Download Introduction to Genetics

Introduction to
Genetics
Genetics = the scientific study of heredity
The Work of Gregor Mendel
The Father of Genetics
Austrian monk
Studied heredity
– Heredity: the passing on of
characteristics from parents to offspring
– Characteristics that are inherited are called
traits.
First person to successfully predict how traits
are transferred from generation to generation
Used garden peas in his experiments
Why pea plants?
– Reproduce sexually, which means that
they produce male and female sex cells,
called gametes.
– In a process called fertilization, the male
gamete unites with the female gamete.
– The resulting fertilized cell, called a zygote,
then develops into a seed.
Mendel’s Experiment
He took pollen from a male plant and dusted it
onto a female plant.
– Parental generation (p) = the original pair of
plants
Pollen
grains
Transfer
pollen
Female
part
Male
parts
Cross-Pollination
• Offspring
• (f1) = first filial
generation
• (f2) = second filial
generation
His first experiments are called
monohybrid crosses because they only
deal with ONE single trait (height, color)
mono means “one”
Conclusions
Each organism has two
factors that control each
of its traits.
These factors are genes
and that they are located
on chromosomes.
Genes exist in different
forms called alleles.
P1
Short pea plant
Tall pea plant
F1
All tall pea plants
F2
3 tall: 1 short
Mendel called the
observed trait
dominant and the trait
that disappeared
recessive.
Mendel concluded that
the allele for tall plants is
dominant to the allele for
short plants.
Short plant
Tall plant
t
T T
t
t
T
F1
All tall plants
T t
Law of Segregation
Tt xTt cross
F1
Tall plant
Tall plant
T
T
t
t
F2
Tall
T T
Tall
T
3
t
Tall
T
t
Short
t t
1
• The way an organism
looks and behaves is
called its phenotype.
Ex. Tall, yellow
• The allele combination
an organism contains
is known as its
genotype. Ex. TT, Tt
• An organism’s
genotype can’t always
be determined by its
phenotype.
An organism is homozygous for a trait
if its two alleles for the trait are the
same. (True-breeding)
– Exp. TT or tt
An organism is heterozygous for a trait
if its two alleles for the trait differ from
each other. (Hybrid)
– Exp. Tt
Genetics & Probability
Probability = the likelihood that a particular event will
occur
– Ex. Coin flipping: 1/2 probability that coin will flip head/tail
– If you flip the coin 3 times what's the probability of flipping
3 heads?
1/2 x 1/2 x 1/2 = 1/8
– *Past outcomes do not affect future ones!!*
The principles of probability can be used to predict the
outcomes of genetic crosses.
– What is the probability of parents having two
male offspring in a row? (1/2 x 1/2=1/4)
Punnett Squares
Are used to predict and compare the
genetic variations that will result from a
cross.
– The types of gametes go on the top and
left sides of the square
– The possible gene combinations appear in
the four boxes
If you know the genotypes of the parents, you
can use a Punnett square to predict the
possible genotypes of their offspring.
T
t
T
TT
Tt
t
Tt
tt
You try this one…
A
A
a
A
Check Your Understanding
Question 1
The passing on of characteristics from parents to
offspring is __________.
A. genetics
B. heredity
C. pollination
D. allelic frequency
The answer is B.
Genetics is the branch of biology that studies heredity.
Check Your Understanding
Question 2
What are traits?
Answer
Traits are characteristics that are inherited. Height, hair color and eye
color are examples of traits in humans.
Check Your Understanding
Question 3
Gametes are __________.
A. male sex cells
B. female sex cells
C. both male and female sex cells
D. fertilized cells that develop into adult organisms
The answer is C.
Organisms that reproduce sexually produce male and female sex cells,
called gametes.
