Genetics: Inheritance, Monohybrid Crosses and Genetic Disorders

What you'll learn

This revision guide covers everything you need to know about genetics for the CXC CSEC Integrated Science examination. You will learn how characteristics pass from parents to offspring, how to predict inheritance patterns using monohybrid crosses, and understand common genetic disorders. These concepts form a critical part of the Biology section and regularly appear in Paper 2 structured questions.

Key terms and definitions

Gene — A section of DNA that codes for a specific characteristic or trait, located at a particular position on a chromosome.

Allele — Different forms of the same gene; for example, the gene for flower colour may have a purple allele and a white allele.

Genotype — The genetic makeup of an organism, represented by letter symbols (e.g., Tt or TT).

Phenotype — The observable physical or biochemical characteristics of an organism, determined by both genotype and environment (e.g., tall or short).

Dominant allele — An allele that expresses its characteristic when present in either one or two copies; represented by a capital letter (e.g., T).

Recessive allele — An allele that only expresses its characteristic when two copies are present; represented by a lowercase letter (e.g., t).

Homozygous — Having two identical alleles for a particular gene (e.g., TT or tt); also called purebred or true-breeding.

Heterozygous — Having two different alleles for a particular gene (e.g., Tt); also called hybrid.

Core concepts

Chromosomes, genes and inheritance

Human body cells contain 23 pairs of chromosomes (46 total), while sex cells (gametes) contain 23 single chromosomes. During sexual reproduction, gametes from each parent fuse during fertilization, restoring the full chromosome number in the offspring.

Each chromosome carries hundreds of genes that control different characteristics. For any characteristic, an individual inherits one allele from each parent. The combination of alleles determines whether the characteristic is expressed.

Key principles:

• Genes occupy specific positions (loci) on chromosomes
• Alleles are alternative forms of the same gene
• Diploid cells have two copies of each gene (one from each parent)
• Haploid gametes carry only one copy of each gene

Dominant and recessive inheritance

When an organism has two different alleles for a characteristic, only one may be expressed. The dominant allele masks the effect of the recessive allele.

Examples relevant to Caribbean agriculture:

• In pigeon peas (Cajanus cajan), purple flowers are dominant over white flowers
• In dasheen (Colocasia esculenta), green leaf colour is dominant over purple
• In tomatoes, red fruit colour is dominant over yellow

For a recessive characteristic to be expressed in the phenotype, the organism must be homozygous recessive (have two copies of the recessive allele). A heterozygous individual will show the dominant phenotype but carries the recessive allele — this individual is called a carrier.

Monohybrid crosses and Punnett squares

A monohybrid cross examines the inheritance of a single characteristic controlled by one gene. Punnett squares provide a visual method to predict the possible genotypes and phenotypes of offspring.

Steps to construct a Punnett square:

1. Identify the characteristic being studied
2. Assign letter symbols (capital for dominant, lowercase for recessive)
3. Determine the genotypes of both parents
4. Work out the possible gametes each parent can produce
5. Draw a grid and place gametes along the top and side
6. Fill in the boxes by combining alleles
7. Count genotypes and phenotypes to determine ratios

Standard ratios to remember:

Cross 1: Homozygous dominant × Homozygous recessive (TT × tt)

• Offspring genotype: 100% Tt
• Offspring phenotype: 100% dominant phenotype
• Ratio: 4:0

Cross 2: Heterozygous × Heterozygous (Tt × Tt)

• Offspring genotypes: 25% TT, 50% Tt, 25% tt
• Offspring phenotypes: 75% dominant, 25% recessive
• Genotypic ratio: 1:2:1
• Phenotypic ratio: 3:1

Cross 3: Heterozygous × Homozygous recessive (Tt × tt)

• Offspring genotypes: 50% Tt, 50% tt
• Offspring phenotypes: 50% dominant, 50% recessive
• Ratio: 1:1

Genetic terminology in practice

When answering exam questions, precise terminology is essential:

• Pure breeding/True breeding — Organisms that are homozygous and produce offspring identical to themselves
• F1 generation — The first filial generation; offspring produced from a cross between two parent organisms
• F2 generation — The second filial generation; offspring produced by crossing F1 individuals
• Test cross — Crossing an organism showing the dominant phenotype with a homozygous recessive individual to determine if it is homozygous or heterozygous

Genetic disorders in humans

Genetic disorders result from mutations in genes or abnormalities in chromosome number or structure. Many genetic disorders follow predictable inheritance patterns.

