Download Mendel’s Laws and Punnett Square Notes

FUNDAMENTALS OF GENETICS
WEDNESDAYS WARMUP?!??
Humans inherit a set of chromosomes
from each parent, yet the
chromosomes are not identical to the
same chromosomes in either parent.
How can it be that we inherit
chromosomes from our parents, but
they’re NOT identical to any of our
parents’ chromosomes?
INHERITANCE & GENETICS
Every living thing –
plant or animal,
microbe or human
being – has a set of
characteristics
inherited from its
parent or parents.
 Genetics – Study of
heredity

CHROMOSOMES & GENES
Chromosome - Very
long, continuous single
piece of DNA, contains
many genes
 Gene - Sequence of
DNA that codes for a
protein and thus
determines a trait

WHAT IS AN ALLELE?

Alleles are the different
possibilities for a given
trait.
 Every
trait has at least
two alleles (one from the
mother and one from the
father)
 Example: Eye color –
Brown, blue, green, hazel
Examples of Alleles:
A = Brown Eyes
a = Blue Eyes
B = Green Eyes
b = Hazel Eyes
HOMOLOGOUS CHROMOSOMES

Homologous Chromosomes - Term
used to refer to chromosomes that
each have a corresponding
chromosome from the opposite-sex
parent.

Both chromosomes have all the same
genes in the same location, but
different ‘versions’ of those genes.
HAPLOID VS. DIPLOID
Haploid - Term used to refer
to a cell that contains only a
single set of chromosomes and therefore only a
single set of genes; “one set”; represented by
N.
 Diploid - Term used to refer to a cell that
contains both sets of homologous
chromosomes; “two sets”; represented by 2N.

GAMETE VS. ZYGOTE

Gamete - A mature sexual reproductive cell that
has a haploid numbers that unites with another
cell to form a new organism.
 Example:

Sperm or egg cell
Zygote - The cell formed by the union of two
gametes.
Sexual Reproduction - Process by which two cells from
different parents unite to produce the first cell of a new
organism.
Haploid
sperm
(gamete)
Haploid
egg
(gamete)
Diploid
zygote
1n
1n
2n
+
=
Remember, in SEXUAL
reproduction…
…to form a
…which
HAPLOID
grows
gametes
into a
DIPLOID
DIPLOID
join…fetus
zygote
SEXUAL REPRODUCTION

Sexual Reproduction – Process by which two
cells from different parents unite to produce
the first cell of a new organism.
MEIOSIS

Meiosis - Process by which the number of
chromosomes per cell is cut in half through the
separation of homologous chromosomes in a
diploid cell; Haploid (N) gamete cells are
produced from diploid (2N) cells.
Begin with Diploid
(2N) Cells
Produce
Haploid
(N) Cells
FOUR GAMETES ARE MADE DURING CELL DIVISION BY
MEIOSIS. THE GAMETE CELLS HAVE HALF THE NUMBER OF
CHROMOSOMES AS THE ORIGINAL CELL.
MEIOSIS

Meiosis usually involves two distinct divisions:
1.
2.
Meiosis 1
Meiosis 2
MEIOSIS 1
Prior to meiosis I, each
chromosome is replicated.
 The cells then begin to
divide in a way that looks
similar to mitosis.
 One big difference:


Each chromosome pairs with
its corresponding
homologous chromosome to
form a structure called a
tetrad and exchange portions
of their chromatids in a
process called crossing-over.
MEIOSIS 1

Tetrad - Structure
containing 4 chromatids
that forms during
meiosis

Crossing Over - Process
in which homologous
chromosomes exchange
portions of their
chromatids or alleles.
CROSSING OVER
Produces new combinations
of alleles.
 Meiosis 1 is similar to mitosis
except:

 The
two cells produced by meiosis I have sets of
chromosomes and alleles that are different from
each other and from the diploid cell that entered
meiosis I.
MEIOSIS 2
Two cells produced by meiosis I now enter a
second meiotic division.
 Unlike the first division, neither cell goes
through a round of chromosome replication
before entering meiosis II.
 Those four daughter cells now contain the
haploid number (N)—just 2 chromosomes each.

GAMETE FORMATION
In male animals, the haploid
gametes produced by meiosis are
called sperm.
 In female animals, generally only
one of the cells produced by
meiosis is involved in reproduction. This female
gamete is called an egg in animals.


