“Genetics: Mendelian Inheritance”

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your WAEC exam.

Key Terms:

• Gene: Segment of DNA that codes for a trait
• Allele: Different versions of a gene
• Dominant: allele that expresses in heterozygote (represented by capital letter)
• Recessive: allele only expresses in homozygote (represented by lowercase)
• Genotype: Genetic makeup (e.g., Tt, TT, tt)
• Phenotype: Physical appearance (e.g., tall, short)
• Homozygous: Two identical alleles (TT or tt)
• Heterozygous: Two different alleles (Tt)
• Carrier: Heterozygous individual with recessive allele but no symptoms

Mendel’s First Law (Law of Segregation): During gamete formation, paired alleles separate so each gamete receives one allele from each pair.

Mendel’s Second Law (Law of Independent Assortment): Alleles of different genes assort independently of one another during gamete formation. (This applies to genes on different chromosomes.)

Monohybrid Cross Pattern: Parents: Tt × Tt Gametes: T, t from each parent Offspring genotypes: TT : Tt : tt = 1 : 2 : 1 Phenotypic ratio: 3 dominant : 1 recessive

⚡ WAEC Tip: When asked to cross a homozygous dominant with homozygous recessive, ALL offspring will be heterozygous and show the dominant phenotype. Example: TT × tt → ALL Tt (tall).

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🟡 Standard — Regular Study (2d–2mo)

For students who want genuine understanding.

Working with Genetics Problems:

Step-by-step approach:

1. Assign letters (choose clear letters — avoid C/c because it looks like G)
2. Write parental genotypes
3. Determine gametes (one allele from each gene per gamete)
4. Use Punnett square to find offspring
5. Calculate ratios

Example - Dihybrid Cross: Cross: TtRr × TtRr (where T = tall, t = short, R = round, r = wrinkled)

Gametes from TtRr: TR, Tr, tR, tr (all equally likely)

Punnett Square Results: Phenotypic ratio = 9 tall round : 3 tall wrinkled : 3 short round : 1 short wrinkled

Genotypic ratio = 1 TT RR : 2 TT Rr : 1 TT rr : 2 Tt RR : 4 Tt Rr : 2 Tt rr : 1 tt RR : 2 tt Rr : 1 tt rr

Test Cross: Used to determine genotype of dominant phenotype. Cross with homozygous recessive (tt or aa).

• If ALL offspring show dominant → parent was homozygous dominant (TT)
• If 50% show dominant, 50% recessive → parent was heterozygous (Tt)

Incomplete Dominance: Neither allele is fully dominant. Heterozygote shows intermediate phenotype.

Example: Snapdragons

• Red (RR) × White (WW) → Pink (RW)
• Pink × Pink → 1 Red : 2 Pink : 1 White

Codominance: Both alleles fully expressed in heterozygote. Example: MN blood group

• M antigen AND N antigen both present on RBCs in MN individuals
• Locus: LM and LN alleles, both expressed

Multiple Alleles: When a gene has more than two alleles in a population. Example: ABO blood groups

• Three alleles: Iᴬ, Iᴮ, i
• Six genotypes: IᴬIᴬ, Iᴬi (Type A); IᴮIᴮ, Iᴮi (Type B); IᴬIᴮ (Type AB); ii (Type O)

⚡ Common Mistake: Students confuse “dominant” with “more common.” In genetics, dominant simply means it masks the recessive allele in a heterozygote. A trait can be rare and still be dominant (e.g., Huntington’s disease).

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🔴 Extended — Deep Study (3mo+)

Comprehensive theory for serious exam preparation.

