CH.06 The Genetics of Sex and Sex-Linked Inheritance

Vocabulary practice flashcards covering wild-type sex determination, chromosomal mechanisms, sex-linked inheritance patterns, X-inactivation, and genomic imprinting.

Chromosomal Sex

The first level of sex determination established at fertilization based on sex chromosome composition, either XX (homogametic) or XY (heterogametic).

Gonadal Sex

The second stage of sex determination involving the development of internal reproductive organs (testes or ovaries) from indifferent gonads.

Phenotypic Sex

The physical appearance and external sexual characteristics of an individual that develop during embryonic development under hormonal control.

Homogametic Sex

The sex possessing two of the same sex chromosomes, which in humans corresponds to females ($XX$).

Heterogametic Sex

The sex possessing two different sex chromosomes, which in humans corresponds to males ($XY$).

SRY Gene

A Y-linked gene encoding a transcription factor that signals the indifferent gonad to develop into testes by stimulating transcription factor Sx9.

Wnt4

A gene essential for female development and maturation; mutation causes an $XX$ female to have high male sex hormones and fail to develop a vaginal canal or uterus.

Müllerian Ducts

Embryonic duct structures that progress to form ovaries and female reproductive structures in the absence of male-directing signals.

Wolffian Ducts

Embryonic duct structures that progress to develop into testes and male reproductive structures.

Dihydroxytestosterone (DHT)

A hormone converted from testosterone that is required for the formation of male external genitalia.

Androgen Insensitivity Syndrome (AIS)

A condition in $XY$ individuals caused by a mutation in the X-linked androgen receptor gene, resulting in cells failing to respond to testosterone and developing as phenotypic females despite the presence of MIH.

Pseudohermaphroditism

A condition characterized by the presence of both male and female structures at different stages of an individual's life.

5-alpha Reductase Deficiency

An autosomal recessive condition preventing the conversion of testosterone into DHT, causing $XY$ individuals to have female-appearing external genitalia at birth but develop functional male organs at puberty.

Congenital Adrenal Hyperplasia (CAH)

An inherited condition caused by 21-hydroxylase deficiency, resulting in an enzyme block that causes testosterone and DHT to accumulate, affecting both males and females.

Intersex

A term used outside the medical community for individuals whose internal sexual structures are inconsistent with their external ones or are ambiguous.

Transgender

An individual whose gender identity does not match the sex assigned to them at birth.

Pseudo-autosomal Regions (PAR1 and PAR2)

Regions at both tips of the Y chromosome comprising 5% of its sequence that contain 63 genes capable of crossing over with counterparts on the X chromosome.

Male-Specific Region of the Y (MSY)

The central 95% of the Y chromosome lying between PAR1 and PAR2 that contains 22 male-specific genes and numerous palindromic sequences essential for fertility.

Sex Ratio

The proportion of males to females in a human population, calculated as \frac{\text{# of males}}{\text{# of females}} \times 1000 for a particular age group.

Hemizygous

The genetic condition of possessing only one copy of a gene or chromosome, such as human males having only one copy of X-linked genes.

Hypophosphatemia

An X-linked dominant disorder characterized by low blood phosphate levels, bowleggedness, bone disease, and spine degradation.

Rett Syndrome

A severe X-linked dominant condition primarily affecting girls, causing severe disability that typically prevents affected females from having children; most cases arise from new mutations.

Incontinentia Pigmenti

An X-linked dominant condition that causes a 25% miscarriage rate in pregnant affected women due to lethality in hemizygous males.

Color Blindness

A condition affecting cone cell proteins in the retina; red-green color blindness is an X-linked recessive trait affecting ~8% of U.S. males, while blue color blindness is autosomal dominant.

Hemophilia B

An X-linked recessive blood-clotting disorder characterized by transmission from heterozygous mothers to hemizygous sons, historically significant among European royalty.

Duchenne Muscular Dystrophy

An X-linked recessive genetic disorder that affects approximately 1 in 3500 males and 1 in 50 million females, often leading to death before reproductive age.

Kallmann Syndrome

An X-linked recessive condition characterized by a poor or absent sense of smell and small testes or ovaries.

Sex-Influenced Trait

A trait expressed in both sexes but differing in phenotypic expression or dominance due to hormonal differences (e.g., pattern baldness being dominant in males and recessive in females).

Sex-Limited Trait

A trait inherited by both sexes but normally expressed in only one sex because it affects a structure or function exclusive to males or females (e.g., pregnancy-related symptoms).

Dosage Compensation

A regulatory mechanism that equalizes the expression of X-linked gene products between females ($XX$) and males ($XY$).

Lyon Hypothesis

The proposal by Mary Lyon that dosage compensation occurs via the random, permanent inactivation of one X chromosome in female somatic cells early in embryonic development.

Barr Body

A darkly staining, condensed, inactivated X chromosome visible in the nucleus of a female cell during interphase.

XIST Gene

A gene located within the X inactivation center that encodes an RNA molecule that coats and silences the inactive X chromosome.

Manifesting Heterozygote

A female carrier of an X-linked recessive trait who expresses the associated clinical phenotype because the normal allele is inactivated in affected tissues.

Hunter Syndrome

An X-linked lysosomal storage deficiency where carrier females remain asymptomatic because enzyme-producing cells share the functional enzyme with deficient neighboring cells.

Fabry Disease

An X-linked deficiency of alpha-galactosidase A where cells do not release the enzyme, leading heterozygous females to exhibit mild symptoms such as abdominal pain, kidney impairment, or skin lesions.

Lesch-Nyhan Syndrome

An X-linked recessive disorder caused by defective or absent HGPRT, resulting in cerebral palsy, intellectual disability, painful urinary stones, and compulsive self-mutilation.

Mosaicism

The presence of two or more genetically distinct cell populations in an individual, illustrated by coat color patches in tortoiseshell cats and Blaschko's lines in humans.

Genomic Imprinting

An epigenetic phenomenon where methyl groups silence specific genes or linked clusters based on whether they were inherited from the mother or the father.

Prader-Willi Syndrome

An imprinting disorder characterized by infancy weight struggle followed by obsessive eating and slow metabolism, occurring when several paternal genes on chromosome 15 are deleted while maternal genes are imprinted.

Angelman Syndrome

An imprinting disorder caused by deletion of the maternal UBE3A gene on chromosome 15 while the paternal copy is silenced in brain neurons, causing autism, intellectual disability, extended tongue, and arm-flapping convulsions.

Beckwith-Wiedemann Syndrome

An imprinting disorder with higher prevalence in children conceived using assisted reproductive technologies such as IVF and intracytoplasmic sperm injection.

Paternal Age Effect

An increased risk of specific disorders (such as Noonan syndrome) in the offspring of older fathers, caused by higher replication rates and mutation accumulation in sperm stem cells.