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Mendelelian Mendelelian GeneticsGenetics

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Generation “Gap”Generation “Gap”•P1, patrial generation = the parents or first

two organisims crossed.F1, first filial generation – the first set of F1, first filial generation – the first set of

offspringoffspringF2, second filial generation = the result of two of F2, second filial generation = the result of two of

the F1 generation being crossed.the F1 generation being crossed.

Developed the terms – gene,allele, Developed the terms – gene,allele, homozygous,heterozygous, dominent, homozygous,heterozygous, dominent, recessive, genotype and phenotype.recessive, genotype and phenotype.

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Mendel’s LawsMendel’s Laws

1. Law of Dominance1. Law of Dominance2. Law of Segregation2. Law of Segregation3. Law of Independent 3. Law of Independent

assortmentassortment

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Law of Segregation

Law of Independent Law of Independent AssortmentAssortment

Law of Independent Law of Independent AssortmentAssortment

Law of Independent Law of Independent AssortmentAssortment

• Traits are inherited independantly and are not changed by other alleles for other traits.

• Each paired gene must seperate during gamete formation so alleles cqn recombine in new pairs.

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Results of Monohybrid Results of Monohybrid CrossesCrosses

InheritableInheritable factors or genesfactors or genes are are responsible for all heritable responsible for all heritable characteristics characteristics

PhenotypePhenotype is based on is based on GenotypeGenotype Each traitEach trait is based onis based on two two

genesgenes, , one from the mother one from the mother and the other from the fatherand the other from the father

True-breeding individuals are True-breeding individuals are homozygous ( both alleles)homozygous ( both alleles) are are the samethe same

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•Back or test cross to test for homo or heterozygous.

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Incomplete DominanceIncomplete Dominanceandand

CodominanceCodominance

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Incomplete DominanceIncomplete Dominance

F1 hybrids F1 hybrids have an appearance somewhat in betweenin between the phenotypes phenotypes of the two parental varieties. Neither gene is dominent – both are expressed equally.

RRRR = = red flowerred flowerrr = white flowerBut Rr makes pink flowers!

r

r

R R

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Incomplete dominance problemsIn Andalusian chickens, the black

Andalusian character is incompletely dominant to the white-splashed Andalusian character. The heterozygous condition produces blue Andalusian chickens. Determine the genotypes and phenotypes of the F1 and F2 generations if a pure breeding, black Andalusian is crossed with a pure breeding, white-splashed Andalusian.

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CodominanceBoth alleles are equally expressed. The offspring have a mix of alleles which are equally expresses.

Eg. Red cows crossed with white will generate Roan cows.

But F2 generation

demonstrate Mendalian genetics (1:2:1)

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Codominance vs Codominance vs IncompleteIncomplete

In incomplete dominance only one In incomplete dominance only one allele is active but is reduced in allele is active but is reduced in effect.effect.

In codominence both alleles are In codominence both alleles are active. In human blood this active. In human blood this produces a whole new blood group produces a whole new blood group - AB.- AB.

1.1.type Atype A = I= IAAIIAA or I or IAAii2.2.type Btype B = I= IBBIIBB or I or IBBii3.3.type ABtype AB= I= IAAIIBB

4.4.type Otype O = ii= ii

How we write it - CrCw

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OVERDOMINANCE

•The phenotype falls outside the range of the parents e.g if one homozygote is tall (TT) and the other is short (tt) then the heterozygote may be extra tall (Tt). If this is a good adpation then it can become common in the population.

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LETHAL GENE

A mutation of a gene that produces a product that is nonfunctional. In some the homozygous dominant is lethal – it dies as an embryo so get a ratio of 2:1 instead of 1:2:1. In some it just affects expression of genes (Manx cats) and it can also be expressed at different stages of development e.g. Huntingtons.

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Manx CatManx Cat – Tailless cat is

another trait caused by an allele that has dominant effect in heterozygous and is lethal in homozygotes.

The Manx and normal alleles are denoted by L and l respectively

MLMl or MLML

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Other example is achondroplasia, the most common form of dwarfism, with a normal length body trunk but shortened limbs. AA – die, aa – normal, Aa - dwarf.

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Multiple Alleles

•More than one gene at a locus coding for a trait. Human blood type is a great example – there are three alleles for blood type, A,B and O. Allele A and B are co-dominant and both are dominant over O.

•Reminder – the truth about eye colour.

