Download 2.1 12 Using genetic diagrams – 2

2.1
12
Using genetic diagrams – 2
By the end of this spread, you should be able to …
1 Use genetic diagrams to solve problems involving codominance.
Sickle-cell anaemia
Key definition
Alleles are codominant if they both
contribute to the phenotype.
Parent
genotypes
Gametes
HAHS
HA
HS
HAHS
HA
HS
HA
HS
HA
HAHA
normal
HAHS
carrier
HS
HSHA
carrier
HSHS
sickle-cell
anaemia
25%
normal
haemoglobin
50%
carrier
3ICKLECELLANAEMIAWASTHElRSTHUMANDISEASETOBEUNDERSTOODATTHEMOLECULARLEVEL
s !LLINDIVIDUALSWITHTHEDISEASEHAVETHESAMEMUTATION
s 4HEB-strands of haemoglobin differ by one amino acid at position 6.
s )NNORMALHAEMOGLOBINGLUTAMICACIDISATPOSITIONBUTINSICKLECELLHAEMOGLOBIN
VALINEISPRESENTINSTEAD
s 7HENTHISABNORMALHAEMOGLOBINISDEOXYGENATEDITISNOTSOLUBLEANDBECOMES
crystalline and aggregates into more linear and less globular structures. This deforms
THEREDBLOODCELLSMAKINGTHEMINmEXIBLEANDOFTENSICKLESHAPEDANDUNABLETO
SQUEEZETHROUGHCAPILLARIES
s !FTERMANYCYCLESOFOXYGENATIONANDDEOXYGENATIONSOMECELLSBECOMEIRREVERSIBLY
sickled. Some are destroyed.
s )FENOUGHSICKLECELLSBECOMELODGEDINCAPILLARIESBLOODmOWISIMPEDED/RGANS
PARTICULARLYBONESDONOTRECEIVEENOUGHOXYGENLEADINGTOAPAINFULCRISIS
s %VENTUALLYORGANSESPECIALLYHEARTLUNGSANDKIDNEYSBECOMEDAMAGED
s 4HEGENOTYPEOFPEOPLEWITHNORMALHAEMOGLOBINCANBEDENOTEDASHAHA.
s 4HEGENOTYPEOFPEOPLEWITHSICKLECELLANAEMIAISHsHs.
s 4HEGENOTYPEOFSYMPTOMLESSHETEROZYGOTESISHAHs.
25%
sicklecell anaemia
Key:
HA allele for normal haemoglobin
HS allele for abnormal haemoglobin
Figure 3 Genotypes and phenotypes of
offspring of two parents, both
HETEROZYGOUSFORSICKLECELLDISEASE
Figure 1 3ICKLEDANDREDBLOODCELLS™
Figure 2 0ATIENTWITHSICKLECELLANAEMIA
EXPERIENCINGAPAINFULCRISIS
)NHETEROZYGOTESREDBLOODCELLSAREMADEINTHEBONEMARROWWITHHALFTHEIR
HAEMOGLOBINNORMALANDHALFSICKLED4HEPRESENCEOFNORMALHAEMOGLOBINPREVENTS
SICKLINGINTHEREDCELLSWHENTHEYAREINTHECIRCULATIONANDDEOXYGENATED4HUS
HETEROZYGOTESARESYMPTOMLESSCARRIERSANDATWHOLEORGANISMLEVELTHISCONDITIONCOULD
BECONSIDEREDTOBERECESSIVE(OWEVERATTHEMOLECULARANDCELLULARLEVELBECAUSE
BOTHALLELESCONTRIBUTETOTHEPHENOTYPEASOBSERVEDINREDBLOODCELLSITISCODOMINANT
What are the possible genotypes and phenotypes of offspring of two parents, both
carriers of sickle-cell disease?
7ECANSEEFROM&IGURETHATWEEXPECTTOHAVENORMALHAEMOGLOBINTOBE
HETEROZYGOUSCARRIERSANDTOHAVESICKLECELLANAEMIA
Roan cattle
/NEOFTHEGENESFORCOATCOLOURINSHORTHORNCATTLEHASTWOALLELESCR codes for red
hairs and CW codes for white hairs.
s (OMOZYGOUSINDIVIDUALSWITHGENOTYPECRCRHAVEREDCHESTNUTCOATS
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Module 1
Cellular control
Using genetic diagrams – 2
s (OMOZYGOUSINDIVIDUALSWITHGENOTYPECWCWHAVEWHITECOATS
s (ETEROZYGOTESGENOTYPECRCWHAVEREDANDWHITEHAIRSANDTHECOATISROAN
)FREDANDWHITESHORTHORNCATTLEAREINTERBREDTHENALLTHEOFFSPRINGAREROANnAMIXTURE
of red and white.
