1.5. Linkage and Sex Determination

Question 1

genes located on the same (linear) chromosome that tend to be inherited together and cannot undergo independent assortment are called ___.

Answer 1

linked genes

Question 2

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

Answer 2

complete linkage

Question 3

estimation of the distance between two genes or loci along the chromosome

Answer 3

chromosome mapping

Question 4

what is a genetic map?

Answer 4

estimates the genetic distance between genes

Question 5

what is a physical gene map?

Answer 5

specifies the physical location and distance between genes

Question 6

what is a cytological map?

Answer 6

chromosomes are distinguished by banding patterns and differences in sizes

Question 7

significance of chromosome maps

Answer 7

(1) differences between different species
(2) understanding of segregation and inheritance of genes
(3) insight to gene function

Question 8

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

Answer 8

thomas morgan and alfred sturtevant

Question 9

exchange between non-sister chromatids in a single tetrad

Answer 9

single crossover

Question 10

two or more exchanges between non-sister chromatids in a single tetrad

Answer 10

multiple crossover

Question 11

results to double exchanges of genetic material; two separate and independent exchanges must occur simultaneously

Answer 11

double crossover

Question 12

formula: chromosome mapping

Answer 12

[(SCO + DCO)/Total Frequency]100

Question 13

when do mapping estimates become inaccurate?

Answer 13

when the distance between two genes increase

Question 14

a phenomenon through which a crossover event in one region of the chromosome inhibits a second event in nearby regions

Answer 14

interference (I = 1 - C)

Question 15

ratio of observed double crossovers to expected double crossovers (O/E)

Answer 15

coefficient of coincidence

Question 16

type of interference: no DCO; I = 1.0

Answer 16

complete interference

Question 17

type of interference: observed DCO < expected DCO; I = positive number

Answer 17

positive interference

Question 18

type of interference: DCOobs > DCO expected; I = negative number

Answer 18

negative interference

Question 19

a threshold for maleness is reached when the X:A ratio is 1:2 (1X:2A)

Answer 19

genic balance theory

Question 20

how does sex determination through temperature work?

Answer 20

incubation temp. of eggs during a critical period of embryonic development

Question 21

houses the genetic information necessary for maleness; the switch that triggers gonadal ridge development into testes or ovaries

Answer 21

y chromosome

Question 22

in human genome project, how many genes have y chromosome? how about x chromosome?

Answer 22

50 genes: y
1000 genes: x

Question 23

a region of the y chromosome that synapses and recombines with the x-chromosome during meiosis; shares homology with the x-chromosome

Answer 23

pseudoautosomal region (par)

Question 24

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

Answer 24

male-specific region of the y (MSY)

Question 25

region of MSY that contains functional genes

Answer 25

euchromatic region

Question 26

region of MSY that lacks genes

Answer 26

heterochromatic regions

Question 27

region of MSY euchromatic region that makes up 15% of its entirety; originally derived from x-chromosomes; contains two genes with x chromosome homologs

Answer 27

x-transposed region

Question 28

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

Answer 28

x-degenerative region

Question 29

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

Answer 29

sex-determining region Y (SRY)

Question 30

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

Answer 30

ampliconic region

Question 31

genetic compensation that balances the dose of x-chromosome gene expression in females and males

Answer 31

dosage compensation

Question 32

process of inactivation of x-chromosome

Answer 32

lyonization

Question 33

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?

Answer 33

lyon hypothesis (mary lyon and liane russell)

Question 34

at what stage do x-chromosomes get activated again?

Answer 34

during oogenesis

Question 35

physical evidences of lyonization

Answer 35

(1) barr body
(2) mosaicism

Question 36

an inactivated x chromosome; also known as sex chromatin body

Answer 36

barr body

Question 37

describe ohno’s hypothesis (susumu ohno)

Answer 37

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

Question 38

formula for calculating number of barr bodies

Answer 38

n - 1; wherein n is the number of x-chromosomes present

Question 39

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

Answer 39

(1) false
(2) true

Question 40

formation of mosaic patterns in heterozygous females

Answer 40

mosaicism