Inheritance

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Inheritance — Key Facts for MDCAT

Mendel’s Laws:

1. Law of Dominance: In a heterozygote, one allele (dominant) masks the other (recessive). Only the dominant trait appears in the phenotype.

   • Example: Tall (T) is dominant over dwarf (t) in pea plants. TT and Tt = tall; tt = dwarf.

2. Law of Segregation: During gamete formation, the two alleles for a gene separate so each gamete receives only one allele.

   • Each gamete carries only one allele for each gene

3. Law of Independent Assortment: Alleles of different genes assort (separate) independently during gamete formation (only TRUE when genes are on DIFFERENT chromosomes).

Key Terminology:

• Genotype: Genetic makeup (e.g., TT, Tt, tt)
• Phenotype: Physical appearance (e.g., tall or dwarf)
• Homozygous (pure): Both alleles same (TT or tt)
• Heterozygous (hybrid): Different alleles (Tt)
• Carrier: Heterozygote who carries a recessive allele but doesn’t show it
• Punnett Square: Diagram to predict offspring genotypes from a cross

Monohybrid Cross (One Gene): Cross: Tt × Tt Offspring ratio: 3 tall : 1 dwarf (phenotypic); 1 TT : 2 Tt : 1 tt (genotypic)

Dihybrid Cross (Two Genes): Cross: TtRr × TtRr (T = tall, t = dwarf; R = round, r = wrinkled) Phenotypic ratio: 9 tall-round : 3 tall-wrinkled : 3 dwarf-round : 1 dwarf-wrinkled

Test Cross: Crossing an individual of unknown genotype with a homozygous recessive individual to determine the unknown genotype.

• If all offspring show dominant trait → unknown was homozygous dominant
• If offspring show both dominant and recessive → unknown was heterozygous

⚡ Exam tip: When a question asks “what percentage of offspring will be tall?”, always check whether the question refers to genotype or phenotype, and whether genes are linked (not assorting independently). Linked genes (on the same chromosome) do NOT follow independent assortment.

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students who want genuine understanding.

Inheritance — Complete Study Guide

Incomplete Dominance: Neither allele is completely dominant — heterozygote shows intermediate phenotype.

• Example: Snapdragons — red (RR) × white (rr) → pink (Rr)
• F₂: 1 red : 2 pink : 1 white

Co-dominance: Both alleles are fully expressed in the heterozygote.

• Example: MN blood group — genotype LM LM = M antigen; LN LN = N antigen; LM LN = both M and N antigens
• Example: ABO blood group I^A and I^B are co-dominant

Multiple Alleles: More than two alleles exist in a population (but each individual has only two).

• ABO Blood Group System:
  • I^A = A antigen (dominant over i)
  • I^B = B antigen (dominant over i)
  • I^A and I^B are co-dominant
  • i = no antigen (recessive)
  • Genotypes: 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
  • ABO compatibility: Type O is universal donor; Type AB is universal recipient

Rh Factor (Erythroblastosis Fetalis):

• Rh⁺ (dominant) vs Rh⁻ (recessive)
• Problem: Rh⁻ mother with Rh⁺ fetus → mother produces anti-Rh antibodies → in subsequent pregnancies, antibodies attack fetal RBCs → hemolytic disease of newborn (HDN)

Pleiotropy: One gene affects multiple phenotypic traits.

• Example: Phenylketonuria (PKU) — single gene mutation affects skin colour, intelligence, hair colour, etc.
• Example: Sickle cell anaemia — one mutation affects RBC shape, oxygen transport, organ function

Polygenic Inheritance: Multiple genes affect one trait — creates continuous variation.

• Example: Human skin colour (multiple genes, each with additive effects)
• Example: Height in humans, wheat grain colour

Epistasis: One gene masks the expression of another gene.

• Recessive epistasis (9:3:4 ratio): e.g., in mice — colour gene (C = pigmented, c = albino) epistatic over pattern gene
• Dominant epistasis (12:3:1 ratio): e.g., squash colour — one gene dominant masks another
• Duplicate recessive epistasis (9:7 ratio): Both genes must be dominant for expression

Chromosome Theory of Inheritance: Sutton (1902) and Boveri — genes are located on chromosomes. Mendel’s laws are explained by the behaviour of chromosomes during meiosis.

