that determine traits are called
genes
The principal of dominance
states that some alleles are
dominant and some are
recessive
This chart shows the
segregation of alleles during a
cross between two hybrid
parents
The capital letter T represents the
dominant allele
The lower case letter t represents
the recessive allele
Punnett squares can be used to predict and compare genetic variations
that may result from a cross
Shows that the genes for traits sort independently
Phenotype and Genotype
 Organisms that have two identical alleles for a trait
are called homozygous (TT), or (tt)



These are known as purebreds
(TT) is homozygous dominant
(tt) is homozygous recessive
 Organisms that have two different alleles for the
same trait are heterozygous (Tt)

These are known as hybrids
 An organisms physical characteristics is called the
phenotype
 The genetic makeup discussed above is called the
genotype
A one factor Punnett square (monohybrid cross) follows one
gene during a cross
A two factor Punnett square (dihybrid cross) follows two
different genes from one generation to the next
Mendel found that traits would follow the laws of probability and show
Independent Assortment
Found that different traits can segregate independently during fertilization
Intermediate inheritance
 Purely dominant and recessive traits are
rare in nature
 Many genes show an intermediate
inheritance
 Incomplete dominance is when neither
allele dominates over the other
 Shows
a blending of the two traits
When different dominant traits are crossed and both are present
in the offspring this is co-dominance
A cross between a black chicken (BB) and a white rooster (WW)
X
Producing a speckled offspring (BW)
a second type of co-dominance is human blood type
Like type AB blood
When two or more alleles for a trait exist in a population the
gene has multiple alleles
Fur color in rabbits has multiple alleles
Blood type in humans also has multiple alleles
Traits controlled by two or more genes are called polygenetic traits
Human skin color is a polygenetic trait, over 4 genes are
involved in determining skin color
Eye color is also polygenetic
The environment can also have an effect on gene expression
Seasonal coat color in
foxes and seasonal wing
coloration on butterflies
Hydrangea flower color to
soil pH, goby fish change
sex in male dominated
populations are all
examples of gene
expression responding to
environmental conditions
Another scientist that tried to advance the study of genetics
was Thomas Morgan
 He used fruit flies called drosophila
melanogaster
 They produced large numbers of offspring in
a very short time
 Only have 8 chromosomes to track
 He found that Mendel’s principals also
applied to the flies
 The basics of Mendelian genetics can also
apply to human genetics
The farther apart two genes are on a chromosome, the more likely they are
separated by a crossover
The frequency of crossing-over between genes can be used to construct
genetic maps