BIOL3304 Ch06

Chapter Overview

Focus on human genetics and the biological factors of sex, delving into the complexities of how genetic, hormonal, and environmental influences shape sexual identity and expression.

Learning Outcomes

Learning Outcomes 1

  • Describe factors contributing to gender identity. Gender identity is influenced by various factors including genetics, socialization, and psychological development.

  • Distinguish between X and Y chromosomes. X chromosomes are larger and hold more genes, while Y chromosomes are smaller and critical for male sex determination.

  • Discuss the societal impact of changing sex ratios. Variations in sex ratios can affect marriage patterns, economic stability, and gender-based violence.

  • Differentiate Y linkage from X linkage. Y-linked traits exclusively affect males and are passed from father to son, while X-linked traits can express variably in males and females.

  • Compare X-linked recessive and dominant inheritance. Recessive traits require two copies in females for expression, while dominant traits can be expressed with one copy.

  • Analyze inheritance patterns unique to one sex. Traits like hemophilia and certain forms of color blindness predominantly affect males due to their hemizygous status for X-linked genes.

  • Define sex-influenced traits. Traits that are more likely to be expressed in one sex due to hormonal differences, exemplified by conditions like pattern baldness.

Learning Outcomes 2

  • Explain necessity and process of X inactivation. This process equalizes gene expression across sexes by silencing one of the two X chromosomes in females.

  • Discuss epigenetic implications of X inactivation. X inactivation can lead to variations in phenotype expression due to differential gene silencing, influencing health outcomes.

  • Impact of X inactivation on phenotypes in female mammals. Variable expression can lead to mosaic patterns where different cells express different alleles, notably in conditions like calico cats.

  • Understand chemical mechanisms of X inactivation. The XIST gene plays a central role in initiating the inactivation process, leading to the formation of a Barr body.

  • Analyze effects of oocyte and sperm formation timetables on parent-of-origin effects. Differences in timing may impact genetic disorders' manifestation and inheritance patterns, notably in conditions where paternal imprinting effects are observed.

Biological Determinants of Sex

Sex Determination at Conception

  • Established by chromosome composition, where the combination of XX results in a female and XY results in a male.

  • Hormones and genetics shape sexual identity and feelings of gender. The intrauterine environment and external stimuli also play critical roles in defining gender identity.

Sexual Development

Prenatal Development

  • By week five: Development of gonads and reproductive ducts; androgens commence male differentiation while the absence leads to female development.

  • SRY gene: Critical in initiating male development, with its activation leading to testicular formation; conversely, the Wnt4 gene is essential for proper ovarian development.

  • Mutation Implications: Mutations in Wnt4 can cause primary amenorrhea and encourage the development of male characteristics in genetically female individuals, leading to hormone imbalances.

Sex Chromosomes

Overview

  • Males: XY (heterogametic).

  • Females: XX (homogametic).

Genomic Differences

  • X chromosome: Over 1500 genes involved in critical biological processes.

  • Y chromosome: Holds around 231 genes, primarily involved in male sex determination, and lacks the ability for full crossover with X during meiosis, contributing to its evolutionary degeneration.

Anatomy of Y Chromosome

  • Comprised of a short arm and a long arm with regions that are pseudoautosomal (5%), allowing for some gene exchange with X during meiosis, which is crucial for male fertility processes.

SRY Gene Function

  • Encodes a transcription factor pivotal for male sexual development, influencing the pathway to male differentiation via androgen production and the ensuing physiological changes.

Disorders of Sex Development (DSD)

Definitions

  • Hermaphroditism: Characterized by the presence of both male and female reproductive structures, posing challenges for gender classification.

  • Intersex: Refers to a spectrum of conditions where individuals may have ambiguous or inconsistent sexual anatomy.

  • Pseudohermaphroditism: Indicates the presence of one sex's structures while developing characteristics of the opposite sex.

Specific Conditions

  • Androgen Insensitivity Syndrome (AIS): A genetic mutation that inhibits the formation of functional androgen receptors, leading to a female phenotype despite a male XY genotype.

  • 5-alpha Reductase Deficiency: Disruption of the conversion of testosterone to dihydrotestosterone (DHT), resulting in underdeveloped male anatomy at birth but potential development during adolescence.

