Mendelelian
Genetics
1
Genetics notes
Gregor Mendel
(1822-1884)
An Austrian Monk
He determined the
Laws governing
Inheritance of
Traits
Gregor Mendel
Between 1856 and
1863, Mendel tested
28,000 pea plants
He was devoted to
finding out how
inheritance worked
He is called the
“Father of Genetics"
Site of
Gregor
Mendel’s
experimental
garden as it
looks today
Genetic Terminology
 Trait - any characteristic that can
be passed from parent to offspring
 For example, the brown hair you
inherit from your dad is a trait
 Heredity/Inheritance - passing of
traits from parent to offspring
 Genetics - study of heredity
Particulate Inheritance
Mendel stated that
physical traits are
inherited as “particles”
Mendel did not know
that the “particles” were
actually gene sections of
DNA on Chromosomes
What and where are “Genes”?
 Genes are sections of DNA
on each chromosome /
chromatid strand that
instruct the body to create a
particular trait
 There are active genes,
inactive genes (Junk DNA)
and switch genes
 In people, we have at least
25,000 active genes,
hundreds of thousands of
switch genes, and almost 1
million inactive sections,
located on 23 different
chromosome pairs
Chromosome 6
Gene for
hair color
Gene that
switches on
fingernail
growth
Gene for gills
(inactive)
Gene for
growth of
fingernails
Gene for
dimples
Chromosomes/chromatid and their
Genes come in pairs
 All DNA chromosomes or
chromatid strands in
eukaryote cells exist in pairs
(except for sex cells).
Chromosome 6
 One chromosome is from a
female parent
 One chromosome is from the
male parent
 That means all genes on
them also exist in pairs (2 of
every gene)
Mom
Dad
Genes come in pairs
 Genes for a given trait
always occur in the same
place on the same
chromosome
 Notice that some genes
are identical, but some
genes are different
(different nucleotide
sequences)
 You can inherit a gene for
blond hair from mom, but
a gene for brown hair
from dad
Mom
Dad
Gene for
hair color
Alleles and Homologous Genes
 Allele – another word
for “gene”. It is used
when discussing heredity
 Homozygous Alleles –
are genes that are
identical from each
parent
 Heterozygous Alleles
– are genes that are
different from each
parent
Mom
Dad
Alleles and Homologous Genes
 For example, this might
be the homozygous
alleles for the presence
of five fingers on each
hand
 Both of your gene copies
are identical
(fortunately)!
Mom
Dad
Alleles and Heterozygous Genes
Mom
 These might be the
heterozygous alleles
for color of your skin
 Both of your gene
copies are different!
Dad
Dominant and Recessive Alleles
 Some gene alleles have a
stronger effect in the
body than others.
 A gene that has a
stronger effect on a trait
than its mate is called a
Dominant Gene or Allele
 The “hidden” or less
expressive gene is called
the Recessive Allele
Mom
Baldness is
Recessive
Dad
Full head
of hair is
dominant
Dominant and Recessive Alleles
 The presence of even one
dominant gene will result in you
expressing that trait
 The recessive gene is not
expressed
 For example, if you inherit one
dominant gene for an allergy to
peanuts from mom, you will be
allergic to peanuts, even if the
other gene (from dad) is not
allergic to peanuts
Mom
Dad
Peanut
allergy
gene
Dominant and Recessive Alleles
 When doing gene
calculations,
Dominant alleles are
indicated by a
capital letter (“H)”
 Recessive alleles
are represented by
a small letter (‘’h”)
 The letter you pick
is up to you
h
c gene for
H gene for
C
blonde hair
brown hair
Dominant and Recessive Alleles
 For example, brown
hair is a Dominant
trait
 Blond hair is a
recessive trait
Dominant and Recessive Alleles
 Hair color can also be reddish or
auburn. Red hair is a Recessive
trait of another gene (the red-or
non-red allele).
 To get red hair, you must have
two recessive “rr” alleles for red,
and 2 recessive blonde genes.
 Whether you inherit pure red
hair or auburn color (brownish
red) depends on the brown or
blond gene. To get auburn hair,
you would have a dominant brown
gene (BB or Bb) and two recessive
red genes (rr)
Mom
B
b
r
Dad
b
r
Blond
hair
gene
Auburn
hair
gene
Dominant and Recessive Alleles
 Purple color in pea
flowers is a
Dominant trait
 White color in Pea
flowers is a
Recessive trait
Dominant and Recessive Alleles
 Freckles is a Dominant
trait
 If you inherit two FF
alleles, or an Ff allele,
you will get freckles
 No freckles is a
recessive trait (you
must inherit
two f genes)
Mom
F
Dad
f
Freckles
gene
Co-Dominant Alleles
 Not all genes
come in dominant
and recessive
varieties.
