Topic 4 3 Theoretical Genetics Definitions Segregation Yellow

Topic 4. 3 Theoretical Genetics

Definitions

Segregation Yellow pea plants must be heterozygous. The yellow phenotype is expressed. Through meiosis and fertilization, some offspring peas are homozygous recessive – they express a green color.

Monohybrid Crossing a single trait F Genotype: Gametes: Punnett square: F Genotypes: Phenotype Ratio:

Monohybrid Crossing a single trait F Genotype: Gametes: Punnett square: F Genotypes: Phenotype Ratio: Key to alleles: Y = yellow y = green

Monohybrid Crossing a single trait F Genotype: Gametes: Punnett square: F Genotypes: Phenotype Ratio: Key to alleles: Y = yellow y = green

Monohybrid Crossing a single trait F Genotype: Gametes: Punnett square: F Genotypes: Phenotype Ratio: Key to alleles: Y = yellow y = green

Test Cross – used to determine the genotype of an unknown individual. The unknown is crossed with a known homozygous recessive F Phenotype: Key to alleles: R = red flower r = white Genotype: Possible Outcomes: Phenotypes: F Unknown parent = RR Unknown parent = Rr

Phenylkentonuria (PKU) Is PKU dominant or recessive? How do you know?

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.

Pedigree Charts Key to alleles: T = has enzyme t = no enzyme

Pedigree Charts Key to alleles: T = has enzyme t = no enzyme

Pedigree Chart Practice Female Male affected Not affected decease d 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.

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 IA I A IA I B IA i IB I B IB i ii Phenotypes (Blood types) A AB A B B O Note: ¢ 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 nonself ¢ 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 © Bioformatica © Biology Labs Online

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 antigens They 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 O cells do not carry antigens for the ABO system Type 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 © 2007 Paul Billiet ODWS 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 © 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 http: //nobelprize. org/nobel_prizes/medicine/articles/lewis/index. html ¢ ¢ Thomas Hunt Morgan in The Fly Room! (Columbia University 1910) Fruit Flies (Drosophila melanogaster) © 2007 Paul Billiet ODWS

The case of the white-eyed mutant Character Eye colour Traits Red eye (wild type) White eye (mutant) P Phenotypes Wild type (red-eyed) female x White-eyed male F 1 Phenotypes All red-eyed Red eye is dominant to white eye © 2007 Paul Billiet ODWS

Hypothesis A cross between the F 1 flies should give us: 3 red eye : 1 white eye F 2 Phenotypes Numbers So far so good © 2007 Paul Billiet ODWS Red eye White eye 3470 82% 782 18%

An interesting observation F 2 Phenotypes Redeyed males Redeyed females Whiteeyed females Numbers 1011 2459 782 0 24% 58% 18% 0% © 2007 Paul Billiet ODWS

A reciprocal cross Morgan tried the cross the other way around white-eyed female x red-eyed male Result All red-eyed females and all white-eyed males This confirmed what Morgan suspected The gene for eye colour is linked to the X chromosome © 2007 Paul Billiet ODWS

A test cross Phenotypes F 1 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 Eye colour Genotypes X RX R X RX r X r X RY X r. Y © 2007 Paul Billiet ODWS Alleles Red eye R White eye r Phenotypes

Genetic diagrams for sex linked genes Character Trait Eye colour Alleles Red eye R White eye r Genotypes Phenotypes X RX R X RX r X r X RY X r. Y Red-eyed female White-eyed female Red-eyed male White-eyed male © 2007 Paul Billiet ODWS

P Phenotypes Wild type (red-eyed) female Genotypes Gametes Fertilisation x White-eyed male X RX R XR X r. Y XR Xr Y XR X RX r X RY © 2007 Paul Billiet ODWS

F 1 Phenotypes Red-eyed female Genotypes Gametes Fertilisation x Red-eyed male X RX r XR X RY Xr XR Y XR X RX R X RY Xr X RX r X r. Y © 2007 Paul Billiet ODWS

F 2 Phenotypes Females Males Redeyed Whiteeyed Expected All None 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/issue 1/colourblindness_print. htm © 2007 Paul Billiet ODWS

LIGHT The retina Optic nerve fibres Ganglion layer Bipolar cells (neurones) Rod cell Synapse layer Nuclear layer Cone cell © 2007 Paul Billiet ODWS Inner segments packed with mitochondria Rod and cone outer segments

PHOTORECEPTION VISION COLOUR MONOCHROME PHOTORECEPTOR CONES: red sensitive 560 nm green sensitive 530 nm blue sensitive 420 nm RODS: max. sensitivity 505 nm DISTRIBUTION Concentrated in the fovea Widely spread over whole retina, absent from fovea PIGMENTS 3 proteins controlled by 3 genes. RHODOPSIN = Retinol Red and green pigments sex linked (Vit A) + Opsin (a protein). Blue pigment autosomal (Chr. 7) 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 Thrombin Active enzyme Prothrombin Inactive enzyme Fibrinogen Globular protein © 2007 Paul Billiet ODWS Fibrin = Clot Fibrous protein

Contact with collagen fibres in blood vessels Factor III Thromboplastin released from blood vessel walls Factor XII (inactive) Factor XII (active) Factor XI (inactive) Factor XI (active) Factor IX (inactive) Factor IX (active) Antihaemophilic factor B Factor VIII Antihaemophilic factor A Factor X (inactive) Factor X (active) Ca 2+ ions and blood platelets Vitamin K precursor © 2007 Paul Billiet ODWS Factor II (inactive) Factor II (active) Prothrombin Thrombin Factor I (inactive) Factor I (active) Fibrinogen Fibrin

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