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GENE INTERACTIONS
Slide 2
Meiosis
Slide 3
Crossing Over
Slide 4
Mendel
Slide 5
Punnet squares
Slide 6
Bears
Slide 7
Dihybrid Crosses
Slide 8
Incomplete/Codominant
Slide 9
Multiple Alleles and Lethal Genes
Slide 10
Linked Genes
MEIOSIS
Meiosis is sex cell division.
It consists of:
1. the DNA replicating normally
2. Homologous chromosomes line up
independently (and may cross over).
3. A meiotic cell division.
4. A mitotic cell division.
This has the effect of halving the
chromosome number and forming
gametes.
Sexual reproduction is important as it
greatly increases variation in a
species.
CROSSING OVER
During meiosis, as the homologous
chromosomes line up before the first cell
division, part of the neighbouring homologues
may swap.
The point at which the crossing over occurs is
called the chiasma.
Instead of two possible gametes, there are
four produced.
Lab manual pages 105/6, (107 opt)
BASIC GENETIC CROSSES
Remember Mendel? And Peas? And Punnet squares?
He found that traits were inherited in chunks, called genes.
Simple monohybrid (one trait)
crosses:
A purple pea is crossed with a
white flowered plant (P
generation). All of the offspring
(F1 generation) are purple.
When the resulting plants were
crossed he found that there was
always a 3:1 ratio in the offspring
(F2 generation).
He correctly deduced that:
• the parents are separately
donating their information to the
offspring
• the purple colour is dominant
to the white flower (recessive)
The cross of the F1 generation:
Gametes
Offspring
PP is homozygous dominant
Also called “pure breeding”
pp is homozygous recessive
Pp is heterozygous
The genotype is a description of the genes contained in the individual
The phenotype is a description of its physical appearance (e.g. Purple)
BEARS
In bears, white ears are recessive to black
Momma bear (white)
What is the genotype of Momma Bear?
Momma bear=__________
Poppa bear (black)
What genotypes could Poppa Bear have?
Poppa bear=________ or _______
This is baby bear (white eared).
What is his genotype?
Baby bear =
Can we say something
more about Poppa’s
genotype?
Incomplete Dominance and Codominance
Some alleles (gene forms) are not simply dominant or recessive.
In Incomplete Dominance an
intermediate phenotype is
produced:
In Codominance both alleles are
expressed at the same time:
Lab manual pages 114 and 115
Incomplete
Dominance
In cases of incomplete dominance,
neither allele dominates and the
heterozygote is intermediate in
phenotype between the two
homozygotes.
In crosses involving incomplete
dominance, the genotype and
phenotype ratios are identical.
Examples of incomplete dominance
include flower color in snapdragons
(right) and sweet peas, where red
and white flowered plants cross to
produce pink flowered plants.
CwCw
CRCw
CRCR
Flower Color in Snapdragons
Flower color in snapdragons exhibits incomplete dominance,
with red flowered and white flowered plants crossing to produce
offspring with pink flowers.
Red flower
White flower
Parents
X
CRCR
CWCW
Gametes
CR
CR
CW
CW
Possible
fertilizations
F1 offspring
CRCW
Pink
CRCW
Pink
CRCW
Pink
CRCW
Pink
Snapdragon Backcross
Determine the phenotype and genotype ratios of the offspring
resulting from a backcross of the F1 heterozygote to the red parent.
50% of the offspring are red (RR) and 50% are pink (Rr).
Pink flower Red flower
Parents
CRCw
CR
Gametes
CRCR
X
CR
Cw
CR
Possible
fertilizations
Offspring
CRCR
Red
CRCR
Red
CRCw
Pink
CRCw
Pink
Codominance
In cases of codominance, both alleles are independently and equally
expressed in the heterozygote. Examples include:
Roan (stippled red and white) coat color in cattle. A cross between a red bull
and a white cow produces all roan offspring.
ABO human blood groups.
