A, B, D are three independently assorting genes with their recessive alleles a, b, d, respectively. A cross wa

NCERT Exemplar · Q29

Q. A, B, D are three independently assorting genes with their recessive alleles a, b, d, respectively. A cross was made between individuals of Aa bb DD genotype with aa bb dd. Find out the type of genotypes of the offspring produced.

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Concept understanding — Mendelian Genetics Basics

Mendelian Genetics Basics

Imagine you have a box of coloured beads — red and white. If you pick one bead from the box, you get either red or white. Now imagine that the colour of your eyes, or the shape of your earlobe, is decided by something like that: a tiny "packet" inside your cells that comes in two versions, and you inherit one from each parent. That is the core idea of Mendelian genetics.

The everyday intuition

You have probably noticed that children often look like their parents — same hair colour, same dimples, same height. But they are never exact copies. Why? Because each parent contributes half of the "instructions" for building a child. Those instructions come in pairs, one from mother and one from father. Sometimes one instruction overrides the other; sometimes they blend. Gregor Mendel, a 19th-century monk, figured out the rules by watching pea plants — tall vs short, yellow vs green seeds — and counting what appeared in the next generation.

The precise meaning

Mendelian genetics is the study of how traits are passed from parents to offspring through genes. A gene is a unit of heredity — a stretch of DNA that codes for a specific characteristic, like flower colour. Each gene comes in different versions called alleles. For every gene, you inherit two alleles: one from your mother, one from your father.

If the two alleles are identical, you are homozygous for that trait. If they are different, you are heterozygous. In a heterozygous pair, one allele may be dominant — it shows up in the appearance — and the other recessive — it stays hidden unless both alleles are recessive.

Note

Mendel's key insight was that traits are not blended like paint. Instead, alleles remain separate and are passed on intact. A recessive allele can skip a generation and reappear later, unchanged.

Why it matters

Mendelian genetics is the foundation of modern biology. It explains:

Why some diseases run in families (like cystic fibrosis or sickle-cell anaemia)
How plant and animal breeders create new varieties
Why you might have your grandmother's eyes but not your mother's

The NCERT textbook states that Mendel's work established the laws of inheritance — the Law of Dominance, the Law of Segregation, and the Law of Independent Assortment. These laws describe how alleles separate during the formation of eggs and sperm, and how different genes are inherited independently of one another.

Key terms at a glance

Gene: a unit of heredity on a chromosome
Allele: a variant form of a gene
Dominant: the allele that expresses itself even when paired with a different allele
Recessive: the allele that expresses itself only when paired with an identical recessive allele
Homozygous: having two identical alleles for a gene
Heterozygous: having two different alleles for a gene …

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Crossing Aa bb DD x aa bb dd yields only two genotypes — Aa bb Dd and aa bb Dd — in a 1:1 ratio. The B locus stays bb and the D locus is uniformly Dd; only the A locus segregates.

Gametes each parent can form

Parent 1 (Aa bb DD): heterozygous at A (gives A or a), homozygous at B (only b) and D (only D) -> gametes A b D and a b D.
Parent 2 (aa bb dd): homozygous recessive at every locus -> a single gamete a b d.

Combining the gametes, locus by locus

Because the genes assort independently, each locus can be analysed on its own:

A locus: Aa x aa -> Aa or aa (1 : 1)
B locus: bb x bb -> bb in every offspring
D locus: DD x dd -> Dd in every offspring …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

KCET 2026Set UNKNOWN1 markMCQ
$Q. Thalassemia and Sickle cell anaemia are due to a problem in globin molecule synthesis. Select the correct statement. (A) Sickle cell anaemia is due to a quantitative problem of globin molecule. (B) Both are due to qualitative defects in globin chain synthesis. (C) Both are due to quantitative defects in globin chain synthesis. (D) Thalassemia is due to less synthesis of globin molecules.$
›Reveal solutionSolution

Distinguishing the two disorders comes down to qualitative versus quantitative defects: sickle cell anaemia changes the structure of the globin chain, while thalassemia reduces the amount of a normal globin chain that is made.

Step 1 — Recall the defect in sickle cell anaemia

Sickle cell anaemia results from a point mutation that changes one amino acid (glutamic acid to valine) in the beta-globin chain — this alters the structure/quality of the globin molecule produced, not the amount made. This is a qualitative defect.

Step 2 — Recall the defect in Thalassemia

Thalassemia is caused by mutations (often deletions or defects in regulatory/synthesis machinery) that reduce or completely stop the synthesis rate of one of the globin chains (alpha or beta) — the globin chains produced are structurally normal, but fewer of them are made. This is a quantitative defect.

Step 3 — Evaluate the options …
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KCET 2025Set C-41 markMCQ
$Q. In a practical examination, the following pedigree chart was given as a spotter for identification. The students identify the given pedigree chart as_____: (A) Autosomal recessive (B) Sex-linked document (C) Sex-linked recessive (D) Autosomal dominant$
›Reveal solutionSolution
Unaffected × unaffected parents giving affected children ⇒ recessive; affected individuals of both sexes, including an affected daughter from an unaffected father ⇒ autosomal, not X-linked.

Step 1 — Read the pedigree.

Generation I: an unaffected male × an unaffected female.

Generation II: five children — an affected son, an affected daughter, and three unaffected children.

