By Momina Shehzadi | BS Zoology Graduate, Pakistan In this article, we will learn Fungi - Morphology, Reproduction, Physiology and Importance - in simple words with diagram. This topic is very important for BS Zoology and Microbiology students. Fungi are found everywhere around us. They are present in soil, in water, in air, on dead plants, on dead animals, on our food, on our skin and even inside our body. When bread becomes old, green or black spots appear on it. These spots are fungi. When we see mushroom growing after rain, that is also a fungus. When yeast is used to make bread soft, that is also a fungus. Fungi are very different from bacteria and plants. They are eukaryotic organisms. It means their cells have a true nucleus and other membrane bound organelles. They do not have chlorophyll, so they cannot make their own food like plants. They get their food from other living or dead things. That is why they are called heterotrophic organisms. The study of fungi is called mycolog...
Down Syndrome (Trisomy 21) - Genetics, Karyotype, Symptoms, Diagnosis and Management | Complete 2000+ Words Guide for Zoology Students
Why I Wrote This Note?
By Momina Shehzadi | BS Zoology Graduate, Pakistan. When I was in BS Zoology, Down Syndrome was one of the most difficult chapters for me because every book had the same tough English. During a hospital visit, I saw a baby with Down Syndrome for the first time and I realized how important this topic is. Today, I have written this complete note for you in simple language with the help of my teacher's notes, my books (Miller and Harley), and my own experience so that you can easily prepare for your exams. This note is 2000+ words and every point is to-the-point.
1. What is Down Syndrome? Basic Definition
Down Syndrome is a chromosomal disorder, not a disease. Every cell of a normal person has 46 chromosomes (23 pairs). But in Down Syndrome, there are 47 chromosomes because there is an extra copy of Chromosome Number 21. That is why it is also called Trisomy 21.
It is the most common genetic cause of intellectual disability in the world. 1 out of every 700-1000 babies is born with Down Syndrome. In Pakistan, it is estimated to be 1 in 800.
History which is asked in exams:
In 1866, a British doctor Dr. John Langdon Down described it for the first time. He gave it the wrong name "Mongolism", which is now banned. Later in 1959, French geneticist Dr. Jerome Lejeune proved that the cause is an extra chromosome 21.
2. Karyotype - The Most Important Topic for Exams
Karyotype means a picture of chromosomes.
Normal Karyotype:
Female: 46, XX
Male: 46, XY
Down Syndrome Karyotype:
Female: 47, XX, +21
Male: 47, XY, +21
Here +21 means one extra chromosome 21.
Types of Down Syndrome - Remember 3 Types:
A) Non-Disjunction Trisomy 21 (95% of cases)
This is the most common. During meiosis, chromosome 21 fails to separate. Non-disjunction occurs in anaphase. This creates a gamete with 24 chromosomes (2 copies of 21). When this gamete fuses with a normal gamete (23), a zygote with 47 chromosomes is formed. Every cell has 47 chromosomes. This is a random event.
B) Robertsonian Translocation (3-4% of cases)
In this type, the long arm of chromosome 21 fuses with chromosome 14 or 21. The total number of chromosomes is counted as 46 but genetically there is material of 3 copies. Karyotype: 46,XX,t(14;21). Parents of this type are carriers, they look normal but they have 45 chromosomes. The risk of Down Syndrome in the next child is 10-15%, which is very high.
C) Mosaic Down Syndrome (1-2% of cases)
In this, some cells are normal (46) and some are trisomic (47). Karyotype: 46,XX / 47,XX,+21. This happens due to non-disjunction in mitosis, after fertilization. Children with this type have slightly higher IQ and milder symptoms.
3. Causes and Risk Factors - Why Does It Happen?
The main cause is Non-Disjunction.
Meiotic Non-Disjunction: In 90% of cases it happens in the mother's egg (oogenesis) and in 10% in the father's sperm (spermatogenesis).
Risk Factors:
1. Advanced Maternal Age - The Biggest Factor
As the mother's age increases, eggs become older and the chance of non-disjunction increases.
20 years: 1 in 1500
30 years: 1 in 900
35 years: 1 in 350
40 years: 1 in 100
45 years: 1 in 30
I asked my teacher if it happens due to food? She said no, it does not happen due to diet, medicine, or mobile. Parents should never be blamed for this.
