Title: Using genetics
Thursday 23rd January 2014
Learning question: How can genetic diagrams be used to solve
problems?
Homework:
Learning package 6 (apart from 1(c) ) for Monday 27th January
What do
these two
famous
people have
in common?
Learning Outcomes
• Explain the terms allele, locus, phenotype, genotype,
dominant, codominant and recessive;
• Explain the terms linkage and crossing-over;
Starter
• Complete the activity sheet on genetics.
• There are three parts to this activity:
– Vocabulary
– Genetic diagrams
– Codomiance
• You will be tested on all three areas in your examination!
Progress tracker
0-5
5-10
10-15
Monohybrid inheritance
• Mendel’s First Law
– principle of segregation
“The alleles of a gene exist in pairs but when gametes are
formed, the members of each pair pass into different
gametes, thus each gamete contains only one of each allele.”
Genetic Cross conventions
• Use symbols to represent two alleles
• Alleles of the same gene should be given the same
letter
– Capital letter represents the dominant allele
– Small letter represents the recessive allele
• Choose letters where the capital and small letter
look different
– The examiner needs to be in no doubt about what you
have written
Inheritance of height in pea plants
• Laying out the cross
–
–
–
–
–
–
–
–
–
–
P phenotype
P genotype
Gametes
F1 genotype
F1 phenotype
F1 self-fertilised
Gametes
Random fertilisation
F2 genotypic ratio
F2 phenotypic ratio
Inheritance of height in pea plants
gene
Allele
relationship
Symbol
Tall
Dominant
T
dwarf
recessive
t
Height of
pea plants
• Follow out the following cross to the F2 generation
– Homozygous tall pea plant with a homozygous dwarf pea
plant
• Write out the genotypic and phenotypic ratios from the
F2 generation
Pupil Activity – example question
(a)In peas the height of the plant is controlled by one gene which has two
alleles.
T represents the dominant allele for tall stems.
t represents the allele for short stems.
True breeding, tall-stemmed pea plants were crossed with short-stemmed
pea plants to produce the F1 generation.
(i)
State the genotypes of the parents.
(ii)
State the phenotype of the F1 plants.
Plants from the F1 generation were crossed to produce the F2 generation of
plants.
(iii) State the phenotypes and their expected ratio in the F2
generation.
Cystic Fibrosis
• Cystic Fibrosis is caused by a mutation to a gene on one of
the autosomes.
• Mutation
– Changes the shape of the transmembrane chloride ion channels
(CFTR protein)
– The CFTR gene is found on Chromosome 7
– The faulty gene is recessive
Inheritance of cystic fibrosis
• Three possible genotypes
– FF
– Ff
– ff
unaffected
unaffected
cystic fibrosis
• Remember gametes can only contain one allele for
the CFTR gene
• At fertilisation, any gamete from the father can
fertilise any gamete from the mother
– This can be shown in a genetic diagram
Genetic diagram showing the chances of a
heterozygous man and a heterozygous woman having
a child with cystic fibrosis.
Phenotype ratio of offspring
• Genotype ratio 1FF: 2Ff: 1ff
• Phenotype ratio 3 unaffected:1cystic fibrosis
• Can also be expressed as
– 25% chance of the child having cystic fibrosis
– Probability of 0.25 that a child will inherit the disease
– Probability that 1 in 4 that a child from these parents will
have this disease.
Mini Plenary
1. In tomato plants the allele for red fruit is dominant to the
allele for yellow fruit. If a heterozygous tomato plant is
crossed with a plant which produces yellow fruit, the
expected phenotype ratio of the offspring would be
A
B
C
D
3 red
1 red
1 red
1 red
:
:
:
:
1 yellow
3 yellow
2 yellow
1 yellow
Mini Plenary
2. Achoo syndrome is a dominant characteristic in humans which
causes the sufferer to sneeze in response to bright light. A woman who
is homozygous for the syndrome and a man who is unaffected have
children.
What proportion of their children would be expected to have Achoo
syndrome?
