Q.Write a short note on Down's syndrome.
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Pedigree Analysis: A Family Tree for Traits
Imagine you're looking at a family photograph album. You notice that your grandfather, your uncle, and your cousin all have the same unusual hairline or the same colour blindness. You start wondering: Is this just a coincidence, or is it something that runs in the family? How does this pattern pass from one generation to the next?
That curiosity is exactly where pedigree analysis begins.
What is a Pedigree?
A pedigree is simply a family tree that records the presence or absence of a particular trait — like a disease, a physical feature, or a genetic condition — across several generations. Instead of writing names and stories, biologists use a standard set of symbols to map out who has the trait and who doesn't, and how they are related.
The NCERT textbook defines it as a method to study the inheritance of a trait in several generations of a family. It is a tool, not a formula. You don't calculate anything; you observe patterns.
Why Do We Need It?
Humans cannot be bred in a laboratory like fruit flies or pea plants. You cannot ask two cousins to have children just to see if a disease is inherited. So pedigree analysis becomes the only ethical, practical way to study how traits pass through human families.
It helps in:
- Predicting the likelihood that a child might inherit a genetic disorder
- Tracing whether a trait is dominant or recessive
- Identifying carriers — people who carry a gene for a disease but do not show it themselves
- Offering genetic counselling to families with a history of inherited conditions
The Basic Symbols (What You Need to Know)
In a pedigree chart:
- Males are represented by squares
- Females are represented by circles
- A horizontal line connecting a square and a circle shows a marriage (mating)
- A vertical line descending from that couple leads to their children
- Shaded (filled) symbols indicate individuals who show the trait
- Unshaded (empty) symbols indicate individuals who do not show the trait
- A half-shaded symbol often indicates a carrier (someone who has the gene but does not express the trait)
In NCERT, you will see that a pedigree is drawn from the top (oldest generation) to the bottom (youngest). The first generation is labelled I, the second II, and so on. Individuals within a generation are numbered from left to right (e.g., I-1, I-2, II-3).
How to Read a Pedigree: The Core Idea
You are essentially looking for a pattern. Ask yourself:
- Does the trait appear in every generation? If yes, it might be dominant — because a dominant trait only needs one copy of the gene to show up, so it rarely skips a generation.
- Does the trait skip a generation? If yes, it might be recessive — because a recessive trait needs two copies (one from each parent), and carriers can pass it on without showing it themselves.
- Does the trait affect mostly males? If yes, it might be sex-linked (carried on the X chromosome). For example, colour blindness and haemophilia are much more common in males because they have only one X chromosome.
The most important rule from NCERT: A recessive trait can skip a generation, but a dominant trait cannot. If you see a child with a trait whose parents do not have it, the trait is almost certainly recessive. The parents are carriers.
A Simple Example (Without Numbers)
Suppose you see a pedigree where:
- A grandfather has a condition (shaded square)
- His daughter does not have it (empty circle)
- But his grandson (the daughter's son) has it (shaded square)
What does this tell you? The trait skipped a generation (from grandfather to grandson, missing the daughter). That suggests it is recessive. Also, it appears only in males in this case, so it might be X-linked recessive — the daughter is a carrier, and she passed the gene to her son. …
Down's syndrome is a chromosomal disorder caused by an extra copy of chromosome 21. …
Down's syndrome results from trisomy 21 (an extra chromosome 21, total 47), causing characteristic physical features and mental retardation.
Down's syndrome is a chromosomal (aneuploidy) disorder first described by Langdon Down.
- Cause: presence of an additional copy of chromosome number 21 (trisomy 21). The affected person has 47 chromosomes instead of 46. This usually arises due to non-disjunction of chromosome 21 during gamete formation, and its chance increases with the mother's age. …
- CBSE 2026Set ANNUAL1 markQ.Symbols used in human pedigree analysis is given below. Identify these symbols used in pedigree chart.