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Try These1. If a male has blood type B and a

female has blood type A, what are the possible blood types in the offspring?

2. Is it possible for a child with Type O blood to be born to a mother who is type AB? Why or why not?

3. A child is type AB. His biological mother is also type AB. What are the possible phenotypes of his biological father?

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Gene Interaction and Epistasis

The phenomenon of two or more genes governing the development of single character is known as Gene interaction. The genes can be on different chromosomes.

When one gene masks or alters the expression of another gene, the phenomenon is called Epistasis. (eg and meaning)

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3.Supplementary Gene Action

This is the masking of a characteristic (which is determined by one pair of alleles) by the action of another pair of alleles.

An example is mouse fur – one allele gives black (B) – lots of melanin, when recessive (b) it gives brown – less melanin. Another gene allows how much melanin is deposited so C allows colour to show but c does not.

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Collaboration• This is where one characteristic is controlled by two or more pairs of

alleles. The two genes interact to produce a novel phenotype. One well known example is comb shape in chickens. The four shapes are single,pea,walnut,rose.

• The ratio is the same as a normal dihybrid cross 9:3:3:1

• P_R_ - two dominants gives walnut

• P_rr – one dominant + one recessive gives pea

• ppR_ - other dominant and other recessive gives rose

• pprr – two recessives gives single.

• The big difference to a normal dihybrid cross is the fact that the phenotypes are four differnt, not variations of the same two traits.

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The ratio is 9:4:3.B_C_ - blackB_cc – whitebbC_ - brownbbcc - white

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Types of Epistasis1. Collaboration2. Complementary 3. Supplementary

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2. Complementary gene action

In this case two dominant allelees from two different genes are needed to get expression of the gene. The genes are complementary to each other. The ratio is 9:7.

As an example: sweet peas the development of purple flowers requires the presence of 2 dominant genes, C and R, e.g., CCRR.

When either C or R alone present purple flowers cannot be produced; as a result white flowers are obtained

e.g., ccRR or CCrr or ccrr

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Single

PeaRose

Walnut

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Linkage and sex determination

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What is so different between the X and Y chromosomes?

X- over 1000 genes identifiedY- 330 genes identified, many are inactive

One gene on the Y is very important: SRY. The SRY gene is the primary determinant of sex.

If SRY is present, testes develop in the early embryo. The testes secrete the hormone testosterone, which causes development as a male.

If SRY is absent (no Y chromosome), ovaries develop instead of testes, and the embryo develops into a female.

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The sex chromosomes• Many organisims like humans with

a homogamteic sex (female XX) and a heterogametic sex (males XY).

• XY- female, XX- Male This system of sex determination operates in birds, reptiles, some insectsThe bird sex chromosomes are called Z and W.

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In some insects there is no Y chromosome so you get XO and XX.

Diploid (2n) female, Haploid (n) maleThis system of sex determination is found mainly in Hymenoptera – honey bees, ants, termites, etc.

The unfertilized haploids develop male and called drones.

These drones carry only half the number (16) of chromosomes of the female (32)

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In reptiles and some fish sex is determined by the environment ( temperature, abilty to change sex etc).

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Family trees• 3 types – sex-linked

– autosomal recessive and autosomal dominant

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Sex Linked InheritanceBecause the X chromosome is larger than the Y

there are parts of the X chromosome that have no matching part on the Y. Any gene carried on the non-homologous part are called sex linked. Examples are red-green colour blindendss, haemophilia, all tortiseshell cats are female.

For males, any faulty gene on the X will show up as there is no gene on the Y to mask the effect. In females both parents must have the recessive trait to pass it on.

We write the alleles above the X and Y symbol.

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Sex-linked notationXBXB normal female XBXb carrier

femaleXbXb affected female XBY normal

maleXbY affected male

SEX LINKED DOMINANCEDominant gene on X chromosomeAffected males pass to all daughters

and none of their sons Genotype= XAY

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If the mother has an X- linked dominant trait and is homozygous (XAXA) all children will be affected

If Mother heterozygous (XAXa) 50% chance of each child being affected

E.g. dwarfism, rickets, brown teeth enamel

SEX LINKED RECESSIVEGene located on the X chromosomeMore males than females affected

(males inherit X from mother)Females can only inherit if the father

is

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Sex linked dominant disorders

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affected and mother is a carrier (hetero) or affected (homo)

An affected female will pass the trait to all her sons

Daughters will be carriers if father is not affected

Males cannot be carriers (only have 1 X so either affected or not)

E.g. colour blindness, haemophilia, Duchene muscular dystrophy

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2. Duchene type Muscular Dystrophy:Is also depends on sex linked recessive

gene. If mother is carrier, about half of the male

children are expected to be affected. Can be identified by chromosome study.It affects male before they reach teens, with

muscular deterioration .Muscles of leg and shoulders become stiff and

the children usually become paralyzed and crippled during their middle or late teens.