Generation III: an unaffected Gen-II daughter married to an unaffected male produces an affected son plus two unaffected daughters.

Step 2 — Dominant or recessive?

The trait skips: both Gen-I parents are unaffected, yet affected children appear. A dominant allele must be expressed in at least one parent (an affected child of a dominant trait must have an affected parent). Since the phenotype disappears in a generation and reappears, it is carried silently by heterozygotes ⇒ the allele is recessive. Both parents are carriers ( $Aa \times Aa \rightarrow \tfrac14\ aa$ affected).

This kills options (B) "Sex-linked dominant" and (D) "Autosomal dominant".

Step 3 — Autosomal or sex-linked (X-linked)?

Test the X-linked recessive hypothesis on the Gen-I family. An affected daughter would need genotype $X^a X^a$ , i.e. one $X^a$ from her mother (a carrier, fine) and one $X^a$ from her father. But a father's only X carries the allele he expresses — so an $X^a$ -bearing father would himself be affected. The Gen-I father is drawn unaffected. Contradiction. …
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KCET 2023Set B-41 markMCQ

Q. Which of the following statements is correct? (A) Female carrier for haemophilia may transmit the disease to sons. (B) Thalassemia is a qualitative problem. (C) Change in whole set of chromosomes is called aneuploidy. (D) Sickle cell anaemia is a quantitative problem.

›Reveal solution

Only (A) survives: haemophilia is X-linked recessive, so a carrier mother transmits it to ~50% of her sons. (B), (C) and (D) each state the opposite of the correct fact.

1. Statement (A) — check the cross

Haemophilia is a sex-linked (X-linked) recessive disorder. Let $X^H$ = normal allele, $X^h$ = haemophilia allele.

A carrier mother is heterozygous $X^H X^h$ (she is normal, because the dominant $X^H$ masks $X^h$ ). Cross her with a normal father $X^H Y$ :

X^H X^h \times X^H Y

	$X^H$ (from mother)	$X^h$ (from mother)
$X^H$ (father)	$X^H X^H$ — normal daughter	$X^H X^h$ — carrier daughter
$Y$ (father)	$X^H Y$ — normal son	$X^h Y$ — HAEMOPHILIC SON

A son receives his single X from his mother and a Y from his father. With no second X to carry a masking dominant allele, he is hemizygous and expresses whatever is on that X. Hence half the sons of a carrier mother are haemophilic — statement (A) is CORRECT. ✓

2. Why the other three are wrong

(B) "Thalassemia is a qualitative problem." ✗ — inverted. The standard contrast is:

Disorder	Defect
Thalassemia	QUANTITATIVE — reduced synthesis (too few globin chains are made) of an otherwise normal $\alpha$ or $\beta$ globin chain
Sickle-cell anaemia	QUALITATIVE — a normal amount of globin is made, but it is structurally abnormal (Glu → Val at position 6 of the $\beta$ -chain)

So thalassemia is quantitative, not qualitative. …

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KCET 2021Set C-31 markMCQ
$Q. Match the Column - I with Column - II Column - I i. Autosomal trisomy ii. Allosomal trisomy iii. Allosomal Monosomy iv. Cystic fibrosis Column - II p. Turner’s Syndrome q. Mendelian disorder r. Klinefelter’s Syndrome s. Down’s Syndrome (A) i-p, ii-q, iii-r, iv-s (B) i-p, ii-q, iii-s, iv-r (C) i-s, ii-r, iii-q, iv-p (D) i-s, ii-r, iii-p, iv-q$
›Reveal solutionSolution
Classify each disorder by which chromosome set is affected and whether a chromosome is gained or lost — that fixes all four matches.

Step 1 — The two vocabularies you need

Autosome vs allosome: autosomes are chromosome pairs 1–22; allosomes (sex chromosomes) are the X and Y.

Trisomy vs monosomy: these are aneuploidies, caused by the failure of chromatids to segregate during cell division (non-disjunction):

Trisomy = one EXTRA chromosome ( $2n+1$ ).

Monosomy = one chromosome MISSING ( $2n-1$ ).

Separately, a Mendelian disorder is one caused by an alteration/mutation in a SINGLE GENE (not a change in chromosome number).

Step 2 — Work through Column I

i. Autosomal trisomy → (s) Down's Syndrome.

An extra copy of chromosome 21 (trisomy 21) — chromosome 21 is an autosome. Karyotype $47$ , i.e. $2n+1$ . Features: short stature, small round head, furrowed tongue, partially open mouth, palm crease, retarded mental and physical development.

ii. Allosomal trisomy → (r) Klinefelter's Syndrome.

Karyotype 47, XXY — an extra X on the SEX chromosomes. The individual is masculine in development but has gynaecomastia (feminine development, e.g. breast development) and is sterile.

iii. Allosomal monosomy → (p) Turner's Syndrome.

Karyotype 45, XO — one of the X chromosomes is MISSING, again on the SEX chromosomes. Such females are sterile, have rudimentary ovaries and lack other secondary sexual characters.

iv. Cystic fibrosis → (q) Mendelian disorder. …
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Q.A, B, D are three independently assorting genes with their recessive alleles a, b, d, respectively. A cross was made between individuals of Aa bb DD genotype with aa bb dd. Find out the type of genotypes of the offspring produced.

Gametes each parent can form

Combining the gametes, locus by locus

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