Heredity

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Heredity — Key Facts for MDCAT

Heredity is the branch of biology that studies how traits are transmitted from parents to offspring. The foundational work was done by Gregor Mendel on pea plants (1856–1863).

Mendel’s Laws:

1. Law of Dominance: In a heterozygote, the dominant allele masks the recessive allele. The dominant phenotype appears.

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

2. Law of Segregation: During gamete formation, the two alleles for a gene separate (segregate) 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 independently during gamete formation (only applies when genes are on DIFFERENT chromosomes).

Key Definitions:

• Genotype: Genetic makeup (e.g., TT, Tt, tt)
• Phenotype: Physical appearance (e.g., tall or dwarf)
• Homozygous: Both alleles same (TT or tt)
• Heterozygous: Different alleles (Tt)
• Carrier: Heterozygote with recessive allele (Tt) but dominant phenotype
• Punnett Square: Diagram for predicting offspring genotypes

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

⚡ Exam tip: In MDCAT heredity questions, always determine which allele is dominant BEFORE setting up your Punnett square. The dominant allele is usually capital letter. In a test cross (unknown genotype × homozygous recessive), if ALL offspring show the dominant trait → unknown was homozygous dominant (TT). If offspring show BOTH dominant and recessive → unknown was heterozygous (Tt).

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students who want genuine understanding.

Heredity — Complete Study Guide

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

• Snapdragon flowers: Red (RR) × White (rr) → Pink (Rr)
• F₂ ratio: 1 red : 2 pink : 1 white

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

• MN blood group: L^M L^M → M antigen; L^N L^N → N antigen; L^M L^N → both M and N antigens
• AB blood group: I^A and I^B are co-dominant → both antigens present

Multiple Alleles (ABO Blood Group): Three alleles: I^A, I^B, i (but each person has only two)

• 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 (co-dominant)
• ii → Type O

Pleiotropy: One gene affects multiple unrelated traits.

• Example: Phenylketonuria (PKU) — single gene affects skin colour, intelligence, hair colour, body odour
• Example: Sickle cell anaemia — affects RBC shape, oxygen-carrying capacity, organ function, resistance to malaria

Polygenic Inheritance: Multiple genes affect one trait → continuous variation.

• Human skin colour (6+ genes, each with additive effect)
• Human height (~400 genes)
• Kernel colour in wheat (3 genes, each with additive effect → 7 phenotypes from 1 to 7 red)

Epistasis: One gene masks the expression of another gene.

• Recessive epistasis (9:3:4): Both recessive alleles at one locus mask the other gene
• Dominant epistasis (12:3:1): Dominant allele at one locus masks the other gene
• Duplicate recessive epistasis (9:7): Both genes must be dominant for expression

Sex Determination:

• Human: XX (female) × XY (male) → 50% chance of each sex
• X and Y are sex chromosomes — all other chromosomes are autosomes
• Male produces X or Y sperm (50:50); Female produces only X eggs
• Sex of child depends on which sperm fertilises the egg

⚡ Common mistakes: Confusing incomplete dominance (pink from red × white) with co-dominance (both colours visible as separate, like roan cattle or MN blood group). Forgetting that sex-linked traits are carried on X or Y chromosome — X-linked recessive traits appear more in males because they have only one X chromosome (no second copy to mask the recessive allele).

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Heredity — Advanced Notes

Sex-Linked Inheritance (X-Linked):

X-linked Recessive:

• Trait appears predominantly in males; females are usually carriers
• Examples: Red-green colour blindness, Haemophilia A (Factor VIII), Duchenne muscular dystrophy
• Cross: Carrier female (X^B X^b) × Normal male (X^B Y)
  • Daughters: 50% carrier, 50% normal
  • Sons: 50% affected, 50% normal
• An affected male cannot pass the trait to his sons (sons get Y from father)
• All daughters of an affected male are carriers (X^b from father → X^B X^b)

X-linked Dominant:

• Both males and females affected but females less severely (if heterozygous)
• Examples: Vitamin D-resistant rickets (X-linked dominant), G6PD deficiency
• Affected male → all daughters affected, no sons affected

Y-linked (Holandric):

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

Chromosome Theory of Inheritance: Sutton (1902) and Boveri proposed that genes are located on chromosomes. This explained Mendel’s laws physically — chromosomes segregate and assort independently during meiosis.

