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GENETICS
Mrs. Harlin
3.2.2

Predict offspring ratios based on a variety of
inheritance patterns (including dominance, codominance, incomplete dominance, multiple alleles,
and sex-linked traits).
MENDEL’S LAWS OF HEREDITY
Gregor Mendel
The first person to succeed in predicting how traits are
transferred from one generation to the next.
Genetics: study of heredity.
Heredity: passing on of traits from parents to
offspring.
Why Mendel Succeeded?

1. Chose his subject carefully.
 Garden
pea plant- > reproduces sexually.
 Used
cross-pollination to breed two different plants.
Normally pea plants self-fertilize.
 Pollination
is transferring pollen grains from a male
reproductive organ in a plant to a female reproductive
organ.

2. Mendel was a careful researcher.
 Studied
one trait at a time in order to control variables.
 The pea plants he worked with were true-breeding.
P
Short X Tall
↓
ALL TALL
F1
↓
↓
↓
F2 Tall
Tall
Tall
3:1 ratio
↓
Short
Why did he get these results?

3 rules:
1. The rule of unit factors: what Mendel called factor,
we call a gene. Genes are located on chromosomes
in cells. Different forms of a gene are called alleles.
Example: the gene for height could have alleles
short and tall.
2.
The rule of dominance:
dominant: trait that appears whenever an allele is
present.
Write as a CAPITAL letter.
recessive: trait that only shows up when 2 forms of
the allele are present.
Write as a LOWERCASE letter.

3. The law of segregation: every individual has 2
alleles of each gene. When gametes (sex cells) are
produced, each gamete receives one allele.


Phenotype: the way an organism looks and
behaves. “Pheno” means to show.
Example: Tall
Genotype: the allele combination.
Example: TT
Homozygous (true-breeding): an organism has 2
alleles for a trait that are the SAME.
Example: TT = tall (dominant)
tt=short (recessive)
Heterozygous (hybrid): an organism has 2 DIFFERENT
alleles for a trait.
Example: Tt=tall (dominant)
Punnett Squares

Diagrams used to predict the outcome of a genetic
cross considering all possible combinations of
gametes.
Complex Patterns of Heredity
Incomplete Dominance

The heterozygous individual is intermediate
between those of 2 homozygotes.
Snapdragon flower
 RR – red
 rr- white


When these 2 are crossed you get: Rr - pink
Incomplete Dominance

Sometimes incomplete dominance is denoted with
an apostrophe, RR’ instead of Rr.
Codominance

Causes the phenotype of both homozygotes to be
produced in heterozygous individuals.
Chickens
 BB- black feathers
 WW-white feathers


When these are crossed (BW), you do not get black,
white, or gray feathers, you get BLACK and WHITE
feathers (checkered).
Multiple Alleles
Traits controlled by more than 2 alleles.
 3 alleles on one gene govern blood type.

Multiple Alleles Govern Blood Type

3 possible alleles: IA, IB, or i.
Genotypes
IAIA or IAi
Phenotypes
A
IBIB or IBi
B
IAIB
AB
ii
O
Sex-linked Traits



Traits specifically located on the X-chromosome (not
chromosomes 1-22).
Ex: color-blindness and hemophilia
Males are more likely than females to exhibit a sexlinked trait because they only have 1 X
chromosome.
Pedigree Tracing a Sex-linked Trait
Polygenetic Inheritance
Inheritance pattern of a trait is controlled by 2 or
more genes.
 Ex: eye color or height

Genetic Disorders

Sometimes genes are damaged or copied
incorrectly, resulting in faulty proteins. These
mutations can cause disorders that may or may not
be lethal.
Sickle-Cell Anemia



Recessive
If homozygous, the oxygencarrying protein hemoglobin
differs by one amino acid than
normal. It changes the shape of
the red blood cells.
Heterozygous individual are
protected from effects of
malaria.
Hemophilia
Recessive sex-linked
 Causes blood not to
clot.
 Mutation of 1 of the
blood-clotting genes on
X chromosomes.

Huntington’s Disease
Dominant
Rare, lethal disorder in which
certain areas of the brain
are broken down.
 Doesn’t show up until
ages 30-50, so an
individual may have
already reproduced.
 There is a test to see if
you have this disease.
Cystic Fibrosis
Recessive

Due to defective
protein in the plasma
membrane. Results in
thick mucus in lungs
and digestive track.
Gene Therapy

Gene therapy may soon
allow scientists to correct
certain recessive genetic
disorders by replacing
defective genes with
copies of healthy ones.
Basically this means
inserting “good” genes
into an organism with
“bad” genes.
Changes in Chromosomes



Caused by nondisjunction.
Occurs when homologues fail to separate during
anaphase I of meiosis, or sister chromatids fail to
separate during anaphase II.
Results in gametes with missing or too many
chromosomes.
Changes in Chromosomes

Normal Karyotype
Changes in Chromosomes

Normal Karyotype
What is different about this karyotype?
How about this one?
How about this one?
How about this one?
How about this one?
Importance of Karyotypes

What can we determine about a person by looking
at a karyotype?


1. Chromosomal abnormalities
2. Gender