  • Congenital Adrenal Hyperplasia: A group of genetic disorders affecting adrenal hormone production, resulting in an overproduction of androgens and ambiguous genitalia in genetically female subjects.

Same-Sex Attraction

Overview

  • Demonstrates consistent patterns across cultures and species, suggesting a biological basis intertwined with social and environmental influences.

Genetic Factors

  • Studies indicate that identical twins exhibit a higher concordance for same-sex attraction, illustrating potential genetic underpinnings alongside environmental factors.

Components of Sexual Identity

  1. Chromosomal Sex: Determined at fertilization (XY for male, XX for female).

  2. Gonadal Sex: Development of testes or ovaries occurs about 6 weeks after fertilization.

  3. Phenotypic Sex: Development of reproductive structures occurs around 8 weeks after fertilization and continues to influence characteristics throughout puberty.

  4. Gender Identity: The internal understanding of oneself as male, female, or nonbinary can develop from a very young age based on a mix of biological and social factors.

  5. Sexual Orientation: Refers to attraction toward the same or opposite sex, often established during childhood or adolescence.

Sex Ratios

  • Defines the proportion of males to females in populations, which can fluctuate due to socio-economic factors, healthcare advancements, and cultural practices.

  • Ratios analyzed at different life stages: primary (at conception), secondary (at birth), and tertiary (maturity). Notably influenced by societal norms and policies.

X and Y-Linked Traits

  • Y-Linked Traits: Rare characteristics transmitted only from male to male, often linked to fertility and virility aspects.

  • X-Linked Traits: Demonstrate different expression patterns based on sex, with males being hemizygous for X-linked genes, resulting in more pronounced phenotypes compared to females who must inherit two defective alleles for expression.

Genetic Inheritance and Disorders

X-Linked Inheritance Comparison

  • X-Linked Recessive Traits: Typically affect males more severely due to their single X chromosome; females can be carriers.

  • X-Linked Dominant Traits: Females can be affected and transmit the condition to offspring, while males generally express these traits more severely.

Trait Examples

  • X-Linked Recessive: Disorders like hemophilia, colorblindness, and ichthyosis.

  • X-Linked Dominant: Conditions such as Rett syndrome and Incontinentia pigmenti raise issues for affected females compared to males.

Solving Genetic Problems

  • Identify inheritance pattern through pedigrees and family history.

  • Construct a pedigree to visualize genealogical relationships and inheritance.

  • List genotypes and phenotypes associated with traits.

  • Assign parental genotypes/phenotypes to predict offspring traits.

  • Utilize Punnett squares for gamete calculations and expected ratios for offspring generations.

Sex-Limited and Sex-Influenced Traits

  • Sex-Limited Traits: These traits manifest in one sex only; examples include mammalian milk production and male secondary sexual characteristics like beard growth.

  • Sex-Influenced Traits: Dominant in one sex but recessive in another, influenced by hormonal variations; pattern baldness exemplifies such traits.

X Inactivation

Overview

  • A critical process that balances gene expression between sexes, especially in mammals, resulting in the silencing of one X chromosome in females, ensuring equal expression of X-linked genes.

Phenotypic Effects of X Inactivation

  • Detectable through the presence of a Barr body in female cells; affects expression levels in heterozygous females, leading to variation in phenotypes.

  • Plays a significant role in human genetic disorders and response to treatment as it can impact the expression of X-linked mutations.

Parent-of-Origin Effects

  • Traits inherited from either maternal or paternal lineage can exhibit differential expression, highlighting the role of genomic imprinting in determining phenotype phenotype.

Genomic Imprinting

  • Mechanisms: Methylation processes can enhance or silence gene expression based on parental origin, resulting in variances, such as central precocious puberty influenced by the paternal gene.

Importance in Disease

  • Over 150 genes are biologically imprinted, implicating them in various health outcomes; they can be associated with disorders like Prader-Willi and Angelman syndromes, with potential links to conditions like diabetes and schizophrenia.

Development Timeline Differences in Sperm and Oocyte Formation

  • Distinct variations in the development timelines for sperm and oocyte can contribute to manifestations of certain genetic conditions, illustrating how paternal factors might affect onset age and severity of disorders like Huntington disease.