Inheritance Patterns and Human Genetic Disease: Mendelian and Non-Mendelian Genetics

Inheritance Patterns in Genetics

Introduction

Inheritance patterns describe how genetic traits are transmitted from parents to offspring. While many traits follow simple Mendelian genetics, numerous human characteristics and diseases exhibit more complex patterns due to multiple genes, environmental factors, and gene interactions.

Nomenclature and Key Concepts

• Incomplete Dominance: Heterozygotes display a phenotype intermediate between both homozygotes.

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

• Pleiotropy: A single gene affects multiple phenotypic traits.

• Epistasis: One gene alters or masks the expression of another gene.

• Polygenic Inheritance: Multiple genes contribute to a single trait, often resulting in continuous variation.

• Epigenetics: Heritable changes in gene expression that do not involve changes to the DNA sequence, often influenced by environmental factors.

• Pedigree Analysis: Tracking inheritance patterns through family trees.

• Recessive Trait: Trait expressed only when two recessive alleles are present.

• Dominant Trait: Trait expressed when at least one dominant allele is present.

Learning Outcomes

• Compare and contrast incomplete dominance, codominance, pleiotropy, and polygenic inheritance.

• Explain how epistasis can lead to mismatches between genotype and phenotype.

• Use pedigrees to track inheritance and predict offspring characteristics.

• Distinguish between recessively and dominantly inherited traits.

• Describe how epigenetics integrates environmental experience to affect phenotype.

Complex Inheritance Patterns

Beyond Mendelian Genetics

Some inheritance patterns are more complex than predicted by simple Mendelian genetics. These include:

• Incomplete dominance

• Co-dominance

• Multiple alleles

• Pleiotropy

• Epistasis

• Polygenic inheritance

• Environmental impact

Incomplete Dominance

Definition and Examples

Incomplete dominance occurs when the heterozygote exhibits a phenotype that is intermediate between the two homozygotes.

• Example: Flower Color in Snapdragons

  • Cross between red (CRCR) and white (CWCW) produces pink (CRCW) offspring.

  • Self-pollination of F1 pink plants yields a 1:2:1 ratio of red:pink:white for both genotype and phenotype.

  • $C^RC^R \times C^WC^W \rightarrow C^RC^W$ (pink)

• Example: Hypercholesterolemia in Humans

  • HH: normal LDL receptors, Hh: reduced receptors (mild disease), hh: no receptors (severe disease).

Co-dominance and Multiple Alleles

Definition and Examples

Co-dominance occurs when both alleles in a heterozygote are fully expressed and distinguishable.

• Example: ABO Blood Groups

  • Three alleles: IA, IB, i.

  • IA and IB are co-dominant; i is recessive.

  • Blood type AB expresses both A and B antigens.

Genotype	Blood Type	Antigens	Antibodies
IAIA or IAi	A	A	Anti-B
IBIB or IBi	B	B	Anti-A
IAIB	AB	A and B	None
ii	O	None	Anti-A, Anti-B

Pleiotropy

Definition and Example

Pleiotropy occurs when a single gene influences multiple, seemingly unrelated phenotypic traits.

• Example: Sickle Cell Anemia

  • Mutation in the hemoglobin gene causes sickling of red blood cells (RBCs).

  • Symptoms include pain, stroke, vascular and kidney problems.

  • One gene affects multiple systems.

Gene Editing Therapies: CRISPR

Application

CRISPR/Cas9 gene editing can target and correct mutations responsible for genetic diseases such as sickle cell anemia. This technology offers potential for curing inherited disorders by directly modifying the DNA sequence in affected cells.

Epistasis

Definition and Example

Epistasis occurs when the expression of one gene is affected by another gene at a different locus.

• Example: Labrador Retriever Coat Color

  • Two genes: one for pigment deposition (E/e), one for pigment color (B/b).

  • E gene is epistatic to B gene; if ee, no pigment is deposited regardless of B/b genotype.

