Sex Determination & Sex Chromosomes Flashcards
What are the two types of reproduction modes found in nature?
Asexual and Sexual
What are the pros and cons of asexual reproduction?
pros: rapid increase in population size
cons: no variation
What are the pros in sexual reproduction?
creates variation, increases resistance to parasites, removes deleterious traits, increase fitness, promotes evolution
What are the cons of sexual reproduction?
slow increase in numbers, sexual differentiation, search for mates, sexual selection
What is isogamy?
similar gametes
What is anisogamy?
different gametes
What are the different types of sex determinators?
environmental sex determination and genetic sex determination
What are the different factor in genetic sex determination?
autosomal factors and sex chromosomes
What are isogametes?
morphologically identical but chemically different
Sex chromosomes are…
- characteristic to one sex or another
- heteromorphic
- different in gene content
- Do NOT recombine during meiosis in the heterogametic gender
What is heteromorphic?
(dissimilar) different size and shape- distinguishable from autosomes and from each other
What are the principle sex chromosome systems?
XY and ZW
Which gender is heterogametic in a XY system?
Males
Which gender is heterogametic in a ZW system?
Females
What type of sex chromosomes do plants have?
Majority have no sex chromosomes; there are a few exceptions
What type of sex chromosome system are roundworms?
XX/X system
Males: X
Hermaphrodite: XX
What type of sex chromosome system are insects?
- plant bugs/fruit flies: XX/XY
- Moths/butterflies: ZZ/ZW
- Bees/Wasps: Haploidy/diploidy: No sex chrom.
- Genomics imprinting: No sex chromosomes
- Environmental: depends on temp.
Bees and wasps have a haploid/diploid sex system. Which gender is haploid and which is diploid?
Males: haploid
Females: diploid
What type of sex system are reptiles?
- XY
- ZW
- Environmental
- combination of genotypic and environmental
What type of sex system are birds?
ZW system
males: ZZ
females: ZW
What type of sex system do mammals have?
XY
What are the characteristics of the conservative X in mammals?
ancestral gene content and order
cytogenetically similar between species
~5% of the genome
What are the characteristics of the inventive Y in mammals?
unique gene content
cytogenetically/genetically different between species
<2% of the genome
What type of sex system are mammals (eutherians)?
XX/XY
What type of sex system are mammals (marsupials)?
XX/XY
How are marsupial sex chromosomes different from eutherians?
Marsupials have XAR and XCR regions on their chromosomes
What is the XAR
X added region autosomal in marsupials
What is the XCR?
X conserved region shared between marsupials and eutherians
What type of sex system are monotremes?
X1-5X1-5/X1-5Y1-5
Are sex chromosomes paired in marsupials?
No
Are sex chromosomes paired in monotremes?
Males: No
Females: Yes
What is the driving force for the evolution of sex chromosomes?
suppressed recombination
What are the characteristics of human sex chromosomes?
pseudoautosomal region 1 (top)
pseudoautosomal region 2 (bottom)
X larger than Y
What is the PAR?
the Pseudoautosomal Region
Why is the PAR important?
XY pairing and segregation in male meiosis
What are the characteristics of the human X chromosome?
5% genome
>800 protein coding genes
~156 mega-base pairs
What are the purpose of the X chromosomes?
Sex and reproduction genes
brain and mental functioning genes
muscle genes
X inactivation
What is the dosage of X chromosomes between males and females?
females have 2x more than males
What is a Barr Body?
genetic mechanism to compensate X chromosome dosage between males and females
What are facultatively heterochromatic X chromosomes?
condensed, late replicating, histones are deacetylated, DNA is methylated
What is the rule of Barr bodies?
inactivation of all but one X chromosome
total number of X chromosomes= N
Number of Barr bodies= N-1
When does inactivation occur?
later in embryonic development, blastocyst stage after sex determination
What are XCI escape genes?
PARs and X-Y gametologs
XCI important facts
- XCI occurs during embryonic development
- XCI in female somatic cells is random
- XCI is clonal: all descendants of that cell have the same parental X inactivated
- All mammalian females are mosaics for maternal (Xm) and paternal (Xp) X-linked alleles
How is XCI different in somatic cell nuclear cloning?
XCI in clones is altered: X reactivation in cloned blastocysts is heterogeneous, X inactivation is later inconsistent across cells
What are the molecular features of XCI?
~6 genes, dominated by noncoding RNA genes
What are XIST?
long noncoding RNA which coats the X with active XIST gene
What is XIST expression?
triggers suppressive chromatin modifications
Is XCI reversible?
Yes, X is reactivated in oocytes before meiosis
What is the structure of the Y chromosome?
palindromic sequences
What the classes of Y chromosomes?
X-degenerate or X-Y ancestral genes
Ampliconic
What are X-degenerate/X-Y ancestral genes?
ancestral homologs between X and Y
usually single copy and broad expression/cellular function
What are ampliconic?
often novel and Y born
often species specific
testes limited expression
function in male fertility
What are the functions of the Y chromosome?
Male sex determination (SRY)
spermatogenesis, male fertility
Predicted genes for male characteristics
What are some specific features of the Y chromosome?
haploid
male specific
inherited exclusively through patrilinies
no pairing/recombination with a homolog
haplotype
What is a haplotype?
combination of alleles at adjacent locations on the chromosome that are transmitted together
