Pedigree Analysis

Pedigree Analysis & Mendelian Inheritance in Humans

Mendelian inheritance in humans describes traits where a specific genotype is necessary and sufficient to produce a phenotype. Humans display over 15,000 known Mendelian traits, including both pathological (disease-causing) and non-pathological characteristics. However, most human traits are not purely Mendelian; instead, they result from interactions between genes and environmental factors. Pedigree analysis is a key tool used to study inheritance patterns by examining family histories, especially when controlled breeding experiments are impossible.

At each autosomal gene locus, humans inherit two alleles—one from each parent. Traits may be dominant or recessive depending on how alleles interact. Dominant traits require only one mutant allele to be expressed and appear in both heterozygotes and homozygotes. Recessive traits require two mutant alleles for expression; individuals with only one mutant allele are carriers and typically show no symptoms. Punnett squares help predict inheritance probabilities, such as a 50% risk for offspring of an autosomal dominant heterozygote or a 25% chance of disease when two carriers of an autosomal recessive trait reproduce.

Sex-linked inheritance adds further complexity. In X-linked recessive conditions, males are hemizygous for the X chromosome, meaning a single mutant allele will always be expressed, while females require two mutant alleles to be affected. As a result, these traits are much more common in males, as seen in red–green colour blindness and haemophilia. X-linked dominant traits affect both sexes but often more females, with affected males passing the trait to all daughters but none of their sons. Y-linked traits are extremely rare and are passed strictly from father to son.

Pedigree analysis identifies five basic Mendelian inheritance patterns: autosomal dominant, autosomal recessive, X-linked recessive, X-linked dominant, and Y-linked. Autosomal dominant disorders, such as Huntington’s disease, often show vertical transmission across generations and may demonstrate anticipation, where symptoms appear earlier in successive generations. Autosomal recessive disorders, like cystic fibrosis, frequently appear in children of unaffected carrier parents and are more common in populations with high consanguinity.

Several special inheritance patterns can complicate pedigree interpretation. Mitochondrial inheritance involves genes passed exclusively from the mother and affects both sexes. Genetic imprinting results in gene expression that depends on whether the allele was inherited from the mother or father. Other complications include non-penetrance, where individuals carry a mutation but show no phenotype; de novo mutations, which arise spontaneously; and X-linked dominant male lethality, where affected males do not survive to birth.

Finally, not all Mendelian traits are harmful. Traits such as PTC taste sensitivity follow simple dominant inheritance without causing disease. In contrast, most common human diseases are multifactorial, involving multiple genes and environmental influences. These traits increase susceptibility rather than guaranteeing a phenotype, making them difficult to analyze using traditional Mendelian pedigrees.

🧬 Fun & Detailed Bullet-Point Summary

Pedigree Analysis & Mendelian Inheritance in Humans

🧠 Mendelian Inheritance in Humans

  • Some traits are necessary and sufficient for a phenotype → Mendelian traits

  • Humans show 15,000+ Mendelian traits (pathological + non-pathological)

  • Most human traits result from genes + environment

  • Pedigree analysis = studying family histories to infer inheritance patterns

🧬 Alleles, Dominance & Recessiveness

  • Humans have two alleles at each autosomal locus

    • One from mother

    • One from father

  • Dominant trait

    • Only one mutant allele needed

    • Expressed in heterozygotes and homozygotes

  • Recessive trait

    • Requires two mutant alleles

    • Heterozygotes = carriers, usually unaffected

🧩 Punnett Squares (Classic Probabilities)

  • Autosomal dominant (affected × unaffected)
    → 50% affected offspring

  • Autosomal recessive (carrier × carrier)
    → 25% affected
    → 50% carriers
    → 25% unaffected

🚹 X-Linked Recessive Traits

  • Males are hemizygous (only one X)

  • Any mutant allele on X → phenotype is always expressed

  • Females need two mutant alleles to be affected

  • Example: Red–green colour blindness

    • ~8% of males

    • ~0.5% of females (Northern European ancestry)

🌳 Pedigree Analysis Basics

  • Used when controlled crosses are impossible

  • Pedigrees show:

    • Family relationships

    • Sex

    • Phenotype

  • Five basic Mendelian pedigree patterns:

    1. Autosomal dominant

    2. Autosomal recessive

    3. X-linked recessive

    4. X-linked dominant

    5. Y-linked

🧠 Autosomal Dominant (AD) Inheritance

Key Features

  • Affects both sexes equally

  • Transmitted by either sex

  • Affected person usually has an affected parent

  • 50% risk for children (if parent is heterozygous)

Example: Huntington’s Disease

  • Neurodegenerative disorder

  • Caused by CAG trinucleotide repeat expansion

40 repeats → disease

  • Shows anticipation (earlier onset each generation)

