Extensions and Modifications to Inheritance

Overview

This chapter explores advanced concepts in genetics, focusing on variations and exceptions to classical Mendelian inheritance. Topics include sex determination, sex-linked traits, single locus factors, gene interactions, and multifactorial traits.

Sex Determination

Chromosome-Based Sex Determination

Sex determination systems vary among organisms and are often governed by specific sex chromosomes or ploidy levels.

• Male Heterogamety (XX:XY system): Found in placental animals (e.g., humans). Males have XY, females have XX.

• Female Heterogamety (ZW:ZZ system): Found in birds. Females are ZW, males are ZZ.

• ZO System: Moths have only one sex chromosome (Z) in females.

• XO System: Grasshoppers have only one sex chromosome (X) in males.

• Ploidy Determines Sex: In some insects (e.g., bees), sex is determined by the number of chromosome sets (haploid males, diploid females).

Example: The haplo-diploid system in bees: males are haploid (from unfertilized eggs), females are diploid (from fertilized eggs).

Genetic and Environmental Sex Determination

• Genic Sex Determination: Sex is determined by specific genes, not entire chromosomes.

• Environmental Sex Determination: Factors such as temperature can influence sex (e.g., turtles).

Human Sex Determination

• X Chromosome: Contains essential genetic information for both sexes; at least one X is required.

• SRY Gene: The sex-determining region Y gene on the Y chromosome triggers male development.

• Phenotypes: Presence of Y (even with multiple Xs) results in male phenotype; absence of Y results in female phenotype.

Sex Chromosome Abnormalities

• Androgen-Insensitivity Syndrome: Caused by defective androgen receptor; genetically male (XY) but phenotypically female.

• Klinefelter Syndrome: XXY males; often have underdeveloped secondary sex characteristics.

• Triple X Syndrome: XXX females; may have mild developmental differences.

Sex-Linked Characteristics and Sex Influence

X-Linked Inheritance

X-linked traits are associated with genes located on the X chromosome. These traits often show distinct inheritance patterns in pedigrees.

• X-Linked Recessive: Trait is rare, often skips generations, affected fathers do not pass to sons, males more frequently affected.

• X-Linked Dominant: Trait is common, affected fathers pass to all daughters, both sexes equally likely to be affected.

HTML Table: Comparison of X-Linked Recessive vs. Dominant Inheritance

Pedigree Analysis

• Reciprocal crosses can distinguish X-linked inheritance from autosomal inheritance.

• Example: Colorblindness is a sex-linked recessive trait.

Dosage Compensation and X-Inactivation

Dosage compensation ensures equal expression of X-linked genes in males and females.

• X-Inactivation: In mammals, one X chromosome in females is randomly inactivated, forming a Barr body.

• Mosaic Coloration: Example: Calico cats show mosaic fur color due to X-inactivation.

HTML Table: Dosage Compensation Mechanisms

Single Locus Factors

Variations of Dominance

• Incomplete Dominance: Heterozygote shows intermediate phenotype (e.g., pink flowers from red and white parents).

• Codominance: Both alleles are fully expressed (e.g., AB blood type).

Penetrance and Expressivity

• Penetrance: Percentage of individuals with a genotype that express the expected phenotype.

• Expressivity: Degree to which a trait is expressed among individuals.

Example: Polydactyly shows variable penetrance and expressivity.

Lethal Alleles

• Some alleles cause death when present in certain genotypes (e.g., homozygous lethal).

Multiple Alleles

• More than two alleles exist for a gene in a population (e.g., ABO blood group).

HTML Table: ABO Blood Group Alleles

Gene Interaction

Epistasis

Epistasis occurs when one gene masks the effect of another gene at a different locus.

• Recessive Epistasis: Homozygous recessive at one locus masks expression at another locus (e.g., coat color in mice).

• Dominant Epistasis: Dominant allele at one locus masks expression at another locus (e.g., squash color).

Example: In mice, genotype at one locus results in yellow fur regardless of other alleles.

HTML Table: Epistasis Ratios

Complementation Test

Used to determine if mutations causing similar phenotypes are in the same or different genes.

• If offspring show wild-type phenotype, mutations are in different genes (complementation).

• If offspring show mutant phenotype, mutations are in the same gene (no complementation).

Multifactorial/Complex Traits

Polygenic Traits

Traits controlled by multiple genes, often showing continuous variation (e.g., height, skin color).

• As the number of genes increases, the number of phenotypic classes increases.

Pleiotropy

One gene affects multiple, seemingly unrelated traits.

• Examples: Sickle cell anemia, Marfan syndrome, PKU.

Sex-Influenced and Sex-Limited Traits

• Sex-Influenced: Expression differs between sexes due to hormones (e.g., pattern baldness).

• Sex-Limited: Trait expressed in only one sex (e.g., milk production in females, beard growth in males).

Cytoplasmic Inheritance

Genes located in mitochondria or chloroplasts are inherited maternally in most species.

• All children inherit cytoplasmic genes from the mother.

Genomic Imprinting

Expression of a gene depends on whether it is inherited from the mother or father.

Conditional Traits and Phenocopy

• Conditional Alleles: Expression depends on environmental conditions (e.g., temperature-sensitive alleles in Siamese cats).

• Phenocopy: Environmental factors produce a phenotype that mimics a genetic mutation.

Summary Table: Types of Gene Interaction and Inheritance

Key Equations

• Penetrance:

Additional info: Some context and examples were inferred and expanded for clarity and completeness.