Mendelian Genetics and Probability

🟢 Lite — Quick Review (1h–1d)

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

Mendel’s Two Laws:

1. Law of Segregation: Each organism has two alleles for each trait. During gamete formation, these alleles separate so each gamete receives only one allele. During fertilisation, the offspring receives one allele from each parent.

2. Law of Independent Assortment: Alleles for different traits segregate independently of each other during gamete formation (true for genes on different chromosomes).

Key Vocabulary:

• Gene: A section of DNA that codes for a specific trait
• Alleles: Different versions of a gene (e.g., $Y$ and $y$ for seed colour)
• Dominant: An allele that expresses its phenotype even when heterozygous (capital letter, e.g., $Y$)
• Recessive: An allele that only expresses in homozygous condition (lowercase, e.g., $y$)
• Genotype: Genetic makeup (e.g., $Yy$, $YY$, $yy$)
• Phenotype: Physical appearance (e.g., yellow seeds, green seeds)
• Homozygous: Both alleles the same ($YY$ or $yy$)
• Heterozygous: Different alleles ($Yy$)

Monohybrid Cross Example (One Trait):

Cross: Yellow seeds ($YY$) × Green seeds ($yy$)

P generation: $YY$ × $yy$ Gametes: $Y$ and $Y$ from first plant; $y$ and $y$ from second F₁ generation: All $Yy$ (yellow — $Y$ is dominant) F₁ × F₁ cross: $Yy$ × $Yy$ F₂ generation: $YY : Yy : yy = 1 : 2 : 1$ Phenotypic ratio: 3 yellow : 1 green

⚡ NECO Tip: When a question says “a plant self-pollinates” or “is crossed with itself,” both parents have the same genotype. When crossing two heterozygotes ($Yy$ × $Yy$), the genotypic ratio is always $1 YY : 2 Yy : 1 yy$ and phenotypic ratio is $3 : 1$ (if dominant).

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🟡 Standard — Regular Study (2d–2mo)

Standard content for NECO Biology students with a few days to months.

Test Cross: To determine if a dominant phenotype is homozygous ($YY$) or heterozygous ($Yy$), cross it with the recessive homozygote ($yy$).

• If all offspring show dominant phenotype → the unknown parent was homozygous dominant
• If offspring are 50% dominant and 50% recessive → the unknown parent was heterozygous

Dihybrid Cross (Two Traits):

Cross: $RRYY$ (round, yellow) × $rryy$ (wrinkled, green)

F₁: All $RrYy$ (round, yellow) F₁ × F₁: $RrYy$ × $RrYy$

For each gene pair: $3 : 1$ phenotypic ratio. Combined phenotypic ratio for dihybrid cross: 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green (9:3:3:1)

Probability Rules:

1. Product rule: Probability of two independent events both occurring = $P(A) \times P(B)$
2. Sum rule: Probability of either of two mutually exclusive events occurring = $P(A) + P(B)$

Example: Probability of getting two boys in two children:

• $P(\text{boy}) = \frac{1}{2}$
• $P(\text{boy and boy}) = \frac{1}{2} \times \frac{1}{2} = \frac{1}{4}$

Incomplete Dominance: Neither allele is fully dominant. Heterozygotes show an intermediate phenotype. Example: Snapdragon flowers — $R$ = red, $R’$ = white. $RR’$ = pink. Cross $RR$ × $R’R’$ → All $RR’$ (pink)

Codominance: Both alleles are fully expressed in heterozygotes. Example: MN blood group — $L^M L^M$ = M antigen; $L^N L^N$ = N antigen; $L^M L^N$ = MN (both antigens present).

Multiple Alleles (Polyallelic): More than two alleles exist in the population, but each individual has only two. Example: ABO blood groups — three alleles: $I^A$, $I^B$, $i$.

• $I^A I^A$ or $I^A i$ → Type A
• $I^B I^B$ or $I^B i$ → Type B
• $I^A I^B$ → Type AB
• $ii$ → Type O

⚡ NECO Common Mistakes:

• Mixing up genotype and phenotype
• Forgetting that recessive alleles can be passed to children without showing in parents
• Confusing incomplete dominance with codominance
• In dihybrid crosses, forgetting that each trait independently segregates

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for NECO and JAMB Biology preparation.

Linkage:

Genes located on the same chromosome tend to be inherited together — they are linked. The degree of linkage depends on how close the genes are on the chromosome.

Sex Determination (XX/XY System):

• Human females: $XX$ (homogametic — all eggs carry $X$)
• Human males: $XY$ (heterogametic — sperm carry either $X$ or $Y$)
• Sex is determined by the father: $X$ from mother + $X$ from father = female; $X$ from mother + $Y$ from father = male
• Ratio of male:female offspring ≈ 1:1

Sex-Linked Inheritance:

Genes located on sex chromosomes (usually X chromosome) show different inheritance patterns in males and females because males have only one X chromosome (hemizygous).

X-linked Recessive Examples (Colour Blindness):

• $X^b$ = colour blind allele, $X^B$ = normal vision allele
• Carrier female: $X^B X^b$ — phenotypically normal
• Affected male: $X^b Y$
• Affected female: $X^b X^b$ (rare — father must be colour blind, mother must be carrier or affected)

Cross: Normal male ($X^B Y$) × Carrier female ($X^B X^b$) Offspring: $X^B X^B$ (normal female), $X^B X^b$ (carrier female), $X^B Y$ (normal male), $X^b Y$ (affected male) Expected: 1 normal female : 1 carrier female : 1 normal male : 1 colour blind male

Pedigree Analysis:

A pedigree chart shows the inheritance of a trait across generations:

• Autosomal recessive traits: often skip generations, affected individuals usually have unaffected parents
• Autosomal dominant traits: appear in every generation, affected individual has at least one affected parent
• X-linked recessive: affected males inherit the allele from their carrier mother; affected females have an affected father

Mutation:

• Gene mutation: Change in DNA sequence. Point mutation (substitution — e.g., sickle cell anaemia where GAG → GTG in the beta-globin gene, causing glutamic acid → valine substitution)
• Chromosomal mutation: Change in chromosome structure (deletion, duplication, inversion, translocation) or number (aneuploidy — e.g., Down syndrome: trisomy 21)
• Karyotype: The complete set of chromosomes of an individual, arranged by size

Sickle Cell Anaemia:

Caused by a point mutation: codon GAG → GTG (one nucleotide substitution). This changes the 6th amino acid of haemoglobin from glutamic acid (hydrophilic) to valine (hydrophobic), causing haemoglobin molecules to polymerise under low oxygen conditions and distort red blood cells into a sickle shape.

Inheritance: Autosomal recessive. Heterozygotes ($Hb^A Hb^S$) have sickle cell trait — generally asymptomatic, but may experience problems under extreme conditions.

⚡ NECO/JAMB Patterns:

• NECO frequently asks: work out monohybrid and dihybrid crosses; interpret pedigree charts; explain the difference between gene and chromosome; describe sex-linked inheritance with specific crosses; calculate probability of offspring having specific genotypes

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