Genetics 3: Sex chromosomes Flashcards
How was sex linked traits shown on Drosophila-
The offspring of a cross between two flies heterozygous for Red eyes (D) x clear eye (r). Produced offspring in expected ratio of 3: red 1: clear, however all of the clear are males.
Explain the inheritance pattern in drosophila for sex linked traits
The male that were heterozygous and being crossed with a heterozygous female to produce this inheritance pattern did not have the gene for this trait on their Y chromosome, therefore they only have the dominant version in their X chromosome. Therefore any gametes where male donates the X, will have dominant phenotype. Therefore a fully recessive type would be one where the males Y chromosome has been donated and therefore be male
What is sex linked traits
Traits that are coded for by genes found on the sex chromosome.
Compare and contrast Heterogametic sex vs Homogametic sex
Heterogametic sex: the sex that produces two kinds of gametes and determines the sex of the offspring (males) whereas Homogametic sex: the sex that produces one kind of gamete. (females)
Compare the phenotypes of different sex chromosome combinations of humans and drosophila
With Humans, any genotype that has a Y will be a male. However with drosophila, it is the ratio of Xs to the autosomes that determine the sex.
Describe the main genetic features of the sex chromosome that make them similar and different
Both chromosomes 1 pseudoautosomal region at the top and bottom of chromsome which allow them to pair at meiosis
However the Y chromosome has a SRY just under the pseudoatosomal region. which isn’t on the X chromosome.
How is the SRY region important for sex determination in humans
SRY is a transcriptional factors produces a protein that binds to DNA and activates the genes leading to development of testes
Can SRY region be on the X and why
Yes, because of crossing over of the close pseudoautosomal region
How can individual with XY genome be phenotypically female (apart from gonads)
AIS: mutations in androgen receptor gene result in partial (go back to males in puberty) or complete inability of cells to respond to androgens
Explain why dosage compensation and X chromosome inactivation can lead to mosaic individuals.
Individuals with more than 1 X chromosome will inactivate the others, leading to only one active, however this is random in all cells throughout the body producing a potentially ‘mosaic individual as the alleles on the X chromsomes could be different.
What is an inactivated chromosome and what is it called and when is it present
Inactivated chromosome is condensed into a densely staining object in nuclei called Barr Body.
Found stable in mitosis but the X is reactivated in cells that give rise to ova.
How are organelle (mitochondria) genes inherited
Mitochondria/ (Chloroplasts) have small circular DNA. Inherited through maternal line as Ovum’s organelles are preserved and only sperm nucleus enters. Therefore not mendelian inheritance
What disorders are associated with mitochondrial DNA
Most disorders affect nervous and muscle system because they are affected by low energy stores. Eg. Myopathy causes muscle deterioration
What are two situations where meiotic non-disjunction happens and what n of chromosomes does resulting 4 daughter cells have
It can happen in Meiosis 1 where replicated chromosomes line up to metaphase plate.
- In non-disjunction, both of the homologous pairs go to one pole instead of both so two resulting gametes have the 2 chromatids in it. As these each split into 2 more, 2= n-1, 2 = n+1
Other situation is where in Meiosis 2: instead of sister chromatids (2-> 4) segregating, both of the sister chromatids go into 1 of the 4 resulting, so 1 has n+ 1, 1 has n-1 and 2 have n
Compare Trisomic, Monosomic and Nullisomic
Trisomic – 1 extra chromosome (3 copies).
Monosomic – 1 less chromosome (1 copy).
Nullisomic – lack both chromosomes (0 copies).
Compare Aneuploidy and Polyploidy
Aneuploidy – abnormal number of certain chromosomes.
whereas
Polyploidy – more than two complete chromosome sets e.g. triploid 3n and tetraploid 4n
What is Turners syndrome, karyotype + phenotype
XO : Phenotypic but
sterile females as sex organs don’t mature. They can get estrogen to get 2ndary sex characteristics
What is Klinefelters syndrome, karyotype + phenotype:
XXY: They have males sex organs, sterile and have abnormally small testes. The extra X is inactivated however some breast enlargement and other female characteristics.
What is trisomy X karyotype + phenotype: vs trisomy 21
XXX: May be taller, but mostly usual female physical characteristics (nondisjunction of sex chromosome)
Trisomy 21: Downs Syndrome (nondisjunction of somatosome)
Why does Aneuploidy happen and compare the difference between autosome disjunction and sex chromosome disjunction
Non-disjunction of chromosomes during Meiosis leads to imbalances in the number of chromosomes present in the resulting gamete.
In sex chromosomes this can be less worse than autosomes because Y carries very few genes and because extra copies of X become inactivated Barr Bodies.
Compare the alterations to chromosome structure : Deletion, Inversion and Duplication with what it is and severity
- Deletion: removes chromosomal segment.
Effect: depends on how early, how much and how many genes affected - Inversion: Reverses a segment in a chromosome by making two breaks and then turning it back to front.
Effect: Doesn’t have major phenotypic effects, but does have consequences in making gametes. - Duplication: repeats a segment of DNA.
Effect: Happens frequently. Many of our genes are duplicated.
What is Translocation: Reciprocal vs non reciprocal and what is a disease related to it
Translocation: Moving a segment of one chromosome to another non homologous one.
Reciprocal is an exchange of fragments. Whereas Nonreciprocal is where one receives but one doesn’t.
Disease: CML, reciprocal translocation from C22 to C9
What genes on the X chromosome cause Barr Bodies
XIST gene in the XIC region of X chromosome to be inactivated is expressed to produce multiple RNA transcripts that bind to that chromosome and effectively cover it up, thereby generating a Barr Body
What is the evidence for existence of 4 pairs of chromosomes in Drosophila
Drosophila genes cluster into 4 distinct groups of linked genes
