What is the cross between the progeny of F1 and the homozygous recessive parent called? How is it useful?

NCERT Exemplar · Q19

Q. What is the cross between the progeny of F1 and the homozygous recessive parent called? How is it useful?

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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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The cross between the F1 progeny and its homozygous recessive parent is called a Test Cross, and it is used to determine the unknown genotype of an individual exhibiting a dominant phenotype.

Gregor Mendel's pioneering work with garden peas established the fundamental principles of heredity. He observed that when two true-breeding parents differing in a single trait were crossed, the first generation of offspring, known as the First Filial (F1) generation, uniformly expressed only one of the parental traits. This expressed trait was termed dominant, while the unexpressed one was recessive. For example, crossing a true-breeding tall pea plant with a true-breeding dwarf pea plant resulted in all tall F1 progeny, indicating tallness is dominant over dwarfness.

However, these F1 tall plants were not genetically identical to the true-breeding tall parent. While both appeared tall, the true-breeding parent carried two alleles for tallness (homozygous dominant), whereas the F1 progeny carried one allele for tallness and one for dwarfness (heterozygous). To distinguish between an individual that is homozygous dominant and one that is heterozygous, both of which display the dominant phenotype, a specific type of cross is employed.

The cross between the F1 progeny (or any individual showing a dominant phenotype) and the homozygous recessive parent is known as a Test Cross.

Note

The homozygous recessive parent is always chosen for a test cross because it can only contribute recessive alleles to its offspring. This ensures that any dominant alleles observed in the offspring must have come from the individual whose genotype is being tested, making the results clear and unambiguous.

The primary utility of a test cross lies in its ability to determine the unknown genotype of an individual that expresses a dominant phenotype. An individual showing a dominant trait could either be homozygous dominant (e.g., TT for tallness) or heterozygous (e.g., Tt for tallness). A test cross helps differentiate between these two possibilities.

Consider an example where we have a pea plant producing yellow seeds, and we know that yellow seed colour is dominant over green seed colour. We want to determine if this yellow-seeded plant is homozygous dominant (YY) or heterozygous (Yy). We would cross this unknown yellow-seeded plant with a homozygous recessive green-seeded plant (yy).

There are two distinct outcomes for the progeny of this test cross, each revealing the genotype of the unknown parent:

If the unknown dominant individual is homozygous dominant (YY):

When a YY plant is crossed with a yy plant, all the offspring will inherit a 'Y' allele from the first parent and a 'y' allele from the second, resulting in all heterozygous (Yy) progeny. Consequently, all the offspring will display the dominant phenotype, meaning all will produce yellow seeds.
If the unknown dominant individual is heterozygous (Yy): …

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Q.What is the cross between the progeny of F1 and the homozygous recessive parent called? How is it useful?

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