COMPLEX GENETICS Flashcards
Describe incomplete dominance.
Refers to a phenotype that is intermediate between the dominant and recessive phenotypes. Example: egg plant colour (dark purple, light purple and white).
Describe co-dominance.
the phenotype of heterozygotes shows the phenotypes of both the 2 homozygotes. Eg human AB blood type.
Describe X inactivation.
Opposite of co-dominance. Each female cell contains 2 X chromosomes, one from the mother and one from the father. Instead of the matched alleles from both chromosomes being expressed, one of the two X chromosomes is randomly inactivated so that only the genes on the other are expressed.
Inactivation occurs early in embryogenesis. Results in some clusters of cells expressing a completely different phenotype to other cells in the individual.
Eg calico cats.
Describe transposable genetic elements.
Pieces of DNA able to move between genome sites. can insert, exit or relocate.
Can be a gene, gene fragment or a small set of linked genes.
Can move within a chromosome or to a new chromosome.
The effect of transposon movement depends on 2 factors:
- nature of the DNA it inserts into: insertion into functional genes is likely to disrupt the gene and result in mutation. Can also cause deletions.
- Kind of genes carried by transposons: may carry regulatory sequences that influence transcription and translation.
Describe Eukaryote transposable elements and retrotransposons.
25-40% of mammalian genomes have transposable elements, often of unknown function.
A large proportion of spontaneous mutations are attributed to the movement and insertion of these elements.
Describe somatic mosaicism.
mutation arises in a single cell in somatic tissue during development. All adult cells descended from this mutant cell will carry the same mutation, but it will not be transmitted to the next generation because it is not present in the germ line.
differentiate between qualitative traits and quantitative traits.
Qualitative traits involve discontinuous variables having only a few possible values (e.g. wrinkled/smooth) and can typically be analysed using basic mendelian principles.
Quantitative traits involve continuous variables, such as height. Can have many overlapping phenotypes. Most quantitative traits are both polygenic and influenced by environment, so are multifactorial.
Define polygenic.
Many quantitative traits are polygenic, meaning they are influenced by many genes. this allows slightly different phenotypes to arise.
What are the limitations of twin studies?
MZ twins are of the same gender, and so are more likely to experience same environment that DZ twins.
Separated twin studies have small numbers, separation is usually only partial, can’t distinguish intrauterine environmental causes from genetic causes.
MZ twins have genetic differences, including in mitochondrial DNA, antibodies and T cell receptors, somatic mutations and random X linked inactivation.
What are the 2 main approaches of adoption studies?
- sample adopted people with a particular trait or disease to determine if the trait runs in their adopted or biological family.
- start with affected subjects whose children have been adopted away, and ask whether this protects the child from the risk of the disease.
