Epistasis/Mutation Flashcards
What is epistasis? Provide an example.
Epistasis occurs when one gene masks or modifies the expression of another gene. Example:
Labrador coat color: Two loci (B for pigment production and E for deposition). B_ produces black (BB or Bb) or brown (bb) pigment, but ee masks pigment deposition, resulting in yellow fur.
Bombay phenotype: H gene (epistatic) must produce H antigen for ABO antigens to form. hh individuals lack ABO antigens despite having A/B alleles.
Compare recessive epistasis and dominant epistasis.
Recessive epistasis: Homozygous recessive (ww) at one locus masks another locus. Example: Labrador coat color (ee hides B locus). Phenotypic ratio: 9:3:4.
Dominant epistasis: Dominant allele (B) at one locus masks another. Example: Squash fruit color (B_ suppresses A locus). Phenotypic ratio: 12:3:1.
Explain duplicate recessive epistasis with an example.
Either homozygous recessive (aa or bb) masks the phenotype. Example: Flower color in sweet peas. Phenotypic ratio: 9:7.
Outline the steps of karyotyping.
Sample collection: Blood or amniotic fluid.
Culture cells with phytohemagglutinin (blood) for 3 days.
Arrest in metaphase using colchicine.
Hypotonic treatment to swell cells.
Fix, stain (e.g., Giemsa for G-banding), and photograph chromosomes.
Arrange chromosomes by size, centromere position, and banding patterns.
Differentiate between G-banding and C-banding.
G-banding: Giemsa stains AT-rich regions; reveals unique banding patterns for chromosome identification.
C-banding: Stains constitutive heterochromatin near centromeres; used to study structural abnormalities.
Define aneuploidy and provide two examples of trisomy disorders.
Aneuploidy is an abnormal chromosome number (2n ± 1). Examples:
Trisomy 21 (Down syndrome): Intellectual disability, characteristic facial features.
Trisomy 18 (Edwards syndrome): Severe developmental defects; most die in infancy.
What is Cri-du-chat syndrome?
Caused by a deletion in chromosome 5p. Symptoms: High-pitched cry, microcephaly, intellectual disability.
Contrast missense and nonsense mutations.
Missense: Single base substitution alters an amino acid (e.g., sickle cell anemia: GAG → GTG, glutamic acid → valine).
Nonsense: Substitution creates a premature stop codon (e.g., TAG in β-globin gene truncates hemoglobin).
How do frameshift mutations affect protein synthesis?
Insertions/deletions (indels) not divisible by 3 shift the reading frame, altering downstream codons and often creating premature stop codons. Example: Huntington’s disease (CAG repeats).
Explain Fragile X syndrome.
Caused by CGG triplet repeats (>200) in the FMR1 gene on the X chromosome. Results in intellectual disability, elongated face, and behavioral issues.
How does a splice site mutation cause phenylketonuria (PKU)?
A G→A mutation in the PAH gene disrupts the splice donor site of intron 12, leading to mis-spliced mRNA and nonfunctional phenylalanine hydroxylase.
What is spectral karyotyping used for?
Fluorescently labels each chromosome with unique colors to detect structural abnormalities (e.g., translocations, deletions).
How is FISH used in prenatal testing?
Fluorescent probes for chromosomes 13, 18, 21, X, and Y detect aneuploidy in uncultured amniocytes. Results in 24–48 hours
Name an ethical issue related to genetic testing.
Pre-implantation embryo screening raises concerns about embryo selection and potential eugenics.