A
B
C
D
0%
25%
50%
100%
Mini Plenary
3. Which term refers to a description of a
characteristic of an organism?
A
B
C
D
genotype
phenotype
allele
natural selection
Mini Plenary
4. Which term refers to forms of a gene
controlling the same characteristic?
A
B
C
D
genotypes
phenotypes
alleles
dominant
Learning Outcome
• Use genetic diagrams to solve problems involving sex-linkage
and codominance.
Sex-Linkage
• Sex-linked genes are genes whose loci are on the X or Y
chromosomes
• The sex chromosomes are not homologous, as many genes
present on the X are not present on the Y.
• Examples
– Haemophilia
– Fragile X syndrome
– Red green colour blindness
Sex Chromosomes
Factor VIII and Haemophilia
• Haemophilia is caused by a recessive allele of a gene that
codes for a faulty version of the protein factor VIII
– XH
– Xh
normal allele
haemophilia allele
Remember, males are XY, females are XX
Possible genotypes and phenotypes
Applying your knowledge
Write out the genotypes for these phenotypes in
hemophilia
•
•
•
•
•
Affected male_____________
Normal male ______________
Affected female ____________
Normal female ____________
Carrier female ____________
Inheritance of Haemophilia
Note! The stages for
writing out a genetic
diagram is the same
Pedigree for a sex linked recessive
Write out the
disease
genotypes for as
many people in the
family tree as
possible.
Mini-plenary
• Collect a “connect 4” board and question sheet.
• In pairs, test each others knowledge of genetics and try to win
the game!
Codominance
• Codominance describes a pair of alleles, neither of which is
dominant over the other.
• This means both have an effect on the phenotype when
present together in the genotype
Codominance example
• Flower colour in plants
– CR
– Cw
red
white
• Write out a genetic cross
between a pure breeding
red plant and a pure
breeding white plant.
• Genotypes
– CRCR
– CRCW
– C WC W
red flowers
pink flowers
white flowers
• Carry out the cross to the
F2 generation.
– Write out the genotype and
phenotype ratio for the F2
generation
Revision Question
• Coat colour in Galloway cattle is controlled by a gene with two
alleles. The CR allele produces red hairs and therefore a red
coat colour. The Cw allele produces white hairs.
• A farmer crossed a true-breeding, red-coated cow with a truebreeding white-coated bull. The calf produced had roan coat
colouring (made up of an equal number of red and white
hairs).
• Explain the result and draw a genetic diagram to predict the
outcome of crossing two roan coloured animals.
Inheritance of A, B, AB and O blood
groups
• Human blood groups give an example of
codominance and multiple alleles
– There are 3 alleles present
• IA
• IB
• Io
• IA and IB are codominant
• Io is recessive
• Remember each human will only have two alleles
Blood Groups
Genotype
Phenotype
IAIA
IA Io
IAIB
IBIB
IB Io
Io Io
Blood Group A
Blood Group A
Blood Group AB
Blood Group B
Blood Group B
Blood Group o
Inheritance of blood groups
• Carry out genetic crosses for the following
examples.
– Two parents have blood groups A and B, the
father is IAIo and the mother is IBIo
– Father has blood group AB and the mother has
blood group O
– Mother is homozygous blood group A and the
father is heterozygous B.
Progress tracker
0-5
5-10
10-15
Learning Outcome
• Describe the interactions between loci (epistasis).
• Predict phenotypic ratios in problems involving epistasis.
Dihybrid Inheritance
• Monohybrid cross
– Inheritance of one gene
• Dihybrid cross
– Inheritance of two genes
Example – dihybrid cross
• Tomato plants
– Stem colour
A
purple stem
a
green stem
d
potato leaves
– Leaf shape
D
cut leaves
• NOTE
– In the heterozygote AaDd due to independent assortment
in meiosis there are 4 possible gamete combinations
AD
Ad
aD
ad
Crosses
• Cross a heterozygous plant with a plant
with a green stem and potato leaves
• Cross two heterozygous tomato plants
Dihybrid Inheritance
• A woman with cystic fibrosis has blood group A
(genotype IAIo). Her partner does not have cystic
fibrosis and is not a carrier for it. He has blood group
O.
