Inheritance...

GCSE Biology Week 9

This week: InheritanceB11.21GenesexistinalternativeformsB11.22KeytermsB11.23MonohybridGeneticDiagram/PunnettsquaresandfamilypedigreesB11.24CalculateandanalyseoutcomesfrommonohybridcrossesB11.25SymptomsofsicklecelldiseaseandcysticfibrosisB11.26PedigreeanalysisscreeningforgeneticdisordersB31.17ThesexofapersoniscontrolledbyonepairofchromosomesB31.18Howthesexofoffspringisdeterminedatfertilization(geneticdiagram)B31.19Howsex-linkedgeneticdisordersareinherited

How do cells divide to become

anything at all?

Somaticcellsaredescribedas................(fromtheGreekmeaning‘double’).…youwillseethiswrittenas“2n”

DIPLOID

Through a process called… Mitosis!

RememberGCSEBiologyWeek4:CellDivision?

Some cells in the human body are not diploid... Gametes contain only 1 copy of each gene as they have only 1 set of chromosomes.

These cells are .............. and are produced by a

special type of cell division called ............Haploid

Meiosis

A male and Female gamete join together at fertilisation...

This 1 cell then divides

by

forming a. zygote

Mitosisto produce a complete

new organism...

Youreceivedonechromatidfromyourfather.

Andonefromchromatid yourmother!

Remember…thatinalllivingthings,characteristicsarepassedoninthe

chromosomesthatoffspringinheritfromtheirparents.

GENEforEyecolour

GENEforEyecolour

Eachchromosomemayhaveadifferentversion ofagene.

GENE/Allele forBlueEyecolour

GENE/Allele forBrownEyecolour

Differentversionsofagene,thatcodefor

differentversionsofacharacteristic,arecalled

ALLELES.

Homozygousindividualsaretruebreeding.Thismeansthattheywillalwaysproducethesamephenotypeofoffspringbecausetheyarenot‘hiding’arecessiveallele.

Ingenetics,differentallelesareoftenrepresentedbyletters,suchasAa.ThecapitalletterstandsfortheDOMINANT allele‘A’;whereas‘a’wouldbetherecessive allele.

Monohybrid inheritance: how single genes are passed on... Medal and his peas…

Monohybrid (single- gene)inheritanceconcernstheinheritanceofdifferentalleles(usually2)ofasinglegene.(e.g.thegeneforheight)

...LikeMendel,we’llstartwiththepeaplants,whichhaveeasilyobservablefeaturesthatarecontrolledbyasinglegene.

e.g.Peaplantshaveonegeneforheight.Theheightgenehas2alleles:

T(TALL)

t(dwarf)

Peaplantsarediploid andsohave2alleles (forheight).Thereforethereare3possiblegenotypes

TT = homozygous dominant (homo = same)

(so homozygous for T)

Tt = Heterozygous(hetero = different)

tt = homozygous recessive (homo = same)

(so homozygous for t)

ConsiderwhathappenswhenahomozygousTallplant(T)iscrossedwithahomozygousdwarf(t)plant.

T

tHomozygousDwarfPlant(tt)

Homozygous TallPlant(TT)

TT

tt

tt

TTT

t

AllgametesfromtheTallplantcontainaTallele,andallthosefromadwarfplantcontainatallele.Thesecombineatfertilisationtogive

offspringallwiththegenotypeTt(thegenes/allelesthatdetermineanorganismsphenotype)

However,althoughtheylookidentical inrespecttotheirTallparentplant,theyareverydifferentinoneveryimportantrespect:theyareHeterozygousandNOT

homozygous.

If2oftheseheterozygousplantsarecrossed,halfofthegametesfromeachparentareTandhalfaret,givingus4

possiblegenotypesinthesecondgeneration(F2)

T

tHeterozygous(Tall)Plant(Tt)

Heterozygous(Tall)Plant(Tt)

TT

tt

TT

ttt

T

The1st 3giveTallplants,the4th givesadwarf,so75%Tall,25%dwarf.