›Reveal solutionSolution
Pedigree charts use squares for males, circles for females; a filled symbol denotes an affected individual, and a horizontal line joining a square and a circle denotes mating between them.
Standard pedigree symbols include:
- Unfilled (open) square = unaffected/normal male
- Filled (solid) square = affected male
- Unfilled (open) circle = unaffected/normal female
- Filled (solid) circle = affected female
- A square joined to a circle by a horizontal line = mating (marriage) between the two individuals shown
Applying this to the given symbols:
- Symbol A is a single filled (solid black) square, which represents an affected male. …
- CBSE 2026Set ANNUAL1 markMCQQ.This pedigree symbol represents : [figure: a small printed pedigree-chart symbol — two squares (offspring symbols) placed side by side and joined by a short horizontal line, with a single line rising from the midpoint of that horizontal line up to a peak/apex above, giving the whole mark a triangular silhouette (a single point of origin branching down to the two squares).](a) Male(b) Monozygotic twins(c) Mating(d) Dizygotic twins
›Reveal solutionSolution
A horizontal bar directly joining two offspring symbols that both hang from a single line from the parents is the pedigree symbol for monozygotic (identical) twins.
In pedigree analysis, squares represent males and circles represent females, and twins are shown as two offspring symbols connected by short diagonal or vertical lines to a single point on the horizontal line coming down from the parents' mating line. The key distinguishing feature between the two kinds of twins in pedigree notation is whether there is an extra horizontal bar directly joining the two offspring symbols to each other: if such a bar is present (as described here -- the two squares are joined to each other by a short horizontal line, in addition to both connecting up to a single point of origin from the parents), it indicates that the twins arose from a single fertilised egg that split into two -- monozygotic (identical) twins. If the two offspring sy …
- CBSE 2026Set ANNUAL1 markQ.State whether True or False: Colour blind people is able to difference between Red and Green colour.
›Reveal solutionSolution
False - red-green colour-blind people cannot tell red and green apart.
Colour blindness (Daltonism) is an X-linked recessive genetic disorder in which a defect in the cone cells of the retina prevents the person from distinguishing certain colours. In red-green colour blindness, the person cannot correctly differentiat …
- CBSE 2025Set 57/4/11 markMCQQ.In the given pedigree chart, a cross between a normal couple resulted in a son who was haemophilic and a normal daughter. In course of time, when the daughter was married to a normal man, to their surprise the grandson was also haemophilic. [Pedigree chart] Choose the option that indicates the correct inheritance of trait in the above pedigree chart : (A) Autosome linked dominant trait (B) Sex-linked dominant trait (C) Autosomal recessive trait (D) Sex-linked recessive trait
›Reveal solutionSolution
The pedigree shows haemophilia skipping a generation and passing from a carrier mother to her son — the classic pattern of an X-linked recessive trait. The correct option is (D).
The concept: why sex-linked recessive fits
Haemophilia is a well-known X-linked recessive disorder. The key signature of such a trait is that it appears almost exclusively in males, and it is transmitted from a carrier mother (who is unaffected) to her sons. A father never passes his X chromosome to his sons — he gives them a Y — so an affected male always inherits the defective X from his mother. In a pedigree, this creates a pattern where the disease seems to skip a generation: an unaffected woman can carry the allele and pass it to her son, who then expresses it.
Let’s map the given family onto this logic.
Step-by-step reasoning
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The first couple: normal parents, one affected son
A normal man (XY with normal X) and a normal woman have a haemophilic son. Since the son is affected, he must have received an X chromosome carrying the haemophilia allele. His father gave him a Y, so the defective X came from his mother. The mother is phenotypically normal, so she must be a carrier — heterozygous for the recessive allele. This immediately rules out any dominant inheritance (if it were dominant, she would be affected) and any autosomal recessive pattern (where both parents would need to be carriers, but the father would then be normal — possible, but we’ll see why X-linked fits better).
