Lecture 7: Sex Chromosomes and Sex Determination Flashcards
In animals, including humans, differentiation of sexes is evident via _____ ______
Phenotypic dimorphism
What type of chromosomes are dissimilar? Provide an example
- Heteromorphic chromosomes
- Sex chromosomes X and Y
What determines the sex? Is it the entire chromosome?
By specific genes on the X and Y, NOT the entire chromosomes
Define Homogametic sex
- Producing like chromosomes
- Zygotes with two X chromosomes
- Results in Female offspring
Define Heterogametic sex
- Producing unlike chromosomes
- Zygotes with one X and one Y chromosome
- Results in male offspring
Describe the Human Karyotype
- 22 pairs of autosomal chromosomes
- 2 sex chromosomes
- Reveals one pair of chromosomes differs in males and females
- Female: XX
- Male: XY
Characterize both Klinefelter and Turner syndrome
- Two human abnormalities
- Characterized by aberrant sexual
development - Both syndromes result from
nondisjunction - Failure of X chromosomes to
segregate during meiosis
Explain Klinefelter syndrome (47, XXY)
– Tall, long arms and legs
– Large hands and feet
– Internal ducts are male, rudimentary
testes fail to produce sperm
– Feminine development NOT suppressed
- Enlarged breasts common, rounded
hips
Explain Turner syndrome (45, X)
- Phenotypically female
- Female external genitalia and internal ducts
- Ovaries are rudimentary
- Underdeveloped breasts
- Short stature
- Cognitive Impairment
Explain 47, XXX syndrome: Triplo X
- Three X chromosomes
- Normal set of autosomes
- Results in female differentiation
- Sometimes women are perfectly
normal - Sometimes underdeveloped secondary sex characteristics occur causing Sterility and Mental retardation
Explain 47, XYY condition
- Only consistently shared characteristic—males are over 6 feet tall
- Subnormal intelligence
- Personality disorders
Describe Gonadal primordia
- The tissues that will form the gonad
- By the fifth week of gestation, a pair of gonadal (genital) ridges associated with each embryonic kidney
- Gonadal phenotype is sexually indifferent
- Primordial germ cells migrate to ridges
- Bipotential gonads: Gonadal ridges can form either ovaries or testes
Define Bipotential gonads
- Gonadal ridges can form either ovaries or testes
Describe the Y chromosome and male development
- Y chromosome has at least 50 genes
- Fewer genes than X chromosome (100 genes)
Define PARs: Pseudoautosomal regions
- Present on both ends of Y
chromosome - Share homology with regions on X
chromosome - Synapse and recombine with X during
meiosis - Pairing region critical to segregation of X and Y chromosomes during male gametogenesis
Define MSY: Male-specific region of the Y
- Nonrecombining region of Y
chromosome
Define SRY: Sex-determining region Y
- Located adjacent to PAR of the short arm of Y chromosome
- Controls male development
- Encodes protein: Testis-
determining factor (T D F)
Compare Euchromatin and Heterochromatin
- Think hetero contains multiple DNA material which is MORE compact and less accessible
- Think “Eu Go Ahead” in the sense of it is more relaxed and more free for expression. AKA a bit more accessible
Define Sex Ratio
Actual proportion of male to female offspring
Define Primary sex ratio
Reflects proportion of males to females conceived in population
Define Secondary sex ratio
Reflect proportion of each sex born
T/F: Male to Female Ratio is ALWAYS favoring the Male
TRUE
Define Dosage Compensation
- Genetic mechanism
- Balances dose of X chromosome gene expression in males and females
- Prevents excessive expression of X-linked genes in humans and other mammals
Define Barr bodies (sex chromatin bodies)
- Genetic mechanism compensates for X dosage disparities
- Inactive X chromosome, highly condensed
- Darkly stained body in interphase nerve cells observed: Barr bodies
- Random inactivation
- Occurs early in embryonic development
Why does X-inactivation NOT affect syndromes such as Turner or Klinefelter?
- Chromosome inactivation not in early stages of development for cells destined for gonadal tissue
- Not all X chromosomes forming Barr bodies are inactivated
- 15% escape inactivation
Define the Lyon Hypothesis
- Inactivation of X chromosome is random
- Occurs in somatic cells at early stage of embryonic
development - All descendant cells have same X-inactivation
- Example: Calico cats and fur color/patterns
