Human Genetics 18
X Chromosome Inactivation in Females
Females express one of two alleles on their X chromosomes, resulting in patchy expression across cells.
Example: Calico cats have orange and black patches due to X-linked genes.
Male vs. Female Expression
Male cats can only express orange or black—determined by X chromosome (either X\text{ orange}, Y or X\text{ black}, Y).
Females (XX) display a mosaic pattern where only one allele of X is active per cell.
More severe effects arise in males due to expression of mutant alleles in all cells, while in females, only half may express the mutant allele.
Expressivity of X-linked Traits
Examples of varying expressivity explained through the phenotype presentation.
Condition of not producing sweat glands (X-linked disorder) showcases variable thermostat regulation relative to the location of patches on the body.
Severity of the phenotype in females with heterozygous alleles depends on where patches without the sweat glands are located.
Inheritance Patterns
Affected heterozygous female crossed with unaffected male can produce:
Affected and unaffected daughters.
Affected and unaffected sons.
Visualizing crosses with Punnett squares is helpful for understanding rare inheritance patterns.
Sex-limited and Sex-influenced Traits
Traits may result from autosomal genes (located on chromosomes 1-22) rather than X or Y.
Sex-limited traits: Present only in one sex (e.g., rooster feathers).
Sex-influenced traits: Found in both sexes but expressed differently due to hormones (e.g., beard growth and male pattern baldness).
Male pattern baldness is influenced by testosterone and differs in expression between genders.
X Chromosome Inactivation Mechanism
In mammals, X inactivation corrects the potential dosage imbalance of X-linked genes.
Early embryogenesis involves X inactivation where one chromosome becomes silenced (bar body formation).
The Xist gene is crucial for X inactivation—it encodes an RNA that binds the X chromosome, leading to transcriptional silencing.
Epigenetic Modifications
Inactivated X chromosome through methylation: Methyl groups are added to cytosine residues in the DNA sequence to inhibit transcription.
X inactivation alters the phenotype (observable traits) but not the genotype (underlying genetic makeup).
Examples of X-linked Traits in Cats
Calico and Tortoiseshell cats exemplify X-linked coloration due to allelic inactivation.
White coloration in Calico cats arises from a different, autosomal gene.
Conditional on heterozygosity, X-linked traits lead to expressed phenotypes in females, termed manifesting heterozygotes.
Chapter 7: Complex Traits
Complex traits: Traits that have Mendelian inheritance combined with additional factors, encompassing both single gene and multi-gene influences.
Polygenic traits are influenced by numerous genes and can exhibit continuous variation (height, skin color).
Environmental factors significantly contribute to phenotypic expression.
Empiric Risk and Heritability
Empiric risk: Probability of a trait recuring based on familial observation. Example: cleft lip incidence increases with family history.
Heritability: Proportion of observed variation in a trait due to genetics. Can be defined as narrow (recessive effects only) or broad (includes dominant alleles and gene interactions).
Twin and Adoption Studies for Trait Analysis
Twin studies help to distinguish genetic influence from environmental factors using concordance rates between monozygotic and dizygotic twins.
Adoption studies compare genetic correlations between biological and adoptive familial relations to assess heritability.
Genome-Wide Association Studies (GWAS)
GWAS identifies genetic variants linked to specific traits or diseases by comparing affected individuals to controls.
Focus often includes SNPs (single nucleotide polymorphisms) and CNVs (copy number variations) to examine genetic contributions to complex traits.