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Topic 4.3
Theoretical Genetics
Definitions
Yellow pea plants must be heterozygous. The yellow phenotype is expressed.
Segregation
Through meiosis and fertilization, some offspring peas are homozygous recessive – they express a green color.
Genotype:
Gametes:
Punnett square:
Genotypes:
Phenotypes:
Phenotype Ratio:
F
Monohybrid CrossCrossing a single trait
F
Genotype:
Gametes:
Punnett square:
Genotypes:
Phenotypes:
Phenotype Ratio:
F
Monohybrid CrossCrossing a single trait
F
Key to alleles:Y = yellowy = green
Genotype:
Gametes:
Punnett square:
Genotypes:
Phenotypes:
Phenotype Ratio:
F
Monohybrid CrossCrossing a single trait
F
Key to alleles:Y = yellowy = green
Genotype:
Gametes:
Punnett square:
Genotypes:
Phenotypes:
Phenotype Ratio:
F
Monohybrid CrossCrossing a single trait
F
Key to alleles:Y = yellowy = green
Key to alleles:R = red flowerr = white
Test Cross – used to determine the genotype of an
unknown individual. The unknown is crossed with a known homozygous recessive
Phenotype:
Genotype:
Phenotypes:
Unknown parent = RR Unknown parent = Rr
F
F
Possible Outcomes:
Is PKU dominant or recessive? How do you know?
Phenylkentonuria (PKU)
Phenylkentonuria (PKU)Phenylketonuria (PKU) is a rare condition in which a baby is born without the ability to properly break down an amino acid called phenylalanine. It is a recessive mis-sense mutation.
Key to alleles:T = has enzymet = no enzyme
Pedigree Charts
Pedigree Charts Key to alleles:T = has enzymet = no enzyme
Pedigree Chart Practice
Dominant or Recessive? - Dominant: A and B are both affected but have produced an unaffected D and F, therefore A and B must be carrying healthy recessive alleles.
Autosomal or Sex Linked?- Autosomal: Male C can only pass on one X chromosome. If it were carried on X then daughter H would be affected as well.
affected
Not affected
deceased
Female Male
MULTIPLE ALLELES
Genes which have more than two alleles
© 2007 Paul Billiet ODWS
Genes and their alleles
About 30% of the genes in humans are di-allelic, that is they exist in two forms, (they have two alleles)
About 70% are mono-allelic, they only exist in one form and they show no variation
A very few are poly-allelic having more than two forms
© 2007 Paul Billiet ODWS
Combinations
Di-allelic genes can generate 3 genotypes Genes with 3 alleles can generate 6
genotypes (3+2+1) Genes with 4 alleles can generate 10
genotypes Genes with 8 alleles can generate 36
genotypes
© 2007 Paul Billiet ODWS
Genes and the immune system
Poly-allelic alleles are usually associated with tissue types
These genes are so varied that they provide us with our genetic finger print
This is very important to our immune system which must tell the difference between our own cells (self) and invading disease causing microbes (non-self)
© 2007 Paul Billiet ODWS
The ABO blood system
This is a controlled by a tri-allelic gene It can generate 6 genotypes
The alleles control the production of antigens on the surface of the red blood cells
Two of the alleles are codominant to one another and both are dominant over the third
Allele IA produces antigen A Allele IB produces antigen B Allele i produces no antigen
© 2007 Paul Billiet ODWS
The ABO blood system
Genotypes Phenotypes (Blood types)
IA IA A
IA IB AB
IAi A
IB IB B
IBi B
ii ONote: Blood types A and B have two possible genotypes –
homozygous and heterozygous. Blood types AB and O only have one genotype each.
© 2007 Paul Billiet ODWS
Blood types and transfusions
Blood types vary and your immune system recognises your own blood type as being self
Other blood types are recognised as non-self
If a blood which is incompatible with your body is transfused it will result in the agglutination of the foreign red blood cells
© 2007 Paul Billiet ODWS
Antigens
© Biology Labs Online
© Bioformatica
Agglutination
© Dr Delphine Grézel, Ecole Nationale Vétérinaire de Lyon
Blood types and transfusions
People who are Type A blood produce antibodies to agglutinate cells which carry Type B antigensThey recognise them as non-self
The opposite is true for people who are Type B Neither of these people will agglutinate blood cells
which are Type OType O cells do not carry any antigens for the ABO systemType O cells pass incognito
What about type AB people?
© 2007 Paul Billiet ODWS
Donor-recipient compatibility
Recipient
Type A B AB O
A
Donor B
AB
O
= Agglutination
= Safe transfusion
Note: Type O blood may be transfused into all
the other types = the universal donor. Type AB blood can receive blood from
all the other blood types = the universal recipient.
