W8 Flashcards
what is meant by heterogametic and homogametic?
heterogametic refers to two different sex chromosomes on an organism whereas homogametic refers to the same type sex chromosome on an organism
in humans, the male is heterogametic and the female is homogametic. however, in budgerigars, homogametic refers to males and is denoted with ZZ and females are heterogametic and are denoted with ZW
x linked traits:
- in heterogametic organisms, transmission is between father and daughter. however in homogametic organisms, it is. between mother and son
what is the function of the XIST locus
- a gene locus in the x inactivation centre (XIC) on the X chromosome which is only transcribed from the inactivated X
- expression of the XIST on active X is suppressed
- RNAi (a large ncRNA) produced from XIST which coats the X and modifies the chromatin
- modification: methylation had histone deacetylation attract chromosomal proteins that form heterochromatin, thus inactivating the chromosome - CIS acting- affects the chromosome from which it is produced.
how is expression of XIST on active x suppressed?
- in mice: it is believed to be by a transcript from the TSIX which us the XIST gene in reverse. TSIX is only expressed from the ACTIVE x
- it is said to cis acting because it only affects the X from which expressed - not all genes on the X are inactivated
- genes on the inactivated X in the pseudoautosomal region such as MIC2, XG and SHOX are not transcribed
- LINKS to Turner syndrome
what is a Barr body
- inactive X= Barr body= heterochromatin
- dark staining on the body on the perimeter of the interphase nuclei
- not seen in males because only have one X
what is x inactivation and what are its consequences in terms of a syndrome
XIST (x inactivation specific transcript) is a gene locus in the x inactivation centre on the x chromosome which is only transcribed from the inactivated x
- expression of exist on the active X is suppressed
- genes in the pseudoautosomal region are not activated, such as MIC2, SHOX AND XG
this could explain some of the symptoms in turners syndrome
- two x’s are active until about 500 cell stage- two active Xs at this stage may be important to normal female development
- not all genes are inactivated so there are loci on the X with balletic expression- these could be important to normal female development
- SHOX gene is essential for the development of the skeleton. it plays a particularly important role in the growth and maturation of bones in the arms and legs
what is a sex influenced trait
trait occurs in both sexes, however, is more frequent in one gender
- this is an example of how gene expression is influenced or limited by the sex of the organism and is often an autosomal loci
eg. pattern baldness
this follows the 5 alpha reductase biochemical pathway
- testosterone becomes dihydrotestosterone through use of the enzyme 5 alpha reductase
- however die to androgen insensitivity, DHT cannot bind to go on and produce external male genitalia, penis and scrotum.
- instead, DHT binds to many receptors including those in cells of the scalp. this leads to over expression, thus, hair falls out
ANOTHER EXAMPLE IS PCOD- polycystic ovary disease
what is a sex limited trait
this is another example of gene expression being limited by the sex of the organisms- often autosomal loci
- the phenotype is limited to one sex bit can be p[assed to offspring by either sex
- zero penetrance in one gender
eg. precocious puberty in males, cock feathering in fowl, androgen insensitivity, 5 alpha reductase deficiency
precocious puberty in males
- autosomal dominant mutation
- in males
HAS A CHARACTERISITC PEDIGREE: where sex limited in in males and autosomal recessive. see notes for this image. think about it. heterozygote parents produce autosomal recessive male in large amounts
what is atypical lyonisation
- occurs in x inactivation of heterozygous females
- for x linked recessive traits
- also called manifesting heterozygote
- we typically expect 50/50 of the two alleles
- however, atypical lyonisation is when this does not occur
- this could be in HMA where there could be more than 50% of XH off leading to signs of HMA
- this could also be in Duchenne muscular dystrophy
- for a female to be affected by a X linked recessive trait, her father has to be affected. due to the nature of dmd, THIS is not possible
what is independent assortment
- outcomes from parental gametes segregation are to occur in equal chance (due to large number of gametes)
- the number of different gametes follows 2^n. where n is the number of different PAIRS of chromosomes
what is gene interaction
when two or more genes influence the same genotype- two genes can interact to determine one trait
ie. two loci
eg. coat colour has a dense/dilute locus that controls the distribution of coloured pigment granules called a modifier gene
the biochemical pathway for these traits are different non interacting genes. ie. there is one big pathway rather than seperate pathways
SPECIAL CASE: epistasis
what is epistasis
a genotype at one gene locus masks the phenotypic expression of the alley of another gene locus
- can have dominant or recessive epistasis; in recessive, the homozygous recessive genotype masks the other genotype. dominant epistasis: the presence of a single allele masks the effect at the second locus ie. A-;–
- two related biochemical pathways
list ways to differentiate human blood groups
ABO
Rh
MN
what are the three genetic consequences of meiosis
- segregation of alleles
- independent assortment of chromosomes
- crossing over and recombination of alleles
what is the dihybrid ratio and what genotypes lead to it
occurs when there are 2 genes that are both heterozygote. these mate to produce the 9:3:3:1 ratio.
- furthermore, these alleles must be completely dominant
what is the definition of pure breeding
if the so called ‘pure breeding’ individual self fertilises, all offspring would be the same genotype as parents
what is the antigen antibody reaction
almost any macromolecule if foreign to the body can induce an immune response
- any molecule that induces an immune response is called an antigen
- antibodies=immunoglobulins
