1. mendelian genetics Flashcards
What is the human genome comprised of
GENOME = all DNA present in the human cell. DNA packed into chromosomes (46) containing histone proteins
- nuclear genome: 20,000 genes
- mitochondrial genome: 37 genes
areas in which genes can differ
- size
- composition and size of introns and exons (small minority of intronless genes)
- base composition between diff chromosomes (av = 41% GC).
- distribution of genes across chromosomes (can overlap)
what does a higher GC content signify
a higher gene density
What is a gene family
A gene family is a set of similar genes, formed from the same progenitor gene (via duplication or retrotransposition).
Describe gene duplication
-process by which a region of DNA coding for a gene is copied
-evolutionary important bcos several mutations can occur from it (which can end up providing a benefit for the individual)
-occurs due to unequal crossing over in meiosis (mismatches in the pairing of homologous chromosomes)
!! mutations in duplication can form psuedogenes and paralogous genes
orthologous vs paralogous gene def
Orthologous genes are found in different organisms, but are derived from a single common ancestral gene present in the common ancestor of those organisms.
Paralogous genes are genes present in a particular organism that are related to each other through a gene duplication event (make proteins of diff structures or functions)
ie. O are genes separated by speciation, P are genes separated by duplication
Pseudogenes def
a segment of DNA that structurally resembles a gene but is not capable of coding for a protein
!! derived over evolutionary process by accummulation of mutations (via duplication and unequal crossing over)
What is non-coding DNA
-98% of genetic material
-previously thought to be ‘junk’ but now known to have important fucntiosn such as transcription and translation regulation
non coding DNA = introns, peudogenes
non coding ELEMENTS = interspersed elements (LINEs/SINEs/transposons), satellite DNA
transposons def and function
interspersed repetitive non coding elements:
mobile DNA sequences that migrate to diff regions of genome
Retrotransposones (LINEs/SINEs): able to use reverse transcriptase to make cDNA copies of RNA transcripts
(hence this copy-paste function and insertion of gene sequences in diff areas of the gene has evolutionary importance)
what are tandem repeating DNA
satellite dna (alpha) –> makes up the centromeres of all chromosomes (repeats for each are of diff lengths)
minisatellite dna –> telomeric found on edges of choromosomes and provide mechanism for replicating ends
microsatellite dna –> interspersed throughout genome mainly within introns, and are mutational hotspots
Allele def
Different variations of homologous genes (Can be either heterozygous or homozygous)
locus def
position occupied by a gene on a chromosome
types of allelic trait control (7)
- complete dominance
- incomplete dominance
- overdominance
- codominance
- multiple allele interaction
- multiple allele epistasis
- lethal alleles (dominant and recessive)
complete dominance or recessiveness + example
dominant trait allele: AA and Aa have the same phenotype and cannot be distinguished without the genotype. exhibits HAPLOSUFFICIANCY: when one copy of gene is a diploid organism is enough to give the phenotype.
recessive trait allele: only aa phenotypically expressed trait
EX: tongue rolling (dom)
Incomplete dominance def + example
the expression of one allele over the other is not absolute and hence heterozygous individuals express an intermediate phenotype
EX. hypercholesterolemia: disease where LDL rises above normal levels due to an inability for cells to make LDL receptors. HH is normal, hh is complete inability to make receptors (most severe) but Hh is an intermediate phenotype where SOME LDL receptors are synthesised and hence the disease is mild type
Overdominance def + example
HETEROZYGOTIC ADVANTAGE: a heterozygote has greater
reproductive success compared with either AA/aa. Due to 2 alleles that produce proteins with slightly diff amino acid sequences. Hence they:
1. provide disease resistance
2. encode for subunits of a homodimer with increased functionality
3. encodes for isoforms of enzymes that work well at a larger range of conditions (like temperature)
EX. sickle cell disease - heterozygote form prodivdes resistance to malaria
Codominance def and example
Heterozygote expresses the phenotypes of both homozygotes
eg. blood groups (both ABO and MN)
Multiple allele traits def and example
-traits which have multiple (2+) alleles existing in the population pool
-since each individual is diploid, they posses 2 out of n of these alleles
-this gives rise to a much higher number of total phenotypes
example: rabbit fur coat colour
calculating posisble number of existing genotypes for multiple allele traits
for number of alleles ‘n’, the total kinds of genotypes = kinds of homozygous + kinds of heterozygous genotypes
- kinds of homozygous = n
- kinds of heterozygous = n(n-1)/2
- total = n(n+1)/2
describe ABO blood types
- IA/IB/i alleles encode for the type of antigens present on RBCs (attached by glucosyl transferase encoded for by I gene). i is the recessive type and produces no antigen.
- i is recessive to A/B which are codominant. This produces the blood types: O/A/B/AB
!! diff antigens are recognised by diff antibodies (antibodies made are the ones that the RBCs do not posses)
considerations for correct transfusions
AB = universal acceptor
O = universal donor
! in the case of an incorrect transfusion, antibodies are produced and bind to multiple blood cells at once which causes agglutination - clogging of vessels and hindered blood flow. Hemolysis of RBCs release heme which causes kidney failure and also decreases potential for O2 transport.
Describe MN blood groups
M = MM
MN = MN
N = NN
(example of codominance bcos MN has antigens of both M and N groups)
describe the rhesus blood group system
Rh+ = DD/Dd
Rh- = dd
issue for Rh- women
ERYTHROBLASTOSIS FETALIS: are able to have a first pregnancy but not a successful second one.
- 1st pregnancy with Rh+ fetus causes mixing of blood through placenta
- mother is sensitised to Rh+ blood, but since she is Rh- she develops Rh+ antibodies
- upon second pregnancy with Rh+ fetus these antibodies enter fetal circulation and cause hemolysis of fetal RBCs
- this leads to spontaneous miscarriage
Epistasis description and example
A type multiple allele interaction where the epistatic gene masks the phenotypic expression of another hypostatic gene and no new phenotype is produced
-can either be dominant (A masks B) or recessive (aa masks B at another locus)
EX. labrador coat colour exhibiting recessive epistasis: controlled by 2 separate genes, B determining colour and E controlling expression of B. B causes black colour and bb causes brow, but if ee is present thenthey will be golden regardless of their B alleles.
Describe the bombay blood group phenotype
EPISTASIS: mutant recessive allele at a second gene (hh) masks the ABO phenotype –> this creates individuals with ‘apparent’ blood group O but in fact contain A/B antigens (this can be seen in pedigrees bcos they produce offsprings with supposedly impossible blood groups).
!! need special bombay blood in transfusions
Describe lethal alleles
Mutations in genes that are essential for life and hence might result in death
DOMINANT: death of both AA and Aa (example is huntingtons)
RECESSIVE: death only in cases of aa (example is tay sachs)
penetrance def
probability of a gene or trait being expressed. In some cases despite the presence of a dominant allele the phenotype isnt present (eg polydactyly).
This causes reduced penetrance (instead of complete) meaning that the genetic trait is only expressed in PART of the population.
!! this is a complication of pedigree diagrams bcos it makes it hard to determine family history and calcualte the probability of trait inheritence
expressivity def
variation in phenotypic expression when an allele is penetrant (ie variability in the phenotype seen)
pleiotropy def
the expression of multiple traits by a single gene (ie single allele which has more than one distinguishable phenotypic effect).
EX sickle cell disease, which is a result of a single amino acid change BUT has multiple effects in the body, including weakness, pneumonia, paralysis, rheumatism, kidney failure, abdominal pain, etc…..