In females, cell divisions at the end of meiosis I and
meiosis II are uneven.
A
single cell, which becomes an egg, receives most of the
cytoplasm.
 The other three cells are known as polar bodies and usually do
not participate in reproduction.
NONDISJUNCTION

The most common error
in meiosis occurs when
homologous chromosomes fail to separate.
 This
is known as nondisjunction.
 Results in abnormal numbers of chromosomes in
gametes.

Example - Down syndrome,
which results when an individual
has three copies of chromosome
21.
MITOSIS VS. MEIOSIS
Mitosis: Produces two genetically identical
diploid cells.
 Meiosis: Produces four genetically different
haploid cells.

MITOSIS VS. MEIOSIS

Mitosis





Allows an organism's body to grow and replace cells.
Used in asexual reproduction to produce a new organism.
New (daughter) cell is identical to the parent cell and to
each other.
Produces two diploid (2N) daughter cells.
Meiosis




Used in sexual reproduction to produce gametes.
New (daughter) cells are genetically different from the
parent cells and from one another.
Produces four haploid (N) cells.
Is responsible for the genetic variation among species.
INHERITANCE & CELL TYPE
You can only inherit a trait from
gametes, not other somatic (body) cells!
 Mutations within somatic (body) cells do not
affect future offspring genes. Whereas,
mutations within gametes do alter offspring
genes.
 For example, if your mother has skin cancer,
you will not inherit this mutation because the
mutation is on her somatic (body) cells and
these are not inherited.


Formation of twins video clip:


http://www.pennmedicine.org/encyclopedia/e
m_DisplayAnimation.aspx?gcid=000058&ptid=
17
GREGOR MENDEL




Mendel carried out his work with
ordinary garden peas.
Mendel studied 7 different pea
plant traits such as seed color or
plant height.
Trait - Specific characteristic
that varies from one individual to
another.
Each of the 7 traits Mendel
studied had two contrasting
characters.

For example, green seed color
and yellow seed color.
GREGOR MENDEL’S EXPERIMENTS

Mendel crossed plants
with each of the seven
contrasting characters
and studied their
offspring.
Parental or P Generation Each original pair of plants
 F1 or First Generation –
The first set of offspring
from the parents

GREGOR MENDEL’S FIRST SET OF EXPERIMENTS
1.
Mendel crossed a tall
and short plant and all
offspring were tall.


All of the offspring had
the character of only
one of the parents.
Mendel concluded that
some alleles are
dominant and others
are recessive (called the
Principle of Dominance)
GREGOR MENDEL’S SECOND SET OF EXPERIMENTS
2.
He then crossed the F1
generation with itself to
produce the F2 offspring
(Tall x Tall)


Some of the traits had
reappeared - some were tall
and some were short.
The Law of Segregation was
concluded (the two alleles
segregate from each other
so that each gamete carries
only a single copy of each
gene).
MENDEL'S LATER EXPERIMENTS: TWO-FACTOR
CROSSES
1.
Crossed round, yellow pea
plants with wrinkled, green
pea plants
 All
offspring were round yellow
peas
2.
He then crossed those
offspring (F1) together to
produce the F2 generation
and the offspring were a
mixture of all traits.
MENDEL'S CONCLUSIONS FROM HIS TWOFACTOR CROSSES

Mendel concluded that genes
for different traits can segregate
independently during the
formation of gametes (called
the Law of Independent
Assortment)
These genes that segregate
independently do not influence
each other’s inheritance.
 Helps account for the many
genetic variations observed in
plants, animals, and other
organisms.

DOMINANT VS. RECESSIVE

Dominant - Masks the other trait; the trait that
shows if present
 Represented

by a capital letter
R
Recessive – An organism with a recessive allele
for a particular trait will only exhibit that trait
when the dominant allele is not present; Will only
show if both alleles are present
 Represented
by a lower case letter
r
DOMINANT & RECESSIVE PRACTICE
T – straight hair
t - curly hair



TT - Represent offspring with straight hair
Tt - Represent offspring with straight hair
tt - Represents offspring with curly hair
HOMOZYGOUS VS. HETEROZYGOUS

Homozygous – Term used to refer
to an organism that has two
identical alleles for a particular trait
(TT or tt)


Sometimes called purebred
Heterozygous - Term used to refer
to an organism that has two
different alleles for the same trait
(Tt)

Sometimes called hybrid
RR
rr
Rr
GENOTYPE VS. PHENOTYPE


Genotype – The genetic makeup of an organism;
The gene (or allele) combination an organism has.
 Example: Tt, ss, GG, Ww
Phenotype – The physical characteristics of an
organism; The way an
organism looks
 Example: Curly hair,
straight hair, blue
eyes, tall, green
PUNNETT SQUARES
Punnett Square – Diagram showing the gene
combinations that might result from a
genetic cross
 Used to calculate the
probability of inheriting
a particular trait

 Probability
– The chance
that a given event will
occur
PUNNETT SQUARE
Parent
Parent
Offspring
HOW TO COMPLETE A PUNNETT SQUARE
Y-Yellow
y-white
Genotype:
1:2:1
(YY:Yy:yy)
25%, 50%, 25%
Phenotype:
3 Yellow, 75%
1 White, 25%
YOU TRY IT NOW!

