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.
Why are males more likely to get X linked traits?
They are homozygous for X linked traits and thus will always the display the phenotype for a recessive allele if they have it.
In females half of the cells will have the “defective gene” and half won’t. However generally the half dosage of the normal protein is enough to make the person asymptomatic.
What does ‘linked genes’ refer to?
Genes carried in the same chromosome and thus are inherited together unless separated.
What are the chromatids which have undergone crossing over called?
Recombinants
What does ‘recombinant chromatids’ refer to?
Chromatids which have undergone crossing over and thus have a new combination of genetic material.
In terms of linked genes, how can the offspring be grouped?
Into ‘parental types’ and ‘recombinant types’
What are ‘parental types’?
Offspring which have the same combination of linked genes as one of their parents.
For example if the parents were YyRr and yyrr the ‘parental type offspring’ would be those with the genotype YyRy or yrrr
What are ‘recombinant types’?
Offspring which have different combinations of linked genes than their parents.
For example if the parents were YyRr and yyrr, the ‘recombinant types’ would be Yyrr or yyRr.
What does ’n+’ refer to (n is arbitrary)?
The wild-type allele of the n gene
How are wild-type alleles denoted?
n+ (superscript +) in which n is the gene i.e n for nose shape
What does ‘recombinant’ refer to?
A ‘recombinant type’
What does ‘recombination’ refer to?
‘Crossing over’
What does ’recombination frequency’ refer to?
To the ratio of recombinants to parental type offspring
What is the ratio of recombinants to parental type offspring called?
‘Recombination frequency’
What does recombination frequency indicate?
If recombination has occurred a chiasma must have formed between the two genes.
Since the location of chiasmata is random a greater recombination frequency indicates a greater distance between the loci
What is the maximum recombination frequency and what does this indicate?
50%. This suggests that either the genes are not linked and thus on different chromosomes or that the loci are so far apart that crossing over is virtually guaranteed.
How are loci that are so far apart that they re virtually unlikes described?
They are ‘physically connected but genetically unlinked’
What is a map of all the loci of genes called?
A ‘genetic map’
What does ’genetic map’ refer to?
A map of all the loci of genes
How can a genetic map be constructed?
By analysing the recombination frequency and thus deducing how far the loci are apart
What is a genetic map constructed based on recombination frequencies called?
A ‘linkage map’
What does ’linkage map’ refer to?
A genetic map constructed based on recombination frequencies.
How is a ‘linkage map’ created?
Take the recombination frequency of two neighbouring loci by finding the number of recombinants divided by the total.
For each percent of recombination frequency the loci are 1 ‘map unit’ apart.
For example a recombination frequency of 18.7% would indicate the loci are 1 ‘map unit’ apart
In what units are linkage maps given?
‘Map units’
What does ’map unit’ refer to?
The unit used in ‘linkage maps’ to indicate a 1% difference in recombination frequency.
Besides a linkage map, what is a special form of gene map?
A ‘cytogenetic map’
What does ’cytogenetic map’ refer to?
A gene map which relates loci to specific structures of the chromosome that can be observed.
For example “locus 23 is 0.3 map units from the second striated band seen during staining” (fiction)
What are the events which occur during cell division which can lead to genetic disorders?
’Nondisjunction’ and alteration of chromosome structure
What does ’non-disjunction’ refer to?
When the homologous chromosomes/sister chromatids fail to separate during meiosis or mitosis.
This leads to one daughter cell with an extra chromatid/chromosome and another one which is missing one
An organism has 3 genes, A B and C in order , on one chromosome. A and B are 10 map units apart and B and C are 7 map units apart.
How far apart are A and C?
Slightly LESS than 17 units. This is because of the possibility of two chiasmata forming between A and C and thus reversing the effect
What are the basic ways chromosome structure can be altered during meiosis/mitosis?
Deletion, duplication, inversion and translocation
What does ’deletion’ refer to?
A section (one gene or more) of a chromosome is deleted i.e. ABCD → ABD
What does ‘duplication’ refer to?
A section of a chromosome is repeated once or more. i.e. ABCD → ABCBCD
What does ’inversion’ refer to?
A section of a chromosome is removed and returned backwards.i.e. ABCD → ACBD
What does ’translocation’ refer to?
A translocation moves a segment from one chromosome to a NONHOMOLOGOUS
chromosome. In a reciprocal translocation, the most common type, nonhomologous chromosomes exchange fragments.
Less often, a nonreciprocal translocation occurs: A chromosome transfers a fragment but receives none in return
What is it called when a section of a chromosome is duplicated and inverted?
Just ‘duplication’
