Ch 4 · Monohybrid Inheritance · O Level Biology 5090

Topic 4 of 4 in Inheritance — O Level Biology 5090 — 74 lessons.

Introduction

Monohybrid inheritance follows one gene with two alleles through a cross. Set out every cross in the same order: parental phenotypes, parental genotypes, gametes, Punnett square, offspring genotypes, offspring phenotypes, ratio.

Key ideas — Definitions

🧑‍🏫 Teacher's explanation

Think of your DNA as a massive instruction manual for building you. Each page in the manual is a gene, and each page can have different versions of instructions—these versions are called alleles. Sometimes you get two identical versions of an instruction, and sometimes you get two different ones.

📖 Definitions

🔍 Worked example — Easy

In pea plants, the allele for tall stems (T) is dominant to the allele for short stems (t). A homozygous tall plant is crossed with a homozygous short plant. Determine the phenotype of the F_1 generation.

🔍 Worked example — Standard

A heterozygous tall plant (Tt) is crossed with a homozygous short plant (tt). Calculate the ratio of tall plants to short plants in the offspring.

🔍 Worked example — Exam-style

In humans, eye color is simplified to two alleles: B (brown) is dominant and b (blue) is recessive. A man with brown eyes marries a woman with blue eyes. They have four children: two with brown eyes and two with blue eyes. What is the most likely genotype of the father?

⚠️ Common errors

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🎯 Exam question

Key ideas — Genetic diagrams and Punnett squares

Heterozygous cross, Tt × Tt. Gametes T or t from each parent.

🧑‍🏫 Teacher's explanation

Think of inheritance like a recipe book. Each parent has two copies of every instruction (alleles), but they only pass one copy down to their child. If one instruction is 'loud' (dominant) and the other is 'quiet' (recessive), the loud one will always be heard in the final dish.

📖 Definitions

🔍 Worked example — Easy

A homozygous dominant plant (TT) is crossed with a homozygous recessive plant (tt). Predict the genotype and phenotype of the offspring.

🔍 Worked example — Standard

In pea plants, stem height is controlled by a single gene where tall (T) is dominant to short (t). If two heterozygous plants (Tt) are crossed, calculate the phenotypic ratio of the offspring.

🔍 Worked example — Exam-style

A researcher wants to identify the genotype of a plant with a dominant phenotype. They cross it with a homozygous recessive plant. If 50% of the offspring show the recessive phenotype, what was the genotype of the parent plant?

⚠️ Common errors

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🎯 Exam question

(a) Determine the genotype of the woman. [1] (b) Using a genetic diagram, predict the phenotypic ratio of the children's eye colors. [3] [4 marks]

Key ideas — The test cross

An organism with the dominant phenotype could be TT or Tt. Cross it with a homozygous recessive (tt), whose gametes all carry t and cannot mask the unknown allele.

🧑‍🏫 Teacher's explanation

Imagine you are trying to figure out if a person's height is determined by two identical 'tall' genes or one 'tall' and one 'short' gene. If they have two tall genes, every child they have will definitely be tall. But if they carry a hidden 'short' gene, there is a chance they might pass that hidden gene on.

📖 Definitions

🔍 Worked example — Easy

A pea plant has purple flowers (the dominant trait). The alleles are P for purple and p for white. If the plant is crossed with a white-flowered plant (pp), what is the phenotypic ratio of the offspring if the parent is homozygous dominant (PP)?

🔍 Worked example — Standard

A plant with dominant purple flowers (P) is crossed with a white-flowered plant (pp). If the resulting offspring show a phenotypic ratio of 1 purple : 1 white, what is the genotype of the purple parent?

🔍 Worked example — Exam-style

In a species of moth, black fur (B) is dominant to white fur (b). A researcher finds a black moth and wants to determine if it is homozygous or heterozygous. The researcher crosses it with a white moth. If 50 offspring are produced and 25 are white, calculate the percentage of offspring that are white.

⚠️ Common errors

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🎯 Exam question

(a) State the genotype of the dominant-flowered parent. [1] (b) Explain how the results of this cross allow you to determine the genotype of the parent. [2] [3 marks]

Key ideas — Codominance and ABO blood groups

codominance. Codominance is a situation in which both alleles in heterozygous organisms contribute to the phenotype, so a heterozygote shows a third phenotype.

