Transcript Complicated Genetics

Genetics Since Mendel
Chapter 5, Section 2
Genetics Since Mendel

What have
we found
out since
Mendel?

Since Mendel’s
experiments, scientists
have found that genetic
inheritance can be much
more complicated than
the simple, dominantrecessive inheritance
that Mendel saw in pea
plants.
Incomplete Dominance

Incomplete
Dominance

Occurs when offspring of
two homozygous parents
show an intermediate
phenotype.

The heterozygous form is
in between the dominant
and recessive
homozygous forms.
Incomplete Dominance

Incomplete
Dominance

Example:

Snapdragons:

Crossing a purebred white
snapdragon with a purebred
red snapdragon produces a
pink snapdragon.
Multiple Alleles

Multiple
Alleles


Trait is controlled by more
than two alleles.
Produce more than 3
phenotypes for the trait.
Multiple Alleles

Multiple  Example:
Alleles
 Blood Type:
Alleles: IA IB and i
 Phenotypes: A, B, AB, O



Type O is recessive to types
A and B.
Types A and B are both
expressed in the phenotype
when paired together.
Polygenic Inheritance

Polygenic
Inheritance

Occurs when a group of
gene pairs act together
to produce a trait.



Traits are controlled by
alleles from more than
one gene.
A wide variety of
phenotypes are
produced.
Examples:

Hair, eye, and skin color
Impact of the Environment

Environmental
influence on
genetics:

Environmental
influences can be
internal or external.
 Examples:


A boy who has genetics
to be six feet tall is
malnourished as a child
and is only five foot eight
inches tall.
Male birds exhibit
brighter feathers than
female birds.
Human Genes and Mutations

What is a
mutation
again?



Error in the DNA
sequence.
Can be harmful, helpful,
or null.
Can be caused by
mistakes in replication or
by external agents called
mutagens.
Human Genes and Mutations

Chromosome
Disorders



An organism receives
too many or too few
chromosomes as a
result of mistakes in
meiosis.
Usually fatal
Example:

Down’s syndrome

3 copies of
chromosome 21.
Recessive Genetic Disorders

Recessive
Genetic
Disorder


Disorder only appears if
an individual has both
recessive alleles.
Parents are carriers
(heterozygous) and do
not show symptoms of
the disorder.
Recessive Genetic Disorders

Recessive
Genetic
Disorder

Example:

Cystic fibrosis


Production of thick instead of
thin mucous in the lungs and
intestinal tract.
Most common genetic
disorder leading to death in
Caucasian Americans.
Determination of Gender

Boy or Girl?




Sperm cells have either
an X or a Y
chromosome.
Egg cells have only an
X chromosome.
Males – XY
Females - XX
Sex-Linked Disorders

Sex-linked
Disorders

Alleles controlling the
genetic disorder are
linked to either the X or
the Y chromosome.
Sex-Linked Disorders

Sex-linked
Disorders

Example:

Color blindness



Recessive allele linked to
the X-chromosome.
All males with the
recessive allele are
colorblind.
Females must have two
recessive alleles to be
colorblind.
Pedigrees

Pedigrees


Family tree for a certain
trait.
Used to determine
inheritance patterns and
probabilities and in
breeding plants and
animals.
Pedigrees

How to
read a
pedigree:



Males – squares
Females – circles
Have the trait – colored
Does not have the trait –
empty
Is a carrier
(heterozygous) – halfcolored