Forensic Serology Questions and Answers

Answer: C

DNA fingerprinting of blood and other body fluids was contributed to by Alec Jeffreys.

Sir Alec Jeffreys of Leicester discovered in 1984 that minisatellite regions of DNA differ among individuals and developed DNA profiling, which was first used in a criminal case in 1986.

Osborn is known for questioned documents, Goddard for firearms comparison and Bertillon for anthropometry.

Before this work the identification of blood relied on groups and enzymes, and DNA profiling raised the power of discrimination to a very high level and it is used in paternity and criminal cases.

Answer: B

Davidson's body is found in the neutrophil.

It is a small projection of chromatin, shaped like a drumstick, that is attached to the nucleus of the neutrophil in the female and is the inactive second X chromosome. It is used for sex determination from blood stains.

Platelets, red blood cells and reticulocytes do not have a nucleus with this structure.

The count of drumsticks is made on a smear stained in the usual way, and a female blood stain has a few drumsticks in each hundred neutrophils, so the test is of limited value.

Answer: A

A marker present in seminal plasma is Peptidase A.

Peptidase A is an enzyme found in seminal plasma, and it has genetic types, which is why it is used in serological typing of semen stains.

Peptidases C and D are found in other body fluids or cells, and transferrin is a blood protein.

Because PepA shows several genetic variants in the population, a typing result on a semen stain can help to include or exclude a suspect, which adds to its value in forensic serology.

Answer: B

The Kastle-Meyer test uses phenolphthalein.

It is a presumptive test for blood. The reduced phenolphthalein is oxidised to pink phenolphthalein by hydrogen peroxide in the presence of haemoglobin, which acts as a catalyst.

Benzidine and o-tolidine are used in other catalytic tests, and luminol is used for chemiluminescence.

A pink colour that appears at once, within a few seconds, is read as positive, and a late colour is disregarded, because other oxidising substances can give the same change slowly.

Answer: B

Takayama's test is based on haemochromogen crystal formation.

Blood heated with Takayama's reagent of pyridine, glucose and sodium hydroxide gives pink feathery crystals of pyridine haemochromogen, which confirms blood.

Haemin crystal formation is Teichmann's test, and ferric sulphate and globulin crystals are not used for confirmation of blood.

The pink crystals are seen under the microscope and the test can be done on a very small stain, which makes it a useful confirmatory test.

Answer: C

The test to find if blood is human or animal is the precipitin test.

An antiserum to human protein, raised in a rabbit, forms a visible precipitate with an extract of human blood, but not with blood of other species, so it gives the species of origin.

The benzidine, phenolphthalein and o-tolidine tests only show that blood may be present.

The test is also done on blood stains that are old and dried, when an extract is made in saline, and it can show the animal group from which the blood has come.

Answer: C

The one that is not a DNA base is threonine (printed 'Thronine').

Threonine is an amino acid, a building block of proteins. DNA has four bases: adenine, guanine, cytosine and thymine.

Guanine, cytosine and adenine are bases of DNA, so they are not the answer to this 'not' question.

Uracil is the base that takes the place of thymine in RNA, so the four letters A, C, G and T should be remembered for DNA.

Answer: D

All of the statements are true of SDS-PAGE, so the answer is (D). The detergent sodium dodecyl sulphate (SDS) denatures the proteins and coats them so that every protein carries about the same negative charge per unit mass. Because the charge-to-mass ratio is then the same, the proteins separate on the gel by size alone, and the smaller proteins migrate more rapidly through the pores of the gel.

Since (A), (B) and (C) are each correct, no single one is the complete answer.

Answer: A

The approximate frequency of blood group AB in a population is 5 to 10 per cent, option (A). AB is the rarest of the four ABO groups; in India it is roughly 8 per cent, and in many populations it is lower.

The figures of 25, 40 and 50 per cent are far too high for any single ABO group other than the commonest ones, such as O or B in some populations, and AB is never that common.

The distribution of the ABO groups varies from one population to another, so a question on frequency expects the approximate range of the rarest group, which is the one named in the key.

Answer: B

The Kastle-Meyer presumptive test for blood is also known as the phenolphthalein test. Reduced phenolphthalein is colourless, and in the presence of blood and hydrogen peroxide it is oxidised to phenolphthalein, which turns pink. The answer is (B).

The benzidine test, the o-tolidine test and the tetramethylbenzidine (TMB) test are other presumptive colour tests for blood that work on a similar catalytic principle, but they are not called the Kastle-Meyer test.