Who is Gregor Mendel?
Introduction to Mendelian
genetics
TRUE OR FALSE?
1.
2.
3.
4.
5.
6.
7.
8.
9.
Girls inherit more traits from their mother than their father
You have inherited traits that are not apparent
Color blindness is more common in males than females
Identical twins are ALWAYS the same sex
A person can transmit genetic traits to their offspring which
they themselves DO NOT show
The father determines the sex of a child
The total number of male births exceeds female births each
year
Acquired characteristics, like mathematical skills, can be
inherited
Fraternal twins are more closely relates to each other than to
other siblings
Answers
1.
2.
3.
4.
5.
6.
7.
8.
9.
False
True
True
True
True
True
True
False
False
Genetics

The field of Biology devoted
to understanding how
characteristics are passed
from parents to offspring
Gregor Mendel

In the 19thcentury, Mendel studied
heredity-which is the transmission of
characteristics from parent to offspring
 Mendel is most famous for studying pea
plants
 He studied what he called “factors” in pea
plants
 Factors would be things like tall or short
(height), or yellow or green (pod color)
Some of Mendel’s Factors
Gregor Mendel

First, Mendel grew true-breeding plants
 According to Mendel, true-breeding
plants are plants that will always
produce offspring with the same
traits
 So a true-bred pea plant with purple
flowers will only produce plants with
purple flowers because it only has the
“factors” for purple (not white).
Gregor Mendel:
P generation

Mendel bred two opposite true-breeding
plants


For example, he bred a true-breeding purple
flower pea plant and a true-breeding white flower
pea plant
He called this his P generation – parent
generation
Gregor Mendel:
F1 generation

All of the offspring of the P generation
(which he called the F1 generation)
turned out purple

Mendel called purple flower color the
dominant factor

He hypothesized that when the dominant
factor was present, the recessive
factor(white color) did not show.
Gregor Mendel:
F2 generation



Next, Mendel crossed
the offspring from the
F1 generation (he called
this the F2
generation)
–He observed that
about 75% of the
flowers were purple and
about 25% were white
–This is equal to about
a 3:1 ratio
Mendel
P Generation(true-breeding parents)
Purple
flowers
F1 Generation(hybrids)
F2 Generation
All plants had
purple flowers
White flowers
Mendel’s Real Results
Mendel’s Laws

Keep in mind that Mendel knew nothing
of Punnett squares, genes, alleles, or
even DNA!!!


All he could do was observe phenotypes
and record ratios and other statistics
He came up with 2 important laws as a
result of his observations.
Mendel’s Laws:
Law of Segregation

Mendel concluded that each plant gets
two factors (alleles) for a characteristic
and when the plant reproduces, these
two factors separate or segregate.
So…


Each gamete (sex cell) gives one factor
(allele) AND therefore…
Each offspring gets one factor from each
parent
Law of Segregation:
Punnett Squares
Alleles
separate
Alleles
separate
Mendel’s Laws: Law of
Independent Assortment

Mendel did experiments using more
than one trait (like height and seed
color)


He noticed that one trait did not influence
the inheritance of another trait
In other words, different factors separate
independently of each other during the
formation of gametes
Mendel’s Laws: Law of
Independent Assortment

Examples:
 Pea plants can be short or tall
 Their seeds can be green or yellow
 Short plants can have green or yellow
seeds
 Tall plants can have green or yellow seeds
 So the inheritance of one does not affect
the inheritance of the other.
 Mendel noticed this with all the traits he
studied
Independent Assortment
Mendel’s Laws
Independent Assortment is not always
true•If different genes are located on the
same chromosome, then they will most
likely be inherited together
•These are called Linked Genes
What were Mendel’s
factors in reality?


We call these alleles today
–Alleles are alternative forms of a gene
 Alleles for flower color were purple and
white
 The characteristics (like height) are
caused by genes on DNA
 Genes are segments of DNA that code
for one protein
 Each gene has 2 alleles, or versions (1
from mom and one from dad)
What were Mendel’s
factors in reality?
The reason alleles come in pairs is because
chromosomes come in pairs (homologous
pairs)!!
•One allele on each chromosome!
•WHAT A COINCIDENCE!!!
The Genetics of Mendel’s
Experiments

Some Vocab
 Dominant trait-masks the recessive
 Shown with capital letters
 Recessive trait-only shows if
dominant is not present
 Shown with lower case letters
 Phenotype-physical appearance
 For example purple, wrinkled, tall, etc
The Genetics of Mendel’s
Experiments

Some Vocab

Genotype-genetic makeup


This is usually abbreviated with letters like Gg,
FF, or hh
Genotypes for a trait are usually2 letters
because you get 2 alleles (1 from mom and
1 from dad)


Homozygous-two of the same alleles (like
HH or hh)
Heterozygous-two different alleles (like
Hh)
The Genetics of Mendel’s
Experiments