 Some gene alleles
are both equally
expressed.
 They are called
“Co-dominant”
Alleles
The alleles for blood type
are co-dominant genes
Incomplete Dominant alleles
Some genes contribute ½ of a trait. No allele is
dominant. The trait is a blend of both genes
Example: In snapdragons (flower) –
A red flower gene + white gene = pink flower
+
=
Example: hair curl in humans. Mate a curly haired girl with
a straight haired man = a wavy haired child
Multiple Alleles
 Many traits are
controlled by several
genes at once.
 The multiple genes or
switches that control a
trait are called the
trait’s “Multiple
Alleles”
 These genes/switches
can be on different
chromosomes
Adult height in people is
controlled by nine different
genes or switches
(multiple alleles)
Let’s Practice:
 These two spinning genes
are Homozygous Alleles,
because they are both
identical.
 These two spinning genes
are Heterozygous Alleles,
because they are different
from each other.
Let’s Practice:
 Genes that have a greater
effect on a trait are called
Dominant gene alleles.
 Genes that seem to be
hidden or overpowered by
the presence of a dominant
allele, are called Recessive
gene alleles.
H gene for
brown hair
is
Dominant
Let’s Practice:
 Genes that contribute ½ of
each trait are called
Incomplete Dominant
gene alleles.
Let’s Practice: – no notes
 The alleles for Brown
bb, rr, Ff
or blond hair color are
Bb, rr, ff
BB, Bb, bb. Brown is
bb, RR, FF
dominant over blond
 The alleles for Red
hair or no red hair are
RR, Rr, or rr. Red
hair is recessive.
 The alleles for
freckles are FF, Ff,
or ff. Freckles are
dominant.
bb, Rr, FF
Bb, rr, ff
bb, RR, ff
Genotypes
 Genotype – the letters representing the genes
that control a particular trait
(e.g. RR, Rr, rr)
 Homozygous genotype - gene combination
involving 2 dominant or 2 recessive genes (e.g.
RR or rr); also called pure
 Heterozygous genotype - gene combination of
one dominant & one recessive allele (e.g. Rr);
also called hybrid
 For example, the genotype for the
speed of hair growth may be the
Homozygous Dominant alleles G G
Isn’t
Genetics
You!
great?
G
h
G
h
 The genotype for brown eyes may be
Heterozygous Dominant alleles B b
B
b
 The genotype for dry skin may be
Co-dominant alleles Ds Ds
Ds
Ds
 The genotype for a pug nose may be the
incomplete dominant alleles Pn Pn
Pn
Pn
 The genotype for blonde hair may be
Homozygous Recessive alleles h h
 Phenotype – the physical traits
My phenotype
that result when the genotype
My phenotype
isis silky
of each gene is expressed
blondhair
hair
 For example, having homozygous
dominant hair growth alleles G G on
chromosome 14 can produce people
with a phenotype of silky hair
 Having recessive alleles h h for
blonde hair on chromosome 11 will
produce people with blonde hair
 Having a hybrid B b genotype for
brown eyes will result in a child with
a brown eyes phenotype
 If you are born with the genotype
Ds Ds for dry skin, your phenotype
of that trait will be dry skin
You!
G
h
G
h
B
b
Ds
Ds
Let’s Practice:
 The Genotype of this trait
for hair color are the
letters _h H_
 The Phenotype of this trait
for hair color, is
____brown hair ___
h
H
Let’s Practice:
 The Genotype of this trait
for flower color in pea plants
are the letters _R r_
 The Phenotype of this trait
for flower color would be
____purple flowers ___
 Why? The Genotype for
flower color in this pea plant
are Heterozygous Dominant
Alleles
R
r
Mendel’s Pea Plant
Experiments
Why peas, Pisum sativum?
