Genetic Disorders: Causes, Classification: NEET PG Pathology (2023)

Chromosomal disorders are disorders in which entire chromosomes are affected by chromosome number or structure. Monogenic diseases/Mendelian diseases are inherited according to the Mendelian inheritance pattern, while multifactorial inheritance is a disease in which, in addition to genetic factors, environmental factors also play a very important role. Non-Mendelian disorders are disorders that do not follow typical Mendelian inheritance patterns, eg trinucleotide repeat disorders, gonadal spotting.

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Chromosomal Abnormalities:

A disorder in which the structure or number of chromosomes is altered. Chromosomes are large chunks of genetic material (humans have a total of 46 chromosomes). Chromosomes may be lost, duplicated, or changed in some way. Down syndrome: Trisomy 21 (3 copies of Chr. 21 instead of 2) and Turner syndrome: 45, XO (female with only one X chromosome)

Single Gene Disorders/Mendelian Disorders

These disorders have a typical Mendelian inheritance pattern. include:

  • Autosomal dominant inheritance (no sex chromosomes involved, somatic chromosomes involved)
  • autosomal recessive
  • X-linked recessive
  • X-linked dominant
  • Y-linked disorders: very rare

non-Mendelian diseases

  • trinucleotide repeat disorder
  • Mitochondrial Diseases / Mitochondrial Inheritance Patterns
  • 印记基因
  • mosaicism

multifactorial disorder

In these diseases, in addition to genetic factors, environmental and host-related factors also play an important role. Examples include cleft lip, cleft palate, neural tube defects, diabetes, high blood pressure, etc.

Mendelian disease

Monogenic disorders following a typical Mendelian pattern of inheritance:

autosomal dominant genetic disease

Autosomal dominant disorders are those that occur even when one allele of a gene is affected (neither alleles need be affected). In a dominant disorder, one parent is also affected by the same disorder (at least one parent is affected). Examples of autosomal dominant disorders:

(Video) Genetic Disorders By Dr. Preeti Sharma

  • hereditary spherocytosis, hypercholesterolemia
  • Elles Danros team
  • cartilage plate
  • von Willebrand's disease
  • pseudohypoparathyroidism
  • Myotonic dystrophy (myotonic dystrophy)
  • Osteogenesis imperfecta (most types are autosomal dominant)
  • Marfan syndrome: Marfan syndrome is clinically characterized by tall stature, increased arm span, arachnids
  • intermittent porphyria
  • Noonan Syndrome
  • Adenomatous polyposis of the large bowel (a precancerous form of colon cancer)
  • Neurodermatological disorders - neurofibromatosis and tuberous sclerosis (familial)

autosomal recessive genetic disease

A disorder that occurs only when both copies/alleles of a gene are affected. The disease will only manifest itself in the child if both parents are carriers. Wilson's disease is caused by the ATP 7B gene, which is only manifested when both copies/alleles of the gene are abnormal. In this case, the parents are usually the carriers. If one parent is affected, all children will carry the disease, even if the other parent is normal. Examples of autosomal recessive disorders:

  • albinism, black urine
  • Beta-thalassemia: If both parents have thalassemia traits, each child has a 25% risk
  • cystic fibrosis, congenital adrenal hyperplasia
  • Deafness (congenital sensorineural deafness)
  • emphysema caused by alpha1antitrypsin deficiency
  • Gaucher disease, galactosemia
  • Homocystinuria (classic variety)
  • Mucopolysaccharidosis - autosomal recessive inheritance
  • Most metabolic disorders are inherited in an autosomal recessive manner.

X-linked recessive disease

These are recessive genetic disorders related to the sex chromosome (X). Tendency: Males are more affected

man woman

46、XY 46、XX

Because males have only one X chromosome, if this chromosome is abnormal, then the disease will manifest itself because there is no other normal copy of the X chromosome. Females have 2 X chromosomes, and even if one X chromosome is affected, the other X chromosome is also affected. A normal copy of the same gene on a different X chromosome. Females are usually not affected by X-linked recessive disorders. All daughters of an affected male are carriers. Since the son receives one Y chromosome from the father and one X chromosome from the mother, there is no detectable inheritance from the father to the son. Father to son excluding X-linked inheritance

Examples of X-linked recessive disorders:

  • G6PD (Glucose-6-Phosphate Dehydrogenase) Deficiency: In this condition, red blood cells are easily hemolyzed, so drugs that are notorious for accelerating hemolysis should be avoided in this condition.
  • Duchenne muscular dystrophy (DMD)
  • Color blindness is more common in men.
  • Fabry disease (lysosomal storage disorder)
  • Chronic granulomatous disease (CGD - an immunodeficiency condition)
  • Hemophilia A y B
  • Hunter disease (MPS type 2)
  • Agammaglobulinemia/Bruton disease
  • Double acting
  • Lesch Nyhan Syndrome: This is a self-inflicted disorder in which uric acid builds up in the body.
  • Medical History: If the child is affected and has a history of the mother's siblings also having the same disorder.

When can a female exhibit X-linked recessive disorder?

