How are a developing fetus and a newborn baby screened for conditions, and how are inheritance patterns used?
Antenatal screening (ultrasound imaging, biochemical marker tests and diagnostic tests such as amniocentesis and chorionic villus sampling), postnatal screening for metabolic disorders such as PKU, and the use of pedigree charts and patterns of inheritance to assess genetic risk.
An SQA Higher Human Biology answer on antenatal and postnatal screening, covering ultrasound dating and anomaly scans, biochemical marker tests, diagnostic amniocentesis and chorionic villus sampling, postnatal screening for metabolic disorders such as PKU, and the use of pedigree charts and patterns of inheritance to assess genetic risk.
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What this dot point is asking
The SQA wants you to describe antenatal screening (ultrasound, biochemical marker tests) and diagnostic tests (amniocentesis, chorionic villus sampling), describe postnatal screening for metabolic disorders such as PKU, and explain how pedigree charts and patterns of inheritance are used to assess genetic risk.
Antenatal screening
Two kinds of screening are used during pregnancy:
- Ultrasound imaging. High-frequency sound waves build an image of the fetus. A dating scan measures the fetus to estimate its age and due date, and an anomaly scan later in pregnancy checks for physical abnormalities.
- Biochemical marker tests. The levels of certain chemicals (markers) in the mother's blood change in particular conditions. Comparing marker levels against the normal range for that stage of pregnancy gives an estimate of risk; markers must be measured at the right time, because their normal levels change as pregnancy progresses.
Crucially, screening tests estimate risk rather than giving a definite answer. A high-risk result leads to a diagnostic test.
Diagnostic tests
The fetal cells obtained can be used to make a karyotype, a display of the chromosomes, which reveals chromosome abnormalities such as an extra chromosome 21 (Down's syndrome).
Postnatal screening and inheritance
A well-known example is phenylketonuria (PKU), an inherited disorder in which a faulty enzyme means the amino acid phenylalanine cannot be broken down and builds up to harmful levels. Newborns are screened with a heel-prick blood test; if PKU is detected, the baby is put on a controlled low-phenylalanine diet that prevents brain damage. This shows why early screening matters: treatment can only prevent harm if the condition is found in time.
Families also use pedigree charts (family trees) together with patterns of inheritance to assess genetic risk. By tracking which relatives are affected, it is possible to work out whether an allele is dominant, recessive or sex-linked, deduce people's genotypes, and calculate the probability that a future child will inherit the condition.
Examples in context
Example 1. The dating scan. An early ultrasound scan measures the fetus to estimate how far the pregnancy has progressed. This dating is essential because biochemical marker results can only be interpreted against the normal range for the correct number of weeks.
Example 2. PKU and diet. A baby found to have PKU by the heel-prick test is given a carefully controlled diet low in phenylalanine. Because the harmful build-up is prevented from birth, the child develops normally, illustrating the value of early postnatal screening.
Try this
Q1. Name one diagnostic test that collects fetal cells for chromosome analysis. [1 mark]
- Cue. Amniocentesis (or chorionic villus sampling).
Q2. Explain why newborn screening for PKU is important. [1 mark]
- Cue. It allows the harmful build-up of phenylalanine to be prevented by an early controlled diet before it causes brain damage.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA Higher 20183 marksDescribe the difference between a screening test and a diagnostic test in antenatal care, using examples.Show worked answer →
A 3-mark answer needs the distinction plus examples of each.
A screening test identifies whether a pregnancy is at higher risk of a condition, but does not give a definite answer. Examples are ultrasound scans and biochemical marker tests on the mother's blood, which estimate the probability of a condition such as Down's syndrome.
A diagnostic test gives a definite answer by testing the fetus's own cells, but it carries a small risk of miscarriage. Examples are amniocentesis and chorionic villus sampling, which collect fetal cells so the chromosomes can be examined directly.
Award (1) screening estimates risk and gives a probability, (2) examples such as ultrasound or marker tests, and (3) diagnostic tests such as amniocentesis or CVS give a definite result from fetal cells.
SQA Higher 20224 marksA condition is caused by a recessive allele. Two unaffected parents have an affected child. Determine the genotypes of the parents and the chance that their next child is affected, explaining your reasoning.Show worked answer →
This is a 4-mark genetics calculation using a pedigree pattern.
The condition is recessive, so an affected child must have two recessive alleles (genotype aa). The child inherited one recessive allele from each parent, yet neither parent is affected. Therefore each parent must be a carrier with genotype Aa.
Crossing Aa with Aa gives offspring in the ratio 1 AA : 2 Aa : 1 aa. Only the aa genotype is affected, so the chance that the next child is affected is 1 in 4, or 25 percent.
Award (1) affected child is aa, (2) each parent must be a carrier Aa, (3) the Aa by Aa cross gives a 1:2:1 ratio, and (4) the chance of an affected child is 1 in 4.
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