Heterozygous test
The heterozygous test is a laboratory method that can be used to determine whether a phenotypically (in appearance) healthy person has a recessive hereditary disease . The genotype of the person is examined. One speaks of a molecular genetic proof of carrier for an autosomal recessive, hereditary disease. The test is used for the early detection of recessive hereditary diseases or the passing on of the disposition to a common child.
target
The aim of the test is to find out whether the person in question only has one functional allele relating to a certain characteristic, for example for the production of a certain enzyme , the production of which is specified and encoded in the genetic code . In the case of heterozygosity , in this example there are hindrances in the production of the enzyme concerned, since in comparison to homozygosity, one allele is missing, which means that half of the production information relating to the amount to be produced is also missing. The production runs at half power, so to speak, which z. B. can lead to an accumulation of toxins in the body, which cannot be broken down quickly enough because the necessary enzymes are missing (see example below).
Example: Phenylketonuria (PKU)
Phenylketonuria is an autosomal , recessive metabolic disorder .
The cause of PKU is a point mutation on chromosome 12 . This means that the enzyme phenylalanine hydroxylase is no longer synthesized (produced) in the liver . However, this enzyme is necessary for the conversion of phenylalanine into tyrosine . As a result, there is an accumulation of phenylalanine in the body and a deficiency of tyrosine.
Symptoms:
- mental defects due to the excess of phenylalanine, negative effects on the formation of the human brain.
- Pigment disorders due to the lack of tyrosine, which is necessary for the conversion of the skin pigment melanin .
- malodor due to the conversion of phenylalanine into phenyl acetate , which is secreted through sweat and urine .
Therapy:
- Special diet with phenylalaninarmer and thyrosinreicher food
Carrying out the heterozygote test (example PKU)
- Test subjects are given a certain amount of phenylalanine. Heterozygotes convert phenylalanine to tyrosine much more slowly, while healthy homozygotes break it down more quickly. The tyrosine content in the blood is measured at intervals of approx. 2 hours ; if it does not exceed a certain amount, the test person is heterozygous.
- The increase in the phenylalanine concentration in the blood is determined with the help of a biological test ( Guthrie test ). For this purpose, phenylalanine-deficient mutants from Bacillus subtilis , which can only grow on a phenylalanine-enriched culture medium, are used. (In a more recent test method, the bacterial culture is prevented from growing by an inhibitor . Phenylalanine from the blood removes the inhibition.) If a drop of blood from the tested person is now added to this bacterial culture, a growth of the bacteria in the area of the blood drop shows the presence from phenylalanine on. If the test result is positive, the person tested is a heterozygous carrier of the recessive gene for phenylketonuria.