Genetics
• Heredity: is the passing of traits from
parent to offspring.
– Every organism is a collection of traits, all
from its parents
• Trait: are physical or genetic
characteristics of an individual organism
• Genes control the traits by producing
proteins.
– A Gene is a small section of DNA that codes
for certain proteins.
• An alternative form of genes are called alleles
– Organisms have at least two genes for every
trait.
– They receive at least one from each parent.
• The study of how traits are inherited
through action of genes is the science of
GENETICS.
Gregor Mendel
• Gregor Mendel is often
referred to as the “father
of genetics”.
– He was a monk that
was born in Austria in
1822.
– He was experimenting
in garden with peas.
Through his experiments he discovered that
some genes were either dominant or
recessive.
• Dominant because it dominated or is
always shows itself (D, a capital letter)
• Recessive because it was hidden when the
dominant gene was present (d, lowercase)
Freckles
No Freckles
Dimples
No dimples
Tongue
roller
Non-roller
From Mom
From Dad
Genotype:
Genotype is the actual genes for a
particular trait expressed with letters
 Homozygous dominant (Purebred)

– TT - two capital letters

Heterozygous (hybrid)
– Tt - one of each letter (big and small)

Homozygous recessive (Purebred)
– tt - two lower case letters
Phenotype:

Phenotype is physical expression of the
genotype
Genotype
Phenotype
TT
tall plant
Tt
tall plant*
tt
short plant**
* dominant allele always is expressed over the
recessive allele
** recessive trait is expressed only if there is no
dominant gene
If R = red, and r = white

What is the genotype of homozygous
dominant?
– RR

What is the heterozygous genotype?
– Rr

What is the homozygous recessive
genotype?
– rr
If R=red and r =white

What is the phenotype of of RR?
– Red

What is the phenotype of Rr?
– Red

What is the phenotype of rr?
– White
Mendel’s Experiments led him to
3 Laws of Inheritance

The Law of Dominance

The Law of Segregation

The Law of Independent Assortment
Law of Segregation
– Segregation is the separation of alleles
during gamete formation.
– Since only one egg or one sperm will
contribute to the new offspring, only one
allele for a trait is passed on
– The chance that any allele will be passed
on is 50%
- Mendel’s Work
Segregation
Law of Independent Assortment

the inheritance of alleles for one trait
doesn’t affect the inheritance of alleles
for another trait
– accounts for the many genetic variation
observed in plants, animals and other
organism.
Predicting Traits
• A tool used to predict and compare the
genetic variations that will result from a
cross is a Punnett Square.
If two parents are crossed (TT X tt), what
are the resulting offspring?
You can determine phenotypes using a Punnet
square.
t
t
T
T
Tt
Tt
Tt
parent
Tt
parent
Genotypes: 4 Tt
Phenotypes: 4 Tall
This is the first or parent generation
Now try on your paper a cross of two of the
offspring from the parent generation. This is called
the first generation or the F1.
T
t
T
t
TT
Tt
Tt
tt
Genotypes: 1TT: 2Tt : 1tt
Phenotypes: 3 Tall: 1 Short.
Dihybrid Crosses

A two-factor cross follows two different
genes as they pass from one generation
to the next.
– Explains the Law of Independent
Assortment
Let’s Solve Together

In guinea pigs, the allele for short hair
(S) is dominant to long hair (s), and the
allele for black hair (B) is dominant over
the allele for brown hair (b). What is the
probable offspring phenotype ratio for a
cross involving two parents that are
heterozygotes for both traits?
Example 1: Dihybrid
Short hair = dominant = SS or Ss
 Long Hair = recessive = ss
 Black coat = dominant = BB or Bb
 Brown coat = recessive = bb
 SsBb x SsBb (gametes done by the
FOIL method)

– SB, Sb, sB, sb and SB, Sb, sB, sb
Example 1: Punnett Square
Parents: SsBb x SsBb
SB
Sb
sB
sb
SB SSBB
SSBb
SsBB
SsBb
Sb
SSBb
SSbb
SsBb
Ssbb
sB
SsBB
SsBb
ssBB
ssBb
sb
SsBb
Ssbb
ssBb
ssbb
Example 1: Answer the Question

What is the probable offspring
phenotype ratio for a cross involving two
parents that are heterozygotes for both
traits?
– 9/16 Black, short coats
– 3/16 Black, long coats
– 3/16 Brown, short coats
– 1/16 Brown, long coats