Sickle cell disease:

Sickle cell disease is particularly relevant in the Caribbean due to its prevalence in populations of African descent. It is caused by a mutation in the gene coding for haemoglobin.

• Inheritance pattern: Recessive
• Genotypes: HH (normal), Hs (carrier/sickle cell trait), ss (sickle cell disease)
• The sickle allele (s) causes red blood cells to form a crescent shape
• Sickled cells block blood vessels, causing pain, organ damage and reduced oxygen transport
• Carriers (Hs) have sickle cell trait — usually no symptoms but some resistance to malaria
• This explains why the allele remains common in tropical regions where malaria was historically prevalent

Other genetic disorders:

Cystic fibrosis:

• Recessive disorder affecting mucus production
• Thick, sticky mucus accumulates in lungs and digestive system
• Breathing difficulties and frequent infections

Haemophilia:

• Sex-linked recessive disorder (carried on X chromosome)
• Blood fails to clot properly due to missing clotting factors
• More common in males

Down syndrome:

• Caused by an extra copy of chromosome 21 (trisomy 21)
• Not inherited through alleles but from chromosome non-disjunction
• Characteristic facial features, developmental delays, and learning difficulties

Pedigree analysis

Pedigree charts show the inheritance of a characteristic through generations of a family. Understanding how to interpret these charts is essential for CSEC examinations.

Symbols used:

• Circle = female
• Square = male
• Shaded shape = individual shows the characteristic
• Unshaded shape = individual does not show the characteristic
• Horizontal line between shapes = mating
• Vertical line = offspring

Identifying inheritance patterns:

For recessive disorders:

• Can skip generations
• Two unaffected parents can have affected children (both parents are carriers)
• Affected individuals are homozygous recessive

For dominant disorders:

• Appear in every generation
• Affected individuals have at least one affected parent
• Cannot skip generations unless mutation occurs

Worked examples

Example 1: Monohybrid cross with complete dominance

Question: In tomato plants, red fruit (R) is dominant to yellow fruit (r). A farmer crosses a heterozygous red-fruited plant with a yellow-fruited plant.

(a) State the genotypes of both parent plants. [2 marks] (b) Using a Punnett square, determine the expected genotypic and phenotypic ratios of the offspring. [4 marks] (c) If the farmer obtains 60 offspring, how many would you expect to have yellow fruit? [2 marks]

Mark scheme answer:

(a) Heterozygous red-fruited plant: Rr [1 mark] Yellow-fruited plant: rr [1 mark]

(b) Punnett square:

[1 mark for correct gametes] [1 mark for correct completion of Punnett square]

Genotypic ratio: 2 Rr : 2 rr or 1:1 [1 mark] Phenotypic ratio: 2 red : 2 yellow or 1:1 [1 mark]

(c) 50% would have yellow fruit [1 mark] 50% of 60 = 30 plants [1 mark]

Example 2: Sickle cell inheritance

Question: Sickle cell disease is caused by a recessive allele (s). The normal allele is H.

