Gene interaction Flashcards
How can genes control trait phenotypes?
They can follow simple dominant/recessive relationships
Combine to form completely new phenotypes
Show both dominant and recessive phenotypes simultaneously
Require multiple genes to fully express phenotypes
What is nonsense mediated decay?
Protein that has premature stop codon can stop being made completely.
In dominant gene disorders what can happen to inheritance?
Heterozygous mutations inherited from affected parent or de novo
Mutation only needs to be on one copy to cause disease
Gain of function mutations can make protein acquire new function
In recessive gene disorders what can happen to inheritance?
2 mutations would be needed or compound heterozygous mutations inherited from unaffected parents.
Loss of function mutations are typically what affects these genes resulting in absent or reduced function protein.
What are modifier genes?
Genes that interact with other genes and influence their genotypes
How are mono and biallelic expression different to each other usually?
Biallelic expression is more prevalent where both alleles of a gene are transcribed
Monoallelic expression is expression where only 1 of the 2 copies of a gene is active.
What are the common non-mendelian inheritance patterns?
Multiple alleles
Incomplete dominance
Codominance
Polygenic traits
Pleiotropy
Sex-linked characteristics
Environment also plays an important role in the phenotype
What is incomplete dominance?
Phenotype not completely like the dominant character
What trait shows incomplete dominance in humans?
Hair texture
How is phenotypic ratio different between complete and incomplete dominance?
Incomplete dominance follows genotypic ratio (1:2:1) whereas complete dominance is 3:1 due to dominant gene being most influential
What is codominance?
2 Equally dominant alleles expressed concurrently. Eg Blood group (ABO)
What are the characteristics of dominance?
Dominance is allelic interaction (result of interaction between genes at the same locus)
Dominance doesn’t alter how genes are inherited but does influence which genes are expressed as a phenotype
Dominance’s allelic interaction is between the products of the genes
Classification of dominance depends on the level which the phenotype is examined
What type of expression does sickle cell anaemia show?
Codominance (A/A = wild type, Aa = only sickle under low O2, aa = always sickle)
What is a pleiotropic effect?
One gene affects many phenotypic characteristics. eg. cystic fibrosis or sickle cell anaemia.
What are polygenic traits?
Multiple genes working together to produce a continuous distribution “Bell Shape” curve.
Most traits follow this pattern
What are some examples of polygenic inheritance?
Body type
Height
Skin colour
Hair colour
Eye colour
Intelligence
How are X and Y chromosomes different to each other?
X chromosome contains > 1500 genes
Y chromosome has approximately 231 protein coding genes with some that are unique to Y
When do sexual differences begin to develop?
At gestational week 7. This is determined by genetic and environmental factors.
What is the genotype of turner syndrome, Klinefelter syndrome, and poly-X?
XO = Turner (circle = turn)
XXY = Klinefelter syndrome
XXX = Poly-X females
What is the male-determining gene in humans?
Y chromosome gene; SRY gene. It causes gonad to develop into testis.
Other genes on autosomes play an important role.
When testes develop they secrete 2 hormones; testosterone and Mullerian inhibiting hormone (MIH)
What does SRY gene control?
Testis formation and in turn the secretion of testosterone and MIH.
What are the 2 duct systems present in early embryos and what do they develop into?
Wolffian (male)
Mullerian (female)
At week 7 developmental pathways activate different sets of genes which cause undifferentiated gonads to develop into testes and ovaries which determines gonadal sex of embryo.
What is androgen insensitivity syndrome and what is it caused by?
SRY gene is being expressed but receptor for testosterone is defective so they look like females externally with some testes.
What is congenital adrenal hyperplasia?
Growth of adrenal cortex resulting in production of lots of androgens. This is a problem in females because of formation of male sex organs
What are the features of true hermaphroditism?
Generally have normal chromosomal composition with XX genotype. Appear female but have ovotestis with both spermatagonia and ovarian follicles.
What are the features of pseudohermaphroditism?
Males have XY genotype with normal chromosomal composition. Phallic hypoplasia due to insufficient hormone production.
Females have XX genotype with normal chromosome composition but have high levels of androgenic hormones and have masculinised external genitalia.
How are post-zygotic mutations distributed?
If cell mutates early in development then it will have descendents in many tissues
Mutation arising later will be more localised.
This results in a mix of cells producing normal and abnormal proteins. Phenotype depends on proportion of cells producing abnormal protein and their distribution.
What does mosaicism do to post-zygotic mutation related diseases?
Results in milder disease due to some of the defective genes being switched off.
What kind of effects can mosaicism affect?
Can be for chromosomal or single gene disorders
May affect somatic or germline tissues
Can result in a range of abnormalities depending on the amount and distribution of normal cells
How does mosaicism apply to Turner syndrome?
Some cells they have are XO but some are XX. Depends on when during development non-disjunction took place.
What is skewed X chromosome inactivation?
X-inactivation is not 50/50. Switching off of X chromosomes is often skewed one way or another. (70/30 is considered skewed)
What does mosaicism do to mutations?
Can lessen the overall effect.
What cells mosaicism?
All clonal descendants of the cell. If germline mutation occurs in mitotic stage prior to meiotic stage then all the descending cells will exhibit the mutation. Somatic cells remain unaffected.
What condition is a important example of germline mosaicism?
Osteogenesis imperfecta which is severe in ~6% of people. Mutations in type I collagen genes lead to abnormal collagen, brittle bones, and frequent fractures.
