Cytogenetics Questions and Answers – Penetrance and Expressivity

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This set of Cytogenetics Multiple Choice Questions & Answers (MCQs) focuses on “Penetrance and Expressivity”.

1. An individual with a homozygous genotype for a particular phenotype ____________ expresses the phenotype.
a) Always
b) Never
c) Sometimes
d) Unknown relation
View Answer

Answer: c
Explanation: It is not that the homozygous genotype will always express the required phenotype. It may or may not express it depending on some factors. This gives the concept of penetrance.
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2. It is known that homozygous dominant gene for a type of cone gives brown colour. The formation of cones with brown, yellow colour expresses the _______________ and white expresses the _______________
a) Penetrance, expressivity
b) Expressivity, genotype
c) Genotype, Phenotype
d) Expressivity, penetrance
View Answer

Answer: d
Explanation: The formation of coloured cones is sure that the cones are penetrant, it is variation in the expressivity that gives them different colour. The uncoloured cone shows no penetrance.

3. Choose the incorrect option about penetrance.
a) If penetrance is 100% all recessive genotype shows one phenotype
b) If penetrance is 100% the expressivity is 100%
c) If penetrance is 100% all the heterozygotes have similar phenotype
d) If penetrance is 100% all the dominant genotypes have a different phenotype from recessive
View Answer

Answer: b
Explanation: In case of penetrance being 100% it makes sure that a particular phenotype that is determined by the genotype is expressed in all individuals. But the limit to which it is expressed in the different individuals can vary so the expressivity may not always be 100%.
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4. If the gene is non-penetrant then which of the following is not right?
a) The pedigree chart will not express the defecting phenotype in that individual
b) The person’s children are not likely to express that phenotype
c) The expressivity is 0%
d) The genotype here didn’t make its proper phenotypic expression
View Answer

Answer: b
Explanation: If the gene is non-penetrant for an individual it is not mandatory that it will also be non-penetrant for his offspring. As a result, the pedigree could appear to skip a generation with the parent being a carrier.

5. Expressivity depends on penetrance.
a) True
b) False
View Answer

Answer: a
Explanation: Penetrance doesn’t depend on expressivity, i.e. it is considered penetrant for an individual even if the expressivity varies among individuals, but it is not possible to measure the expressivity unless the gene is penetrant. So the expressivity depends on penetrance.
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6. Which of the following trait shows only variation in expressivity?
a) Brachydactyly
b) Osteoporosis imperfectica
c) Pea seed colour
d) Polydactyly
View Answer

Answer: b
Explanation: In case of pea seed colour the penetrance and expressivity are both 100%. In case of brachydactyly and polydactyly the penetrance varies as well as the expressivity. But in the case on osteoporosis imperfectica it is almost 100% penetrant but the expressivity is different.

7. Neurofibrilomatosis varies in ________________
a) Penetrance
b) Expressivity
c) Both penetrance and expressivity
d) It is 100% penetrant and expressive
View Answer

Answer: c
Explanation: Neurofibrilomatosis is an autosomal dominant trait that causes tumour lie growth all over the body. It is 50-80% penetrant and varies in expressivity.
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8. Expessivity doesn’t depend on which of the following factors?
a) Environment
b) Penetrance
c) Genotype
d) Linkage
View Answer

Answer: d
Explanation: Expressivity of a trait depends on all three of environment, penetrance and genotype but it doesn’t depend on linkage.

9. Expressivity may be thought of as a result of different degrees of function of the protein encoded by the gene.
a) True
b) False
View Answer

Answer: a
Explanation: Expressivity means that the individual is able to make the protein due to the penetrance and the genotype. But the degree of functionality of that encoded protein may vary giving different expressivity.
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10. The genes that Mendel worked with were__________________
a) Variable in penetrance and expressivity
b) Variable in penetrance only
c) Variable in expressivity only
d) 100% penetrant with 100% expressivity
View Answer

Answer: d
Explanation: The 7 sets of pea genes that Mendel worked with showed 100% penetrance and 100% expressivity. Due to this the dominant and recessive genes always expressed the different phenotype to the full degree of expression.

Sanfoundry Global Education & Learning Series – Cytogenetics.

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Manish Bhojasia - Founder & CTO at Sanfoundry
Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He is Linux Kernel Developer & SAN Architect and is passionate about competency developments in these areas. He lives in Bangalore and delivers focused training sessions to IT professionals in Linux Kernel, Linux Debugging, Linux Device Drivers, Linux Networking, Linux Storage, Advanced C Programming, SAN Storage Technologies, SCSI Internals & Storage Protocols such as iSCSI & Fiber Channel. Stay connected with him @ LinkedIn | Youtube | Instagram | Facebook | Twitter