Sex Linkage And Chromosome Rearrangements Flashcards
(43 cards)
Which aneuploid sex chromosome combinations are viable in drosophila and humans?
Drosophila- XXY (female) XO (male)
Humans- XO XXX (female), XXY (male)
What are the long and short arms of a chromosome called?
Short P
Long Q
What are metacentric, acrocentric and telocentric chromosomes?
Metacentric- centromere at centre
Acrocentric- closer to one end
Telocentric- near the very end
Where does proof of the chromosome theory come from?
Thomas Hunt Morgan made reciprocal crosses in Drosophila
-> Gave different result depending on whether Male/female carried the mutant allele
(X^W is recessive for white eyes)
X^WY mutant Male
X^WX^W mutant female
-> Pattern of inheritance follows that of X chromosome (physical object)
What was definite proof of the chromosome theory?
Aneuploid cross
- > females that produce only X^WX^W eggs or O eggs
- > Cross with XY
Define sex linkage and give examples
Genes carried on X chromosome. When recessive:
- Transmitted mainly through females
- Manifests mainly in males
- Colour blindness
- Lesch-Nyhan syndrome- fatal error in purine metabolism
- Haemophilia/factor VII deficiency
Outline Lesch-Nyhan syndrome
- Mutation in HPRT gene on X chromosome- sex linked
- HGPRT deficiency causes Utica acid build up in body fluids
- Fatal error in purine metabolism
- Leads to self mutilation and death
- Affects 1/380,000 live births
Outline haemophilia A
- X-linked recessive
- Deficiency in factor VIII, causes increased bleeding as blood can’t clot properly
- Mild haemophilia A can occur in heterozygote females due to XCI
- 1/5000 males
Define lyonisation and give examples
X-chromosome inactivation. Females randomly inactivate one X in each cell early in development- Barr body in cell nucleus
- Tortoiseshell cat- patches of orange and black fur
- Colour blindness in human females- patches of normal and colour blind cells in retina
- Duchenne muscular dystrophy- heterozygous female twins, one with, one without.
What determines sex in humans, insects and birds and butterflies?
Insects- number of Xs
Humans- presence of Y
Birds and butterflies- males are homogametic WW and females are heterogametic ZW
What is cytogenetics?
The study of chromosomes
Why do we study whole chromosomes?
- Many human diseases are associated with whole chromosome changes
• Cancers often associated with chromosome arrangements
• Patau syndrome- Trisomy 13 - Evolutionary genetics
• Chromosome 2 in humans is result of fusion of two primate chromosomes
Define centromere
Section where the chromosome attaches to the spindle during cell division
Define telomere
Repeated sequence at the end of each chromosome.
Protect ends of chromosomes- prevent from binding to others.
Define deletion and give an example
Section of chromosome is removed (deletion loop is formed)
- Notch wing in Drosophila- dominant X chromosome mutation. Homozygotes lethal
- Cri-du-chat in humans- tip of chromosome 5 is lost- causes pinched facial features, mental retardation, cat-like cry
Define duplication and give examples
Common. A section of chromosome is doubled up. Important force in evolution.
Tandem ABCBCD, reverse ABCCBD
- Globin gene families- alpha and beta groups on different chromosomes
- Odorant gene family
What is replication slippage? Give an example
• tandem duplication
• chiasma forms
• results in further duplication
Important force in mutation
Drosophila bar eye:
• Results from tandem duplication
• Cross two bar eye flies together and can result in normal and double bar eyed flies
• Due to chiasma forming and crossing over
Huntington’s disease:
• increase in number of CAG repeats
Define inversion
Rare. Section of chromosome rotated and reinserted. Often viable as no change in amount of material.
Prevents crossing over of inverted chromosomes with unconverted chromosomes as a dicentric and acentric (centromeres) fragment are formed
Dicentric breaks
Acentric lost at cell division
In heterozygotes for the inversion, abnormal gametes will be formed. Excess of some genes, deficient of others. Small-> viable gametes, abnormal child, large-> lethal
Give an example of inversion
Some drosophila species have many overlapping inversions all over their chromosomes- all multiple inversion heterozygotes.
-> No recombination at all. One fixed set of genes
Define translocation and give an example
Two non-homologous chromosomes exchange parts
• Associated with cancers- chronic myeloid leukaemia- 9-22 translocation, Burkitt’s lymphoma- 8-14 translocation
Outline Burkitt’s lymphoma
- Disease of lymphatic system
- extended lymph glands in the neck
- Translocation from c8 to c14
- Translocation of oncogene next to an active immune system gene- position effect variegation
- Lymphoma gene is more active than it should be
What has translocation been used for?
- Releasing insects with translocations means it will become more and more common-> most of offspring will be heterozygous and will die
• Was used to wipe out tsetse flies in parts of Africa, responsible for transmitting sleeping sickness. - Used in mapping genes such as Testis-determining Factor (TDF/SDY). XX males have crucial part of Y translocated onto X so search narrowed down.
- Suggested position effect variegation
Define position effect variegation and give an example
A gene’s effect can be modified by it’s position on a chromosome
Red eye/white eye Drosophila
Translocation of recessive allele to somewhere else= mixture of red and white eye phenotype
Translocation of oncogenes to somewhere on the genome near a more active promoter- spreads faster
• Burkitt’s lymphoma
What is a Giemsa stain?
G-banding- staining of condensed chromosomes to produce a visible karyotype