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The daughter: normal phenotype, but a carrier
The couple also has a normal daughter. She received one X from her father (normal) and one X from her mother. Since her mother is a carrier, the daughter has a 50% chance of being a carrier herself. The pedigree tells us she later marries a normal man and has a haemophilic son — so she must indeed be a carrier. This is exactly what we expect for an X-linked recessive: the daughter is unaffected because she has one normal X, but she can pass the defective X to her children.
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The grandson: affected son of a carrier mother and normal father
The daughter (carrier) marries a normal man. Their son inherits his mother’s defective X and his father’s Y — and therefore has haemophilia. The father is normal, so he could not have contributed the disease allele. This pattern — affected son born to unaffected parents, where the mother is the carrier — is the hallmark of X-linked recessive inheritance.
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Why the other options fail
- (A) Autosome linked dominant: If it were autosomal dominant, at least one parent of every affected child would be affected. Here, both parents of the affected son are normal — impossible. …
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- CBSE 2025Set 57/6/11 markMCQQ.Study the pedigree chart of a family sharing the inheritance of sickle cell anemia. [Pedigree chart figure] The trait traced in the above pedigree chart is : (A) Dominant X-linked (B) Autosomal dominant (C) Recessive X-linked (D) Autosomal recessive
›Reveal solutionSolution
Sickle cell anemia is an autosomal recessive disorder, requiring two copies of the mutant allele for expression and affecting both sexes equally without sex-linkage.
Pedigree analysis is a powerful tool in human genetics that allows us to trace the inheritance pattern of a trait through generations of a family. When we examine a pedigree chart for sickle cell anemia, we're looking for clues that reveal whether the trait is dominant or recessive, and whether it's carried on an autosome (any chromosome except the sex chromosomes) or on a sex chromosome (X or Y).
Sickle cell anemia is caused by a mutation in the gene coding for the beta chain of hemoglobin. The mutant hemoglobin (HbS) causes red blood cells to assume a sickle shape under low oxygen conditions, leading to various complications. The key to identifying its inheritance pattern lies in understanding how the trait appears across generations and between sexes.
Distinguishing the pattern
To determine the correct inheritance pattern, we need to consider four possibilities:
Dominant versus Recessive:
A dominant trait appears in every generation when present — an affected individual has at least one affected parent. A recessive trait can skip generations because carriers (heterozygotes) don't show symptoms but can pass the allele to their children. Sickle cell anemia typically appears in children of two apparently healthy parents who are carriers, which immediately suggests a recessive pattern.
Autosomal versus Sex-linked:
An autosomal trait affects males and females equally because the gene is on one of the 22 pairs of autosomes. An X-linked trait shows a characteristic pattern — recessive X-linked traits predominantly affect males (who have only one X chromosome), while females are usually carriers. Dominant X-linked traits affect more females than males and show no male-to-male transmission.
NoteIn sickle cell anemia, both males and females are affected with equal frequency. If you see affected daughters born to unaffected fathers in a pedigree, the trait cannot be X-linked recessive, because fathers pass their X chromosome only to daughters, and an unaffected father cannot carry a recessive X-linked allele.
Why sickle cell anemia is autosomal recessive
The inheritance pattern of sickle cell anemia reveals several telltale features:
- Both sexes are equally affected — there's no male or female predominance, ruling out X-linkage.
- The trait can skip generations — two carrier parents (genotype HbA HbS) who are phenotypically normal can have affected children (genotype HbS HbS).
- Affected individuals typically have unaffected parents — both parents are heterozygous carriers. …
- CBSE 2025Set X11 markMCQQ.Which one among the given is a symbol used in human pedigree analysis for consanguineous mating?(a) A square (male) joined by a single horizontal line to a circle (female)(b) A square (male) joined by a double horizontal line to a circle (female)(c) A single square (male)(d) A single circle (female)
›Reveal solutionSolution
In pedigree analysis a consanguineous (related) mating is shown by a double horizontal line joining the male and female symbols.