Females have 2 X chromosomes, and even if 1 X chromosome is affected, there is another normal copy of the same gene on a different X chromosome. If a woman has an X-linked recessive disorder, if she has only one X chromosome, which is 45,XO, this may indicate Turner syndrome, which is 45,XO. Usually, in females, one X chromosome is randomly inactivated while the other remains active. Thus, the disease can manifest in females if the X chromosome carrying the abnormal allele remains active while the normal chromosome is inactive.


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X-linked dominant disease

This is more common in women. The female karyotype is 46, XX. The disease manifests itself only when one of the alleles is abnormal/affected. Therefore, if one X chromosome is affected and the other is normal, the disease will continue to manifest. The male karyotype is 46, XY. The condition can be fatal if the only X chromosome in males is affected and they die in utero. All daughters of the affected man were ill, but all sons were normal. There is no father to son. The pedigree does not include X-linked inheritance, ie.

Examples of X-linked dominant disorders:

  • Charcot-Marie Dental Disease - Peripheral Neuropathy, Pes cavus
  • Alport syndrome (controversial)
  • entitlement syndrome
  • Rickets: X-linked hypophosphatemic rickets.


Schematic diagram showing the inheritance of different genetic traits in different generations in a family.

pedigree chart

Genetic Disorders: Causes, Classification: NEET PG Pathology (1)
Genetic Disorders: Causes, Classification: NEET PG Pathology (2)
Genetic Disorders: Causes, Classification: NEET PG Pathology (3)

Inheritance mode:

Genetic Disorders: Causes, Classification: NEET PG Pathology (4)

for 1urGenerations: When a woman is affected, all her children will be affected. 2 peopleNorth DakotaGeneration - A man suffers but not his children, but a woman suffers and all her children suffer. So your explanation is mitochondrial inheritance.

Inheritance mode:

Genetic Disorders: Causes, Classification: NEET PG Pathology (5)

Males are affected but not parents, i.e. the father of the recessive pedigree father is affected. There is no parent-child transmission, so interpretEsX-linked recessive inheritance X-linked recessive inheritance is more common in men, such as hemophilia, Duchenne muscular dystrophy and so on.

non-Mendelian diseases

trinucleotide repeat complex

These diseases are caused by an increase in the number of trinucleotide repeats above a certain threshold. The more repetitions, the greater the severity of the disease.

(Video) Understanding Autosomal Dominant and Autosomal Recessive Inheritance

The "harbinger" phenomenon: In this condition, the symptoms of the disease worsen or appear at an earlier age in the offspring. This is because the number of nucleotides increases with each generation.


disease namerepeat
Huntington's diseaseAll-Around Sports Federation
myotonic dystrophyChina Three Gorges Group
Fragile X SyndromeCGG

Fragile X Syndrome

The gene that causes Fragile X syndrome is the FMR-1 mental retardation gene, which is located on the X chromosome:

genetic basisCGG repetitions
normal people5-55
operator55-200 (pre-mutation)
Fragile X Syndrome> 200 repetitions

The premutative stage has no clinical features. In women, it is associated with premature ovarian failure. 10% of women may have intellectual disability/low IQ. It is more common in men.

  • clinical manifestations
    • long face
    • big ear
    • large/protruding jaw
    • The upper jaw is highly arched.
    • hyperextended joints
    • mitral valve prolapse
    • Large testes/big testicles: observed in the adolescent age group
    • Intellectual disability (developmental delay, mental retardation)

Mitochondrial disease

All the children of a sick woman are sick. All cells in the body have a nucleus containing nuclear DNA and mitochondria containing mitochondrial DNA present in the cytoplasm.

When the sperm enters the egg, only the head of the sperm enters the egg. The sperm head contains DNA material from the father and the rest of the whole cell comes from the mother. When the zygote forms a complete egg, it comes from the mother and only the sperm head comes from the father, i.e. all the mitochondrial DNA/nuclear material comes from the baby's mother.

Heteroplasmy: presence of wild-type (normal) and mutant mtDNA in the same individual

(Video) Genetic Disorders - Sex Determination and Genetic Disorders | Biology Crash Course | NEET 2020

Threshold effect: minimum percentage of mitochondrial DNA required for disease manifestation


  • Karnsell's syndrome
  • Leber hereditary optic neuropathy
  • MELAS (mitochondrial myopathy, lactic acidosis, stroke-like episodes)
  • MERRF (myoclonic epilepsy, red dentate fibers)
  • NARP (neuropathy, ataxia, retinitis pigmentosa)
  • CPEO (chronic progressive external ophthalmoplegia)
  • Pearson syndrome: bone marrow involvement, pancytopenia, pancreatic involvement

imprinted gene

This is due to the specific inactivation of certain genes by the parents. Genomic imprinting refers to changes in the DNA without any changes in the nucleotide sequence. It is the result of epigenetic phenomena. Epigenetic changes are changes in the DNA without changes in the nucleotide sequence. There are differences in gene expression depending on the parent of origin. Epigenetic changes are often the cause of DNA methylation.

