4. Gene for fungal resistance in certain organism resides in chloroplast. If a susceptible female and resistant male are crossed, the progeny will be
(1) All resistance
(2) All susceptible
(3) Half resistance and half susceptible
(4) Cannot be predicted

In this question, the fungal resistance gene is located in the chloroplast DNA, which is inherited cytoplasmically (non‑Mendelian). In most flowering plants, chloroplast DNA is inherited maternally, i.e., only from the female parent. Therefore, when a susceptible female is crossed with a resistant male, all progeny will inherit the chloroplast (and hence the susceptibility allele) from the female parent, and so all progeny will be susceptible.​

Correct answer: (2) All susceptible.​

Background: Chloroplast DNA Inheritance

In eukaryotes, genes located in organelles like mitochondria and chloroplasts show non‑Mendelian (cytoplasmic) inheritance because they are not on nuclear chromosomes.​

• Chloroplast DNA (cpDNA) is typically inherited maternally in most angiosperms (flowering plants).​

• This means that the chloroplast genome (and any gene in it) is passed on only through the egg (female gamete), not through the pollen (male gamete).​

• As a result, the phenotype of the progeny for a chloroplast gene depends only on the genotype of the female parent.​

The Given Cross

• Gene location: Fungal resistance gene is in the chloroplast DNA.​

• Parents:

  • Female parent: Susceptible (so her chloroplasts carry the susceptibility allele).​

  • Male parent: Resistant (his chloroplasts carry the resistance allele).​

• Cross: Susceptible female × Resistant male.​

Because chloroplast DNA is maternally inherited:

• All progeny receive their chloroplasts (and hence the fungal resistance gene) from the female (susceptible) parent.​

• They do not inherit the resistant chloroplasts from the male parent.​

Therefore, all F₁ progeny will be susceptible to the fungus.​

Analysis of Each Option

Let us now evaluate each of the four options given in the question.

(1) All resistance

This would be true only if the resistance gene were inherited paternally or biparentally.​

• In most flowering plants, chloroplast DNA is not inherited from the male parent; it is strictly maternal.​

• Since the female parent is susceptible, her chloroplasts carry the susceptibility allele, so progeny cannot be resistant.​

Hence, option (1) is incorrect.​

(2) All susceptible

This matches the maternal inheritance pattern of chloroplast DNA:

• The female parent is susceptible → her chloroplasts carry the susceptibility allele.​

• All progeny inherit chloroplasts only from the female parent → all progeny are susceptible.​

This is the expected outcome in standard angiosperms where cpDNA is maternally inherited.​

Therefore, option (2) is the correct answer.​

(3) Half resistance and half susceptible

This would be expected in a Mendelian nuclear gene with complete dominance, where:

• Female is homozygous recessive (susceptible)

• Male is homozygous dominant (resistant)

• F₁ progeny are all heterozygous → all resistant (if resistance is dominant).​

Even in a test cross (heterozygous × homozygous recessive), we get 1:1 ratio, but that is for nuclear genes.​

However, here the gene is in the chloroplast, not in the nucleus, so it does not follow Mendelian segregation.​

Thus, option (3) is incorrect.​

(4) Cannot be predicted

This would be true only if the mode of chloroplast inheritance were unknown or variable (e.g., paternal or biparental in that species).​

• In standard genetics problems (especially in CSIR NET‑type questions), unless specified otherwise, chloroplast genes are assumed to show maternal inheritance.​

• Given that, the progeny phenotype can be predicted: all will be like the female parent.​

So, option (4) is incorrect.​

Why Maternal Inheritance of Chloroplasts?

In most flowering plants:

• The egg cell contributes the bulk of the cytoplasm (including chloroplasts) to the zygote.​

• Pollen contributes mainly the nucleus; its cytoplasm (and chloroplasts) are usually excluded or degraded after fertilization.​

• As a result, the chloroplast genome in the offspring comes entirely from the maternal parent.​

A classic example is the inheritance of leaf color in Mirabilis jalapa (four‑o’clock plant), where the leaf color phenotype of the progeny is determined solely by the maternal parent.​

Exception: When Inheritance Might Be Different

Although maternal inheritance is the rule in angiosperms, there are exceptions:

• In some species (e.g., gymnosperms like pine), chloroplasts are inherited paternally (from the male).​

• In a few angiosperms (e.g., some Passiflora species), chloroplast inheritance can be paternal or biparental.​

However, in standard exam questions like this one, if no specific organism is mentioned, we assume the typical pattern: maternal inheritance of chloroplast DNA.​

Final Answer

In the cross:

• Susceptible female (♀) × Resistant male (♂)

• Gene for fungal resistance is in chloroplast DNA

All progeny will inherit the chloroplasts (and hence the susceptibility allele) from the female parent → all progeny are susceptible.​

Correct option: (2) All susceptible.​