15 The Chromosomal Basis of Inheritence Flashcards
What is the typical phenotype found in the wild called?
‘Wild type’
What does ‘wild type’ refer to the?
The phenotype typically found in the wild i.e. not a mutant
What is ‘HD’?
The allele for Huntington’ disease.
What are the basic methods of sex determination?
The ‘X-Y system’, ‘X-O system’, ‘Z-W system’ and the ‘haplo-diploid system’
What is the ‘X-Y system’?
In mammals, the sex of an offspring depends on whether the sperm cell contains an X chromosome or a Y.
What is the ‘X-O system’?
In grasshoppers, cockroaches, and some other insects, there is only one type of sex chromosome, the X. Females are XX; males have only one sex chromosome (X0).
Sex of the offspring is determined by whether the sperm cell contains an X chromosome or no sex chromosome.
What is the ‘Z-W system’?
In birds, some fishes, and some insects, the sex chromosomes present in the egg (not the sperm) determine the sex of offspring.
The sex chromosomes are designated Z and W. Females are ZW and males are ZZ.
What is the ‘hallo-diploid system’?
There are no sex chromosomes in most species of bees and ants.
Females develop from fertilized eggs and are thus diploid. Males develop from unfertilized eggs and are haploid; they have no fathers.
What are characteristics that are encoded on the ‘X chromosome’ called?
‘X-linked genes’
What are ‘X-linked genes’?
Genes found on the X-chromosomes.
What is it called when an organism has only one allele for a gene?
‘Hemizygous’
What does ’hemizygous’ refer to?
When the organism has only one allele for a gene.
This is found in ‘X-linked’ genes in males as they have only one X chromosome.
What are some examples of ‘X-linked traits’?
‘Duchenne muscular dystrophy’, ‘colour blindness’ and haemophilia.
What is ‘Duchenne muscular dystrophy?’
An X-linked disorder which causes muscular weakness due to the absence of the muscle protein ‘dystrophin’
What is colour blindness?
An X-linked trait
What is ‘haemophilia’?
An X-linked recessive disorder in which the proteins needed in blood clotting are absent.
How could females having two X chromosomes be problematic?
Since they have twice as many X chromosomes as males each gene will be transcribed twice as often Thus the gene dosage would be abnormally high.
What is ‘gene dosage’?
The level of gene product i.e. protein produced
What is the level of gene product produced called?
‘Gene doasage’
How are the issues with ‘gene dosage’ prevented in females that are XX?
With ‘X Inactivation’ in which the one X chromosome is deactivated by a ‘Barr body’
What are ‘Barr bodies’?
The structures which inactivate X chromosomes
What determines which X chromosome is inactivated?
It is random and decided per cell. Therefore half of the cells will have one chromosome active and the other half with have the other one active.
Who do ‘Barr bodies’ inactivate X chromosomes?
Inactivation involves modifying the DNA and the histone proteins bound to it, including attachment of methyl groups (—CH3) to one of the nitroge- nous bases of DNA nucleotides.
A particular region of each X chromosome contains several genes involved in the inactivation process. The two regions, one on each X chromosome, associate briefly with each other in each cell at an early stage of embryonic development. Then one of the genes, called XIST (for X-inactive specific
ranscript) becomes active only on the chromosome that will become the Barr body.
Multiple copies of the RNA product of this gene apparently attach to the X chromosome on which they are made, eventually almost covering it. Interaction of this RNA with the chromosome initiates initiate X inactivation The RNA products of other on there other X chromosome help regulate the process.