25%=TT50%=Tt25%=tt

T

tHeterozygous(Tall)Plant(Tt)

Heterozygous(Tall)Plant(Tt)

TT

tt

TT

ttt

T

B11.22Keyterms:Youhavetobeabletodefine thefollowing

terms…

HomozygousRecessive

Monohybrid inheritance in humans:

Clear-cutexamplesofmonohybridinheritanceinhumansarerelativelyrare,andofteninvolvegeneticdiseasewherepeopleinherit1ormorefaultyalleles.

Geneticdiseasesareoftenrecessive;thisisbecausefaultyallelesthatfailtomakeanimportantproteincanbe

maskedbynormalonesthatfunctionproperly.

i.e.RecessiveallelesaremaskedbyDominantAlleles.

Incontrastsomegeneticdisease,e.g.Huntington'sdisease,arecausedbyDOMINANTALLELES.

Theallelesconcernedcodeforaproductthatactivelycausesdamage;symptomsarenotduetoanallelenotdoingitsjob.Suchallelesaredominant becausethepresenceofanormalallelecannotmaskthesymptoms.

Someexamplesofmonohybridinheritanceinhumans:Traits Features

Dominant traits

Huntington'sdisease Symptoms in30’sand40’s,degenerative.

Freckles Freckles

Dimpleinchin Dimpleinchin

Recessivetraits

Sickle cellanaemia(B11.25Symptomsofsicklecelldiseaseandcysticfibrosis)

Haemoglobinpolymerises,distortingredbloodcellsintosickleshape – blockageofcirculatorysystemandanaemia.

Albinism Inabilitytomakepigment melanin.

Lactoseintolerance Inabilitytobreakdownthedisaccharidelactoseintoglucoseandgalactose.Leadstovomiting,diarrhoea, flatulence.

Cystic fibrosis(B11.25Symptomsofsicklecelldiseaseandcysticfibrosis)

Excessivemucous production,especiallyinthelungsandpancreas.Breathinganddigestionarealsoaffectedandsuffersareverysusceptibletolunginfections.

(B11.26Pedigreeanalysisscreeningforgeneticdisorders)

P Q

R S

Affected male

Unaffected male

Affected female

Unaffected female

Key

e.g.Huntington'sdisease:arareinheriteddisorderofthenervoussystem.CausedbyadominantalleleH.Therecessivealleleofthisgeneisrepresentedbyh.

ThediagramshowstheinheritanceofHuntington’sdiseaseinafamily.

P Q

R S

Affected male

Unaffected male

Affected female

Unaffected female

Key

UseageneticdiagramtoshowtheinheritanceoftheHuntington'sDiseaseallelebythechildrenofparentsPandQ.

hh

Hh

Hh

hh

hh Hh

h h

h hh h

H h

hH hH hh hh

Father: Mother:

Genotype:

Alleles: (ineggandsperm)

Poss.CombinationsofAlleles

Phenotype: normalsuffer

meiosis

normalsuffer50%Chance

B11.24Calculateandanalyseoutcomesfrommonohybridcrosses

Q)ExplainwhynoneofthechildrenofRandSinheritedHuntington’sdisease.

P Q

R S

Affected male

Unaffected male

Affected female

Unaffected female

Key

hh

Hh

Hh

hhhh

hhhh hh hh

A)Bothparentsareunaffected,Thustheydon'thavethedisease.TheGenotypeofparentsis...hhhomozygousrecessive(orneitherparenthasHdominantgene/allele).

Whatifbothparentsareheterozygous?OrganiseyourresultsintoaPunnetsquare:

H

hHeterozygousMother(Hh)

HeterozygousFather(Hh)

HH

hh

HH

hhh

H

25%=HH50%=Hh25%=hh

B11.24Calculateandanalyseoutcomesfrommonohybridcrosses

3:1

Ff Ff

F f

F Ff f

F f

FF fF Ff ff

Father: Mother:

Genotype:

Alleles: (ineggandsperm)

Poss.CombinationsofAlleles

Phenotype: normal carrier carrier suffer

meiosis

B11.23MonohybridGeneticDiagramPunnettsquaresandfamilypedigrees…Cysticfibrosisisarecessivedisorder,socross2carrierswithgenotypesFf

OrganiseyourresultsintoaPunnetsquare:

F

fHeterozygousMother(Ff)

HeterozygousFather(Ff)

FF

ff

FF

fff

F

25%=FF50%=Ff25%=ff

3:1

Meiosis: The production of Gametes (sex cells)

Remember sperm and egg (ova) are Hapliod – having just “n” half the number of

chromosomes.