Can be grown in a
small area
Produce lots of
offspring
Produce pure plants
when allowed to
self-pollinate
several generations
Can be artificially
cross-pollinated
Reproduction in Flowering Plants
Pollen is plant sperm
Produced by the Stamen
The Ovary produces an
Ovule (egg) found inside
the flower (ovum is
plural)
Pollen is carried to the
ovule for fertilization
through the Pistil
Self-fertilization can
occur in the same flower
Cross-fertilization can
occur between flowers
Mendel’s Experimental Methods
Mendel hand-pollinated
flowers using a brush rubbed
across the stamen of one
plant to pick up pollen, then
brushed over the pistil of
another plant
He snipped the stamens of
the target plant to prevent
self-pollination
He tracked various traits
through several generations
Eight Pea Plant Traits
Flower color --- Purple (P) or white (p)
Seed Color ---- Yellow (Y) or Green (y)
Seed shape --- Round (R) or Wrinkled (r)
Pod Shape --- Smooth (S) or wrinkled (s)
Pod Color --- Green (G) or Yellow (g)
Seed Coat Color ---Gray (G) or White (g)
Flower position---Axial (A) or Terminal (a)
Plant Height --- Tall (T) or Short (t)
How Mendel Began
Mendel
produced pure
strains by
allowing the
plants to selfpollinate for
several
generations
Mendel’s Experimental Results
Phenotype
Phenotype
Types of Genetic Crosses
 Monohybrid cross - cross involving
a single trait ( e.g. flower color only)
 Dihybrid cross - cross involving
two or more traits
(e.g. flower color & plant height)
Why did Mendel get a 3:1 ratio
PP
PP
Assume each of these two purple
What plants
would is
bepure
their(both
individual
flower
of their
“Genotype”
for flower
genes for flower
colorcolor?
are
Dominant “Purple” alleles
Purple is DOMINANT
Why did Mendel get a 3:1 ratio
What would be the outcome if these two
Because the color Purple in
plants
mated?
Consider
this
cross
pea plant flowers is a
Dominant trait, the genes or
Genotype for this trait in two
homozygous PURE plants
PP
PP
would be written as the
capital letters PP x PP
Let’s use a “square graph” to
predict it!
It’s called a “Punnett
Square”
Why did Mendel get a 3:1 ratio
PP
PP
Step 1 – Write down the four
possible arrangements along the
top and bottom of the square
“Punnett Square”
Why did Mendel get a 3:1 ratio
Step 2 – List all the possible
genotypes within the square
P
P
P
P P
P P
P
P P
P P
“Punnett Square”
Why did Mendel get a 3:1 ratio
Step 3 – Predict the color of the
flowers given their genotype
Purple is a dominant color
trait, so P x P would produce
only Purple flowers
(the homozygous Pure Purple
phenotype).
P
P
P
P P
P P
P
P P
P P
“Punnett Square”
This is a purebred cross
between two
homozygous
purple flowers
P generation
F1 generation
P
P
“Phenotype”
P
P P
P P
The parents are called the
Parent or P generation, and the
offspring are called the 1st Filial
or F1 generation
P
P P
P P
“Genotype”
Now let’s consider this cross
x
pp
pp
White is a recessive color
trait, so p x p would produce
only White flowers
(the Pure White phenotype).
What is the Phenotype?
Genotype?
pp
pp
pp
pp
“Punnett Square”
Now consider this cross
x
pp
PP
Purple is a dominant trait, so
its presence in even one allele
will produce all purple
flowers (the Heterozygous
Hybrid Purple phenotype).
What
What is
is the
the Genotype?
Phenotype?
Pp
Pp
Pp
Pp
“Punnett Square”
Now consider a cross between
two F1 hybrid offspring
F1 generation
Pp
Pp
Purple is a dominant trait, so
its presence in 3 of the four
offspring will produce purple
flowers. However, one of the
flowers will be recessive white
(pure white phenotype)
What
What is
is the
the Genotype?
Phenotype?
Pp
pp
F2
generation
PP
Pp
“Punnett Square”
Gregor Mendel realized that this 3:1 ratio in single monohybrid
traits can only occur if the factors of Heredity occur in pairs,
and are governed by the principle of Dominance
F1 generation
F2 generation
Pw
ww
Pw
Pw
“Punnett Square”
Mendel’s Law of Segregation
During the formation of gametes (eggs or
sperm) in Meiosis, the two gene alleles
responsible for a trait randomly
separate from each other.
Alleles for a trait are then "recombined"
at fertilization
Mendel’s Law of Dominance
Some gene alleles are dominant. In a
cross of parents that are pure for
different phenotypes of the same trait,
one trait may dominate the other and only
it may appear in the next generation.
All the offspring will be heterozygous and
express only the dominant phenotype.
RR x rr yields all R r (round seeds)
Law of Independent Assortment
Alleles (genes) for different
traits that are located on
different chromosomes are
distributed randomly to sex cells
(& offspring), independently of
one another.
