2. Consider the following pedigree chart
(1) X-linked recessive
(2) X-linked dominant
(3) Sex limited recessive
(4) Autosomal dominant
The pedigree chart is most consistent with an X‑linked recessive mode of inheritance, so the correct option is (1) X‑linked recessive.
Understanding the pedigree chart
In the given pedigree, an affected female is born to two unaffected parents, which clearly indicates a recessive trait because the phenotype appears in the offspring despite both parents being phenotypically normal. Both males and females can be affected, but affected males are fewer and the trait can skip generations, a classical indication of X‑linked recessive inheritance rather than autosomal dominant or X‑linked dominant.
Another key observation is that unaffected parents can have an affected son or daughter, implying that both parents are carriers; in X‑linked recessive disorders, carrier mothers and normal fathers often produce affected sons, and homozygous daughters can occasionally appear when the father is affected or the mother is a carrier of the mutant allele.
Option (1) X‑linked recessive – why it fits
In X‑linked recessive inheritance, males are hemizygous for the X chromosome, so a single mutant allele causes the disease, whereas females must be homozygous mutant to be affected. Unaffected carrier females can transmit the allele to sons (who may be affected) and to daughters (who often become carriers), explaining how unaffected parents can produce affected offspring as seen in the pedigree.
The pattern in the question is compatible with:
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unaffected parents producing an affected child,
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more frequent expression in males,
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absence of father‑to‑son transmission but possible transmission from mother to son,
all hallmarks of X‑linked recessive disorders such as haemophilia A and red–green colour blindness.
Option (2) X‑linked dominant – why it is wrong
In X‑linked dominant inheritance, every affected male must pass the trait to all his daughters and to none of his sons, because daughters receive his only X while sons receive his Y. The CSIR NET June 2008 question does not show the obligatory transmission from an affected father to all daughters in each generation, so the pattern does not match X‑linked dominant rules.
Moreover, X‑linked dominant traits usually do not skip generations; at least one affected parent is seen in every generation, which is not the case here where unaffected parents have affected offspring, again arguing against dominance.
Option (3) Sex‑limited recessive – why it is wrong
Sex‑limited traits are expressed in only one sex even though the genes can be present in both sexes; classic examples include traits limited to male secondary sexual characters in animals. In the given pedigree, both males and females can be affected, so the phenotype is not restricted to a single sex and therefore cannot be classified as sex‑limited recessive.
Additionally, sex‑limited traits often involve hormonal or anatomical differences rather than simple Mendelian X‑linked transmission, while the exam question clearly aims at distinguishing standard Mendelian modes such as autosomal vs X‑linked dominant or recessive.
Option (4) Autosomal dominant – why it is wrong
Autosomal dominant traits characteristically appear in every generation, and every affected individual has at least one affected parent, because a single mutant allele on a non‑sex chromosome is enough to cause the phenotype. The pedigree in this CSIR NET problem includes affected individuals born to unaffected parents, which contradicts this essential feature of autosomal dominant inheritance.
Furthermore, autosomal dominant traits usually affect males and females equally and show vertical transmission without generational skips; the presence of skipped generations and unaffected carriers instead suggests a recessive pattern, consistent with X‑linked recessive rather than autosomal dominant.
Introduction (SEO‑optimized)
Pedigree analysis questions are a favourite in CSIR NET Life Science, and the June 2008 paper includes a classic problem on identifying the correct mode of inheritance from a small family tree. By carefully examining which individuals are affected, whether unaffected parents can have affected offspring, and how the trait passes between males and females, the pedigree clearly points to X‑linked recessive inheritance, not X‑linked dominant, sex‑limited recessive, or autosomal dominant patterns.