Other factors: If previous child had Down Syndrome then 1% risk, If father's age is 50+ then a small risk.
4. Pathophysiology - What Happens Inside?
Chromosome 21 is the smallest autosome and it contains 200-300 genes. Due to the extra copy, 50% more proteins are made. This Gene Dosage Imbalance disturbs development.
Some important genes:
DYRK1A: Reduces brain growth, causes low IQ.
APP Gene: This is why people with Down get Alzheimer's after age 40.
DSCAM: Cause of heart defects.
COL6A1: Cause of soft muscles (Hypotonia).
SOD1: Causes oxidative stress.
5. Clinical Features - How Does a Doctor Identify?
A) Facial Features (Seen First):
Flat facial profile, Depressed nasal bridge, Eyes slanted upwards (Upslanting palpebral fissures), Skin fold at inner corner of eye (Epicanthal folds), Small round head (Brachycephaly), Small, low-set ears, Open mouth with protruding tongue (Tongue is normal but mouth is small), Short neck with extra skin at back.
B) Muscles and Limbs:
Generalized Hypotonia - Baby is very soft, called floppy baby. This is the most confirmed sign in a newborn. Joints are very flexible. Short height (Male 157 cm, Female 144 cm). Short hands, Single palmar crease (Simian crease) in 45%, Bent 5th finger (Clinodactyly), Wide gap between big toe and second toe (Sandal gap).
C) IQ and Behaviour:
Average IQ 45-50. Delay in sitting, walking, speaking. But these children are very social, cheerful, and love music.
D) Internal Medical Problems:
- Heart: 40-50% children have a hole in the heart.AVSD is most common.
- Stomach: Duodenal atresia (Double bubble sign), Hirschsprung disease.
- Blood: Risk of Leukemia is 10 times higher.
- Thyroid: 30% have hypothyroidism.
- Eyes/Ears: Cataract, hearing loss.
6. Diagnosis
A) During Pregnancy (Prenatal):
_Screening Tests (Only Tell Risk):_
1. First Trimester (11-13 weeks): NT Scan + Blood test (Beta-hCG high, PAPP-A low). If NT is more than 3mm then high risk.
2. Quadruple Test (15-20 weeks): AFP low, HCG high.
3. NIPT: The best one. Baby's DNA is checked from mother's blood. 99% accurate. Can be done after 10 weeks.
_Confirmatory Tests (100% Confirm):_
1. CVS (11-13 weeks): Piece of placenta.
2. Amniocentesis (15-20 weeks): Sample of amniotic fluid. Karyotyping is done from it which is the Gold Standard.
B) After Birth:
Suspicion from clinical features and confirmation by Karyotyping. 20 cells are checked from blood.
7. Management - No Cure, But Better Life
There is no cure for the extra chromosome but life can be made much better. Previously people lived for 25 years, now they live up to 60 years.
- Early Intervention (0-5 years): Physiotherapy, Speech therapy, Occupational therapy.
- Medical: Heart surgery within 6 months, Thyroid test every year, Eye/Ear test every 6 months.
- Education: Special schools, IEP plan. Children with mosaic type can also go to normal schools.
- Family Support: Genetic counseling. To explain to parents that it is not their fault.
- Adult Care:Alzheimer screening after age 40, Care for cervical spine instability.
8. My Final Conclusion for Students
Friends, an easy way to remember Down Syndrome is - 47, Extra 21, Happy Baby. These children don't need sympathy, they need respect and opportunities. I hope you liked my note. If you need a diagram for this, let me know.
FAQs for Exam:
Q1: Why are males with Down Syndrome infertile?
Ans: Almost 100% males are infertile. Females can get pregnant but there is 50% chance the child will have Down Syndrome.
Q2: What is the life expectancy in Down Syndrome?
Ans: Previously it was 25 years, now with heart surgery and better care it is up to 60 years.
Q3: Is there any prevention for Down Syndrome?
Ans: No prevention, but risk is lower if pregnancy is before 35 years and with folic acid. Prenatal screening can detect it early.
Q4: What is the karyotype of Down Syndrome?
Ans: Female: 47,XX,+21 and Male: 47,XY,+21. Normal karyotype is 46,XX and 46,XY. There is one extra chromosome 21.
Also read previous complete topic Klinefelter syndrome
Written by Momina Shehzadi, BS Zoology Graduate from Pakistan.
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