Ch.6 Flashcards
Chromosomal Sex
established at fertilization
gonadal sex
- development of ovaries or testes from undifferentiated gonads
- initiated by gene expression pathways to form either male or female gonads
Phenotypic sex
development of external reproductive structures
Why are the stages of sexual differentiation not always correlated?
genetic variations, hormonal imbalances, timing issues, environmental issues, gene mutations, intersex conditions
During the fifth week of prenatal development, all embryos develop 2 sets of
unspecialized gonads & reproductive ducts (Mullerian: female-specific & Wolffian: male-specific)
SRY gene
sex-determining region of the Y chromosome
The testis secretes which 2 hormones?
Testosterone & Anti-Mullerian Hormone (AMH)
Testosterone
- stimulates the Wolffian ducts to develop into the male duct system
- stimulates the development of external male structures
Anti-Mullerian Hormone (AMH)
stimulates regression of the Mullerian ducts
SrY gene on Y chromosome
codes for a protein that directs the development of male anatomical features
mutations can uncouple chromosomal sex from phenotypic sex
mutations can uncouple chromosomal sex from phenotypic sex
ex. gonadal intersexuality, complete androgen insensitivtiy
X Chromosome
present in females and males, larger of the two sex chromosomes
Anatomy of the Y Chromosome
Y Chromosome
The Y Chromosome has a very short arm and a long arm, present in males only, has few genes, contains the SRY gene
Pseudoautosomal Regions (PAR1 and PAR2)
- 5% of the chromosome
- contain 63 genes shared with X chromosome (control bone growth, cell division, in both sexes)
Male Specific Region (MSY)
- most of the chromosome
- three classes of DNA sequences (sequence almost identical to the X chromosome, somewhat identical to the X chromosome, palindromes)
- many of the genes are essential to fertility (including SRY)
Hermaphroditism
individual w/ both male and female sexual structures
Intersex
individuals whose internal structures are inconsistent with external structures, or whose genitalia are ambiguous
Pseudohermaphroditism
individuals whose external genitalia appear to be of one sex, but they have the internal reproductive organs of another
includes:
- androgen insensitivity syndrome
- 5-alpha reductase deficiency
- congenital adrenal hyperplasia (enzyme bock causes androgens to accumulate, cause precocious puberty in males or male secondary sex characteristics to develop in females)
Mutations that Affect Male Sexual Development
5- alpha Reductase deficiency
unable to convert testosterone to DHT, child has inside male anatomy but looks like girl on the exterior
Mutations that Affect Male Sexual Development
Androgen Insensitivity Syndrome
mutation on androgen receptor
Androgens
Include testosterone (T) and dihydrotestosterone (DHT)
Steroid Receptor Family
sex steroid receptors:
androgen receptor (AR), estrogen receptor (ER), progesterone receptor (PR)
glucocorticoid recpetor (GR) and mineralocorticoid receptor (MR)
Steroid hormone
steroid hormone - cytoplasm where steroid receptor connects w hormone (hormone-receptor complex) - nucleus where complex binds to sites on chromatin, activating mRNA transcription (nucleus response)
Ligand
a molecule that binds to another (usually larger) molecule
ex. estrogen, progesterone, testosterone, RU 486 (abortion med- blocking progesterone action)
Agonist
a substance that fully activates the receptor that it binds to
ex. estrogen, progesterone, testosterone
Antagonist
a substance that binds to a receptor but does not activate and can block the activity of other agonists
ex. RU 486
X Inactivation
balances inequality (females have 2 alleles for x chromosome genes but males only have 1) in expression of genes on the x chromosome
alters the phenotype and not the genotype
XIST gene
encodes an RNA binds to and inactivates the X chromosome
Epigenetic Change
an inherited change that DOES NOT alter the DNA base sequence
inactivated DNA is methylated
Manifesting heterozygote
a female that expresses the phenotype corresponding to an X-linked gene ALWAYS
Gene Imprinting
genes are expressed only from a single allele, not both. the other allele is silenced by DNA methylation
DNA methylation
gene silencing = no/low gene expression
the addition of methyl groups to cytosine bases in DNA
- turns transcription off (binding to non-histone protein forms silent chromatin)
- on CpG sites
CpG Island
promoter to the gene for gene expression
Imprinting
deletion on chromosome 15 reveals imprinting, can cause Prader-Willi syndrome if deletion comes from father or Angelman syndrome if it comes from the mother
Prader-Willi Syndrome (PWS)
different alleles, caused by no gene expression on PWS region
Angelman Syndrome (AS)
no gene expression on AS region; different alleles from father
X inactivation normally occurs only in males.
False; X inactivation is a process that occurs in females
X inactivation is a change in an individual’s genotype
False; One of the X chromosomes becomes inactivated, which affects the expression of genes on that chromosome. In this way, the individual’s phenotype, but not the genotype, is altered.
The inactivated X chromosome is called a Barr body.
True
The XIST gene encodes a protein that binds to the X chromosome.
False; The molecular basis of X inactivation involves the RNA produced from the XIST gene.This RNA does not encode a protein. Instead, the XIST RNA binds to the X chromosome and initiates the process of inactivation.
Females are mosaics for the expression of most genes on the X chromosome.
True
X inactivation is an example of an epigenetic change.
True
Genomic imprinting is permanent in the somatic cells of a given individual.
True; Genomic imprinting silences the contribution of the allele inherited from one parent. After an imprinting pattern is established in an individual, it is permanent in his or her somatic cells.
Genomic imprinting is permanent and affects future generations
False; If a gene is imprinted, the offspring can express either the maternal or the paternal allele, but not both.
Methylation is the chemical change underlying genomic imprinting.
True; Methylation of genes is the molecular mechanism underlying genomic imprinting.
If a gene is imprinted, the offspring can express both the maternal and paternal allele.
False
