Human Anatomy
Chromosomal Aberrations and Clinical Genetics
Chromosomal aberrations and clinical genetics for MBBS: aneuploidy, structural changes, Down, Edward, Patau, Turner and Klinefelter syndromes and counselling, mapped to NMC codes AN75.1 to AN75.5.
MedNext Academy | 3 min read
Chromosomal Aberrations and Clinical Genetics
Chromosomal aberrations and clinical genetics for MBBS: aneuploidy, structural changes, Down, Edward, Patau, Turner and Klinefelter syndromes and counselling, mapped to NMC codes AN75.1 to AN75.5.
This chapter classifies numerical and structural chromosomal aberrations and gives the genetic basis of important clinical syndromes. It covers mosaics and chimeras, mutation and polymorphism, and the principles of genetic counselling.
High-yield: Chromosomal Aberrations and Clinical Genetics
- Chromosomal aberrations are either numerical, involving a change in chromosome number, or structural, involving a change in chromosome form.
- Aneuploidy is an abnormal number of chromosomes and usually arises from non-disjunction during cell division.
- Trisomy 21, or Down syndrome, is the commonest autosomal trisomy compatible with survival.
- Trisomy 18 is Edward syndrome and trisomy 13 is Patau syndrome, both with severe malformations.
- Turner syndrome is 45,X, a monosomy of the X chromosome, giving a short female with ovarian failure.
- Klinefelter syndrome is 47,XXY, giving a tall male with small testes and infertility.
- Structural aberrations include deletion, duplication, inversion, ring formation and translocation.
- A Robertsonian translocation fuses two acrocentric chromosomes and can lead to translocation Down syndrome.
- A mosaic has two or more cell lines with different chromosome constitutions arising from one zygote.
- A chimera has cell lines derived from more than one zygote.
- Prader-Willi syndrome results from loss of the paternal copy of a region of chromosome 15.
- The same region lost from the maternal copy gives Angelman syndrome, an example of genomic imprinting.
- A mutation is a permanent change in the DNA sequence, while polymorphism is a common variation in the population.
- Genetic counselling gives families information on risk, testing and options in a non-directive way.
- Advanced maternal age increases the risk of trisomy through a higher rate of non-disjunction.
Key chromosomal syndromes
- **Down syndrome:** Trisomy 21. Commonest autosomal trisomy; risk rises with maternal age.
- **Edward and Patau:** Trisomy 18 (Edward) and trisomy 13 (Patau); severe malformations, poor survival.
- **Turner and Klinefelter:** 45,X Turner (short female, ovarian failure); 47,XXY Klinefelter (tall male, infertility).
- **Prader-Willi:** Loss of paternal chromosome 15 region; genomic imprinting, contrast with Angelman.
NMC competencies in this chapter
- **AN75.1:** Structural and numerical chromosomal aberrations
- **AN75.2:** The terms mosaics and chimeras with examples
- **AN75.3:** Genetic basis and features of Prader-Willi, Edward and Patau syndromes
- **AN75.4:** Genetic basis of variation: polymorphism and mutation
- **AN75.5:** Principles of genetic counselling
Frequently Asked Questions
What is the difference between numerical and structural chromosomal aberrations?
Numerical aberrations involve a change in the number of chromosomes, such as trisomy 21, while structural aberrations involve a change in chromosome form, such as deletions, inversions and translocations.
What is the difference between a mosaic and a chimera?
A mosaic has two or more cell lines with different chromosome constitutions that all arise from a single zygote, whereas a chimera has cell lines derived from more than one zygote.
How do Prader-Willi and Angelman syndromes illustrate imprinting?
Both involve the same region of chromosome 15. Loss of the paternal copy gives Prader-Willi syndrome and loss of the maternal copy gives Angelman syndrome, showing that the parent of origin matters.
What is the aim of genetic counselling?
To give families clear, non-directive information about the nature and risk of a genetic condition, the testing available and the options open to them, so they can make informed decisions.
Continue reading
MBBSAll Human Anatomy chapters
Continue through the Human Anatomy chapter map.
Continue studying Human Anatomy
Explore clinician-written learning resources, structured revision and practice across the MedNext platform.
Open in the MedNext appSee plans