Q.37 Which of the following are autosomal dominant traits?
(B) Myotonic dystrophy
(C) Phenylthio Carbamide Tasting
(D) Phenylketonuria
(E) Brachydactyly
Choose the most appropriate answer from the options given below:
Myotonic dystrophy, Phenylthiocarbamide (PTC) tasting, and brachydactyly are autosomal dominant traits.
These traits require only one copy of the dominant allele on an autosome for expression, unlike recessive or X-linked conditions.
Option Analysis
(A) Colour Blindness
Colour blindness, especially red-green type, is X-linked recessive, not autosomal dominant.
It primarily affects males due to inheritance on the X chromosome.
Blue-yellow variants are rare and autosomal dominant, but “colour blindness” typically refers to the recessive form.
(B) Myotonic Dystrophy
Myotonic dystrophy is a classic autosomal dominant disorder caused by CTG repeat expansion in the DMPK gene.
One mutated allele from either parent suffices for symptoms like muscle weakness.
It shows anticipation, worsening across generations.
(C) Phenylthiocarbamide Tasting
PTC tasting (ability to taste bitterness) is autosomal dominant, controlled by the TAS2R38 gene.
Tasters have at least one dominant allele (T); non-tasters are homozygous recessive (tt).
This trait aids in studying taste genetics and food preferences.
(D) Phenylketonuria
Phenylketonuria (PKU) is autosomal recessive, due to PAH gene mutations blocking phenylalanine metabolism.
Both alleles must be mutated for intellectual disability risk.
Newborn screening and diet manage it effectively.
(E) Brachydactyly
Brachydactyly (short fingers/toes) exemplifies autosomal dominant inheritance, often via IHH or other genes.
A single dominant allele causes variable expressivity in digit length.
Type A1 is the most studied form.
Correct Answer
(4) (B), (C), (E) only
Autosomal dominant traits express with one dominant allele on non-sex chromosomes, key for genetics exams like GATE Life Sciences or CUET PG. This guide analyzes colour blindness, myotonic dystrophy, phenylthiocarbamide (PTC) tasting, phenylketonuria (PKU), and brachydactyly to identify true autosomal dominant traits.
Understanding Autosomal Dominant Inheritance
In autosomal dominant traits, a single mutated allele (e.g., from one parent) triggers the phenotype, unlike recessives needing two.
Penetrance varies, but 50% offspring risk per affected parent applies.
Examples aid molecular biology and plant sciences students prepping competitive exams.
Detailed Trait Breakdown
| Trait | Inheritance Type | Gene/Chromosome | Key Feature |
|---|---|---|---|
| Colour Blindness (A) | X-linked recessive | OPN1LW/MW (X) | Males > females; red-green common |
| Myotonic Dystrophy (B) | Autosomal dominant | DMPK (19q) | Muscle stiffness; anticipation |
| PTC Tasting (C) | Autosomal dominant | TAS2R38 (7q) | Bitter taste perception |
| Phenylketonuria (D) | Autosomal recessive | PAH (12q) | Metabolic disorder; screening vital |
| Brachydactyly (E) | Autosomal dominant | IHH (2q) | Short digits; variable expression |
Exam Relevance for Life Sciences
For CUET PG Botany or GATE, recognize myotonic dystrophy, PTC tasting, and brachydactyly as autosomal dominant traits (option 4).
Colour blindness confuses as X-linked; PKU as recessive.
Master pedigrees: dominant shows in every generation, unaffected parents can’t pass it.
This breakdown equips you for genetics MCQs in molecular biology and human heredity sections.