Mendel’s P generation had the
genotypes FF (for purple) and ff
(for white)
 True breeding is also
homozygous
 FF is homozygous dominant
 ff is homozygous recessive
The Genetics of Mendel’s
Experiments


We can show the results Mendel
observed using a Punnett Square:
 A Punnett Square shows possible
genetic combinations in the zygotes
 Mendel crossed his true breeding
purple and white flower pea plants
 We write this as FF x ff
LET’S DO THIS ON THE BOARD
The Genetics of Mendel’s
Experiments

What Mendel did not know:
 All of F1 pea plant flowers heterozygous
(two different alleles), or Ff
 That is why they were all purple
 Remember dominant alleles mask
recessive alleles
 So with one purple allele present and
one white, only purple would show as it
is dominant
F2 generation

LET’S EXAMINE EACH RATIO FOR EACH
CROSS:

F2 Generation
 What genotypes do you start with?
 How are they crossed?
 What are your results?
 What is the genotypic ratio (genes)?
 What is the phenotypic ratio of purple
(F) to white (f)?
Punnett Square Examples
•Let’s do a Punnett square for BB x Bb
•B= black fur in bunnies
•b= white fur in bunnies
•Black fur is dominant
•What is the genotypic ratio?
•What is the phenotypic ratio?
•What are the chances for a white bunny?
Punnett Square Examples
•Let’s look at a heterozygous cross
•Bb x Bb
•What is the genotypic ratio?
•What is the phenotypic ratio?
•What are the chances for a white or
black bunny?
Predicting the Results of
Heredity

What do these ratios and percents mean?
 If we flip a coin, there is a 50% chance that it will
land on heads. But it is still possible to get 5 tails
in a row (although it is highly UNLIKELY!)
 The more times you flip it, the more likely your
results will be 50:50
 If Bb and Bb bunnies mate, there is a 1:4 chance
the offspring will be white (this does NOT mean
that they will or will not have white bunnies)
 If they have LOTS of children, about 25% of them
will be white
REMEMBER…



Homozygous dominant means 2 BIG
letters
Heterozygous means one big one little
Homozygous recessive means 2 little
letters
 If an organism shows the dominant trait,
then the can be either heterozygous OR
homozygous dominant
Test Cross

When genotypes are not known, a test
cross can be performed to figure it
out


The organism with an unknown genotype
is crossed with a homozygous recessive
individual.
Test crosses are often used in breeding
(like dog breeding) to determine is
organisms are really “pure bred”
(homozygous) for desired characteristics
Test Cross

Problem:


Let’s say you want to breed black bunnies
and you do not want any white bunnies
What would be the only parents’ genotypes
to produce black bunnies?


BB x BB
There are 2 ways to know for sure which
black bunnies are homozygous and which
are heterozygous: expensive genetic
testing, or test crosses
Test Cross

Solution:
 We take some black bunnies and
mate them with white bunnies
(homozygous recessive)
 Let’s look at the Punnett Square
results to see the possible results
 Remember, black bunnies can be
either BB or Bb
Test Cross – Punnett Squares
(try each cross)




If a BB is crossed with bb, no white
bunnies are produced
If a Bb is crossed with bb, then white
bunnies may be produced
If a test cross produces white bunnies,
we know the unknown genotype is Bb;
if not the genotype is BB
The cross would be performed multiple
times to be sure of the results
Predicting Dihybrid Crosses

When 2 traits are being looked at…
 Let’s do a cross between two
heterozygous tall, heterozygous
purple flowered pea plants
 So, TtFfx TtFf

For each plant, we now look at
genotype for color and height
Predicting Dihybrid Crosses



Instead of 2 possible gametes, there will be 4
 So, the Punnett Square will be 4 x 4
Phenotypic Ratios
 Tall, purple : tall, white : short, purple : short,
white
 Keep same letters together, capitals 1st
 You will not be asked for genotypic ratios for
dihybrid crosses
 What are the phenotypic ratios?
LET’S DO IT ON THE BOARD
Complex Inheritance

Mendel observed
monogenic traits and no
linked genes…It’s not
usually that simple….
Other Types of Inheritance

Incomplete Dominance



The phenotype of the heterozygote is intermediate
between phenotypes of the dominant and
recessive traits
Example: when a homozygous red carnation is
crossed with a homozygous white carnations, then
pink carnations are produced
We usually don’t use lower case letters in this type
of inheritance because nothing is really dominant
Incomplete Dominance
•RR = Red
•RW= pink
•WW= white
Let’s look
at the
cross on
the board
Other Types of Inheritance

Codominance
 Occurs when both alleles for a
trait are expressed in
heterozygous offspring
 Codominant alleles are often
symbolized with different letters
Codominance




BB = Brown
BW= Roan
WW= White
Notice both brown and white are
present in the heterozygous genotype
Codominance


