Chapter 6 Notes
Sections 6.4, 6.5 & 6.6
Mendel’s “discrete units” are
now called genes.
 Genes are pieces of DNA that
provides a set of instructions
to a cell to make a certain
protein.
 Most genes exist in many forms
 When looking at Mendel’s
experiments this is evident:

 Yellow or green seed color
 Tall of short plant height

Any alternative form of a gene
is an allele and we represent
these alleles with letters.
 T = tall
 t= short



Every trait has 2 genes (alleles),
one from mom and one from
dad.
When both of the alleles are
the same (TT) or (tt), we call
this a homozygous condition.
When the alleles are different
(Tt), we call this a
heterozygous condition.
When we talk about an organism’s gene
make-up, we are talking about its genotype.
 While both (TT) or (Tt) plants would be tall,
they have a different genotype.
 This refers to the actual alleles that make-up
a trait.

When we talk about an
organisms physical
appearance, we are talking
about its’ phenotype.
 In the previous example,
both TT and Tt would be
tall, so we don’t care that
the alleles are different
 We only care about the
physical appearance when
talking about phenotype.


In Mendel’s studies, he found that when a trait
was in the heterozygous condition, the
phenotype of the plant showed only one of
the two traits
◦ NOT BOTH AND NOT A MIXTURE OF BOTH!
Mendel quickly learned from his crosses that
certain genes masked other genes.
 He called genes that masked other genes
dominant genes.
 The genes that are being masked are the
recessive genes.

Trait
Dominant
Recessive
Seed Shape
Round
Wrinkled
Seed Color
Yellow
Green
Pod Shape
Smooth
Constricted
Pod Color
Green
Yellow
Flower Position
Axial
Terminal
Flower Color
Purple
White
Plant Height
Tall
Short
•Remember- dominate alleles are not necessarily better or occur more. It simply means
when 2 different alleles are together, one masks the other.
We use an uppercase letter for dominant
traits and a lowercase letter for recessive
traits.
 Because some alleles are dominant over
others, 2 different genotypes can produce the
same phenotype!

◦ TT= tall and Tt =tall

As you know not all plants are tall or short or
just have purple or white flowers.
Environmental factors like water and sun
affect plants as well.
Mendel soon concluded after
some of his initial findings that
through probability laws he
could determine possible
outcomes.
 A scientist by the name of
R.C. Punnett took some of
Mendel’s work and developed
a grid system to figure
possible outcomes.
 The punnett square is a grid
system used to predict all
possible genotypes and
phenotypes resulting from a
cross.

By using this grid
system, possible
outcomes of certain
crosses can be
predicted.
 Remember the
punnett square is a
prediction of
possible outcomes,
it is not what will
actually happen.


The first type of cross is
called a monohybrid
cross or one-factor cross.
◦ This deals with only one
trait!!


The traits can be either
homozygous or
heterozygous.
Remember, every trait
ALWAYS has 2 genes, you
get one gene from mom
and one gene from dad.
When doing actual crosses, it is
important to know what type of
organism you are starting with.
 If you don’t know the genotype
of an organism, you can do a
test cross

◦ A test cross involves crossing an
organism with an unknown
genotype with a recessive
organism.

If you cross an unknown plant with a
recessive plant and get offspring that are
recessive, than you know your unknown is
hybrid!
Dihybrid crosses
involve 2 traits!
 Up until now we have
dealt with crosses
involving one trait.
 You can, however, cross
two traits at a time- this
is called a dihybrid cross.


Mendel did dihybrid crosses and wondered:
◦ “would traits always stick together or if they would
express themselves independently?”

In other words- Mendel wanted to know if you
took a plant that was both tall and axial and
crossed it with a short, terminal plant would you
only get offspring that look like the parents, or
would you get recombination (tall & terminal)
and (short & axial).

When Mendel did his first
dihybrid cross, his results
were similar to the results he
got in his first monohybrid
crosses
◦ Crossing a purbred (Tall, Axial)
x (Short, Terminal) = ALL tall,
axial

He then let the F1 fertilize
themselves (TtAa x TtAa)
◦ His results showed that all
possible combinations are
possible




Tall, axial
Short, axial
Tall, terminal
Short, terminal
This led him to his 2nd law which is the Law
of Independent assortment.
 This law states that given 2 traits each of
these traits separate independently so all
combinations are possible.

◦ Traits don’t stick together!
After years of study, Mendel started to figure
out that genetics has a probability component
to it.
 Probability is the likelihood that a particular
event will occur

◦ Probability= # of desirable outcomes/total #
of all possible outcomes
This type of probability can be applied to
gamete formation.
 Suppose you have a plant hybrid for height
(Tt).
 The likelihood of a (T) is 1 over the total
possible outcomes (T) & (t)is two

◦ So probability = ½



Genetic variation is the
essence of all life.
Sexual reproduction
allows for different
genetic variations daily.
When chromosomes
line up in the middle
during metaphase I of
meiosis, there is about 8
million different
combinations that can
be formed.
Since an egg & sperm combine, the possible
different combinations increase to about 70
trillion
 In other words:

◦ A human couple can produce a child with one of
about 70,000,000,000 different combinations of
genes!
Genetic diversity is affected by other things as
well.
 A lot of variation comes from a process called
crossing-over
 This is when chromosomes exchange parts
during metaphase I of meiosis.

As you can see, genes have
exchanged places on like
chromosomes.
 What this does is increase the
number of different combinations
possible now that genes have
crossed-over.
 What Mendel didn’t know was that
some genes are on the same
chromosomes!

◦ How will that affect crosses???
 FIND OUT NEXT CHAPTER!!!!