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Part Four…
Determination of Phenotype
AS 90715
Describe the role of
DNA in relation to
gene expression
Take me to NZQA Documents relating to
this standard
Contents
 Monhybrid and Dihybrid crosses
 Incomplete/Codominant
 Multiple Alleles and Lethal Genes
 Linked Genes
 Recombination
 Chromosome Mapping
 Sex Linked genes
 Pedigree charts
 Pleiotropy and Polygeny
 Epistasis
 Epigenetics
Some words you may recognise
 Phenotype
 Genotype
R
RR
Red
hair
r
 Recessive
 Dominant
 Homozygous
 Heterozygous
Rr
 Punnet Square
 Allele
 Gene
R
Hair
Colour
MONOHYBRID CROSSES
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)
L2 Revision
In bears, white ears are recessive to black
Momma bear (white)
What is the genotype of Momma Bear?
Poppa bear (black)
What genotypes could Poppa Bear have?
Momma bear=__________
Poppa bear=________ or _______
This is baby bear (white eared).
What is his genotype?
Baby bear =
Can we say something
more about Poppa’s
genotype?
DIHYBRID CROSSES
Not linked (not on the same chromosome).
In “Quarks” Two eyes (E) is dominant to one eye and Triangular shape (T) is
dominant to Pentagonal.
2 Quarks both EeTt are crossed:
Page 115
Alleles that are not dominant and
recessive!
In yr 10, yr 11 and yes again in yr 12 we lead you to believe that alleles were either
recessive or dominate. But now that you are big grown up yr 13s – it is time we
expanded you understanding!
Yeah sure some
alleles are dominant
and recessive
Some alleles are:
•Co-dominate
•Incomplete dominant
•Lethal
•Multiple
•Linked
•Sex linked
INCOMPLETE DOMINANCE
AND CODOMINANCE
In Incomplete Dominance an
intermediate phenotype is
produced:
In Codominance both alleles are
expressed at the same time:
MULTIPLE ALLELES
It is possible to have more than 2
alleles for a particular trait.
Example – In humans the blood group
is determined by 3 alleles
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).
LINKED GENES
Linked genes are on the same chromosome.
This means that when cell division occurs the 2 genes are 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…
Draw the gametes each could form.
Draw a punnet square for the cross.
Explain these results:
BD
Bd
bD
bd
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.
Page 110
RECOMBINATION
Recombination (crossing over) during meiosis increases gamete
variation.
1. Homologous
chromosomes
line up
A
B
A
3. Meiotic
division
A
B
B
A
b
a
b
2. Chiasma forms,
segments swap
Expected gamete
A
a
a
B
a
b
a
b
B
Recombinants
b
Expected gamete
4. Mitotic
division
The closer the genes, the more likely they stay together
Page 111
CHROMOSOME MAPPING
We can “map” the distance between 2 genes by comparing how often the
cross over.
No. of recombinants
Crossover value (%) =
 100
No. of offspring
Low cross over value = not often recombined = close together
Page 114
SEX LINKED GENES
Carried on the X-chromosome.
As opposed to autosomal.
Males only have 1 allele – more susceptible to recessive diseases,
e.g. colour blindness. DO NOT SAY WHAT YOU SEE.
X is normal, Xc is colour blind. Y carries no information.
So…
XX is a normal female
What other possibilities can you have?
XcX
= normal female
XcXc
= colour blind female
XY
= normal male
XcY
= colour blind male
Explain why it is less common for women to be colour blind.
In cats one aspect of coat colour is controlled by a sex-linked gene with alleles
that are codominant.
These 2 females (XoXo) and (XbXb) are crossed with a male (XbY)…
Draw Punnett squares to find the offspring of each cross.
What is the XbXo offspring called?
How do we get male Tortoiseshell?
Page 121-2
PEDIGREE CHARTS
Dead
Males
Normal
Females
Affected (but
not dead – yet)
Page 128-9
GENE-GENE INTERACTIONS
Pleiotropy: One gene ( one protein) controls many phenotypes
40% of cats with white fur and blue eyes are deaf.
Marfan syndrome: one gene is responsible for thinness, joint hyper
mobility, limb elongation, lens dislocation, and increased susceptibility to
heart disease.
The p53 gene directs damaged cells to stop reproducing, thereby
resulting in cell death… helps avert cancer. BUT it also suppresses the
division of stem cells, preventing replacement of deteriorating tissues
during aging.
Polygeny: Many genes control one phenotype (e.g. Human skin
colour, cat coat colour)
Epistasis: Two genes interact, mask, or modify. There are a number of
types…
Lab manual page 133
EPISTASIS
One gene alters the outcome of another
Substance
Product A
Product B
Complementary genes: Both need to be present for either to work.
In a flower, 2 genes control the production of a purple pigment. The
intermediate product has no colour.
White
White
Purple
Can we cross 2 white plants to get purple offspring?
What will the genotype ratio of the dihybrid cross be?
PC
PPcc
ppCC
PpCc
Pc
pC
pc
PC
9:7
Pc
Think 9:3:3:1, but group
the last 3 sets.
pC
pc
Supplementary genes: the second gene adds more to the first.
Coat color in Labrador retrievers: two genes (B and E)
A black dog is B_E_
Yellow
Brown
Black
a. What are the genotypes of two black parental dogs
that, when mated, produce black puppies, yellow
puppies and brown puppies?
b. What proportions of black and yellow puppies do you
expect from this cross?
c. This cross is an example of a what type of gene
interaction?
Lab manual page 135/6
COLLABORATION
This is where 2 genes interact to make a product different to that which
either could make independently.
The most common example is comb types in chickens.
rose
single
Walnut
pea
rrpp
rrP_
See more chickens…
R_pp
R_P_
Page 132
Summary 138
EPIGENETICS
This is where gene expression is affected without changing the
underlying DNA code.
e.g. a zygote (toti-potent) becomes more and more specialised by
turning different genes on or off.
e.g. Genomic Imprinting.
Some genes can be activated or silenced depending on which parent
they came from. The ‘switch’ used to silence or activate a gene is often
methylation of cytosines – this inhibits that region of the DNA.
GENE-ENVIRONMENT INTERACTIONS
This is where gene expression is affected by the environment.
Some organisms are affected by temperature, altitude or the presence
of members of the opposite sex or competitors/predators.
Page 125-6 (opt)