LET’S EXAMINE THE PUNNETT SQUARE
ON THE BOARD
Roan x Roan


BW x BW
What are the ratios for each
phenotype?
Other Types of Inheritance

Multiple Alleles:
 Genes with 3 or more alleles
(or variations)
 Human blood type shows
codominance and it has
multiple alleles-A, B, and O
Blood Type

Human blood types have 3 alleles A, B, and O.
 Each person still only gets 2 alleles, but there are
3 possibilities
 O is recessive to A and B,
 A and B are codominant:
 Genotype AO or AA = A blood
 Genotype BO or BB = B blood
 Genotype OO = O blood
 Genotype AB = AB blood (both alleles expressed)
Blood Type
Terminology






Heterozygous B
Heterozygous A
Homozygous recessive
Homozygous A
Homozygous B
AB(technically heterozygous)
Genotype
BO
AO
OO
AA
BB
AB
Codominance Punnett Square


LET’S EXAMINE THE PUNNETT SQUARE
ON THE BOARD
Heterozygous A with Heterozygous B


AO x BO
What are the ratios for each
phenotype?
Other Types of Inheritance

Sex-Linked Genes and Traits



Remember sex chromosomes are the
chromosomes that determine the sex of an
organism
So these are traits/genes carried on sex
chromosomes
These traits are symbolized using a
superscript on the X or Y, such as Xr or XR
Other Types of Inheritance

Sex-Linked Genes and Traits Examples:
 In fruit flies, the gene for eye color is on the X
chromosome. Red (XR) is dominant, white (Xr) is
recessive.
 To have white eyes, females must have the
genotype XrXr, or in other words TWO white
alleles
 To have white eyes, males must have the
genotype XrY, or in other words ONE white allele
 This is why X chromosome sex-linked traits are
more common in males
Try the Punnett Square


Homozygous red eyed female x
white eyed male
R R x XrY
X X
What are the ratios for each
phenotype?
Other Types of Inheritance

Polygenic Inheritance:
 Traits that are controlled by
more than one gene
 Most human traits are polygenic
 Examples are height, skin color,
eye color, and hair color
Other Types of Inheritance

Complex Characters:
 Characters that are influenced by
genetics AND the environment
 Skin color and height are
examples
Other Types of Inheritance

Sex-Influenced Traits:



Traits in which males and females show
different phenotypes even though they
have the same genotypes
Baldness is an example- it is dominant in
men, but recessive in women
The differences are mainly due to males
and females producing different hormones
(chemical signals)
Other Types of Inheritance

Single Allele Traits
 Traits where there is only one
allele
 If you have the allele you have
the trait-there is no recessive
 Huntington’s disease is an
example
Pedigrees
Another way to
show heredity….

Pedigree is a chart or “family tree” that
tracks which members of a family have a
particular trait.
Pedigrees

In pedigrees, carriers have one copy
of the recessive allele
 So they CARRY the trait, but they do
not show it
 Pedigrees can be used to make
predictions about
 Future offspring
 The genotype of individuals in the
pedigree
Pedigrees

The first pedigree tracks the widow’s
peak, so the filled in shapes have a
widow’s peak
 Widow’s peak is a dominant trait
 Carriers are not always shown on
pedigrees
 Think about what alleles their parents
can give them
First generation(grandparents)
Ww
ww
ww
Ww
Second generation(parents
plus aunts and uncles)
Ww
ww ww Ww
WW
Ww
ww
or
ww
Third
generation
(two sisters)
Ww
Dominant trait (widow’s peak)
ww= no widow’s peak
WW= widow’s peak
Chromosome Mutations




Chromosome mutations
involve changes in the
structure of a chromosome
or the loss or gain of a
chromosome.
–Deletion: The loss of a
piece of chromosome due to
breakage
–Inversion: A chromosomal
segment breaks off, flips
around, and reattaches
-Missence: A change in
chromosomal arrangement
by insertion of DNA
segment
Translocation-A piece of chromosome breaks off
and reattached to a nonhomologous chromosome
Chromosome Mutations


Nondisjunction-When a
chromosome fails to detach from
its homologue during meiosis, so
one gamete gets an extra
chromosome
Instead of a haploid number (n) or
diploid (2n), the gamete has 3
chromosomes (3n)
Chromosome Mutations
(Examples)


Down’s syndrome
 Nondisjunction of chromosome 21 in the egg cell
produces 3 copies of chromosome 21
 Symptoms include: heart defects, stunted growth,
mental retardation
Cystic fibrosis
 Can be caused by several mutations on
chromosome 7 (insertion, missence)
 Symptoms include: problems with respiratory and
digestive systems
Chromosome Mutations
(Examples)


Klinefelter’s syndrome
 A male receives an extra X chromosome (XXY)
because of nondisjunction of egg cell
 Symptoms: Boyish, rounded look (despite age)
and often infertility
Trisomy 18 (Edward’s syndrome)
 Nondisjunction of chromosome 18 results in 3
copies of chromosome 18
 Much more severe problems than Down’s
syndrome. Only 10% of births survive to their
first birthday