Linkage:

• Genes on the SAME chromosome are linked → tend to be inherited together
• Complete linkage: No recombination → parental phenotypes only (e.g., male Drosophila)
• Incomplete linkage: Crossing over produces recombinant gametes → new combinations
• Morgan’s experiments: White eye gene in Drosophila is X-linked

Crossing Over and Genetic Maps:

• Frequency of crossing over between genes on the same chromosome → map distance
• 1% recombination frequency = 1 map unit (centimorgan)
• Genes further apart have higher recombination frequency
• Maximum recombination = 50% (independent assortment, same as genes on different chromosomes)

Human Genetics and Genetic Disorders:

Disorder	Inheritance	Chromosome/Gene	Features
Down syndrome	Trisomy 21	Extra chromosome 21	Intellectual disability, flat face
Turner syndrome	Monosomy X	XO (female)	Short stature, infertility
Klinefelter syndrome	Trisomy XXY	XXY (male)	Infertility, feminine features
Cri-du-chat	Deletion	Deletion on chromosome 5	Cat-like cry, microcephaly
Sickle cell anaemia	Autosomal recessive	Point mutation in β-globin	Misshapen RBCs
Cystic fibrosis	Autosomal recessive	CFTR gene mutation	Lung disease, digestive problems
Phenylketonuria	Autosomal recessive	PAH gene mutation	Intellectual disability if untreated
Huntington disease	Autosomal dominant	CAG repeat expansion in HTT	Progressive neurodegeneration
Marfan syndrome	Autosomal dominant	FBN1 gene	Tall, long limbs, aortic aneurysm
Haemophilia A	X-linked recessive	Factor VIII deficiency	Bleeding disorder
Red-green colour blindness	X-linked recessive	Opsin gene mutations	Cannot distinguish red/green

Probability in Genetics:

• Product rule: P(A and B) = P(A) × P(B) — for independent events
• Sum rule: P(A or B) = P(A) + P(B) — for mutually exclusive events
• Example: Probability of having two daughters = P(first daughter) × P(second daughter) = ½ × ½ = ¼

Pedigree Analysis:

1. Identify which family members are affected (shaded)
2. Look for vertical transmission (autosomal dominant), horizontal clustering (autosomal recessive), or X-linked pattern
3. Males predominantly affected → think X-linked recessive
4. Affected father → all daughters carriers → think X-linked
5. Skip generations → think recessive

Non-Mendelian Inheritance:

• Mitochondrial inheritance: All mitochondria from mother (egg); affects males and females but only passed by females (e.g., mitochondrial myopathies)
• Genomic imprinting: Expression depends on parent of origin — Prader-Willi (paternal deletion) vs Angelman (maternal deletion) — same chromosome region!
• Anticipation: Genetic disorders become more severe or appear earlier in successive generations (e.g., Huntington disease, myotonic dystrophy) — due to trinucleotide repeat expansion
• Cytoplasmic inheritance: Traits coded by mitochondrial DNA, transmitted through cytoplasm

MDCAT Question Patterns: MDCAT Pakistan heredity questions frequently test: (1) Mendel’s laws and monohybrid/dihybrid crosses, (2) incomplete dominance vs co-dominance, (3) multiple alleles (ABO blood groups), (4) sex-linked inheritance (especially X-linked recessive), (5) test cross to determine genotype, (6) probability calculations, (7) pedigree analysis, (8) epistasis ratios, (9) human genetic disorders and their inheritance patterns. 2–3 questions per paper. X-linked inheritance and ABO blood groups are extremely high-yield topics.

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