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Genetics
Mr. Cobb
Biology
Genetics


Genetics is the study of heredity.
It deals with the transmission of
traits or characteristics from one
generation to the next.
History of Genetics


Little is known about when
humans first realized the
importance of genetics and
inheritance.
The breeding of cattle, horses, and
dogs began around 8000 B.C.
History of Genetics


Plants such as corn, wheat, and
rice were cultivated in Mexico
around 5000 B.C.
Around 500 B.C. Pythagoras (a
Greek philosopher) stated that
human life began with male and
female fluids.
History of Genetics

Aristotle continued this idea when
he suggested that these fluids (or
semens) were actually purified
blood, and therefore that blood
must be important to heredity.
Genetic Theories

Over the years, scientists and
philosophers suggested various
theories to explain how
characteristics were passed from
parent to child.
Homunculus Theory


The theory of
Homunculus states
that sex cells
contained a complete,
but miniature adult, in
human form.
This theory was
popular until the 18th
century.
Blending Theory


In the 1800’s, scientists suggested
that the Blending Hypothesis
better explained heredity.
They stated that genetic material
from both the mother and the
father were blended to produce
offspring.
Blending Theory

To support their Blending Theory,
scientists looked at flowers:

Parent:
Red flower+Yellow Flower
Offspring:
Orange Flower
Blending Theory




Parent:
Tall Plant + Short Plant
Offspring:
Medium Plant
Parent:
Blue Bird + Yellow Bird
Offspring:
Green bird
Blending Theory

If the blending theory were
correct, then eventually all
extreme characteristics would
disappear from the population.
Gregor Mendel


Gregor Mendel
was an Austrian
monk, teacher,
and
mathemetician.
He is considered
to be the “father
of genetics”
Gregor Mendel



In the 1800’s he worked with garden
peas to determine a pattern of heredity
He bred different types of pea plants
to determine how characteristics were
spread from one generation to the
next.
Characteristics that are passed on are
called traits.
Alleles



Alleles are alternative forms of a
gene that determines a trait.
Genes encode for different traits
(or characteristics).
Ex: Tall, short
Alleles




Alleles can either be dominant or
recessive.
Dominant alleles “overshadow”
recessive alleles.
Uppercase letters are used to show
dominant traits.
Lowercase letters are used to show
recessive traits.
Alleles


Ex: Tall trait is dominant to short
trait.
Tall = T, short = t
Alleles are expressed in pairs.
Ex: TT, Tt, tt
Definitions




Phenotype – Physical appearance
Genotype – Genetic makeup
Dominant – easily observed
Recessive – often masked or
overshadowed.
Definitions


Homozygous – The two alleles for
a trait are the same (“homo”
means same). Ex: TT, tt
Heterozygous – The two alleles
for a trait are different (“hetero”
means different). Ex: Tt
Genotype vs. Phenotype



A plant is homozygous dominant
for plant height.
Genotype: TT
Phenotype: Tall
Genetics
Mr. Cobb
Biology
Gregor Mendel


Gregor Mendel
was an Austrian
monk, teacher,
and
mathemetician.
He is considered
to be the “father
of genetics”
Gregor Mendel

Mendel worked with garden peas
to determine how traits were
passed on.
Gregor Mendel


Mendel used
peas because
they are fast
growing and
easily available.
Peas are also
capable of selffertilization.
Fertilization

In crossfertilization,
one plant is
fertilized by
another plant.
Definitions





Self-pollinated – pure bred plants, or
homozygous plants.
Cross-pollinated – hybrid plants, or
heterozygous plants.
P generation – Parent generation
F1 – first filial (generation)
F2 – second filial (generation)
Generations
Mendel’s Experiment

Mendel looked at seven pea traits:

Flower color, flower position, pea
color, pea shape, pod color, pod
shape, and plant height.
Mendel’s Experiment
Mendel’s Experiment


Mendel looked at pea plants that were
self-pollinated (pure bred, or
homozygous), and those that were
cross-pollinated (hybrids, or
heterozygous).
Mendel crossed a pure-bred purple
flower plant with a pure-bred white
flower plant. (the P generation)
Mendel’s Experiment


All F1 plants were purple, because
purple color is dominant to white.
Mendel crossed the F1 generation
by self-pollination. The resulting
F2 generation was ¾ purple plants
and ¼ white plants.
Probability


A probability is a fraction or ratio that
is used to predict that an event will
occur.
Ex: What is the probability of rolling
a dice and getting a five?