© 2007 Paul Billiet ODWS
SEX LINKAGE
Characters which are associate more with one gender
© 2007 Paul Billiet ODWS
Characters associated with gender
Anhiorotic ectodermal dysplasia Small teeth, no sweat glands, sparse
body hair Occurs primarily in men Never transmitted from father to son Unaffected daughters may pass the
condition onto their sons (the grandsons)
© 2007 Paul Billiet ODWS
Sex linkage explained
Thomas Hunt Morgan in The Fly Room! (Columbia University 1910)
Fruit Flies (Drosophila melanogaster)
http://nobelprize.org/nobel_prizes/medicine/articles/lewis/index.html
© 2007 Paul Billiet ODWS
The case of the white-eyed mutant
Character TraitsEye colour Red eye (wild type)
White eye (mutant)
P PhenotypesWild type (red-eyed) female x White-eyed male
F1 Phenotypes All red-eyed
Red eye is dominant to white eye
© 2007 Paul Billiet ODWS
Hypothesis
A cross between the F1 flies should give us: 3 red eye : 1 white eye
F2 Phenotypes Red eye White eye
Numbers 3470
82%
782
18%
So far so good
© 2007 Paul Billiet ODWS
An interesting observation
F2 Phenotypes Red-eyed males
Red-eyed
females
White-eyed males
White-eyed
females
Numbers 1011 2459 782 0
24% 58% 18% 0%
© 2007 Paul Billiet ODWS
A reciprocal cross
Morgan tried the cross the other way aroundwhite-eyed female x red-eyed male
ResultAll red-eyed females and all white-eyed males
This confirmed what Morgan suspectedThe gene for eye colour is linked to the X
chromosome
© 2007 Paul Billiet ODWS
A test cross
Phenotypes F1 Red-eyed female x White-eyed male
Expected result
50% red-eyed offspring: 50% white-eyed offspring Regardless of the sex
Observed Results
Red-eyed Males
Red-eyed Females
White-eyed Males
White-eyed Females
132 129 86 88
© 2007 Paul Billiet ODWS
Genetic diagram for sex linked genes
Character Trait Alleles
Eye colour Red eye R
White eye r
Genotypes Phenotypes
XRXR
XRXr
XrXr
XRY
XrY© 2007 Paul Billiet ODWS
Genetic diagrams for sex linked genes
Character Trait Alleles
Eye colour Red eye R
White eye r
Genotypes Phenotypes
XRXR
XRXr
XrXr
Red-eyed female
Red-eyed female
White-eyed female
XRY
XrY
Red-eyed male
White-eyed male © 2007 Paul Billiet ODWS
P Phenotypes Wild type (red-eyed)
female
x White-eyed male
Genotypes XRXR XrY
Gametes XR XR Xr Y
Fertilisation Xr Y
XR XRXr XRY
XR XRXr XRY
© 2007 Paul Billiet ODWS
F1 Phenotypes Red-eyed female
x Red-eyed male
Genotypes XRXr XRY
Gametes XR Xr XR Y
Fertilisation XR Y
XR XRXR XRY
Xr XRXr XrY
© 2007 Paul Billiet ODWS
F2 Phenotypes Females Males
Red-eyed
White-eyed
Red-eyed
White-eyed
Expected All None 50% 50%
Observed 2459 0 1011 782
This gene has its LOCUS on the X-chromosome
It is said to be SEX-LINKED
© 2007 Paul Billiet ODWS
X-linked genes
In sex linked characteristics the reciprocal crosses do not give the same results
For X-linked genes fathers do not pass the mutant allele onto their sons
For X-linked genes fathers pass the mutant allele onto their daughters who are carriers
Carrier mothers may pass the allele onto their sons (50% chance)
Females showing the trait for an X-linked mutant allele can exist but they are rare
Female carriers may show patches of cells with either trait due to X chromosome inactivation
© 2007 Paul Billiet ODWS
Tortioseshell Cats are Female
© 2007 Paul Billiet ODWS
Daltonism = Red-Green Colourblindness
Normal vision Colour blind simulation
http://www.onset.unsw.edu.au/issue1/colourblindness/colourblindness_print.htm
© 2007 Paul Billiet ODWS
LIGHT
Optic nerve fibres Ganglion layer
Bipolar cells (neurones)
Synapse layer
Nuclear layer
Inner segments packed with mitochondria
Rod and cone outer segments
Rod cell
Cone cell
The retina
© 2007 Paul Billiet ODWS
PHOTORECEPTION
VISION COLOUR MONOCHROME
PHOTORECEPTOR CONES:red sensitive 560nmgreen sensitive 530nmblue sensitive 420nm
RODS: max. sensitivity 505nm
DISTRIBUTION Concentrated in the fovea Widely spread over whole retina, absent from fovea
PIGMENTS 3 proteins controlled by 3 genes. Red and green pigments sex linkedBlue pigment autosomal (Chr.7)
RHODOPSIN = Retinol (Vit A) + Opsin (a protein). Also called visual purple
BLEACHING Slow Fast (very sensitive)
REGENERATION Slow (after images in bright light, complementary colours)
Fast
USE Daylight vision Light adaptation 5 min
Night vision Dark adaptation 20 min or wear red goggles!© 2007 Paul Billiet ODWS
Blood Clotting and Haemophilia
A simplified scheme of the important steps
Damaged blood vessels
ProthrombinInactive enzyme
ThrombinActive enzyme
FibrinogenGlobular protein
Fibrin = ClotFibrous protein
© 2007 Paul Billiet ODWS
Contact with collagen fibres in blood vessels
Factor XII (inactive) Factor XII (active)
Factor XI (inactive) Factor XI (active)
Factor IX (inactive) Factor IX (active)Antihaemophilic factor B
Factor X (inactive) Factor X (active)
Factor II (inactive) Factor II (active)Prothrombin Thrombin
Factor I (inactive) Factor I (active)Fibrinogen Fibrin
Factor IIIThromboplastin released from blood vessel walls
Factor VIIIAntihaemophilic factor A
Ca2+ ions and blood platelets
Vitamin K precursor
© 2007 Paul Billiet ODWS
The antihaemophilic factors
The blood clotting reaction is an enzyme cascade involving Factors XII, XI, IX, X and II
Each of these enzymes are proteases that cut the next protein in line
Other factors including proteins like Factor VIII are essential as coenzymes
© 2007 Paul Billiet ODWS
Heamophilia
About 85% of haemophiliacs suffer from classic haemophilia (1 male in 10 000)
They cannot produce factor VIII The rest show Christmas disease where they
cannot make factor IX The genes for both forms of haemophilia are
sex linked Haemophiliacs do clot their blood slowly
because there is an alternative pathway via thromboplastin
© 2007 Paul Billiet ODWS