Prader-Willi syndrome: it is caused by silencing or imprinting of the paternal gene

Angelman syndrome: This is caused by silencing or imprinting of the maternal gene copy.


Purdue syndrome

angel syndrome

  • The paternal copy of the gene.
    • deleted
    • Mute
  • Two maternal bodies
    • Two copies of the maternal allele are produced, with no copy from the father.
  • The maternal copy of the gene.
    • deleted
    • Mute
  • paternal disomy
    • Two copies of the father but not the mother.
  • Involved gene: UBE3A gene on chromosome 15
  • Involved gene: UBE3A gene on chromosome 15
  • clinical manifestations
    • obesity
    • short and small
    • Intellectual disability
    • hypotonia
    • Hypogonadism (delayed puberty)
    • typical deformity
      • almond eyes
      • tapered fingers/numbers
  • Also known as Happy Puppet Syndrome: The child has a cheerful behavior
  • Clinical features: ANGELMAN
    • ataxia
    • lack of intelligence/intellectual disability
    • OWS
    • Excessive laughter (laughter episodes similar to gelatin episodes in hypothalamic hamartomas)
    • Maternal genes None

Mozaika gonad

This is a condition that occurs due to mutations that occur after zygote formation, somatic cells remain intact but gonad/germ cells are affected. If the somatic cells are intact and the individual is asymptomatic, and the gonad cells are affected, many children may be affected.

(Video) NEET PG | Mnemonics for Genes and Chromosome Number by Nikita Nanwani | Unacademy

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What causes genetic abnormalities? ›

Genetic disorders can be caused by a mutation in one gene (monogenic disorder), by mutations in multiple genes (multifactorial inheritance disorder), by a combination of gene mutations and environmental factors, or by damage to chromosomes (changes in the number or structure of entire chromosomes, the structures that ...

What are the general causes of chromosomal diseases? ›

Abnormal chromosomes most often happen as a result of an error during cell division. Chromosome abnormalities often happen due to 1 or more of these: Errors during dividing of sex cells (meiosis) Errors during dividing of other cells (mitosis)

What do you mean by genetic disorders describe any two of them? ›

There are three types of genetic disorders: Single-gene disorders, where a mutation affects one gene. Sickle cell anemia is an example. Chromosomal disorders, where chromosomes (or parts of chromosomes) are missing or changed. Chromosomes are the structures that hold our genes.

What is a single trait genetic disorder? ›

A single gene disorder is caused by variations (or mutations) in the DNA sequence of a specific gene. The DNA changes affect the product that the gene codes for—usually a protein—causing it to be altered or missing.

What are the 4 types of genetic disorders? ›

Four of the main types are:
  • Single-gene inheritance diseases.
  • Multifactorial genetic inheritance disorders.
  • Chromosome abnormalities.
  • Mitochondrial genetic inheritance disorders.

What is the classification of a genetic disorder? ›

In medical genetics, diseases are classified according to the nature (hypothetical nature) of the underlying genetic defect. The classification is “gene-centric” and “factor-centric”; a disease may be, thereby, designated as monogenic, oligogenic or polygenic/multifactorial.

What are 7 genetic disorders? ›

Genetic Disorders
  • Genetic Disorders. Sickle Cell Disease.
  • Cystic fibrosis. Cystic Fibrosis Liver Disease.
  • Brain, Nerves and Spine. Huntington's Disease.
  • Cleft lip and palate. Cleft Lip and Palate.

What are 3 other genetic disorders caused by an abnormal of chromosomes? ›

Chromosomal disorders
  • Down syndrome (Trisomy 21).
  • FragileX syndrome.
  • Klinefelter syndrome.
  • Triple-X syndrome.
  • Turner syndrome.
  • Trisomy 18.
  • Trisomy 13.
Aug 20, 2021

Which genetic disorders are caused by mutation? ›

Some of the more common single-gene disorders include cystic fibrosis, hemochromatosis, Tay-Sachs, and sickle cell anemia. Even though these diseases are primarily caused by a single gene, several different mutations can result in the same disease but with varying degrees of severity and phenotype.

What is the most common genetic abnormality? ›

The most common type of chromosomal abnormality is known as aneuploidy, an abnormal chromosome number due to an extra or missing chromosome. Most people with aneuploidy have trisomy (three copies of a chromosome) instead of monosomy (single copy of a chromosome).

What causes genetic abnormalities in pregnancy? ›

We don't know all the causes, but these things may increase the risk of your baby being born with a health condition or birth defect: Smoking, drinking alcohol or taking street drugs during pregnancy. Taking certain medicines before or during pregnancy. Getting certain infections during pregnancy.

How can you prevent genetic abnormalities during pregnancy? ›

Commit to Healthy Choices to Help Prevent Birth Defects
  1. Plan ahead. Get 400 micrograms (mcg) of folic acid every day. ...
  2. Avoid harmful substances. Avoid alcohol at any time during pregnancy. ...
  3. Choose a healthy lifestyle. Keep diabetes under control. ...
  4. Talk with your healthcare provider.


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