Meiosis is a special kind of cell division in which there are 2 successive divisions

that result in the production of gametes –

the sex cells!

Meiosis: The production of Gametes (sex cells)

B3 1.17 The sex of a person is controlled by one pair of chromosomes.

B3 1.18 How the sex of offspring is determined at fertilization (genetic diagram)

XY XX

X Y

X XY Y

X X

XX XY XX XY

Father: Mother:

Genotype:

Gametes

Poss.Combinations

Phenotype:i.e.Sex Female Male Female Male

meiosis

B3 1.17 The sex of a person is controlled by one pair of chromosomes.

B3 1.18 How the sex of offspring is determined at fertilization (genetic diagram)

OrganiseyourresultsintoaPunnetsquare:

X

X

Female=XX

Male=XYXX

XX

XX

YYY

X

50%=XX50%=XY

1:1

Sex linked Genetic disorders…

B31.19Howsex-linkedgeneticdisordersareinherited

Sex linked Genetic disorders…

Somegeneticcharacteristicsaresexlinked– meaningtheyarecarriedononeoftheSexChromomsomes.(XorY)

Colour Blindness is inherited, and is caused

by a faulty allele on the X chromosome…

Since the Y chromosome is smaller than the X chromosome

it carries fewer sex-linked genetic disorders…

So, colour blindness, for

example is much more common in

men than in women – because men only

require 1 (recessive) faulty allele – whereas women need 2. XnY

XnXnThis genotype is

rare…

This genotype is more common…

XNY XNXn

XN Y

XN XNY Y

XN Xn

XNXN XNY XNXn XnY

NormalMale:

ParentsGenotype:

GenotypeofGametes

Poss.Combinations

Offspring'sPhenotype:i.e.colourblind/normalorcarrier

NormalFemale

NormalMale

NormalFemaleCarrier

ColourBlindMale

meiosis

NormalFemale(carrier)ParentsPhenotypeN=AlleleforNormal

colourvision

n=Alleleforcolourblindness– (Faultyallele)

OrganiseyourresultsintoaPunnetsquare:

XN

XN

Female=XNXn

Male=XNY

XNXN

XNXN

XnXn

YYY

Xn2:1:1

3:1

Theratiois3:1(normal:Colourblind)

butcanalsoberepresentedas2:1:1(Normal:Carrier:

ColourBlind)

Sex linked Genetic disorders…

Haemophilia(adiseasewhereblooddoesn'tclotproperly)isageneticdiseasethatisinheritedinexactlythesamewayascolour blindness.

Haemophilia is inherited, and is caused by a faulty

allele on the X chromosome…

Homozygous:Pairofallelesthatproduceacharacteristicarethesame,e.g.Homozygous dominant=HHHomozygous Recessive=hh

Heterozygous:Pairofallelesthatproduceacharacteristic;theallelesaredifferent,e.g.Hh.

Recessive:Anallelethatwillonlybeexpressedwhenbothallelesareofthistypee.g.hh.

Dominant:Anallelethatwillalwaysbeexpressedevenwhenthereisonlyoneoftheseallelespresent, representedbyacapitalletter.e.g.HHorHh.– theH(dominateallele– willbeexpressed)

Gene:SectionofDNAthatcodesforaparticulartraitorcharacteristic.

Allele:Adifferentformofagenethatcodesforadifferentversionofacharacteristic.

Genotype:Adescriptionofthepairofallelespresentforacharacteristic.

Phenotype:Thephysicalexpressionofthealleles.