Incomplete Dominance – in which one
allele is not completely dominant over
another; a blending in the
heterozygous
= red
Incomplete Dominance RR
RW = pink
WW = white
RR x WW
R
W
W
R
RW
RW
RW
RW
What are the phenotypes of the offspring?
All the offspring will be Pink

Codominance – can see both alleles
at the same time.
Codominance
Feathers in chickens
FWFW – White Feather
FBFB – Black Feather
FWFB – Half Black Half White
FWFW xWFBFB
F
FB
FB
FW
FWFB
FWFB
FWFB
FWFB
What are the phenotypes of the offspring?
All of the offspring will be half black and half white
Sex-linked traits
Are traits that are controlled by the X or
Y chromosomes.
 Sex-linked disorders, which are
disorders caused by abnormal sex
chromosomes.

– Occurs more often in men than women.
• Most are recessive
– Two common disorders
• Colorblindness and hemophilia
Colorblindness Test
Hemophilia
A person’s blood clots very slowly or not at all.
 Danger from small bumps and cuts
 A recessive gene on the X-chromosome

– Occurs more in males than females
Multiple Alleles – three or more
alleles of the same gene.

Blood Types in Humans
– Single gene, but four phenotypes
•
•
•
•
Type A  can be IAIA or IAi
Type B  can be IBIB or IBi
Type AB  only IAIB (codominant pattern here)
Type O  only ii (both recessive)
– All 3 blood types are dominant to O
Practice Problem

Cross a person with AB blood with a
person with B blood. (2 crosses). What are
the possible blood types of the children?
Blood tranfusions
Universal
Donor
Universal
Acceptor
Polygenic Traits
Traits controlled by two or more genes
 Examples

– Eye color
– Skin color
– Hair color
- Human Genetic Disorders
A Pedigree
• A pedigree is a chart or “family tree” that
tracks which members of a family have
a particular trait.
Pedigree analysis
• Pedigree analysis reveals Mendelian patterns
in human inheritance; tracks certain traits
through a family
– data mapped on a family tree
= male
= female
= male w/ trait
= female w/ trait
Simple pedigree analysis
1
3
3
4
2
1
4
5
2
5
6
6
Genetic counseling
• Pedigree can help us understand the past
& predict the future
The importance of the
environment

An individual phenotype depends on
environment as well as on genes.
– Temperature
– Nutrition

The product of a genotype is generally
not single, rigidly defined phenotype but
a range of possibility influenced by the
environment.
Human Genetic Diseases
1
2
2006-2007
Human Genetics Disorders
• Genetic Disorder: is an abnormal condition
that a person inherits through genes or
chromosome.
– Caused by mutations in the DNA of genes, or
the overall structure and number of
chromosomes
Recessive disorders
• Your genotype is homozygous recessive (aa)
– Heterozygotes (Aa) have a normal phenotype
because one “normal” allele produces enough of the
required protein
– A carrier is a person who doesn’t have the disease
but carry the gene for the disease
Heterozygote crosses
• Heterozygotes as carriers of recessive alleles
Aa x Aa
A
female / eggs
male / sperm
A
a
A
a
AA
AA
Aa
Aa
Aa
a
carrier
Aa
Aa
aa
carrier
disease
A
Aa
a
Cystic Fibrosis
• The body produces
abnormally thick
mucus in the lungs
and intestines.
• This mucus makes it
hard to breathe
– Caused by a removal
of 3 bases from DNA
Tay-Sachs
• Primarily Jews of eastern European (Ashkenazi)
descent & Cajuns (Louisiana)
– non-functional enzyme fails to breakdown lipids in
brain cells
• fats collect in cells destroying their function
• symptoms begin few months
after birth
• seizures, blindness &
degeneration of muscle &
mental performance
• child usually dies before 5yo
Sickle-Cell Disease
• Affects hemoglobin, a protein in red blood
cells that carries oxygen
• Cells have an unusual sickle-shape
• They clog blood vessels, make it difficult for
blood flow
Huntington’s disease
• Dominant inheritance
– repeated mutation on end of
chromosome 4
– build up of “huntingtin” protein in brain
Testing…
Would you causing cell death
want to
know?
•
•
•
•
memory loss
muscle tremors, jerky movements
starts at age 30-50
early death
– 10-20 years after start
1872