1/6
What is the probability of rolling two
fives?
1/6 X 1/6 = 1/36
Probability

What is the
probability
of flipping
a coin and
it landing
on heads?

½ or 50%
Punnett Square

A punnett square is a diagram that
can be used to show the possible
outcomes of a cross.
Monohybrid Cross


A monohybrid cross is done when
only one trait is being examined.
Ex: Trait to be examined is plant
height, so crossing a homozygous
tall plant with a homozygous short
plant.
Monohybrid Cross

Ex: Crossing a
pure dominant
tall plant with a
pure recessive
short plant.
Monohybrid Cross

Ex: Crossing two heterozygous black
(Bb) rabbits.
Test Cross

Test crosses are when trying to
determine the unknown genotype
of an organism. The unknown
organism is crossed with an
organism that shows the recessive
trait.
Test Cross
Crosses Involving Two Traits:
Dihybrid Crosses
Color: Yellow, Green
Shape: Round, Wrinkled
Yellow-Round
Yellow-Wrinkled
Green-Round
Green-Wrinkled
Dihybrid
cross
Using 2
different
traits
Genetics
Mr. Cobb
Biology
Law of Segregation

The law of segregation states that
alleles of a gene occur in pairs.
These pairs of alleles are separated
during meiosis and are
recombined during fertilization.
Law of Dominance

The law of dominance states that
when organisms pure for
contrasting traits are crossed, all
their offspring will show the
dominant trait.
Law of Independent Assortment

During meiosis, genes (alleles) for
different traits are separated and
distributed to gametes
independently of one another.
Mendel’s Laws
Mendel’s Laws

There are exceptions to Mendel’s
Laws.



Incomplete Dominance
Codominance
Multiple Alleles
Incomplete Dominance


In incomplete dominance, a
heterozygous genotype results in
an intermediate phenotype that is
not like either parent.
Ex: Snapdragons, carnations
Incomplete Dominance


Alleles for both traits are written
in capital letters. Ex: Flower
color
RR (red)X WW (white)
Codominance


Two dominant
alleles are
expressed at the
same time.
Ex: Cows
(Roan)
Codominance


Alleles are
written as capital
letters with
superscripts
CRCR (red)
CWCW (white)
CRCW (roan)
Multiple Alleles


More than two alleles for a trait
exist.
An individual cannot have more
than two alleles for a trait, but
more than 2 allele types may be
present in the population.
Blood Types

One example of multiple alleles is
blood types. There are several
possibilities of blood types:




A
B
AB
O
Blood Types






Blood type alleles are written as a
capital I with a superscript A or B, OR
as a lowercase i.
IA
IB
i (type O blood)
IA, IB – both dominant over i
IA, IB – codominant with each other.
Blood Types





Type A – IAIA or IAi
Type B – IBIB or Ibi
Type AB – IAIB
Type O – ii
Each blood group may be positive
or negative
Blood Types


Ex: A man homozygous for type
B blood marries a woman who is
heterozygous for type A blood.
What are the possible genotypes
and phenotypes of their children?
IBIB X Iai - Parents



Offspring will be…
Genotypes: IAIB, IBi
Phenotypes: AB and B
Blood Types

Ex: A couple has a child with
type O blood. If one parent is type
O, what are the possible genotypes
of the other parent?
O blood-type Baby, O Type
Parent…
A
B
 Genotypes: I i, ii, I i
Polygenetic Inheritance & Sexlinked Traits
Mr. Cobb
Biology
Polygenic Inheritance


Many characteristics are not
determined by one gene.
Some characteristics, like eye
color and skin color, are
determined by many genes.
Polygenic Inheritance

Multiple gene (or polygenic)
inheritance occurs when two or
more independent genes affect one
characteristic.
Polygenic Inheritance

In humans, eye
color is
determined by a
cluster of genes,
rather than by a
single gene.
Polygenic Inheritance

Ex: Instead of having completely
brown eyes, humans can have
brown eyes with green flecks. The
brown pigment comes from one
gene, and the green pigment
comes from a different gene.
Sex-Linked Traits



