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NCERT's Class 12 genetics chapter describes phenylketonuria as an "inborn error of metabolism". That phrase is more than a hundred years old, and it began with one doctor's careful look at a rare condition. This post tells how Archibald Garrod linked chemistry to Mendel's laws, works through the probability he relied on, and ends with what you can learn from how he worked.
Who was Archibald Garrod?
The Royal College of Physicians biography records that Garrod was born in London on 25 November 1857 and died in Cambridge on 28 March 1936. He worked at the Hospital for Sick Children, Great Ormond Street, and at St Bartholomew's Hospital, gave the Croonian Lectures in 1908, and was Regius Professor of Medicine at Oxford from 1920 to 1927.
What did Garrod notice about alkaptonuria?
In alkaptonuria, the urine contains homogentisic acid. Garrod's 1902 paper in The Lancet, The incidence of alkaptonuria: a study in chemical individuality41972-6), is freely available as a reprint in the Yale Journal of Biology and Medicine. In it he made three observations.
First, a person was either "frankly alkaptonuric" or normal; he found no in-between cases. Second, of 40 recorded cases, 29 were male and 11 female. Third, and most striking, families often had several affected children while the parents were unaffected, and the parents were often first cousins. He had already pointed out in 1901 that of four British families with 11 affected members, three came from first-cousin marriages. In his table of six first-cousin families, 12 of 36 children were alkaptonuric.
He compared this with an estimate by G. Darwin that perhaps 3% at most of all marriages in England were between first cousins. Far more alkaptonuric children came from such marriages than chance would suggest.
How did Mendel's laws explain it?
Garrod wrote that "it has recently been pointed out by Bateson that the law of heredity discovered by Mendel offers a reasonable account of such phenomena." When two recessive gametes meet, the recessive character appears. A rare recessive trait is most likely to show up in a child when both parents carry it, and relatives are more likely to carry the same one. He concluded that "a peculiarity of the gametes of both parents is necessary for its production."
He then went further. If alkaptonuria was an inherited chemical difference, maybe smaller chemical differences are everywhere: "just as no two individuals of a species are absolutely identical in bodily structure neither are their chemical processes carried out on exactly the same lines."
Worked example: the probability behind Garrod's families
Let A be the normal allele and a the recessive one. Two unaffected carrier parents are Aa × Aa.
Each parent passes A or a with probability 1/2.
The chance a child is aa is 1/2 × 1/2 = 1/4.
Carriers (Aa) occur with probability 2 × 1/2 × 1/2 = 1/2, and AA with 1/4.
So in carrier × carrier families you expect about one affected child in four. Garrod's six first-cousin families had 12 affected out of 36, which is 1/3. That is in the same range but higher. With small numbers, a gap like this is common, and families tend to be noticed in the first place because they have an affected child, so they are not a random sample. Our post on solving genetics problems for NEET drills these ratios.
What were the inborn errors of metabolism?
In the 1908 Croonian Lectures to the Royal College of Physicians, published in The Lancet78482-6), Garrod named the idea. A centenary review by Charles Scriver (2008) says he used albinism, alkaptonuria, cystinuria and pentosuria to illustrate it, and that his work showed normal metabolic pathways "made variant by Mendelian inheritance". The review adds that the idea was taken up mainly by biochemists and was controversial among geneticists at the time.
The NCERT Class 12 chapter Principles of Inheritance and Variation uses the same framing for phenylketonuria: an inborn error of metabolism inherited as an autosomal recessive trait, in which the affected person lacks the enzyme that converts phenylalanine into tyrosine. The direct line from one gene to one enzyme was later made by Beadle and Tatum; see our post on the one gene one enzyme idea.
Common mistakes students make here
Treating "recessive" as "rare". Recessive describes how a trait is inherited, not how common it is.
Using 1/4 for every family. The 1/4 applies only when both parents are carriers.
Mixing up phenotype and genotype ratios. Aa × Aa gives a 3:1 phenotype ratio but a 1:2:1 genotype ratio.
If a pedigree question keeps defeating you at the first step, Fermi's AI tutor can work out what you already know and build a plan for the session, so you start from the step you are missing.
What can a student learn from Garrod's way of working?
Garrod collected every case he could, wrote to other doctors for missing family details, and laid the numbers out in tables before reaching a conclusion. Then he connected his findings to a new idea from another field. When you study, look for links between chapters: genetics and biochemistry were separate subjects until Garrod joined them.
Today, draw a pedigree for two carrier parents with four children and work out the expected number who are affected.