Let’s try it (1 chip – no notes)
F1 generation
Pp
pp
What is the Genotype?
pp
pp
F2
generation
What is the Phenotype of the
four offspring?
Pp
Pp
“Punnett Square”
Let’s try it (1 chip – no notes)
F1 generation
Bb
bb
What is the Genotype?
bb
bb
F2
generation
What is the Phenotype of the
four offspring?
Bb
Bb
“Punnett Square”
Your turn – complete the
Punnett Square handout
Let’s try it with Co-Dominant traits
A person can have alleles of
A, B or O for blood type
AO
BO
(A and B are dominant over O)
What is the Genotype?
B
Mom
Dad
Type
A AB
AB
O
Type
AO
A
F1
generation
What is the Phenotype of the
four offspring?
O
Type
BO
B
Type
OO
O
“Punnett Square”
Dihybrid Cross
A breeding experiment that tracks
the inheritance of two traits.
Mendel’s “Law of Independent
Assortment” states that each pair of
alleles segregates independently during
sex cell formation, and will be inherited
separately
Dihybrid Cross
Traits: Eye color and hair curl
Alleles:
BB brown eyes
Eye color is a
Bb brown eyes
dominant trait
bb blue eyes
CC = curly hair
Hair curl is an
Incomplete
Cc = wavy hair
dominant trait BC
cc = straight hair (a blend)
FOIL Rule
Bc
100% (all) of the kids will have brown
First of each pair
eyes (Bb) (check it OUT!)
BC
Outside of each pair
50% of the kids will have wavy hair (Cc)
Inside of each pair
50% will have straight hair (cc)
Bc
Last of each pair
bb, cc
BB, Cc
bc
bc
bc
bc
BbCca BbCc
BbCc BbCc
To construct
Di-hybrid cross 16 box
BbccPunnett
Bbcc listBbcc
Bbcc Square,
each combination as
follows:
BbCc BbCc BbCc BbCc
Bbcc
Bbcc
Bbcc
Bbcc
Dihybrid Cross
Traits: Body color and teeth
Alleles:
BB Yellow body Yellow body is
a dominant
Bb Yellow body
trait
bb blue body
TT = 2 teeth
Teeth is a
dominant trait
Tt = 2 teeth
BT
tt = no teeth
FOIL Rule
Bt
the
16
will
be
recessive
in the
the
dominant
in
13
16
will
be
homozygous
1ofof
of
the
16kids
kids
will
be
homozygous
9the
ofFirst
the
16
be
dominant
in
ofwill
each
pair
first
trait
and
dominant
the
first
trait
and
pure
for
Dominant
traits
pure
forboth
bothrecessive
recessive
traits
both
traits
(either
pure orin
hybrid).
bT
Outside
of
each
pair
second.
They
will
be
blue
with
second.
They
will
be
yellow
with
(yellow
body
(BB)
and
teeth
(TT)
(blue
body
(bb)
and
no
teeth
(tt)
They will be yellow with teeth.
teeth.
teeth.
Insideno
of
each pair
bt
Last of each pair
Bb, Tt
Bb, Tt
BT
Bt
bT
bt
BBTT
BBTt
BbTT
BbTt
BBtT
BBtt
BbtT
Bbtt
bBTT
bBTt
bbTT
bbTt
bBtT
bBtt
bbtT
bbtt
Dihybrid Cross
When two heterozygous parents are mated
(RrSs and RrSs), the ratio of Phenotypes is:
Dominant in both traits (RS)
= 9
Dominant in first but not second = 3
Dominant in second but not first = 3
Recessive in both traits
9:3:3:1
= 1
Remember this
Dihybrid ratio – it
occurs when two
hybrids mate
Sex-linked Traits
Traits (genes) located on the sex
chromosomes
Sex chromosomes are X and Y
XX genotype for females
XY genotype for males
Many sex-linked traits carried on X or Y
chromosome only (only 1 gene is
present)
Sex-linked Traits
Example: Eye color in fruit flies
Sex Chromosomes
fruit fly
eye color
XX chromosome - female
Xy chromosome - male
Sex-linked Trait Problem
Example: Eye color in fruit flies
(red-eyed male) x (white-eyed female)
XRY
x
XrXr
Remember: the Y chromosome in males does
not carry that trait (allele) at all.
RR = red eyed
Rr = red eyed
rr = white eyed
XY = male
XX = female
Xr
Xr
XR XR Xr XR Xr
Y
Xr Y
Xr Y
Your turn – complete the
Dihybrid Cross handout