Chromosomal Genetics Flashcards
What are the key principles of Chromosome Theory?
- Chromosomes are the fundamental carriers of genetic material
- They are linear structures with genes located at specific loci
What are the 3 key aspects of chromosomal behaviour?
- Are present in pairs in somatic cells
- Segregate during formation of gametes and rejoin during fertilisation
- Different chromosomes segregate independently
What was the main proof of Chromosomal Theory?
The reciprocal Drosophila crosses of Thomas Hunt Morgan
What characteristic did Morgan focus on?
Eye colour. Red is WT. White is mutant.
What was the ratio of F1 and F2 flies eye colour if the parentals were the following:
Red female X White male
F1 = all red F2 = all red females, 0.5 red male, 0.5 white male
What was the ratio of F1 and F2 flies eye colour if the parentals were the following:
White female X Red male
F1 = all red females, all white males. F2 = 0.5 red female, 0.5 white female, 0.5 red male, 0.5 white male
What was the pattern of inheritance for eye colour in Drosophila?
X-linked Dominant. The locus for eye colour only on X chromosome.
Define Sex-linked Inheritance.
The inheritance of a gene located on a sex chromosome.
Describe the pattern of inheritance of Sex-linked recessive alleles.
The allele lies on the X-chromosome so is transmitted in mainly through females but manifests mainly in males.
Give an example of Sex-linked recessive inheritance.
Colour blindness
Give an example of Sex-linked recessive inheritance.
Haemophilia.
Define Lyonization.
Also known as X-Chromosome Inactivation- early in development one of the Xs in a particular cell switches off.
What is a Barr Body?
The inactivated X chromosome that appears as a dark staining blob
Give 3 characteristics of Lyonization
- X-chromosome are inactivated at random
- In het females, different patches of tissue have one of the other alleles expressed
- Every patch is a clone descending from an early embryonic cell with one X-chromosome activated
Give an example of Lyonization in humans.
Females who are hets for colour blindness have patches of normal colour vision and colour blind cells in the retina.
Give an example of Lyonization in humans
Duchenne Muscular Dystrophy (DMD) is X-linked recessive. Het twins- one has the disease (normal X activated early in development) the other normal.
What is the purpose of Lyonization?
It equalizes the dose of X-linked alleles in males and females.
Give an example of Lyonization in mammals.
The tortoiseshell cat- has patches of black and orange. Half of cells have orange allele inactivated the other half black allele.
Define Cytogenetics
The study of chromosomes
What is a Centromere?
The section where the chromosome attaches to the spindle during cell division. The centromere separate the chromosome into a short P arm and a long Q arm.
Define the terms Metacentric, Acrocentric and Telocentric chromosomes.
Meta- centromere is at the centre of chromosome Aneu
Acro- centromere closer to one end
Telo- centromere near the very end
What are the 7 classes of Chromosomal Rearrangement?
Deletion,
Duplication, Inversion, Translocation, Aneuploidy, Nondisjunction Polyploidization
What is involved in a Deletion mutation
A deletion involves two breaks and a re-joining of DNA. Only the section of the chromosome containing the centromere survives.
Give an example of a deletion.
Cri du Chat Syndrome- a deletion in chromosome 5p.
How are Deletions used in mapping chromosomes?
They can be used to find out where the coded regions on a chromosome are. If deletion genotype produced wild phenotype then the deleted region is unlikely to be coding.
What are the important features of Duplication?
They have increased the size of the genome, are responsible for Gene Families.
Define the term Gene Family
Groups of adjacent genes with similar sequence and related function.
What is the difference between a Tandem Duplication and a Reverse Duplication?
A Tandem = ABCD –> ABCBCD
A Reverse = ABCD –> ABCCBD
How are Gene Families likely to have been formed?
By duplication of an original gene or short segment of chromosome a long time ago.
Give an example of a Gene Family.
Globin gene family. A range of globin types present at distinct times in development as embryos demand for O2 changes. Alpha globin cluster on C16 and Beta globin cluster on C11.
What is the cause of Tandem Duplication?
Unequal Crossing over during meiosis. Mispairing of regions causing uneven chiasma formation.
Give an example of Tandem Duplication.
Bar Eye in Drosophila- on X chromosome, it reduces the number of facets on eye.
Give an example of Tandem Duplication.
Huntington’s Disease- is due to an increase in the number of “CAG” repeats codes for glutamine.
What is meant by a chromosomal Inversion?
A section of chromosome has been rotated and reinserted. ABCD –> ACBD. Involves two simultaneous breaks in two positions - rare occurrence.
Why are Inversions known as Crossover Suppressors between inverted and non-inverted chromosomes?
- mechanical strain of making the inversion loop
- crossing over may result in one section having two centromeres and the other having none.
What is meant by a Translocation?
Where two non-homologous chromosomes exchange parts.
How can Translocations cause problems?
When a person with translocation mates with a normal there will be unbalanced translocation during crossing over (excess of one C and shortage of another)
Give two examples of Translocations
Chronic Myeloid Leukemia (C9-C22) and Burkitts Lymphoma (C8-C14)
Define Position Effect Variegation.
A gene on euchromatin (coding) shifted by translocation next to heterochromatin (non-coding) causing a change in expression of the gene (inactivation)
How did work on Drosophila describe PEV
Drosophila white eye gene usually recessive on X. Translocated to C4 adjacent to heterochromatin makes dominant red allele lose dominance. - red and white variegation in eyes.