🧠 Autosomal Recessive (AR) Inheritance

Key Features

  • Affected individuals usually born to unaffected parents

  • Parents are carriers

  • Increased incidence with consanguinity

  • 25% recurrence risk per pregnancy

Example: Cystic Fibrosis

  • Caused by mutations in a chloride transporter

  • Thick mucus → lung infections

  • Ireland has the highest CF incidence worldwide

    • 1 in 19 people are carriers

🚹 X-Linked Recessive Inheritance

Key Features

  • Affects mostly males

  • No male-to-male transmission

  • Females often carriers

  • Affected females are rare

Example: Haemophilia

  • Defective blood clotting

  • Famous in European royal families

  • Originated from Queen Victoria

🚺 X-Linked Dominant Inheritance

Key Features

  • Affects both sexes, more females

  • Females show milder symptoms

  • Affected male → all daughters affected, no sons

  • Rare

Example: Vitamin D-resistant rickets

🚹 Y-Linked Inheritance

  • Affects only males

  • Always passed father → son

  • No recombination

  • Very rare

  • Mostly associated with male fertility traits

⚠ Complications in Pedigree Patterns

  • Common recessive traits

  • Mitochondrial inheritance

  • Genetic imprinting

  • X-linked dominant male lethality

  • Non-penetrance

  • De novo (new) mutations

🧫 Special Inheritance Patterns

🧠 Common Recessive Traits

  • Can mimic dominant inheritance

  • Example: ABO blood group

    • O allele is recessive but common

🔋 Mitochondrial Inheritance

  • Passed only from mother

  • Affects both sexes

  • High mutation rate

  • Example: Leigh syndrome

🧬 Genetic Imprinting

  • Gene expression depends on parent of origin

  • Involves epigenetic modifications

  • Example: Prader-Willi & Angelman syndromes

💀 X-Linked Dominant Male Lethality

  • Mutations lethal in males before birth

  • Only females survive

  • Example: Incontinentia pigmenti

🚫 Non-Penetrance

  • Individual carries mutation but shows no phenotype

  • Causes skipped generations

  • Common in autosomal dominant traits

✨ De Novo Mutations

  • New mutation appears with no family history

  • Makes recurrence risk difficult to predict

🍬 Non-Pathogenic Mendelian Traits

PTC Taste Sensitivity

  • Controlled by TAS2R38 gene

  • Dominant inheritance

    • TT → intensely bitter

    • Tt → mildly bitter

    • tt → non-taster

🌍 Multifactorial Inheritance

  • Caused by multiple genes + environment

  • Includes most common diseases

  • Mutation increases risk, not certainty

  • Difficult to track in pedigrees

🧠 40 Flashcards (Q → A)

  1. What is pedigree analysis? → Study of inheritance using family records

  2. What defines a Mendelian trait? → Genotype sufficient for phenotype

  3. Dominant trait requires how many alleles? → One

  4. Recessive trait requires how many alleles? → Two

  5. What is a carrier? → Heterozygous recessive individual

  6. Autosomal traits affect which sexes? → Both

  7. X-linked recessive affects mostly? → Males

  8. Why are males hemizygous? → Only one X chromosome

  9. Colour blindness inheritance? → X-linked recessive

  10. % of colour-blind males (N Europe)? → ~8%

  11. Number of basic pedigree patterns? → Five

  12. Huntington’s disease inheritance? → Autosomal dominant

  13. Cause of Huntington’s disease? → CAG repeat expansion

  14. Anticipation means? → Earlier onset in next generation

  15. Cystic fibrosis inheritance? → Autosomal recessive

  16. CF carrier frequency in Ireland? → 1 in 19

  17. AR traits often skip generations because? → Carriers unaffected

  18. Haemophilia inheritance? → X-linked recessive

  19. Why no male-to-male transmission in X-linked traits? → Father gives Y to sons

  20. X-linked dominant affects which sex more? → Females

  21. Affected X-linked dominant father passes trait to whom? → All daughters

  22. Y-linked traits affect whom? → Only males

  23. Mitochondrial DNA inherited from whom? → Mother

  24. Leigh syndrome inheritance? → Mitochondrial

  25. Genetic imprinting involves what mechanism? → Epigenetics

  26. Imprinted genes are expressed from how many parents? → One

  27. X-linked dominant male lethality causes what pattern? → Only affected females

  28. Example of X-linked dominant male lethal disease? → Incontinentia pigmenti

  29. Non-penetrance means? → Genotype without phenotype

  30. Non-penetrance common in which inheritance? → Autosomal dominant

  31. De novo mutation means? → New mutation

  32. ABO blood group located on which chromosome? → 9

  33. O allele is dominant or recessive? → Recessive

  34. Why common recessive traits mimic dominance? → High frequency

  35. PTC tasting inheritance? → Dominant

  36. TT PTC genotype tastes how? → Intensely bitter

  37. Tt PTC genotype tastes how? → Mildly bitter

  38. tt PTC genotype tastes how? → No taste

  39. Multifactorial inheritance includes what factors? → Genes + environment

  40. Are multifactorial diseases strictly genetic? → No

📝 40 MCQs

(Answers below)

  1. Mendelian traits are determined by:
    A Environment only
    B Multiple genes
    C Single genes
    D Epigenetics

  2. A dominant trait is expressed when:
    A Two alleles are present
    B One allele is present
    C Environment triggers it
    D Gene is imprinted

  3. Recessive traits often skip generations because:
    A They mutate often
    B Carriers are unaffected
    C They are lethal
    D They are rare

