Allele

A gene controls one trait, such as height in a pea plant. It can come in different forms: T for tall and t for dwarf. Each form is an allele. A body cell has two alleles for each gene, one from each parent.

• Homozygous: both alleles are the same, as in TT or tt.
• Heterozygous: the two alleles differ, as in Tt.
• Dominant: the allele that shows even when paired with a different one.
• Recessive: the allele that stays hidden unless both alleles are recessive.

A small example

A pea plant with Tt is tall, because T is dominant. Its genotype, the alleles it carries, is Tt. Its phenotype, the trait you can see, is tall. A tt plant is dwarf. Only two recessive alleles let the dwarf trait show.

Where marks go

The 2025 ICSE Biology paper asked for a one-mark definition of allele. The marking scheme accepted "alternative forms of a gene". CISCE's report on the 2025 ICSE Biology paper calls answers such as "form of a gene" vague. They did not say that alleles are alternative forms of a gene at the same locus, that is, the same position.

The report adds that some answers explained an allele as a dominant or recessive trait. An allele is a form of a gene, not a trait. Write "alternative forms of the same gene".

About the author

Deepti Sardana

Founder Member & Academic Lead

An IIT alumna (M.Sc., M.Phil.) with over 25 years in teaching, child psychology and learning assessment. As Academic Lead at EduBrahma, she leads the method TRUtesting uses to find where and why a child loses marks.

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