Give the genotype and phenotype for the
following cross: TT x tt (T = Tall and t = Short)
TT X TT
Step One: Set Up Punnett Square (put one parent on
the top and the other along the side)
T
t
t
T
TT X TT
Step Two: Complete the Punnett Square
T
t
t
T
Tt
Tt
Tt
Tt
TT X TT
Step Three: Write the genotype and phenotype
T
t
t
T
Tt
Tt
Tt
Tt
Remember: Each box is 25%
Genotype:
4 – Tt or 100%
Phenotype:
100% Tall
YOU TRY IT NOW!

Give the genotype and phenotype for the
following cross: Tt x tt
TT X TT
Step One: Set Up Punnett Square (put one parent on
the top and the other along the side)
T
t
t
t
TT X TT
Step Two: Complete the Punnett Square
T
t
t
t
Tt
tt
Tt
tt
TT X TT
Step Two: Complete the Punnett Square
T
t
t
t
Tt
tt
Tt
tt
Remember: Each box is 25%
Genotype:
Tt - 2 (50%)
tt - 2 (50%)
Phenotype:
50% Tall
50% Short
INCOMPLETE DOMINANCE

Incomplete Dominance - Situation in which
one allele is not completely dominant over
another.
 Example
– Red and white flowers are crossed
and pink flowers are produced.
CODOMINANCE

Codominance - Situation in which both alleles
of a gene contribute to the phenotype of the
organism.


Example – A solid white bull is crossed with a solid
brown cow and the resulting offspring are spotted
brown and white (called roan).
+
MULTIPLE ALLELES

Multiple Alleles- Three or more alleles of
the same gene.
 Even
though three or more alleles exist for a
particular trait, an individual can only have two
alleles - one from the mother and one from the
father.
EXAMPLES OF MULTIPLE ALLELES
1.
Coat color in rabbits is
determined by a single
gene that has at least
four different alleles.
Different combinations
of alleles result in the
four colors you see
here.
EXAMPLES OF MULTIPLE ALLELES
2.
3.
Blood Type – 3 alleles
exist (IA, IB, and i),
which results in four
different possible blood
types
Hair Color – Too many
alleles exist to count
 There
are over 20
different shades of
hair color.
MULTIPLE ALLELES

There Are Always Multiple Alleles!
Genetic inheritance is often presented with
straightforward examples involving only two alleles
with clear-cut dominance. This makes inheritance
patterns easy to see.
 But very few traits actually only have two alleles with
clear-cut dominance. As we learn more about
genetics, we have found that there are often hundreds
of alleles for any particular gene.

 We
probably know this already - as we look around at
other people, we see infinite variation.
POLYGENIC TRAIT

Polygenic Trait - Trait
controlled by two or more
genes.
Polygenic traits often show a
wide range of phenotypes.
 Example: The wide range of
skin color in humans comes
about partly because more
than four different genes
probably control this trait.
 Human eye color