Humans have 23 pairs of
chromosomes (or 46 chromosomes).
22 Pairs of autosomal (or non-sex
chromosomes)
One pair of sex chromosomes: X Y
Sex-Linked Traits



Females have two X
chromosomes, while males have
one X and one Y.
Females: XX
Males: XY
Other Genes…


Linked Gene – these genes are
found on the same chromosome.
Non-disjuction – during meiosis
there is separation error that
causes the gene to “read”
incorrectly.
Karyotype
Sex-Linked Traits


Sex linked traits are traits that are
controlled by a gene located on a
sex chromosome.
Most sex-linked traits are found on
the X chromosome.
Sex-Linked Traits


Sex-linked traits are written as an
X with a superscript letter.
Ex: Baldness is a sex-linked trait.
Baldness = B, so the trait would be
written as XB.
Sex-Linked Traits

Sex-linked genes may be
transmitted from:



Mother or father to daughter
Mother to son
NOT father to son
Sex-Linked Traits


Inheritance of sex-linked traits is
different from the inheritance of
autosomal (non-sex linked) traits.
With autosomal genes, traits can
be inherited from either father or
mother, to either a son or a
daughter.
Sex Determination

Female (XX) x Male (XY)
Sex Determination


So which parent determines the
sex of the child?
Which is more significant, the X
chromosome, or the Y
chromosome?
Sex Determination



The Y chromosome is necessary for
male sexual characteristics to develop.
Offspring without a Y chromosome
develop into females. Offspring with
Y chromosomes develop into males.
Ex: Turner’s Syndrome (X) vs.
Klinefelter’s Syndrome (XXY)
Genetic Diseases


Diseases may be sex-linked
(caused by genes carried on the X
chromosome), or autosomal
(caused by genes NOT on the Xchromosome).
Ex of Autosomal Diseases: Cystic
Fibrosis, Sickle Cell Anemia
Sex-Linked Traits in Humans


Many human diseases are caused
by abnormal alleles on the X
chromosome.
These diseases are more often
observed in males than females.
Sex-linked Diseases


Diseases that are caused by
abnormal alleles on the X
chromosome include:
Color blindness, hemophilia,
Duchenne’s Muscular Dystrophy
Color Blindness
Color Blindness (Sex-Linked)





X X = Normal
XCXC = Color Blind
XCX = Normal, Carrier
X Y = Normal
XCY = Color Blind
Color Blindness Example


A woman with normal vision, but
carries a gene for color blindness,
marries a man with normal vision.
Describe their expected offspring.
Pedigrees



A pedigree is a diagram that shows
relationships between people and
generations.
Females are symbolized by circles,
males are symbolized by squares.
Lines (vertical or horizontal) show
genetic relationships.
Pedigrees

This pedigree shows a male and
female. The horizontal line shows
they are married.
Pedigrees

Vertical lines show the offspring.
Pedigrees
Pedigrees
Pedigrees


Pedigrees can also show which
individuals are normal, which are
carriers, and which have a certain
disorder.
Pedigrees are very helpful in
seeing sex-linked diseases.
Pedigrees
Pedigrees
Pedigrees
Pedigrees


With sex-linked disorders, there
are usually NO male carriers.
Males are either normal, or have
the disorder.
So, when looking at a pedigree, if
you see male carriers, you know
the disorder is NOT sex-linked.
Disorders caused by Genetics

Remember, these can occur in
anyone and we are lucky to be so
‘normal’.
Huntington’s Brain- illustrating
marked striatal atrophy. Caused by a
mutation on the 4th chromosome.
Trisomy 21 – extra
chromosome
on the 21 chromosome
Downs Syndrome:
An infant with Down syndrome, illustrating typical
features of this disorder: upslanting palpebral fissures,
redundant skin of the inner eyelid (epicanthic fold),
protruding tongue, and low nasal bridge.
Marfan
Syndrome:
Marfan syndrome
is an inherited
connective-tissue
disorder
transmitted as an
autosomal
dominant trait.
A young man with Marfan syndrome,
showing characteristically long limbs
and narrow face.
Marfans: Note the
hands
The hand at the left is
that of a young woman
with Marfan's
syndrome, while the
hand at the right is a
normal male. Both
persons were of the
same height, 188 cm.
However, note that the
hand at the left
demonstrates
arachnodactyly.