• Write down the genotypes of these two people.
• With the help of a full and correctly laid out genetic
diagram, determine the possible genotypes and
phenotypes of any children that they may have.
Autosomal linkage
• Each Chromosome carries a large number of
linked genes
• If two genes are on the same chromosome
then independent assortment can not take
place.
• The genes are transmitted together and are
said to be linked.
Linked Genes
• Where linked genes are involved the offspring
of a dihybrid cross will result in a 3:1 ratio
instead of the 9:3:3:1 ratio.
• Example:
– In peas, the genes for plant height and seed
colour are on the same chromosome (i.e. linked)
Learning Outcome
• Describe the interactions between loci
(epistasis).
• Predict phenotypic ratios in problems
involving epistasis.
Flower colour in sweet pea
• Flower colour
– Colourless precursor of a pigment C
– Gene that controls conversion of this pigment to purple
P
– Both dominant alleles need to be present for the purple
colour to develop
• Cross
– Cross two white flowered plants with the genotypes CCpp
and ccPP
– Follow this cross through to the F2 generation
Interactions of unlinked genes
• A single character maybe influenced by two or more unlinked
genes.
• E.g. determination of comb shape in domestic poultry
–
–
–
–
Dominant allele P
Dominant allele R
Two dominant alleles
No dominant alleles
pea comb
rose comb
walnut comb
single comb
Genetic Crosses
• Carry out a genetic cross between a true-breeding pea comb
and a true breeding rose comb
• Follow this cross through to the F2 generation
Inheritance of coat colour in mice
• Wild mice have a coat colour that is referred to
as “agouti”.
– Agouti (A) is dominant to black (a)
– C is a dominant gene required for coat colour to
develop
– A homozygous recessive cc means that no
pigment can be formed and the individual is albino
Inheritance of coat colour in mice
• Carry out a cross between a pure-breeding black mouse
(aaCC) and an albino (AAcc)
• Follow this cross through to the F2 generation.
Epistasis
• This is the interaction of different gene loci so that one gene
locus masks or suppresses the expression of another gene
locus.
• Genes can
– Work antagonistically resulting in masking
– Work complementary
Epistasis ratios
• 9 : 3 : 4 ratio
– Suggests recessive epistasis
• 9 : 7 ratio
– Suggests epistasis by complementary action
• 12 : 3 : 1 ratio or 13 : 3 ratio
– Suggests dominant epistasis
Predicting phenotypic ratios
• Read through pages 132 and 133 of your textbook
– Answer questions 1 – 7
– Complete the stretch and challenge question on “eye colour in
humans”
• Read through and complete the worksheet provided for you
on epistasis
Learning objectives
(e) use genetic diagrams to solve problems involving sex linkage
and codominance;
(f) describe the interactions between loci (epistasis). (Production
of genetic diagrams is not required);
(g) predict phenotypic ratios in problems involving epistasis;
Learning objectives
(e) use genetic diagrams to solve problems involving sex linkage
and codominance;
(f) describe the interactions between loci (epistasis). (Production
of genetic diagrams is not required);
(g) predict phenotypic ratios in problems involving epistasis;
Genetics
• Genetics is the study of inheritance
• Allele
– different varieties of the same gene
• Locus
– position of a gene on a chromosome
Genetics
• Dominant
– An allele whose effect is expressed in the
phenotype if one copy present
• Recessive
– An allele which only expresses as a homozygote
• Co-dominant
– Both alleles have an effect on the phenotype
Genetics
• Genotype
– genetic constitution of the organism
• Phenotype
– appearance of character resulting from
inherited information
• Homozygous
– Individual is true breeding
– Possesses two alleles of a gene e.g. RR or rr
• Heterozygous
– Two different alleles for a gene e.g. Rr