  4. Males express X-linked recessive traits because they are:
    A Homozygous
    B Diploid
    C Hemizygous
    D Polyploid

  5. Colour blindness is inherited as:
    A Autosomal dominant
    B Autosomal recessive
    C X-linked recessive
    D X-linked dominant

  6. How many Mendelian pedigree patterns exist?
    A 3
    B 4
    C 5
    D 6

  7. Huntington’s disease shows which feature?
    A Imprinting
    B Anticipation
    C Mosaicism
    D Non-penetrance

  8. Cystic fibrosis is caused by:
    A Neurological degeneration
    B Ion transport defect
    C Blood clotting defect
    D Bone malformation

  9. CF inheritance pattern is:
    A Autosomal dominant
    B Autosomal recessive
    C X-linked
    D Mitochondrial

  10. Ireland has the highest incidence of:
    A Huntington’s disease
    B CF
    C Haemophilia
    D Leigh syndrome

  11. X-linked recessive traits mostly affect:
    A Females
    B Males
    C Both equally
    D Children only

  12. Haemophilia was common in which families?
    A Irish
    B African
    C European royal
    D Asian

  13. X-linked dominant inheritance affects:
    A Only males
    B Only females
    C Both sexes
    D Only children

  14. Affected X-linked dominant father passes trait to:
    A All children
    B All sons
    C All daughters
    D No children

  15. Y-linked traits are transmitted from:
    A Mother to son
    B Father to son
    C Mother to daughter
    D Father to daughter

  16. Mitochondrial inheritance is:
    A Biparental
    B Paternal
    C Maternal
    D Random

  17. Mitochondrial diseases affect which sexes?
    A Males only
    B Females only
    C Both
    D Neither

  18. Genetic imprinting depends on:
    A DNA sequence
    B Parent of origin
    C Age
    D Sex

  19. Prader-Willi syndrome involves:
    A Paternal UPD
    B Maternal UPD
    C X-linked mutation
    D Autosomal dominant

  20. X-linked dominant male lethality results in:
    A Affected males only
    B No affected individuals
    C Only affected females
    D All children affected

  21. Incontinentia pigmenti affects mainly:
    A Males
    B Females
    C Both equally
    D Children only

  22. Non-penetrance means:
    A Trait always expressed
    B Trait never expressed
    C Genotype without phenotype
    D Environment-dependent gene

  23. Non-penetrance most commonly affects:
    A Autosomal recessive
    B Autosomal dominant
    C X-linked recessive
    D Y-linked

  24. A de novo mutation is:
    A Inherited
    B Environment-induced
    C Newly arisen
    D Epigenetic

  25. ABO blood group locus is on chromosome:
    A 1
    B 9
    C 15
    D X

  26. O blood group allele is:
    A Dominant
    B Codominant
    C Recessive
    D Lethal

  27. PTC tasting shows what inheritance?
    A Autosomal recessive
    B Autosomal dominant
    C X-linked
    D Mitochondrial

  28. TT genotype tastes PTC as:
    A No taste
    B Mildly bitter
    C Intensely bitter
    D Sweet

  29. Multifactorial traits are influenced by:
    A Single gene
    B Genetics only
    C Environment only
    D Genes + environment

  30. Multifactorial diseases are:
    A Fully predictable
    B Rare
    C Complex
    D Mendelian

  31. Pedigree analysis is used because humans lack:
    A DNA
    B Genes
    C Controlled crosses
    D Chromosomes

  32. Which inheritance shows no male-to-male transmission?
    A Autosomal dominant
    B Autosomal recessive
    C X-linked
    D Y-linked

  33. Affected individuals with AR traits usually have parents who are:
    A Affected
    B Carriers
    C Homozygous normal
    D Unrelated

  34. Which pattern shows affected individuals in every generation?
    A Autosomal recessive
    B Autosomal dominant
    C X-linked recessive
    D Mitochondrial

  35. Leigh syndrome inheritance is:
    A Autosomal
    B X-linked
    C Mitochondrial
    D Y-linked

  36. Imprinted genes are regulated by:
    A Mutation
    B Recombination
    C Epigenetics
    D Splicing

  37. PTC non-tasters have genotype:
    A TT
    B Tt
    C tt
    D TS

  38. Mitochondrial DNA has what mutation rate?
    A Low
    B Moderate
    C High
    D Zero

  39. Which inheritance pattern affects only males?
    A X-linked recessive
    B Autosomal dominant
    C Y-linked
    D Mitochondrial

  40. Which is NOT a complication of pedigree analysis?
    A Non-penetrance
    B Genetic imprinting
    C Crossing-over
    D De novo mutations

✅ MCQ Answer Key

1 C
2 B
3 B
4 C
5 C
6 C
7 B
8 B
9 B
10 B
11 B
12 C
13 C
14 C
15 B
16 C
17 C
18 B
19 B
20 C
21 B
22 C
23 B
24 C
25 B
26 C
27 B
28 C
29 D
30 C
31 C
32 C
33 B
34 B
35 C
36 C
37 C
38 C
39 C
40 C

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