Moon pt.1 Flashcards
What is the blending theory?
- Traits of parents get mixed like fluid
- Results in a NEW trait that is a mixed middle of both parents
- The original parent traits are lost in the blend (can’t be recovered)
What was Mendel’s experiment to disprove the blending theory
What does it mean to be monohybrid?
Heterozygous of 1 gene
What is the phenotypical result/ratio of a monohybrid cross of a completely dominant trait?
3 : 1
What is a testcross?
The cross of an individual to a fully recessive individual
Useful for determining the genotype of the gametes a testee can produce
ex: cross a purple heterozygous (purple dominant, white recessive) with full white → 50% purple, 50% white
What is a loss of function mutation?
Often recessive (phenotype) because a single WT copy can be enough for normal function/phenotype → happens when the gene is haplosufficient →
What did Thomas Hunt Morgan’s experiments confirm?
Some genes are not simply inherited in a 50/50 segregation manner
Ex: flie eyes are inherited following the X chromosomes sorting during meiosis
Mendel’s experiment only works for traits encoded by 1 autosomal gene
Confirmed the Chromosomes theory of inheritance → genes are stored in chromosomes inside the cell nuclei, also discovered the crossover phenomen (different chromosomes can trade places with one another)
What is the result of an independent assortement of a dihybrid during meiosis?
Equal ratio of the four different genotypes of a gamete
Start with an AaBb cell → meiosis → gametes = AB, Ab, aB, ab
During meiosis, what is the probability that the paternal chromosomes 1 and 2 are transmitted to a single gamete in a human?
1/2 * 1/2 = 1/4
How many different genotypes of a gamete a human can produce during meiosis via independent assortement alone?
2^23
If you cross 2 dihybrid peas (R/r; Y/y) together, what phenotypic ratio will you get?
*Complete dominance
4 phenotypes: 9:3:3:1 = YR:Ry:rY:ry (FOR THE PHENOTYPE), 16 different genotypes
What does a dihybrid testcross produce when there is independent assortement?
Equal number of parental and recombinant type
*Parental = parents of the F1 dihybrid
1:1:1:1 ratio of parental and recombinant
What is the Chi-square test?
x^2 = Σ (O-E)^2 /E
Used to determine if the degree of deviation from the expected valiue in significant → used 95% confidence level
p = 0.05 → you have 5% chance that rejecting the hypothesis independent assortement is wrong
If p <= 0.05, you can reject the hypothesis with confidence level of 95% or greater
How is the degree of freedom determined
The number of different phenotypes used in the calculation - 1
What is a tetrad?
Four spores produced by meiosis of a haploid organism
What is an ascus?
Physical structure (ascus wall) keeps the tetrad together → can be isolated to study a single meiotic event
What are the different options for crossover between 2 haploid AB x ab
*Crossing over between chromatids, not chromosomes (chromatid = 1/2 of a replicated chromosome)
2-chromosome stage (before replication):
2x Ab and 2x aB
4-chromatid stage (after replication):
AB, Ab, aB, ab
What are the possible results for a multiple crossover between 2 haploids: ABC x abc (ABC are on the same chromosome)
2 chromatids → ABC, AbC, aBc, abc
3 chromatids → ABc, AbC, aBC, abc
4 chromatids → ABc, Abc, aBC, abC
How does distance between 2 genes on a chromosome affect crossover likelyness?
The farther apart the genes are, the more likely they are to crossover and produce recombinants
Relative distance between 2 genes reflected by recombinant frequency
What is a centimorgan?
1 genetic map unit = frequency at which 1% meiosis produce a recombinant
What is the maximal relative frequency of crossover for 2 genes?
50%
What is a molecular marker?
*Good approach to map a gene
Small DNA sequence differences (polymorphisms) within a specie that are presen at specific chromosomal locations
- Present through out the genome
- Most don’t have biological functions, no phenotype
- Molecular markers are often seen as bands on a gel
- Can be used to map a gene (ex: disease causing gene) by determining the linkage between the gene of interest and a molecular marker
What is the utility of Simple sequence length polymorphisms (SSLPs)?
- Human genomes contain a lot of repetitive DNA sequences
- Unrelated people likely have different numbers of repeats
- A child has 1 set of paternal and 1 set of maternal SSLPs
- The lengths of SSLP regions can be quantified by PCR amplification using primers binding to flanking sequences → resolve in DNA gel
Length of the segment is associated with the personne, not with phenotype
What are Single Nucleotide Polymorphisms (SNPs) ?