DIHYBRID CROSSES
Let’s cross a homozygous tall (TT), homozygous
yellow seed (YY) plant with a short (tt), green
seed (yy) plant. Tall is dominant to short and
green is dominant to yellow.
TTYY x ttyy
These are the genotypes of the two plants.
INDEPENDENT ASSORTMENT
Mendels’ principle of Independent Assortment
states that genes for different traits can
segregate independently during the formation
of gametes (eggs & sperm in animals, eggs and
pollen in plants). We must first get all the
possibilities of gametes for each parent!
First T
TTYY
TY
Gamete 1 = sperm, egg, pollen . . .
with first Y
INDEPENDENT ASSORTMENT
Mendels’ principle of Independent Assortment
states that genes for different traits can
segregate independently during the formation
of gametes (eggs & sperm in animals, eggs and
pollen in plants). We must first get all the
possibilities of gametes for each parent!
First T
TY
Gamete 1
TTYY
TY
Gamete 2
with second Y
INDEPENDENT ASSORTMENT
Mendels’ principle of Independent Assortment
states that genes for different traits can
segregate independently during the formation
of gametes (eggs & sperm in animals, eggs and
pollen in plants). We must first get all the
possibilities of gametes for each parent!
Second T
TY
Gamete 1
TY
Gamete 2
TTYY
with first Y
T Y
Gamete 3
INDEPENDENT ASSORTMENT
Mendels’ principle of Independent Assortment
states that genes for different traits can
segregate independently during the formation
of gametes (eggs & sperm in animals, eggs and
pollen in plants). We must first get all the
possibilities of gametes for each parent!
Second T
TY
Gamete 1
TY
Gamete 2
TTYY
with second Y
TY
Gamete 3
T Y
Gamete 4
DIHYBRID PUNNETT SQUARE
Put all the possible combinations of alleles for Parent 1 along the top
P1 = TTYY
TY
P2 = ttyy
Put all the
possible
combos of
alleles for
Parent 2
along the
side
TY
TY
ty
ty
ty
ty
Will be F1 Generation
TY
DIHYBRID PUNNETT SQUARE
Complete the punnett square.
TY
ty
TTYy
ty
TTYy
ty
TTYy
ty
TTYy
TY
TY
TY
DIHYBRID PUNNETT SQUARE
TY
TY
TY
TY
ty
TtYy
TtYy
TtYy
TtYy
ty
TTYY
TtYY
TtYY
TTYY
ty
TTYY
TTYY
TTYY
TTYY
ty
TTYY
TTYY
TTYY
TTYY
DIHYBRID PUNNETT SQUARE
TY
TY
TY
TY
ty
TtYy
TtYy
TtYy
TtYy
ty
TtYy
TtYy
TtYy
TtYy
ty
TtYy
TtYy
TtYy
TtYy
ty
TtYy
TtYy
TtYy
TtYy
DIHYBRID PUNNETT SQUARE
TY
Ty
tY
ty
TY
Ty
tY
ty
TTYY
TTYy
TtYY
TtYy
When you pair up the gametes from
the two plants, always put like
letters together and within the like
letters, put the CAPITAL letter in
front of the lowercase letter.
DIHYBRID PUNNETT SQUARE
Genotype ratio: TtYy - 16/16 or 100%
Phenotype ratio:
Tall, Yellow ty
16/16 or
100%
ty
TY
TY
TY
TY
TtYy
TtYy
TtYy
TtYy
TtYy
TtYy
TtYy
TtYy
ty
TtYy
TtYy
TtYy
TtYy
ty
TtYy
TtYy
TtYy
TtYy
DIHYBRID PUNNETT SQUARE
Let’s cross two of the plants from the F1 generation.
We need to pair up the genes which can be given to
each gamete (egg and pollen).
TtYy
T Y
T y
t Y
x
TtYy
t y
DIHYBRID PUNNETT SQUARE
TY
TY
Ty
tY
ty
Ty
tY
ty
Both the plants can give the same
gene combinations to their
gametes, so the pairs along the top
and down the side are the same.
DIHYBRID PUNNETT SQUARE
TY
Ty
tY
ty
TY
TTYY
TTYy
TtYY
TtYy
Ty
????
????
????
????
tY
????
????
????
????
ty
????
????
????
????
Your Turn!!
DIHYBRID PUNNETT SQUARE
TY
Ty
tY
ty
TY
TTYY
TTYy
TtYY
TtYy
Ty
TTYy
TTyy
TtYy
Ttyy
tY
TtYY
TtYy
ttYY
ttYy
ty
TtYy
Ttyy
ttYy
ttyy
F2 generation
DIHYBRID PUNNETT SQUARE
Genotype and phenotype ratios?
TY
Ty
tY
ty
TY
TTYY
TTYy
TtYY
TtYy
Ty
TTYy
TTyy
TtYy
Ttyy
tY
TtYY
TtYy
ttYY
ttYy
ty
TtYy
Ttyy
ttYy
ttyy
PHENOTYPE RATIO
TTYY - 1
TTYy - 2
TtYY - 2
TtYy - 4
TTyy - 1
Ttyy - 2
ttYY - 1
ttYy - 2
ttyy - 1
Tall, Yellow – 9 or 9/16 or ~56%
chance that the offspring will be tall
and yellow
Tall, Green – 3 or 3/16
Short, Yellow – 3 or 3/16
Short, Green – 1 or 1/16