Sex Determination:

In mammals:

• Female: XX

• Male: XY

• XX × XY → 50% female (X from mother + X from father) : 50% male (X from mother + Y from father)

• So sex is determined by the FATHER (which sperm fertilises the egg)

Sex-linked Traits:

• Genes located on X or Y chromosome
• X-linked recessive: more common in males (they only have one X)
• Examples: Colour blindness, Haemophilia

X-linked Recessive Inheritance Pattern:

Cross	Female	Male
XᴮXᴮ × XᵇY	All normal	All normal
XᴮXᵇ × XᴮY	All normal	50% normal, 50% affected
XᵇXᵇ × XᴮY	All carriers	All affected

Haemophilia in Royal Families:

• Recessive X-linked
• Queen Victoria was carrier (XᴴXʰ)
• Passed to affected sons and carrier daughters
• Tsar Nicholas II’s son Alexei was haemophiliac
• Princess Victoria Eugenie (Queen’s granddaughter) was also carrier

Autosomal vs Sex-linked Traits:

Feature	Autosomal	X-linked
Affects both sexes equally	Yes	No
Father-to-son transmission	Yes	No (males get Y from father)
Pattern in family tree	Vertical (parent to child)	Diagonal (affected males through carrier mothers)

Pedigree Analysis:

Pattern	Likely Inheritance
Affects males and females equally, vertical transmission	Autosomal dominant
Affects males and females equally, can skip generations	Autosomal recessive
Affects mostly males, transmitted through carrier mothers	X-linked recessive
Every affected person has affected parent	Dominant

Probability in Genetics:

Product Rule: Probability of two independent events both occurring = P(A) × P(B)

• Example: Probability of being male AND having haemophilia = ½ (male) × ½ (if carrier mother, chance of receiving Xʰ) = ¼

Sum Rule: Probability of either of two mutually exclusive events = P(A) + P(B)

• Example: Probability of being TT or Tt from Tt × Tt = ¼ + ½ = ¾

Calculating Ratio from Probability: For cross Tt × Tt:

• P(TT) = ¼
• P(Tt or tT) = ½
• P(tt) = ¼ Phenotypic ratio = ¾ dominant : ¼ recessive

Linkage:

• Genes on the same chromosome tend to be inherited together
• Crossing over can separate linked genes
• Frequency of crossing over indicates distance between genes
• Thomas Hunt Morgan’s work with Drosophila (fruit flies)

Mutation:

Gene mutations:

• Point mutation: single base change (e.g., sickle cell anaemia — GAG → GTG)
• Frameshift: insertion or deletion shifts reading frame

Chromosomal mutations:

• Deletion, duplication, inversion, translocation

Sickle Cell Anaemia:

• Autosomal recessive
• Single point mutation in beta-globin gene
• Glutamic acid → Valine at position 6
• Causes sickle-shaped RBCs that block capillaries
• Heterozygotes (HbA HbS) have sickle cell trait — more resistant to malaria

Chromosomal Sex Disorders:

Condition	Chromosomes	Cause
Turner syndrome	XO	Missing X (female, sterile)
Klinefelter syndrome	XXY	Extra X (male, often sterile)
Trisomy 21 (Down)	47, +21	Extra chromosome 21

⚡ WAEC Previous Year Pattern:

Year	Question	Concept
2023	Monohybrid cross ratio	3:1 ratio
2022	Sex-linked inheritance	Colour blindness pedigree
2021	Blood group genetics	Multiple alleles, codominance

Dihybrid Cross Extended:

When genes are LINKED (on same chromosome), they do NOT assort independently.

• Parental types more common than recombinant types
• Morgan found this with Drosophila eye colour and body colour genes

Calculating Genetic Distance:

• 1% recombination frequency = 1 map unit (centimorgan)
• Genes farther apart on chromosome have higher recombination frequency
• Maximum recombination = 50% (behave as if on different chromosomes)

Genetic Crosses with Two Genes (autosomal):

Example: Flower colour (P = purple, p = white) and seed shape (R = round, r = wrinkled) Cross: PpRr × ppRr

Gametes from PpRr: PR, Pr, pR, pr Gametes from ppRr: pR, pr (only, because first parent is pp)

⚡ Exam Strategy: Always start genetics problems by writing out what you’re told clearly. Use a systematic approach: identify which allele is dominant, assign letters, write parental genotypes, work through gametes, and construct Punnett square. Check your work by confirming ratios add up to the right total.

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