- The genomic sequences or 2 unrelated people ~ 99.9% identical
- SNP found in at least 1% of any population (common), if not, variant may be a mutation
- Some SNPs are more common in a specific population/ethnicity
- Can be found in intergenic region, within a gene or in regulatory region near a gene
- Most of time, has no effect on health, but some may play a role in susceptibility of a disease or sensitivity to an external factor
How can mapping of disease gene can be done using SNP?
Some SNP always inherit with a disease gene (not same as SSLP that are related with the person, not the disease gene)
What SNP is associated with Cystic Fibrosis?
Loss of Phe508 → 3 nucleotide deletion
What is a haplotype?
Physical grouping of genomic variants (or polymorphisms) that tend to be inherited together, as a single group
How can the mutations be qualified in a haplosufficient gene?
Recessive → if 1 copy has a loss-of-function mutation → WT phenotype
How can we qualify a gene that has a dominant mutation?
Haploinsufficient
*often gain of function mutations, hyperactive
How does the p53 mutant allele affect health?
p53 is a TF that binds DNA as a homotetramer
Dominant mutation → affects DNA binding domain → the mutant p53 can teramerize → the tetramer can’t bind DNA → no transcription (dominant-negative effect)
What is incomplete/partial dominance?
Phenotype of heterozygous = halfway between the 2 homozygous
ex: white and red → pink
What is codominance?
Phenotype of the heterozygous shows the presence of both homozygous fully
ex: blood types → A and B are codominant and dominant over O
Can see both bands separatly in the gel
*Dominance is determined by the phenotype
What is the dominance class of sickle cell anemia?
HbA and HbS are codominant → both alleles can be clearly discerned at the protein level
Some RBC are sickled and some a normal
How are recessive lethal alleles maintained in the population?
Maintained as heterozygous in haplosufficient genes
Homozygous mutation causing lethality can be recessive or dominant mutations
Ex: homozygous yellow = lethal, yellow is dominant
Yy = yellow, vital, yy = dark coat, YY = yellow, lethal
Yellow x Normal = Yy x yy → 1:1
Yellow x Yellow = Yy x Yy → 2:1 (Yellow : normal)
What are examples of conditional alleles?
Alleles that depend on external factors:
Temperature sensitive mutants → low temp functions like WT, high temp is lethal
Auxotrophs → minimal media + arginine function like WT, minimal media is lethal
What are auxotroph organisms?
organisms that lost ability to synthesize certain substances required for their growth → conditional alleles
What is expressivity?
The degree to which a given allele is expressed at the phenotypic level, intensity of the phenotype
Give an example of conditional allele temperature controlled phenotype
Tyrosine kinase that is active at a lower temperature → extermitis of the rabbit are colder so appear black id raised in colder temperature
What is the complementation test?
To assess the presence of recessive mutations
When you can get a mutant crossing to organisms that apear ar WT (to know if both heterozygous)
What does it indicate on the genotypes if you get a 9:7 phenotypic ratio?
2 genes act on the same pathway → if on of them is homozygous recessive → recessive phenotype
Ex: Interaction between a regulator gene and a target gene (TF and the gene it affects)
9: R/- ; A/-
3: r/r ; A/-
3: R/- ; a/a
1: r/r ; a/a
3+3+1 = 7 recessive phenotypes
What is recesive epistasis?
Epistasis = when phenotypic manifestation of an allele is dependent on the genotype of a different gene
w is epistatic to m if recessive genotype of gene w masks phenotype associated with gene m
Ex: sequence of enzymes for flour → white → E1 (gene w) → pink → E2 (gene m) blue
Without w, no way to have the m phenotype
What ratio is associated with a dihybrid cross in the context of recessive epistasis?
9:3:4
Ex: sequence of enzymes for flour → white → E1 (gene w) → pink → E2 (gene m) blue
9 blue (W/- ; M/-) : 3 pink (W/- ; m/m) : 4 white (w/w ; r/r + w/w -/-)
What are suppressors?
mutant allele of another gene that REVERSES the effect of an original mutation of a 1st gene
Mutation + Supressor → WT
What is a modifier?
a second mutation that changes the degree of expression of a mutated gene (phenotype)
Ex: in regulatory sequences
B code for transcription factor/is in regulatory sequence, A is the actual gene of the protein
A/B → WT
A/b → defective (low transcription)
a/B → defective (defective protein A)
a/b → extremely defective
What does it mean for a gene interaction to be synthetic lethal?
Mutations in 2 genes, indivudally have weak mutant phenotype, together → lethality
Ex: multiprotein complexes
What is the ratio for a dominant epistasis relation?
12 : 3 : 1