🧑‍🏫 Teacher's explanation

In standard Mendelian inheritance, one allele usually 'wins' (dominates) and hides the other. Think of it like a light switch: it is either ON or OFF. However, codominance is more like mixing paint. If you have one blue allele and one yellow allele, you don't get a 'dominant' blue or a 'recessive' yellow; you get green. Both instructions are being followed simultaneously.

📖 Definitions

🔍 Worked example — Easy

A person has blood type AB. What is their genotype?

🔍 Worked example — Standard

A man with blood type O marries a woman with blood type AB. Predict the phenotypic ratio of the offspring's blood groups.

🔍 Worked example — Exam-style

In a population, two individuals with blood type A and blood type B have a child with blood type O. Determine the genotypes of the parents and the probability that their next child will have blood type AB.

⚠️ Common errors

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🎯 Exam question

Key ideas — Sex-linked characteristics

sex linkage. A sex-linked characteristic is a feature in which the gene responsible is located on a sex chromosome, which makes the characteristic more common in one sex than in the other. The Y chromosome lacks most X genes, so a male has one allele and any recessive allele on his X is expressed.

🧑‍🏫 Teacher's explanation

Think of sex chromosomes like a specialized instruction manual. Females have two copies of the 'X' manual, while males have one 'X' manual and one 'Y' manual. The 'Y' manual is very thin and mostly contains instructions for male physical traits, missing most of the instructions found in the 'X' manual.

📖 Definitions

🔍 Worked example — Easy

A female is a carrier for red-green colour blindness (X^B X^b) and a male has normal vision (X^B Y). Draw a Punnett square and determine the probability that a son will be colour blind.

🔍 Worked example — Standard

A man who is colour blind (X^b Y) marries a woman with normal vision whose father was colour blind. Determine the percentage of their daughters that will be carriers.

🔍 Worked example — Exam-style

In a population, the frequency of the recessive allele for colour blindness (X^b) is 0.08. If we assume the population is in Hardy-Weinberg equilibrium (though not required for IGCSE, use this to find the frequency of males with the trait), what is the probability that a random male is colour blind?

⚠️ Common errors

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🎯 Exam question

(a) Using a genetic diagram, show the possible genotypes of their offspring. [3] (b) State the probability that they will have a daughter who is colour blind. [1] [4 marks]

Key ideas — Pedigree diagrams

Squares are males, circles females; shading shows the characteristic; lines join parents to their children. To deduce genotypes:

🧑‍🏫 Teacher's explanation

Think of a pedigree diagram like a family tree, but instead of just tracking names, it tracks a specific 'trait'—like eye color or a genetic condition. It is a visual map that shows how a specific characteristic is passed down through generations. Imagine you are a detective looking at a crime scene; the pedigree is your evidence. You aren't just looking at who looks like whom, you are looking for the 'hidden' instructions (alleles) that caused that appearance.

📖 Definitions

🔍 Worked example — Easy

In a pedigree diagram for a recessive skin disorder, an unaffected male (square) and an unaffected female (circle) have a child who is shaded (affected). Identify the genotypes of the parents.

🔍 Worked example — Standard

A pedigree shows a heterozygous parent (Aa) mating with a homozygous recessive parent (aa). Calculate the probability that the offspring will show the recessive phenotype.

🔍 Worked example — Exam-style

In a pedigree, two parents are both heterozygous (Aa) for a recessive trait. If they have a child, what is the probability that the child is a carrier (heterozygous) but does NOT show the trait?

⚠️ Common errors

🧠 Quick check 1

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🎯 Exam question

(a) State the genotypes of the two parents. [1] (b) If these parents have another child, calculate the probability that this child will be a carrier of the disorder. Show your working. [2] [3 marks]

Key ideas — Observed and expected ratios

Inheritance is the transmission of genetic information from generation to generation. A genetic diagram gives the expected ratio, for example 3 : 1 from two heterozygous parents. Real results, the observed ratio, are often different, because fertilisation is random: which sperm fuses with which egg is a matter of chance, and every offspring is an independent event.

🧑‍🏫 Teacher's explanation

Think of alleles as different versions of a recipe. If you have two recipe books, and one has a 'strong' instruction (dominant) and the other has a 'weak' instruction (recessive), the strong one usually wins out in the final dish. However, if you see a 'weak' version appearing in the children, you know for a fact that both parents must have carried that 'weak' instruction.

Ch 4 · Monohybrid Inheritance · O Level Biology 5090

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