Virtually all die before age of 21.

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Sex-influenced Genes• Some traits even though not on sex

chromosomesappear differnntly in men and women e.g. pattern baldness. It is an inherited trait and controlled by one gene. In females it acts as a recessive so a woman needs two recessives to show baldness. In men only one is needed – BB – full hair both sexes, Bb – bald in men not in women, bb – bald in both.

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The Adams family and Baldness

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Colour blindnessWe have 3 color

receptors in the retinas of our eyes. They respond best to red, green, and blue light.

Each receptor is controlled by a gene. The blue receptor is on an autosome, while the red and green receptors are on the X chromosome (sex-linked).

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Features of Colour blindnessColour blindness is a sex linked character discovered by

WilsonIt is a hereditary disease and the affected person cannot

distinguish green and red colour. The red blindness is called Protonopia, these persons

cannot see red colourWhile, green blindness is called deuteronopia, such a

persons cannot see green colour. Colour blindness is a recessive character, represented

by ccThe genes are for colour blindness is located on X

chromosome. It is common in male but rare female.Colour blindness follows criss- cross inheritance as

transmitted from father to grandson through daughter. It is never transmitted from father to son

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It is universally accepted that genes are located in chromosomes

Linkage: The tendency of two or more genes to stay together during inheritance because they are located on same chromosome.

Linkage

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Recombination During Meiosis

Recombinant gametes

Parental gametes

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Complete and Incomplete linkage

Gene show a linkage because they are located on a chromosome.

For e.g., C and Sh are present in one chromosome, while their recessive alleles c and sh are situated in the homologous chromosome.

C sh

C sh c Sh

c Sh

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Each chromosome behaves as a unit during cell division.

Therefore, C and Sh would move to one pole to while c and sh move to opposite pool.

If this always happened, F1 generation (CcShsh) would produce 2 gametes viz., C Sh and c sh

When only parental character combinations are recovered in test progeny , it is called complete linkage.

However, sometime, allele recombine to produce recombinant types like C sh, cSh

Such a type are called Incomplete linkage.

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Linkage is never 100%. No matter how tightly two genes are linked, if you observe enough individuals, you will find some recombinants.

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Linkage MappingEach gene is found at a fixed position on a

particular chromosome. Making a map of their locations allows us to identify and study them better.

In modern times, we can use the locations to clone the genes so we can better understand what they do and why they cause genetic diseases when mutated.

Thus, the percentage of gametes that had a crossover between two genes is a measure of how far apart those two genes are.

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Pleiotrophy – one gene with many different effects. Often seen in genetic diseases such as sickle cell aneamia.

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Sickle Cell AnemiaUnder conditions of low oxygen tension, hemoglobin S will precipitate, causing cells to sickle Mutations in one amino acid HH – normal and gets maleriaHh – protected from maleria but gets sickle cells in low O2Hh – dies young from anaemia.

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Sickle cell anemia may be the result of a genetic mutation that happened in malaria-prone regions like Africa thousands of years ago.

People with sickle cell trait may have been more likely to survive malaria epidemics - and because they survived when others did not, this allowed the trait to be passed down through generations.

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Polygenes – two or more genes have similar and additive effects on the same characteristic. E.g. intelligence, height, skin colour. The phenotypes for these genes form a bell shaped curve and show continuous variation.

e.gs

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Genes and Environment Genes and Environment Determine CharacteristicsDetermine Characteristics

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Gene/Environment InteractionsGenes determine range of genotypes but

environmental factors fine tune which phenotype is displayed.

Internal and External Environment affectInternal: modifier genes – sometimes the

expression of the gene at one locus is affected by alleles at another locus.

Sex-limited genesSexlinked genes – sex hormaones affect.

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External environmentDrugsStartvation or malnutritionLack of lightTemperatureIonising radiationPoluuting chemicals