Q88. The expression of holandric genes causes which of the following genetic trait
in humans?
(A) Haemophilia
(B) Sickle cell anaemia
(C) Down’s Syndrome
(D) Hypertrichosis
The correct answer is (D) Hypertrichosis. Holandric genes, located exclusively on the Y chromosome, express traits passed directly from father to son in humans, and hypertrichosis (excessive ear hair growth) exemplifies this pattern.
Option Analysis
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(A) Haemophilia: This X-linked recessive disorder affects blood clotting and follows a criss-cross inheritance pattern, where affected males pass the gene to carrier daughters, not sons.
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(B) Sickle cell anaemia: An autosomal recessive condition on chromosome 11, it requires two mutated alleles from either parent and impacts both sexes equally.
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(C) Down’s Syndrome: Caused by trisomy 21 (extra chromosome 21), this chromosomal aneuploidy arises from nondisjunction, not Y-linked genes.
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(D) Hypertrichosis: Known as hypertrichosis pinnae auris (hairy ears), this Y-linked trait appears only in males and transmits father-to-son via holandric genes.
Holandric Inheritance Explained
Holandric genes reside on the non-homologous region of the Y chromosome, absent in females (XX), so traits manifest solely in males (XY) and pass unbroken from fathers to sons. Examples include certain male-specific features like ear hair growth, though true Y-linked traits remain rare due to the Y chromosome’s limited gene count.
Holandric genes hypertrichosis represents a classic Y-linked inheritance in human genetics, vital for CSIR NET aspirants studying sex-linked traits. These genes, confined to the Y chromosome, ensure father-to-son transmission without female involvement.
Understanding Holandric Genes
Holandric genes hypertrichosis traits arise from the Y chromosome’s differential region, coding for male-specific phenotypes like excessive auricular hair (hypertrichosis pinnae). Unlike X-linked conditions, no homologous X counterpart exists, preventing female expression or carriage.
Comparing Genetic Traits
| Trait | Inheritance Type | Chromosome | Affects Both Sexes? |
|---|---|---|---|
| Haemophilia | X-linked recessive | X | Yes (males severe) |
| Sickle Cell Anaemia | Autosomal recessive | 11 | Yes |
| Down’s Syndrome | Chromosomal (trisomy) | 21 | Yes |
| Hypertrichosis | Holandric (Y-linked) | Y | Males only |
This table highlights why only hypertrichosis matches holandric expression.
Relevance for CSIR NET
In exams like CSIR NET Life Sciences, questions test differentiation of inheritance modes—holandric genes hypertrichosis exemplifies Y-linkage, contrasting autosomal or X-linked disorders. Mastery aids pedigree analysis and genetic counseling applications.
