1.5. Linkage and Sex Determination Flashcards
• Linked Genes • Chromosome Mapping • Interference • Sex Determination
genes located on the same (linear) chromosome that tend to be inherited together and cannot undergo independent assortment are called ___.
linked genes
produces only parental gametes in equal proportions; rarely observed, only when (1) genes are very close together and the (2) no of progeny is relatively small
complete linkage
estimation of the distance between two genes or loci along the chromosome
chromosome mapping
what is a genetic map?
estimates the genetic distance between genes
what is a physical gene map?
specifies the physical location and distance between genes
what is a cytological map?
chromosomes are distinguished by banding patterns and differences in sizes
significance of chromosome maps
(1) differences between different species
(2) understanding of segregation and inheritance of genes
(3) insight to gene function
scientist(s) who came up with the idea of using crossing over as basis of distance between genes in their drosophila experiments wherein they concluded that variable amount of exchange occurs between any two gametes during agmete formation
thomas morgan and alfred sturtevant
exchange between non-sister chromatids in a single tetrad
single crossover
two or more exchanges between non-sister chromatids in a single tetrad
multiple crossover
results to double exchanges of genetic material; two separate and independent exchanges must occur simultaneously
double crossover
formula: chromosome mapping
[(SCO + DCO)/Total Frequency]100
when do mapping estimates become inaccurate?
when the distance between two genes increase
a phenomenon through which a crossover event in one region of the chromosome inhibits a second event in nearby regions
interference (I = 1 - C)
ratio of observed double crossovers to expected double crossovers (O/E)
coefficient of coincidence
type of interference: no DCO; I = 1.0
complete interference
type of interference: observed DCO < expected DCO; I = positive number
positive interference
type of interference: DCOobs > DCO expected; I = negative number
negative interference
a threshold for maleness is reached when the X:A ratio is 1:2 (1X:2A)
genic balance theory
how does sex determination through temperature work?
incubation temp. of eggs during a critical period of embryonic development
houses the genetic information necessary for maleness; the switch that triggers gonadal ridge development into testes or ovaries
y chromosome
in human genome project, how many genes have y chromosome? how about x chromosome?
50 genes: y
1000 genes: x
a region of the y chromosome that synapses and recombines with the x-chromosome during meiosis; shares homology with the x-chromosome
pseudoautosomal region (par)
a region of the y-chromosome that makes up 95% of its entirety; non-recombining region of the y chromosome; some portions share homology with genes on x-chromosomes
male-specific region of the y (MSY)
region of MSY that contains functional genes
euchromatic region
region of MSY that lacks genes
heterochromatic regions
region of MSY euchromatic region that makes up 15% of its entirety; originally derived from x-chromosomes; contains two genes with x chromosome homologs
x-transposed region
region of MSY euchromatic region that makes up 20% of MSY; contains dna sequences that are more distancly related to those on the x chromosome; share some homology with x chromosome; 27 single-copy genes and a number of pseudogenes
x-degenerative region
region in the y-chromosome that controls male sexual development and encodes the testis-determining factor (TDF) responsible for the formation of testes from undifferentiated gonadal tissue of the embryo
sex-determining region Y (SRY)
region of MSY euchromatic region that makes up 30% of its entirety; includes most of the genes closely associated with the development of testes; lack counterparts on the X chromosome; expression is limited to the testes
ampliconic region
genetic compensation that balances the dose of x-chromosome gene expression in females and males
dosage compensation
process of inactivation of x-chromosome
lyonization
inactivation of x chromosomes occurs randomly in somatic cells at a point early in embryonic development. once inactivation has occurred, all descendant cells have the same x chromosome inactivated as their initial progenitor cell. what is this idea called?
lyon hypothesis (mary lyon and liane russell)
at what stage do x-chromosomes get activated again?
during oogenesis
physical evidences of lyonization
(1) barr body
(2) mosaicism
an inactivated x chromosome; also known as sex chromatin body
barr body
describe ohno’s hypothesis (susumu ohno)
barr body arises from one of the two x-chromosomes, providing a possible mechanism for dosage compensation. if one of the two x chromosome is inactive in female cells, the dosage of genetic information that can be expressed in males and females will be equivalent
formula for calculating number of barr bodies
n - 1; wherein n is the number of x-chromosomes present
true or false
(1) barr body occurs in the very early stages of development
(2) not all genes on each x chromosome forming a barr body are inactivated
(1) false
(2) true
formation of mosaic patterns in heterozygous females
mosaicism
