genetics for college

1. SCIENCE 2 - GENETICSSCIENCE 2 - GENETICS
INHERITED DISEASES ANDINHERITED DISEASES AND
CODOMINANCECODOMINANCE

2. INHERITED DISEASESINHERITED DISEASES
• There are several important medical examples ofThere are several important medical examples of
monohybrid inheritance, including cystic fibrosis,monohybrid inheritance, including cystic fibrosis,
Huntington’s chorea and sickle cell anaemia.Huntington’s chorea and sickle cell anaemia.
• Cystic fibrosis is caused by a recessive allele.Cystic fibrosis is caused by a recessive allele.
Heterozygous individuals are not affected by theHeterozygous individuals are not affected by the
condition, but they are carriers of the allele. People withcondition, but they are carriers of the allele. People with
cystic fibrosis have an imbalance of chloride ions acrosscystic fibrosis have an imbalance of chloride ions across
the membranes of cell lining some of the majorthe membranes of cell lining some of the major
passageways of the body-causes mucus to build up.passageways of the body-causes mucus to build up.
Congesting the lungs and airways and blocking theCongesting the lungs and airways and blocking the
pancreatic duct so that digestion is limited.pancreatic duct so that digestion is limited.

3. • Huntington’s chorea is caused by a dominant allele soHuntington’s chorea is caused by a dominant allele so
that heterozygous individuals are affected by thethat heterozygous individuals are affected by the
condition. The disease affects the central nervouscondition. The disease affects the central nervous
system. There are no symptoms until middle age, butsystem. There are no symptoms until middle age, but
then there is a rapid loss of coordination, leading tothen there is a rapid loss of coordination, leading to
death.death.
• Sickle cell anaemia is a condition in which aSickle cell anaemia is a condition in which a
homozygote has the disease, but a heterozygoushomozygote has the disease, but a heterozygous
individual may gain some benefit in certainindividual may gain some benefit in certain
environmentsenvironments

4. YOUR TASKYOUR TASK
• Cystic fibrosis is a common inherited diseaseCystic fibrosis is a common inherited disease
caused by a recessive allele. A blood test cancaused by a recessive allele. A blood test can
detect this allele. A man and his wife were bothdetect this allele. A man and his wife were both
found to be carriers of cystic fibrosis allele.found to be carriers of cystic fibrosis allele.
a. What is meant by a carrier of the allele?a. What is meant by a carrier of the allele?
b. Draw a genetic diagram to show theb. Draw a genetic diagram to show the
inheritance of cystic fibrosis in any children ofinheritance of cystic fibrosis in any children of
this couple. What is the chance that a child willthis couple. What is the chance that a child will
have cystic fibrosis?have cystic fibrosis?

5. Let N = normal allele and n = mutant (cystic fibrosis, CF allele)
If both parents are carriers (heterozygous):
Parents Nn X Nn
N n N nGametes
F1, generation
Genotype
NN Nn Nn nn
Phenotype No CF No CF No CF Has CF
Ratio 3 : 1

6. Let N = normal allele and n = mutant (cystic fibrosis, CF allele)
If both parents are carriers (heterozygous):
Parents Nn X Nn
N n N nGametes
F1, generation
Genotype
NN Nn Nn nn
Phenotype No CF No CF No CF Has CF
Ratio 3 : 1