)FTWOROANSHORTHORNCATTLEAREINTERBREDTHENTHEPROBABLEPHENOTYPESARERED
ROANANDWHITE&IGURE
STRETCH and CHALLENGE
Patients with sickle-cell anaemia can be treated by blood transfusions about every
three months. Some patients have been treated with chemotherapy. They are given
hydroxyurea that causes an increase in fetal haemoglobin formation and produces an
improvement in red blood cell function.
Some patients have been treated with bone marrow transplants, although this is
reserved for severely affected patients as there is a risk of graft-versus-host reaction.
Questions
A Explain why an increase in fetal haemoglobin improves red cell function in sickle-cell
patients. (Hint: revisit AS work on transport in animals.)
B Suggest what is meant by ‘graft-versus-host reaction’. (Hint: revisit AS work on the
immune system.)
C Suggest why blood transfusions are given every three months. (Hint: revisit AS work
on transport in animals.)
There are various ways to diagnose sickle-cell disease. A blood smear can be exposed to
very low oxygen concentrations to see if red cells become sickled. However, this can
give false positives for carriers, as under these laboratory conditions, red blood cells
from heterozygotes may also sickle. A more accurate diagnosis involves electrophoresis
of haemoglobin. A sample of extracted haemoglobin is placed in a well in an
electrophoresis gel and a buffer solution added. Electrodes are placed at either end and
the protein migrates towards the positive electrode, according to the overall electric
charge on the protein. Glutamic acid has a negative charge and valine is neutral.
Figure 4 Red shorthorn bull
Parental
phenotypes
Cow ( )
Red
White
Genotypes
CRCR
CWCW
CR
CW
Gametes
Offspring genotypes
All CRCW
Offspring phenotypes
All roan
Figure 5 Genetic diagram of cross
between red shorthorn cow and white
shorthorn bull
Parent
phenotypes
Roan
Roan
Parent
genotypes
CWCR
CWCR
Gametes
An analysis of the gene encoding the B-globulin from the individual’s DNA can diagnose
the presence of the mutation. This can also be done on fetal cells obtained prenatally
by chorionic villus sampling or amniocentesis.
Question
E Explain why cells such as skin cells, and not red blood cells, are used for DNA analysis
for the presence of a mutated B-globulin gene.
Questions
1 !SHORTHORNBULLISRUNWITHAHERDOFREDSHORTHORNCOWS/FTHECALVESPRODUCEDINTHE
lRSTYEARAREREDANDAREROAN7HATISTHEGENOTYPEANDPHENOTYPEOFTHEBULL
2 $UCKSOFTHEBREEDCALLED3WEDISHBLUEHAVEGREYPLUMAGE)FTWO3WEDISHBLUE
DUCKSMATESOMEOFFSPRINGAREGREYSOMEAREWHITEANDSOMEAREBLACK%XPLAIN
THESEOBSERVATIONSANDDESCRIBEHOWAPOULTRYBREEDERCOULDMAKESUREHEPRODUCES
all Swedish blue ducks.
3 (a) Suggest what sort of mutation causes sickle-cell disease.
(b) %XPLAINWHYONEOFTHESYMPTOMSOFSICKLECELLDISEASEISSEVEREANAEMIA
CW
CR
CW
CR
CW
CR
CW
CWCW
white
CWCR
roan
CR
CRCW
roan
CRCR
red
25%
white
50%
roan
25%
red
Question
D Predict how the migration patterns of normal and sickle-cell haemoglobins will differ
from each other.
Bull ( )
Figure 6 Genetic diagram showing
probable outcome from interbreeding
roan shorthorn cattle
Examiner tip
Always provide a key with your genetic
diagrams. Indicate which symbol relates
to which allele.
You may be told the genotype of the
female parent and of the male parent.
Don’t get confused and assume that the
gene in question is sex-linked.
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