Red bull
CRC
Parents
R
Gametes
White cow
CWCW
X
C
C
C
R
C
R
W
W
Possible
fertilizations
F1 offspring
CRC
CRC
CRC
CRC
W
W
W
W
Roa
Roa
Roa
Roa
Codominance in Cattle
In a cross between two heterozygous (roan) shorthorn cattle, red, roan, and
white offspring are produced in a 1:2:1 ratio.
Roan bull
Roan cow
CRC
Parents
Gametes
CRCW
X
W
C
C
C
C
R
W
R
W
Possible
fertilizations
CC
CC
Red
Roan
R
R
R
W
CRCW
CWCW
Offspring
Roan
White
Crosses Involving Codominance
In examples of codominance where a true breeding red or white parent is
crossed with a roan parent, the offspring will occur in a 1 : 1 ratio of the
parental types (i.e. roan and red, or roan and white)
Roan cow
Red bull
Parents
Gametes
CRCW
CRC
X
R
C
CR
R
C
C
R
W
Possible
fertilizations
CC
R
R
CRCW
CRCR
Roa
n
Red
CRCW
Offspring
Red
Roa
n
Crosses Involving Codominance
White bull
Roan cow
Parents
X
CWC
W
Gametes
CRCW
C
C
C
C
W
W
R
W
Possible
fertilizations
CRCW
CWCW
Roan
Whi
te
CRCW
CWCW
Offspring
Roan
Whi
te
Multiple alleles
It is possible to have more than 2 alleles for a particular trait.
A common example is the ABO blood groups in humans:
O is non-functional
A forms a protein with A antigen
B forms a protein with B antigen
A and B are codominant
Lethal Genes
Lethal genes are ones that cause death in the individual. The lethal gene
may be dominant or recessive.
In the heterozygous individual there may be some observed difference, e.g.
Manx (tailless) cats. Even when dominant the lethal gene may be passed on
if it does not have onset until after reproductive age (e.g. Huntington’s).
Lab manual pages 116/117 and 120
Lethal Alleles
Lethal alleles are gene mutations that result in a gene product which
is not only non-functional, but affects organism survival. Some lethal
alleles are fully dominant and are therefore lethal in the
heterozygote. Dominant lethal alleles are usually eliminated rapidly,
because their expression is fatal.
Exceptions occur when the expression of the allele is delayed until after
reproductive maturity, as occurs in Huntington disease.
In other cases (e.g. Manx cats), the allele
mutation results in a viable heterozygote
with a recognizable phenotype.
Recessive lethal alleles are fatal only in
the homozygote since their effect is
masked in the heterozygote carrier.
Examples of Lethal Alleles
When lethal genes prevent full
Parents
term development of the
embryo, offspring are
produced in a 2:1 ratio (2
heterozygotes to one normal).
In the inheritance of coat color
in yellow mice, offspring
phenotype ratios depart from
the expected Mendelian 3:1
when yellow mice are mated
together.
About two thirds of the
offspring are yellow, and one
third are non-yellow (right). A
testcross reveals the yellow
colored mice to be
heterozygotes.
Gametes
X
Yy
Y
Yy
Y
y
y
Possible
fertilizations
YY
Not viable
Yr
Yr
Offspring
yy
Incidence of Lethal Alleles
The average human is heterozygous for 3 to 5 lethal recessive genes.
Example: brachydactyly in humans
Shortening of the finger bones is caused by a lethal allele; heterozygotes have
shorter fingers, but homozygotes for the lethal allele die in infancy from other
skeletal defects.
Of the offspring of two brachydactylic people, one in four will die in infancy, one
half will show brachydactyly, and one in four will be normal (1:2:1 ratio).
X-ray of a normal hand
Brachydactyly: note the shortened bones
The Manx Mutation
Cats produce a gene
Parents
controlling spine length and
therefore production of a tail.
MML
The allele for taillessness
is incompletely dominant over
the allele for a normal tail (M).
