Topic 4.3 Theoretical Genetics. Definitions Yellow pea plants must be heterozygous. The yellow phenotype is expressed. Segregation Through meiosis and

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


F

Key to alleles:Y = yellowy = green

Genotype:

Gametes:

Punnett square:

Genotypes:

Phenotypes:

Phenotype Ratio:

F


F


Genotype:

Gametes:

Punnett square:

Genotypes:

Phenotypes:

Phenotype Ratio:

F


F


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


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


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)


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


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.


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


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?


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.


SEX LINKAGE

Characters which are associate more with one gender


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)


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


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


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


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%


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


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


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


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


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


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


Tortioseshell Cats are Female


Daltonism = Red-Green Colourblindness

Normal vision Colour blind simulation

http://www.onset.unsw.edu.au/issue1/colourblindness/colourblindness_print.htm


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


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


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


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


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


Documents

Topic 4.3 Theoretical Genetics. Definitions Yellow pea plants must be heterozygous. The yellow phenotype is expressed. Segregation Through meiosis and