13. During development, many gene products are provided by the females to the eggs which are needed for normal development of the zygote. Such genes are called as maternal-effect genes. The following are a set of crosses between parents carrying a recessive mutant allele (m) and the offspring obtained:
    Phenotype of offspring

Cross No.	Genotype of parents	Phenotype of offspring
I	m/+♂ X m/+♀	All normal
II	m/m♂ X m/+♀	All normal`
III	m/+♂ X m/m♀	All mutants
IV	m/m♂ X m/m♀	All mutants
V	m/+♂ X m/+♀	Both normal and mutant

Which of the above cross(es) is/are indicative that the mutation is in a maternal-effect gene?
(1) Cross III only            (2) Cross V only
(3) Cross I, II and III      (4) Cross II and V

The mutation is a maternal‑effect gene, and the “diagnostic” crosses are Cross I, II and III, so the correct option is (3).

Concept of maternal‑effect gene

In maternal‑effect genes, the phenotype of the offspring is determined by the mother’s genotype, because the mother deposits critical gene products (mRNA or proteins) into the egg during oogenesis. Even if an embryo’s own genotype is mutant, it can develop normally if its mother has at least one wild‑type allele and provides enough normal product in the egg.

For a recessive maternal‑effect mutation m, the rule is:

• Any mm mother produces all mutant offspring, regardless of the father’s or the zygote’s genotype.

• Any m/+ or +/+ mother produces all normal offspring, even if some embryos inherit m from one or both parents.

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Given crosses and maternal‑effect logic

Notation: first genotype is male (♂), second is female (♀).

• Cross I: m/+ ♂ × m/+ ♀ → all normal

  • Mother is m/+ (heterozygous).

  • Under standard zygotic recessive inheritance, 1/4 mm zygotes should be mutant.

  • But all offspring are normal, meaning mm zygotes are phenotypically rescued by mother’s wild‑type product.

  • This matches a recessive maternal‑effect gene where heterozygous mothers give all normal progeny.

• Cross II: m/m ♂ × m/+ ♀ → all normal

  • Mother again is m/+ (heterozygous).

  • Half the zygotes are expected to be m/m, yet all are normal.

  • This again shows that the mother’s genotype (m/+) determines the normal phenotype, not the embryo’s genotype.

• Cross III: m/+ ♂ × m/m ♀ → all mutants

  • Mother is m/m (homozygous mutant).

  • Zygotes will be 50% m/+ and 50% m/m, but every offspring is mutant.

  • This is the hallmark of a recessive maternal‑effect gene: an mm mother produces only mutant progeny, whatever their own genotype.

• Cross IV: m/m ♂ × m/m ♀ → all mutants

  • Here both the mother and all zygotes are m/m.

  • Such a result is compatible with either an ordinary recessive zygotic gene (all embryos mm) or a recessive maternal‑effect gene (mm mother).

  • Therefore, Cross IV alone is not diagnostic; it does not uniquely point to maternal effect because Mendelian recessive inheritance also predicts all mutants here.

• Cross V: m/+ ♂ × m/+ ♀ → both normal and mutant

  • This is the classic Mendelian 3:1 or similar segregation expected when the mutant phenotype depends on the embryo’s own genotype (zygotic recessive).

  • If m were purely a recessive maternal‑effect allele, an m/+ mother should give all normal progeny; observing both normal and mutant offspring contradicts a simple maternal‑effect pattern.

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Evaluating each option

The question asks: Which of the above cross(es) is/are indicative that the mutation is in a maternal‑effect gene?

Option (1) Cross III only

• Cross III (mm mother → all mutant) is indeed fully consistent with maternal effect, but it is not the only informative cross.

• Cross I and II are also incompatible with simple zygotic recessive inheritance and strongly support maternal effect.

• Hence Option (1) is incorrect.

Option (2) Cross V only

• Cross V shows both normal and mutant progeny from an m/+ mother, which fits zygotic recessive inheritance, not maternal‑effect inheritance.

• Thus, Cross V actually argues against a pure maternal‑effect model.

• Therefore Option (2) is incorrect.

Option (3) Cross I, II and III

• Cross I and II: heterozygous mothers (m/+) give all normal offspring regardless of the father’s genotype, which contradicts simple zygotic recessive expectations and agrees with maternal‑effect behavior.

• Cross III: homozygous mutant mother (m/m) gives all mutant offspring, again independent of zygotic genotype, a classic sign of recessive maternal‑effect mutation.

• Together, these three crosses form a consistent pattern where the mother’s genotype fully determines progeny phenotype, so Option (3) is correct.

Option (4) Cross II and V

• Cross II supports maternal effect (m/+ mother → all normal), while Cross V supports ordinary zygotic recessive inheritance (m/+ mother → mixture of normal and mutant).

• These two crosses are logically inconsistent under a single simple maternal‑effect model.

• Hence Option (4) is incorrect.

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Take‑home points for exam preparation

• In recessive maternal‑effect genes, check whether changing the mother’s genotype (but keeping similar zygotic combinations) switches all offspring from normal to mutant or vice versa.

• Crosses with heterozygous mothers giving all normal progeny and homozygous mutant mothers giving all mutant progeny, despite mixed zygotic genotypes, are strong evidence for a maternal‑effect mutation, as illustrated by Cross I, II and III.