(a) Explain why a person with genotype Hs does not suffer from sickle cell disease. [2 marks] (b) Two parents both have sickle cell trait (Hs). Draw a Punnett square to show the possible genotypes of their children. [3 marks] (c) What is the probability that their child will have sickle cell disease? [1 mark] (d) Explain why the sickle cell allele remains common in regions where malaria is prevalent. [3 marks]

Mark scheme answer:

(a) The person is heterozygous [1 mark] The normal/dominant allele (H) masks the effect of the sickle cell allele/the recessive allele is not expressed [1 mark]

(b) Punnett square:

[1 mark for correct gametes, 1 mark for correct Punnett square, 1 mark for correct genotypes]

(c) 1 in 4 or 25% or 0.25 [1 mark]

(d) People with sickle cell trait (Hs) have some resistance to malaria [1 mark] They are more likely to survive in areas where malaria is common [1 mark] They pass the sickle allele to their children, keeping it in the population [1 mark]

Example 3: Interpreting a genetic cross

Question: When two pink snapdragon flowers are crossed, the offspring consist of 26 red flowers, 54 pink flowers, and 28 white flowers.

(a) What type of inheritance is shown by flower colour in snapdragons? Justify your answer. [3 marks] (b) If R represents the allele for red and W represents the allele for white, what are the genotypes of: (i) the pink parent flowers (ii) the red offspring (iii) the white offspring? [3 marks]

Mark scheme answer:

(a) Incomplete dominance/codominance [1 mark] Neither allele is completely dominant [1 mark] The ratio of approximately 1:2:1 shows both alleles are expressed in heterozygotes [1 mark]

(b) (i) Pink parents: RW [1 mark] (ii) Red offspring: RR [1 mark] (iii) White offspring: WW [1 mark]

Common mistakes and how to avoid them

• Using the same letter in different cases randomly: Always use capital letters for dominant alleles and lowercase for recessive alleles of the SAME gene. Don't write Tt for one characteristic and Gg using unrelated genes in the same problem without clear definition.

• Forgetting gametes contain only ONE allele: When determining gametes from a Tt parent, the gametes are T and t (not Tt). Each gamete receives only one allele from each pair during meiosis.

• Confusing genotype with phenotype: The genotype is the allele combination (letters); the phenotype is the observable characteristic (tall, red, smooth). Always read the question carefully to provide the correct information.

• Incorrect Punnett square setup: Place one parent's gametes across the top and the other parent's gametes down the side. Each box should contain one allele from the top and one from the side. Don't mix up which parent's gametes go where.

• Not simplifying ratios: If your Punnett square shows 2 Tt and 2 tt, express this as a 1:1 ratio, not 2:2. Similarly, 1 TT, 2 Tt, 1 tt should be written as 1:2:1.

• Assuming carriers show recessive characteristics: A carrier (heterozygous individual) possesses one recessive allele but shows the dominant phenotype. Only homozygous recessive individuals (two recessive alleles) show the recessive characteristic.

Exam technique for Genetics: Inheritance, Monohybrid Crosses and Genetic Disorders

• Command word "State" requires a concise answer without explanation. For "State the genotype," simply write "Tt" — no need to explain what this means. "Explain" requires reasoning and mechanisms, usually worth 2-3 marks.

• Show all working in genetic crosses: Even if you know the answer, draw the Punnett square. Examiners award method marks even if your final answer is incorrect. Always label gametes clearly and show where each offspring genotype comes from.

• Use genetic terminology precisely: Don't write "normal gene" when you mean "dominant allele" or "genes inherited" when you mean "alleles inherited." The CSEC mark scheme rewards accurate biological terminology.

• Probability questions require fractional or percentage answers: If a question asks for probability, convert your ratio to a fraction, decimal, or percentage. "1 in 4" or "25%" or "0.25" are acceptable; "1:4 ratio" is not answering the question asked.

Quick revision summary

Inheritance involves passing alleles from parents to offspring. Dominant alleles are expressed in both homozygous and heterozygous genotypes, while recessive alleles only appear in homozygous recessive individuals. Monohybrid crosses predict offspring ratios using Punnett squares; remember the key ratios: 3:1 for heterozygous × heterozygous crosses and 1:1 for heterozygous × homozygous recessive crosses. Genetic disorders like sickle cell disease follow predictable inheritance patterns. Master Punnett square construction, use correct terminology, and always show your working for maximum marks.