If a genetic disease is transferred from a phenotypically normal but carrier female to only some of the male p

NCERT Exemplar · Q4

Q. If a genetic disease is transferred from a phenotypically normal but carrier female to only some of the male progeny, the disease is:

(a) Autosomal dominant

(b) Autosomal recessive

(d) Sex-linked recessive

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Concept understanding — Pedigree Analysis

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)

Note

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.

Important

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. …

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The disease is sex-linked recessive because a phenotypically normal carrier female can transmit the trait to only some of her male offspring, who express the disease due to having only one X chromosome.

Understanding how genetic traits are passed down through families is crucial in genetics, a field often explored through pedigree analysis. This involves studying the inheritance pattern of a particular trait or disease across generations within a family, using standardized symbols to represent individuals and their relationships. By observing who is affected, who is a carrier, and the sex of the individuals, we can deduce the likely mode of inheritance.

Let's break down the characteristics of the disease described in the question:

"transferred from a phenotypically normal but carrier female": This tells us two important things. First, the female herself does not show the disease symptoms (she is "phenotypically normal"). Second, she carries the gene for the disease (she is a "carrier"). This immediately suggests that the disease allele must be recessive, because if it were dominant, a carrier would typically express the trait.
"to only some of the male progeny": This is a critical clue. The disease specifically affects males, and not all of them, but only a portion. This strong sex-bias in inheritance points towards a gene located on a sex chromosome, specifically the X chromosome, as males have only one X chromosome while females have two.

Now, let's consider the different modes of inheritance:

Autosomal Dominant: In this pattern, the disease allele is located on an autosome (any chromosome other than the sex chromosomes), and only one copy of the allele is sufficient to cause the disease.
- Why it doesn't fit: A person carrying a dominant disease allele would typically be affected, not a phenotypically normal carrier. Also, autosomal traits affect males and females with roughly equal frequency, which contradicts the observation that only male progeny are affected.
Autosomal Recessive: Here, the disease allele is also on an autosome, but two copies of the allele are required for the disease to manifest. Individuals with one copy are carriers and are phenotypically normal.
- Why it doesn't fully fit: While a female can be a phenotypically normal carrier, if she passes the recessive allele to her offspring, and the father also contributes a recessive allele (or is a carrier), then both male and female offspring would have an equal chance of being affected. The scenario specifically states "only some of the male progeny" are affected, implying a sex-linked pattern rather than an autosomal one where both sexes are equally susceptible.
Sex-linked Dominant: The disease allele is on the X chromosome and is dominant. Only one copy of the allele on the X chromosome is enough to cause the disease.
- Why it doesn't fit: Similar to autosomal dominant, a female carrying a dominant disease allele on her X chromosome would be affected, not phenotypically normal. Also, sex-linked dominant traits often show higher prevalence in females than males, which is opposite to the given scenario.
Sex-linked Recessive: The disease allele is on the X chromosome and is recessive.
- Why it fits perfectly: …

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Q.If a genetic disease is transferred from a phenotypically normal but carrier female to only some of the male progeny, the disease is:

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