Check Your Understanding
Question 4
Which of the following genotypes represents a
animal that is homozygous dominant for a trait?
a. KK
b. Kk
c. kk
Check Your Understanding
Question 5
Which of the following genotypes represents a
plant that is homozygous recessive for height?
a. TT
b. Tt
c. tt
Mendel’s Dihybrid Crosses
Dihybrid Cross
round yellow x wrinkled green
P1
Round yellow
Wrinkled
green
All
round yellow
F1
F2
9
Round yellow
3
Round green
3
Wrinkled yellow
1
Wrinkled green
Law of Independent Assortment
• He found they appeared in a definite ratio
of phenotypes 9:3:3:1
•
9 round yellow: 3 round green: 3 wrinkled
yellow: 1 wrinkled green.
• He concluded that the seed shape and seed
color—are inherited independently of each
other.
• This conclusion is known as the law of
independent assortment.
Dihybrid Crosses
Gametes from RrYy parent
Ry
RY
rY
ry
RRYY
RRYy
RrYY
RrYy
RRYy
RRYy
RrYy
Rryy
RrYY
RrYy
rrYY
rrYy
RrYy
Rryy
rrYy
rryy
Gametes from RrYy parent
RY
Ry
rY
ry
A Punnett square
for a dihybrid
cross will need to
be four boxes on
each side for a
total of 16 boxes.
Exceptions to Mendel
Not all genes show simple patterns of
dominant and recessive alleles.
– Because the majority of traits are
controlled by more then one
gene/alleles.
Incomplete Dominance
Cases in which one
allele is not
completely
dominant over
another.
Neither allele is
dominant
– Ex. Red flower x
White flower =
Pink flower
Codominance
Cases in which both alleles are expressed.
– Ex. White cow x Black cow = Speckled (white and
black) cow
Cow-1. au
Multiple Alleles
When a gene has
more than two alleles
•
Ex. Mice coat color is
determined by one
gene with four
different alleles.
Three different colors
result from the
combinations of these
alleles.
Blood Types
Multiple Alleles & Codominance
In humans, there are four blood
types (phenotypes): A, B, AB,
and O
Blood type is controlled by three
alleles. A, B, O
O is recessive, two O alleles must
be present for the person to
have type O blood
A and B are codominant. If a
person receives an A allele and a
B allele, their blood type is type
AB
Crosses involving blood type
often use an I to denote the
alleles - see chart.
Type O's are automatically OO and type AB is
automatically AB. Type A and B can be
homozygous or heterozygous.
Polygenic Traits
Traits controlled by two or more genes
– Ex. At least three genes are involved in
making the reddish-brown pigment in the
eyes of fruit flies. Different combinations
of alleles for these genes yield different
eye colors.
Sex Linked
Some genes are located on the X chromosome. Females receive two
alleles (XX) for these genes, but males only receive one (y).
When doing a punnet square, use large X's and Y's to denote male
and female, use superscript letters to designate the alleles.
– Ex. hemophilia (bleeding) and color blindness are recessive sex-linked
traits (XcXc or XcY)
If the parent is a male, the genotype is automatically known. A
colorblind male has to be b (recessive), since he only has one allele
and colorblindness is recessive. A normal male must then be B
(dominate)
Females can be heterozygous for the colorblindness trait - they are
called carriers. A female can be XBXB - normal, XBXb - carrier, or XbXb
- colorblind
The following
shows a cross
between a normal
man and a woman
who is a carrier.
What…
MORE
practice
problems?
Homer_imeanwoohoo.mp3
Check Your Understanding
Question 1
In a certain cactus, prickly spines
can be two pronged or one
pronged. If a true breeding
(homozygous) one-pronged cactus
is crossed with a true breeding twopronged cactus, the F1 generation
has a mixture of spines, some are
two-pronged, some are onepronged.
Is this an example of codominance or incomplete dominance?
codominance
Check Your Understanding
Question 2
A nurse at a hospital removed the wrist tags of three babies in
the maternity ward. She needs to figure out which baby belongs
to which parents, so she checks their blood types. Using the
chart below, match the baby to its correct parents.
Parents
Blood Types
Baby
Blood type
Mr. Hartzel
O
Jennifer
O
Mrs. Hartzel
A
Rebecca
A
Mr. Simon
AB
Holly
B
Mrs. Simon
AB
Mr. Peach
O
Mrs. Peach
O