⚡ Common mistakes: Confusing incomplete dominance (pink from red × white) with co-dominance (both red and white separately visible, e.g., roan cattle). Forgetting that human blood type genetics has three alleles (I^A, I^B, i) but each person has only two. Thinking linkage means genes are on the same chromosome — yes, but linked genes do NOT assort independently, which affects ratios.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Inheritance — Advanced Notes

Linkage:

• Genes on the SAME chromosome tend to be inherited together
• Complete linkage: No crossing over → parental phenotypes only (rare, e.g., male fruit flies)
• Incomplete linkage: Crossing over produces recombinant gametes → new combinations
• Linkage groups: All genes on one chromosome form a linkage group; humans have 23 linkage groups

Morgan’s Experiments (Fruit Flies — Drosophila):

• White eye gene (w) is X-linked (sex-linked)
• Male: X^w Y (white-eyed); Female: X^W X^W (red-eyed)
• Cross: Red-eyed female × White-eyed male → F₁ all red; F₂: 1/4 red females, 1/4 red males, 1/4 white males
• This inheritance pattern is characteristic of X-linked recessive traits

Sex-Linked Inheritance:

• Genes located on sex chromosomes (X and Y)

• X-linked recessive in humans:

  • Colour blindness (red-green): more common in males (males need only one copy to express; females need two)
  • Hemophilia A: Factor VIII deficiency; X^h Y → affected male; X^H X^h → carrier female
  • Duchenne muscular dystrophy

• X-linked dominant:

  • Vitamin D-resistant rickets (X-linked dominant)
  • Rare — affected males usually die before reproduction

• Y-linked (holandric):

  • Genes on Y chromosome → only males affected, passed father → all sons
  • Example: Hairy ear pinna (some populations)

Pedigree Analysis:

• Autosomal dominant: Affected individual has at least one affected parent; vertical transmission
• Autosomal recessive: Can skip generations; often appears in offspring of unaffected carriers
• X-linked recessive: Predominantly affects males; carrier females pass to sons
• X-linked dominant: Both males and females affected; father cannot pass to sons

Probability in Genetics:

• Product rule: Probability of two independent events both occurring = P(A) × P(B)
• Sum rule: Probability of either of two mutually exclusive events occurring = P(A) + P(B)

Dihybrid Cross with Independent Assortment (9:3:3:1): Parents: TTRR × ttrr F₁: All TtRr (tall, round) F₂ (TtRr × TtRr):

• 9 tall-round (T_R_)
• 3 tall-wrinkled (T_rr)
• 3 dwarf-round (ttR_)
• 1 dwarf-wrinkled (ttrr)

Non-Mendelian Genetics:

• Mitochondrial inheritance: All mitochondria come from mother’s egg; paternal mitochondria are destroyed. Affects both males and females but only passed by females.
• Genomic imprinting: Expression of allele depends on which parent it came from (e.g., Prader-Willi syndrome vs Angelman syndrome — same deletion but different parent of origin).

Heredity in Plants (Special Cases):

• Plastid inheritance: Chloroplast DNA (maternal in most plants) — variegated plants show non-Mendelian inheritance
• Self-incompatibility alleles: S-gene prevents self-pollination — heterozygotes produce two types of gametes

Human Genetic Disorders:

Disorder	Inheritance	Cause
Sickle cell anaemia	Autosomal recessive	Single nucleotide mutation in β-globin gene
Phenylketonuria (PKU)	Autosomal recessive	Cannot metabolise phenylalanine
Cystic fibrosis	Autosomal recessive	CFTR gene mutation; common in Caucasians
Huntington disease	Autosomal dominant	CAG trinucleotide repeat expansion
Marfan syndrome	Autosomal dominant	FBN1 gene mutation
Down syndrome	Trisomy 21 (aneuploidy)	Non-disjunction of chromosome 21

MDCAT Question Patterns: MDCAT Pakistan inheritance questions frequently test: (1) Mendel’s laws and monohybrid/dihybrid crosses, (2) incomplete dominance vs co-dominance, (3) multiple alleles (ABO blood group), (4) X-linked inheritance patterns, (5) test cross to determine genotype, (6) probability calculations, (7) pedigree analysis, (8) epistasis ratios. 2–3 questions per paper. X-linked inheritance and ABO blood groups are extremely common MCQ topics.

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