(ML)
ML
The Manx allele
interferes
with spinal development and
heterozygotes (ML M) have no
tail (the Manx phenotype).
In ML ML homozygotes, the
double dose of the allele
produces an abnormal embryo,
which does not survive.
Gametes
M
MML
X
M
ML
ML
Possible
fertilizations
MM
Normal
MML
MML
Manx
Manx
Offspring
MLML
Not
viable
Multiple Alleles in Blood
The four common blood groups of the human ABO blood group
system are determined by three alleles: A, B, and O (also represented
in some texts as IA, IB, IO or just i). This is an example of a multiple
allele system for a gene.
ABO antigens consist of sugars attached to the red blood cell surface.
These sugars provide the individual antigenic properties. The alleles
code for enzymes that join these
sugars together.
Allele O produces a non-functional enzyme
that is unable to make changes to the basic
antigen (sugar) molecule.
The other two alleles (A, B) each produce
a different enzyme that adds a different,
specific sugar to the basic antigen.
Any one individual possesses only two
alleles and they are expressed equally.
RBC
RBC
Multiple Alleles in Blood
Phenotype
(blood group)
O
Genotypes
Allele codes for molecule
OO
Precursor
A
B
AB
Precursor
antigen
without extra
sugar at end
AA, AO
acetylgalactosamine
added to
precursor
BB, BO
galactose
added to
precursor
AB
Contains both
sugars
Antigen
Antibodies
in serum
None
(also
called
universal
donor)
None
(also called
universal
recipient)
Multiple Alleles in Blood
Blood donors must be compatible otherwise the red blood cells of the
donated blood will clump together (agglutinate) and block capillaries.
Phenotype Genotypes
Antibodies
in serum
Results from adding RBCs from groups
below to serum from groups at left
A
A
AA, AO
anti-B
B
BB, BO
anti-A
AB
AB
none
O
OO
anti-A
anti-B
B
AB
O
Multiple Alleles in Blood
EXAMPLE 1:
For both parents with AB
blood type, half of the
offspring will be the same
as the parents (AB), one
quarter will be type A and
one quarter will be type B.
Blood
group:
AB
Blood
group:
AB
Parent
genotyp
es
X
AB
Gametes
A
Possible
fertilizatio
ns
Children'
AA
AB
AB
BB
A
AB
AB
B
s
Blood
genotype
sgroup
s
B
AB
A
B
Multiple Alleles in Blood
Blood
Blood
group:
A
EXAMPLE 2:
group:
Two parents with blood
B
groups A and B respectively,
may produce offspring with Parent
all four possible blood
genotyp
groups: AB, A, B and O.
X
BO
AO
es
This may only occur if both
parents are carrying the
allele for group O.
Gametes
B
Possible
fertilizatio
ns
Children'
AB
s'
Blood
genotype
group
s
s
AB
O
A
O
BO
AO
OO
B
A
O
DIHYBRID CROSSES
This involves two traits that are not linked (not on the same chromosome).
Each of the traits are inherited independently.
In “Quarks” Two eyes (E) is dominant to one eye and Triangular shape (T) is
dominant to Pentagonal.
2 Quarks both EeTt are crossed:
Lab manual page 113
LINKED GENES
Linked genes are on the same chromosome.
This means that when cell division occurs the 2 genes are very
likely to stay together.
So where we might expect a offspring phenotype ratio of 1:1:1:1, we
actually get something else.
Two genes B (Bent) and D (Dark) are linked.
For a cross between BbDd and bbdd…
BD
Bd
bD
bd
Draw the gametes each could form.
Draw a punnet square for the cross.
Explain these results:
bd
BbDd
Bbdd
bbDd
bbdd
Bent Dark: Bent Light: Straight Dark: Straight Light
24
1
3
22
B and D (and b and d) are linked. The 1 Bbdd and 3
bbDd individuals are due to crossing over. The different
numbers are due to random chance.
Lab manual page 108