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Chapter 6. Mendelian Genetics: Patterns of Gene Transmission

In chickens there is a known gene that determines the type of tail and neck plumage. Cock-feathering is more long and curved, while hen-feathering is more short and rounded. This gene is not on a sex chromosome. The genotype–phenotype relationship is described below:

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Genotype Phenotype in females Phenotype in males
HH Hen-feathered Hen-feathered
Hh Hen-feathered Hen-feathered
hh Hen-feathered Cock-feathered

In this example, the inheritance that most likely exhibits cock feathering is

A. recessive with sex-limited phenotype.

B. dominant with sex-limited phenotype.

C. semi-dominance.

D. co-dominance.

E. mitochondrial.

A is the correct answer.

The condition is recessive because homozygous individuals are affected. Among the homozygotes, only males are affected. The other answers are incorrect because:

  • B. Heterozygotes are not affected so it is not dominant.

  • C. Heterozygotes are not affected so it is not dominant.

  • D. Heterozygotes are not affected so it is not dominant.

  • E. There is nothing that suggests maternal/mitochondrial inheritance.

Parental age effects on the incidence of genetic disorders include

A. lower incidence of genetic disorders in the offspring of older parents.

B. increased incidence of chromosome aneuploidy with advanced paternal age.

C. increased incidence of single gene mutations with advanced maternal age.

D. increased incidence of non-disjunction with advanced maternal age.

E. decreased incidence of transcription errors with advanced paternal age.

D is the correct answer.

The increased incidence of chromosome aneuploidy seen with advanced maternal age is due to an increased rate on chromosomal non-disjunction (Chapter 5). The other answers are incorrect because:

  • A. Older parents have a higher incidence of congenital anomalies.

  • B. Older fathers have an increased incidence of single gene mutations.

  • C. Older mother have an increased incidence of chromosome aneuploidy (Chapter 5).

  • E. Transcription errors increase with advancing paternal age.

Patients with Smith-Lemli-Opitz (SLO) syndrome exhibit many clinical features including mental retardation, an unusual facial appearance, genital abnormalities, and syndactyly (fusion of the digits). Patients with SLO tend to be similar in their features. SLO is an autosomal recessive condition. All heterozygotes will have expression. There is only one gene known to be responsible for this condition. This condition shows

A. highly variable phenotype.

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