Genetics Flashcards
What are chromosomes?
The basic packages of DNA in the nucleus of a cell
What does DNA consist of?
Deoxyribose sugar
Phosphate backbone
Covalently bonded bases
How many chromosomes are in a human cell?
22 pairs of autosomal
2 sex chromosomes
What are the telomeres?
Regions at the end of each chromosome
What are DNA strands wound around to be packaged into chromosomes?
Histones
At which stage in the cell cycle does DNA replication occur?
S phase
What is mitosis?
A cell divides to produce two daughter cells
What are the stages of mitosis?
Interphase, prophase, metaphase, anaphase, telophase
When does meiosis occur?
Gamete formation
How many daughter cells are produced from one diploid parent cell in meiosis?
Four haploid daughter cells
What is mosaicism?
When there are two or more populations of cells with different genetic constitutions within the same individual
What is chromosomal aneuploidy?
The presence or absence of a whole extra chromosome
What is the most common aneuploidy?w
Trisomy 21 (Down’s syndrome)
What kind of aneuploidy is generally tolerated better by a fetus?
Sex chromosome aneuploidy e.g. Individuals with 47, XXX or 47, XYY show no obvious phenotypic features
What happens to the majority of conceptions with aneuploidy?
Miscarriage
What are chromosome translocation a?
Exchanges of chromosomal material between two or more chromosomes
What is a balanced chromosomal translocation?
A translocation in which there is no net loss or gain of chromosomal material
Which phenotypic effects can be expected in a child with an unbalanced chromosome translocation that is carried to term?
Renal, gastrointestinal and cardiac malformations
Developmental delay
What determines the likelihood of chromosomal imbalance leading to miscarriage?
The size of the imbalance
What is a Robertsonian translocation?
The long arms of two chromosomes become joined together, with the loss of the short arms
Why does loss of short arms of acrocentric chromosomes not result in clinical phenotype?
Short arms contain highly repetitive DNA sequences and ribosome RNA genes
What is reciprocal translocation?
Exchange of material between two, usually non-homologous chromosomes e.g. tip of chromosome 9 may translocate to chromosome 8
What may a patient with a balanced reciprocal translocation present with?
Infertility
Multiple miscarriages
A child with multiple malformations
At what size might deletion of a segment of a chromosome become visible microscopically?
If it is bigger than approx. 5 megabases
What can fluorescence in-situ hybridisation be used to detect?
Microdeletions
Subtelomere deletions
Gross alterations in chromosome number
What is the procedure in FISH?
A fluorescent dye is attached to a probe DNA that attaches to the chromosomal region of interest
What happens in FISH if the region under investigation is present?
The dye fluoresces and can be visualised microscopically
What is the drawback of FISH?
It can only be used when the chromosomal region for analysis is known
What does the use of microarray comparative genomic hybridization allow for?
Analysis of the entire genome for small loses or gains
What is Mendelian inheritance?
Single gene inheritance
What is an autosomal gene?
A gene that is carried on one of the autosomes (chromosomes 1 to 22)
What is an allele?
A specific version of a gene in an individual
What is autosomal dominant inheritance?
A pattern of inheritance where an affected individual possesses one copy of a mutant allele and one copy of a normal allele
What is the chance, in individuals with an autosomal dominant disease, of passing the mutation on to offspring?
50%
What is autosomal recessive inheritance?
Both copies of a gene must harbour mutations for the disease to arise
What is the chance of passing an autosomal recessive disease on to a child if both parents are carriers?
25%
