18 - genetic basis of disease Flashcards
what are alleles?
variant of a gene at a specific location on a chromosome
Humans are diploid organisms: two alleles at each genetic locus.
what is a genotype?
the genetic ‘make-up’ of an organism
what is a phenotype?
the observable physical characteristics of an organism
what does homozygous mean?
identical alleles at a specific genetic location (i.e. AA or aa)
what does heterozygous mean?
different alleles at a specific genetic location (i.e. Aa)
what does dominant mean?
effect of allele expressed even if individual is heterozygous (i.e. AA or Aa)
what does recessive mean?
effect of allele expressed only if individual is homozygous (i.e. aa)
how is a homologous chromosome made?
When DNA replicates for cell division during mitosis or meiosis the chromatin condenses; the sister chromatids (produced during S-phase replication) are joined together at the centromere.
what is the law of segregation?
alleles are separated into gametes during meiosis, and there is equal frequency in the numbers of egg or sperm cells that contain each allele.
four main concepts:
1. Genes exist in more than one form or allele.
2. Organisms inherit a pair of alleles; one from each parent during sexual reproduction
3. These alleles separate during meiosis.
4. An organism with at least one dominant allele will display the effect of the dominant
allele (heterozygous alleles exhibit complete dominance)
this law will only hold true for unlinked alleles.
what is complete or strict dominance?
an allele that is dominant will be expressed in the phenotype regardless of any conflicting instructions from the other allele.
what is incomplete dominance?
the phenotype of a heterozygote that is
intermediate between the phenotypes of the two homozygotes (e.g. pink
flowers on heterozygotic progeny from red & white homozygotic parents).
what is co-dominance?
the heterozygote simultaneously expresses traits of both homozygotes (e.g. a speckled chicken)
what does recessive mean?
phenotype expressed only if the same allele is present in both chromosomes.
what is penetrance?
proportion of people with a disease carrying mutation who also exhibit symptoms of this disease.
what is complete penetrance?
all people carrying a particular gene for a disease also exhibit its clinical symptoms.
i.e. Huntington’s disease has a 100% penetrance
what is incomplete penetrance?
some people carrying a particular gene for disease exhibit clinical symptoms whilst others don’t.
i.e. breast cancer caused by mutation in BRCA1 gene.
what happens in cases of low penetrance?
it can be difficult to distinguish environmental from genetic factors.
what is expressivity?
the extent to which a particular allele is expressed when it is present.
what is an example of incomplete penetrance and expressivity?
polydactyly: multiple digits on hands and/or feet.
what does genetic recombination do during meiosis?
During meiosis genetic recombination produces
variations in the chromosomes of gametes.
what is homologous recombination?
it is essential for an evolutionary perspective:
- allows the segregation and creation of new combinations of DNA sequences (an astronomical number of possibilities !)
what is crossing over?
when tetrads form during meiosis, non-sister chromatids may overlap.
what is genetic linkage?
the tendency of certain alleles to be grouped together during recombination. the closer the genes are, the more likely they are to be linked.
what is sex linkage?
sex linkage is phenotypic expression correlated to sex i.e. colour blindness, Duchenne muscular dystrophy.
colour blindness is most commonly caused by a defective gene on the X-chromosome.
- females need to inherit two defective copies of the gene to be colour blind.
- males need only inherit one.
sex linked genes can also be genetically linked.