  • Phenotypic ratio deviates from classic Mendelian 9:3:3:1.

Polygenic Inheritance

Definition and Example

Polygenic inheritance involves multiple genes contributing to a single trait, resulting in continuous variation.

• Example: Human Skin Color

  • Three genes (A, B, C) each with two alleles contribute to skin color.

  • Crosses between intermediate individuals produce a wide range of phenotypes, forming a normal distribution.

Environmental Impact and Multifactorial Traits

Norm of Reaction

The phenotype produced by a genotype can vary depending on environmental conditions. The range of possible phenotypes is called the norm of reaction.

• Some traits (e.g., blood type) have a fixed phenotype.

• Polygenic traits (e.g., skin color, height) show broad norms of reaction.

• Environment contributes to the quantitative nature of multifactorial traits.

Epigenetics

Definition and Mechanisms

Epigenetics refers to heritable changes in gene expression that do not involve changes to the DNA sequence. These changes are often mediated by chemical modifications to DNA or histone proteins.

• Gene "off": DNA tightly wound around histones, inaccessible for transcription.

• Gene "on": DNA loosely wound, accessible for transcription.

• Environmental factors (e.g., diet) can influence epigenetic tags and gene expression.

Examples

• Royal jelly in bees turns on "queen" genes, producing queen bees from genetically identical larvae.

• Dietary supplements (e.g., folic acid) in mice can prevent abnormal gene expression and phenotypes.

Mendelian Inheritance Patterns

Pedigree Analysis

Pedigree analysis is used to track inheritance of traits through generations in families. It helps predict the probability of offspring inheriting specific traits.

• Symbols: Squares (males), circles (females), shaded (affected), unshaded (unaffected).

• Widow's peak, PTC tasting, and other traits can be analyzed using pedigrees.

Recessive Traits

• Albinism: Caused by homozygosity for a recessive allele. Carriers (heterozygotes) do not express the trait but can pass it to offspring.

Dominant Traits

• Polydactyly: Extra fingers or toes, expressed when at least one dominant allele is present.

• Huntington's Disease: Neurodegenerative disorder caused by a dominant allele.

Special Inheritance Cases

Lethal Alleles

Some alleles are lethal in homozygous condition, altering expected Mendelian ratios.

• Agouti gene in mice: AA = agouti coat, AB = yellow coat, BB = lethal (die early).

• Expected ratio from Aa x Aa cross: 2:1 yellow:agouti among live offspring.

Color Vision and Sex-Linked Traits

Colorblindness

Color vision is determined by cone cells in the retina. Mutations in genes encoding cone cell proteins can lead to colorblindness, which is more common in males due to X-linked inheritance.

• Incidence: 5-8% of Caucasian men, ~0.5% of women.

• X-linked traits are expressed more frequently in males (XY) than females (XX).

Practice Questions

• Three Character Cross: In a cross PpRRYy x PpRryy, what fraction of offspring will have purple flowers and round, yellow seeds? (Answer: 3/4)

• Agouti Gene in Mice: What is the expected phenotypic ratio of live mice from Aa x Aa cross? (Answer: 2:1 yellow:agouti)

Summary Table: Inheritance Patterns

Pattern	Definition	Example
Incomplete Dominance	Heterozygote intermediate phenotype	Snapdragon flower color
Co-dominance	Both alleles fully expressed	ABO blood group
Pleiotropy	One gene affects multiple traits	Sickle cell anemia
Epistasis	One gene masks another	Labrador coat color
Polygenic Inheritance	Multiple genes, continuous variation	Skin color
Epigenetics	Heritable changes in gene expression	Bee caste, mouse coat color

Additional info:

• Gene editing (CRISPR) is a modern approach to treating genetic diseases by directly modifying DNA.

• Environmental factors can influence gene expression through epigenetic mechanisms, affecting phenotype without altering genotype.