Chapter 4 Flashcards
Wild-type
the most common allele in natural populations
can be dominant or recessive (dom not always more common)
can change over time
7 types of mutations
a. wild type
- both alleles produce functional protein
b. null/amorphic mutation (lf)
- null allele produces no protein
c. leaky/hypomorphic (lf)
- leaky allele produces less protein
or
a protein with diminished function
d. dominant negative mutation (lf)
- result from impaired interaction between two genes
- relevant for multimeric proteins that form from multiple genes
(peptide interactions)
e. hypermorphic mutation (gf)
- more gene activity than wild-type
f. neomorphic mutation (gf)
- produce product with a novel function
Incomplete/partial dominance
dominance of one allele may not completely mask the recessive allele
phenotype is intermediate, a blended version of each homo phenotype
ex. red+white = pink snapdragons
Complexities of genetics
- incomplete dominance
- polygenetic traits
- 2+ alleles for some genes
- gene-environment trait expression
Allelic series
result of a hierarchy of dominance and partial dominance across multiple alleles
ex. rabbits
C > c^ch > c^h > c
depends on tyrosinase activity (melanin)
Co-dominance
heterozygous pheno different than homo phenos
not blended, instead independent
ex. different blood groups
Lethal alleles
some single-gene mutations are lethal
are recessive and can be carried by heterozygotes
low frequency in a population
Incomplete penetrance
a given genotype DOESN’T always produce the same phenotype
traits that are nonpenetrant in some individuals but penetrant in other individuals
seen in every generation
Penetrant
when the phenotype of an organism matches its genotype
Nonpenetrant
when an individual with a certain genotype does no produce the associated phenotype
Fully penetrant
when the genotype always expresses an associated phenotype
Sex-limited traits
gene carried in both sexes but only expressed in one
result of hormones interacting with gene expression
horns, lactation
Sex-influenced traits
both sexes carry a gene, but the phenotype varies between the sexes
can result in phenotypes found in both sexes
ex. goat beards
Variable expressivity
same genotype but variable phenotypes
ex. Waardenburg syndrome
Gene-environment interactions
the influence of the environment on expression of genes and on the phenotype of an organism
ex. treating PKU by not feeding phenylalanine-rich foods
Pleiotropy
when one gene affects many phenotypes
ex. JH in drosophila, sickle cell in humans
Sickle-cell anemia
pleiotropy: one gene contributes to many symptoms
- a single point mutation affects translated protein
Epistasis
the result of genes interacting along a common biosynthetic pathway
the effect of one gene is dependent on the alleles of other genes
causes a deviation from the expected 9:3:3:1
9: AABB
3: A_bb
3: aaB_
1: aabb
Dihybrid cross
heterozygous cross
AaBb x AaBb
6 different types of epistasis
- complementary gene interaction
- duplicate gene interaction
- dominant gene interaction
- recessive epistasis
- dominant epistasis
- dominant supression
Complementary gene interaction
9:7
presence of any recessive causes recessive phenotype
Need Dom of each to compliment and get the desired result
Duplicate gene interaction
15:1
- recessive only shown in homo recessive
- both genes play a redundant role at the same step in a biochemical pathway
they are duplicates so you only need one for the pheno
Dominant gene interaction
9:6:1
three different phenotypes
1. dom present in both genes
2. at least one homo recessive
3. both homo recessive
Recessive epistasis
9:3:4
ex. dogs genes for pigment and pigment deposition
Dominant epistasis
12:3:1
dominant allele can mask other allele
Dominant suppression
13:3
dom for one and recessive for other required
Beadle and Tatum
came up with one-gene-one-enzyme hypothesis
- irradiate neurospora to generate mutants
- transport to nutrient media where both prototrophs (wild type) and auxotrophs (mutant) grow
- transfer to minimal medium where proto grow but but auxo don’t
- transfer auxo to media supplemented with different nutrients and controls
- transfer auxo to media supplemented with one amino acid to identify the defective biochem pathway
when methionine was supplemented in minimal media, auxotrophs were able to grow
= had a blockage in met synthesizing pathway
Horowitz
genetic dissection to identify methionine synthesis pathway steps and their order
Methionine pathway
Homoserine
met 4
Cysteine
met 3
Cystathionine
met 2
Homocysteine
met 1
Methionine
buildup of following compounds if mutation in earlier methionine
Complementation analysis
see how many genes are responsible for the same mutation
look at (-) signs
