Monohybrid Inheritance

What monohybrid inheritance studies

Monohybrid inheritance follows how a single characteristic, controlled by a single gene with two alleles, is passed from parents to offspring. It rests on the basic vocabulary of genetics: an allele is dominant if it is always expressed in the phenotype when present, and recessive if it is only expressed when no dominant allele is present.

Key terms you need together

• Genotype, the allele combination an organism carries for a gene, written using letters, for example Tt.
• Phenotype, the observable characteristic that results from the genotype, for example tall or short.
• Homozygous, carrying two identical alleles for a gene (TT or tt).
• Heterozygous, carrying two different alleles for a gene (Tt).

A simple cross and the 3:1 ratio

Consider plant height in pea plants, where tall (T) is dominant over short (t). Crossing two heterozygous plants, Tt x Tt, gives gametes T and t from each parent.

Combining these in a Punnett square produces offspring genotypes in the ratio 1 TT : 2 Tt : 1 tt.

Since both TT and Tt produce the tall phenotype, the phenotype ratio is 3 tall : 1 short. This 3:1 ratio is the signature result of crossing two heterozygotes for a single gene, and recognising it is central to solving monohybrid inheritance problems.

How this is examined

SPM questions typically give a scenario describing a dominant and a recessive characteristic, ask you to assign letter symbols to the alleles, determine the genotypes and phenotypes of parents and offspring, complete a Punnett square, and state the resulting genotype and phenotype ratios.

Finding an unknown genotype: the test cross

A tall pea plant with genotype TT looks identical to a tall pea plant with genotype Tt, because the dominant phenotype hides which alleles are actually present. To find out which genotype a tall plant has, it is crossed with a plant that is homozygous recessive (tt), this is called a test cross.

If the tall parent is TT, every offspring will be Tt and show the tall phenotype. If the tall parent is Tt, about half the offspring will be Tt (tall) and half tt (short), so a mixture of phenotypes in the offspring reveals that the original tall parent was heterozygous.

Common mistakes to avoid

Worked exam-style question

Question. In a species of flower, purple petal colour (P) is dominant over white petal colour (p). A heterozygous purple flower is crossed with a white flower.

(a) State the genotypes of both parents. (b) Complete a Punnett square for this cross.

(c) State the genotype and phenotype ratios of the offspring.

Model answer. (a) The heterozygous purple parent is Pp; the white parent is pp. (b) The Punnett square below combines the gametes P and p from the purple parent with p and p from the white parent.

(c) The genotype ratio is 1 Pp : 1 pp, and the phenotype ratio is 1 purple : 1 white.

Practice question

In guinea pigs, black fur (B) is dominant over white fur (b). Two heterozygous black guinea pigs (Bb) are crossed.

(a) State the genotypes of the gametes each parent can produce. (b) Predict the genotype and phenotype ratios of the offspring.

Exam tip

Key terms

• Allele, an alternative form of a gene, such as T or t.
• Dominant allele, an allele that is expressed in the phenotype whenever it is present.
• Recessive allele, an allele expressed only when no dominant allele is present.
• Genotype, the allele combination an organism carries, such as Tt.
• Phenotype, the observable characteristic resulting from the genotype, such as tall or short.

Source:SRC-DSKP-EN

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