In pedigree charts a square represents a male and a circle a female. A single horizontal line connecting them denotes a normal mating between unrelated individuals, whereas a double horizontal line denotes a consanguineous mating (between blood relativ …
- CBSE 2022Set ANNUAL1 markQ.Which colours cannot be discriminated by person in colour-blindness ?
›Reveal solutionSolution
In the common form of colour blindness the person fails to distinguish red from green.
Colour blindness is an X-linked recessive defect in which the cones sensitive to red or green light are defective or missing. As a result the affected individual cannot discriminate between red and green c …
- CBSE 2022Set ZOOLOGY1 markMCQQ.A colour-blind person cannot distinguish ______ colour/colours.(i) all(ii) red(iii) green(iv) red and green
›Reveal solutionSolution
A red-green colour-blind person cannot distinguish red and green colours.
Colour blindness is an X-linked recessive disorder caused by defects in the genes for the red or green cone photopigments of the retina. The most common form is red-green colour blindness, in which the individual is unable to discriminate between the red and green regions of the spectrum. It is far more frequent in males because they have only one X chr …
- CBSE 2020Set ANNUAL1 markMCQQ.From the following, find out the symbol used in the human pedigree analysis representing male.(a) An unfilled circle(b) An unfilled diamond(c) An unfilled square(d) A solid (filled) black circle
›Reveal solutionSolution
In pedigree charts, squares represent males and circles represent females; unfilled = unaffected, filled = affected.
Pedigree analysis uses a standard set of symbols to trace the inheritance of a trait through a family:
- An unfilled (open) square = unaffected/normal male
- A filled (solid) square = affected male
- An unfilled (open) circle = unaffected/normal female
- A filled (solid) circle = affected female
- A diamond = individual of unknown/unspecified sex …
- CBSE 2020Set ANNUAL1 markMCQQ.Which of the following indicates the sign [DIAMOND-2] in human pedigree analysis ?(a) Twins(b) Second Sibling(c) Two unaffected sons(d) Two normal offspring
›Reveal solutionSolution
A diamond stands for an individual of unspecified/unknown sex, a number inside a symbol gives the count of individuals grouped there, and an unfilled symbol means unaffected — so a diamond-with-2 denotes two normal (unaffected) offspring of unspecified sex, i.e. option (d).
In pedigree analysis, standardised symbols carry precise meanings:
- A square represents a male, a circle a female, and a diamond an individual whose sex is not specified or not known.
- A number written inside a symbol indicates the count of individuals of that type grouped together at that position (used to save space when several siblings share the same status).
- An unshaded/unfilled symbol represents an individual who is unaffected (normal) for the trait being tracked; a filled (shaded) symbol represents an affected individual. …
- CBSE 2019Set ANNUAL1 markQ.Why a colourblind man is not selected as a driver in railways?
›Reveal solutionSolution
A colour-blind person cannot tell red from green, and railway signals are red/green — so he could misread signals and is not chosen as a driver.
Colour blindness (red-green colour blindness) is an X-linked recessive genetic defect in which the person's eye lacks the ability to distinguish between red and green colours because of a defect in the cone cells of the retina.
…
- CBSE 2018Set ANNUAL1 markMCQQ.A normal vision female whose father is colour-blind marries a normal vision male. What would be the probability of her sons and daughters to be colour blind?(a) 25% sons colour blind and all daughters with phenotypically normal vision.(b) 50% sons colour blind and 50% daughter normal.(c) 50% sons colour blind and 50% daughter colour blind.(d) All sons normal and 50% colour blind daughter.
›Reveal solutionSolution
Carrier mother × normal father → half the sons colour-blind (25% of total children), all daughters phenotypically normal.
Colour blindness is an X-linked recessive trait. The woman has normal vision but her father was colour blind (X^c Y), so she must have inherited his X^c — she is a carrier, genotype X^C X^c. She marries a normal male, X^C Y.
Cross: X^C X^c × X^C Y
- Daughters: X^C X^C (normal) and X^C X^c (carrier, but phenotypically normal vision) → all daughters have normal vision. …
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