A plant with red flowers was crossed with another plant with yellow flowers. If F1 showed all flowers orange i

NCERT Exemplar · Q38

Q. A plant with red flowers was crossed with another plant with yellow flowers. If F1 showed all flowers orange in colour, explain the inheritance.

Chhattisgarh CgbseShort· 2mImportance★★★★★

64% · 54/85 Questions

🔒 Locked · start free trial →

You're viewing a preview — the full solution, concept, methods & PYQ mapping are locked.

Start your 14-day free trial to unlock the full solution →

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 …

Start your 14-day free trial to unlock the full solution →

The orange flowers in F₁ result from incomplete dominance, where neither the red nor yellow allele is fully dominant, so the heterozygote expresses a blended intermediate phenotype.

When we cross a red-flowered plant with a yellow-flowered plant and observe that all offspring bear orange flowers, we are witnessing a departure from the classical Mendelian pattern of dominance. Mendel's pea experiments led him to conclude that one allele completely masks the effect of another—tall plants crossed with short plants gave only tall offspring in F₁, because the tall allele was dominant. But nature does not always follow this script.

In this cross, neither the red allele nor the yellow allele dominates the other. Instead, both contribute to the phenotype in the heterozygous condition, producing an intermediate colour: orange. This phenomenon is called incomplete dominance (or partial dominance). The heterozygote does not look like either parent; it displays a new, blended appearance.

Let us denote the allele for red flowers as R and the allele for yellow flowers as Y. The parental plants are homozygous: the red parent is RR, the yellow parent is YY. When these two are crossed, every offspring in the F₁ generation receives one R allele from the red parent and one Y allele from the yellow parent, making them all RY. Because neither allele is dominant, the RY genotype produces orange flowers—a mix of red and yellow pigments, or an intermediate expression.

Note

Incomplete dominance is distinct from codominance, where both alleles are fully expressed simultaneously (as in human ABO blood groups, where both A and B antigens appear together in AB individuals). Here, the phenotype is truly intermediate, not a side-by-side expression of both traits. …

Start your 14-day free trial to unlock the full solution →

←Previous questionQ37 — How was it concluded that genes are located on chromosomes?→Next questionQ39 — What are the characteristic features of a true-breeding line?

Q.A plant with red flowers was crossed with another plant with yellow flowers. If F1 showed all flowers orange in colour, explain the inheritance.

🎓Unlock everything free for 14 days

Unlock everything free for 14 days