Genetics Chapter 5 Flashcards
Wild-type allele
the most prevalent version of a gene in wild populations (i.e., the “normal” version of a gene)
Genetic polymorphism
Genes can have more than 1 common allele that can be considered wild-type
Recessive Alleles/Mutant allele
- A less common version of a gene
- Due to random mutations that
occur in DNA
Most random mutations produce alleles that are inherited in a recessive fashion
Ex: recessive alleles in pea plants were the mutant alleles
Recessive mutant alleles typically produce less functional protein
Either because the protein is defective or they produce lower levels of the functional protein
Dominant mutant alleles
Gain-of-function mutations - The gene gains a new or abnormal function
May be overexpressed, producing higher levels of the protein
Dominant mutant alleles example
Example: p53
p53 is a tumor suppressor gene
if mutated, p53 can cause cancer
p53 upregulates genes inappropriately, changing the “identity” of the cell
Haploinsufficiency
Sometimes mutant alleles are dominant
The mutant is a loss-of-function allele, and one wild-type copy is not enough to provide function
Example: Polydactyly in humans (extra fingers in babies)
The dominant mutant allele loses its function so that a heterozygous individual shows an abnormal phenotype or disease phenotype.
Loss of function mutations
May have no effect on phenotype
Inactive or a less active protein
One gene can compensate for the loss of another
Gain of function mutations
Gene gains a new or abnormal function
May be overexpressed, producing higher levels of proteins
Example: p53 gene
The product of the dominant mutant allele is expressed in cells where it is not normally active, leading to a change in phenotype.
Penetrance
portion of individuals with a given phenotype associated with a specific allele
→a population
Example: incomplete penetrance→polydactyly
Expressivity
The degree (intensity) of a certain phenotype for a given genotype
→ single individual
Incomplete dominance
when one allele is not completely expressed
The heterozygous will have an intermediate state
Overdominance
when a heterozygous produces a phenotype more extreme or better than that of the homozygote
Heterozygous has greater reproductive success than either homozygous
It is also called: heterozygote advantage or hybrid vigor
Codominance
is when the heterozygous genotype expresses both alleles (red X white= red + white spots
how overdominance can occur
- Sickle-cell anemia
Autosomal recessive disorder
Affected individuals produce abnormal form of hemoglobin - Two alleles:
HbA→normal hemoglobin, hemoglobin A
HbS→ abnormal hemoglobin, hemoglobin S - Malaria incidences correlates with increased HbS allele frequency
Heterozygotes are malaria-resistant
Sex-linked
associated with one sex or the other, carried by a sex chromosome
Sex-limited
traits that occur only in one of the two sexes
Responsible for sexual dimorphism (differences in appearance between the sexes of an animal in addition to difference between the sexual organs themselves).
Might be autosomal or sex-linked
Sex-influnced
an allele is dominant in on sex but recessive in the other
Sex influence is a phenomenon of heterozygotes
This doesn’t mean sex-linked
Most sex-influenced traits are autosomal
Essential genes
are required for survival
Lethal allele
Lethal allele: has the potential to cause the death of an organism (usually in essential genes)
Examples: mutations in genes that promote cell division
Hungtington’s disease
Conditional lethal alleles
kill an organism only under certain environmental conditions
Temperature -sensitive (ts) lethals: proteins misfold at higher temperatures, becoming nonfunctional
Semi Lethal alleles
kill some individuals in a population, but not all of them
Environmental factors and other genes may help prevent detrimental effects of semi lethal genes
The enzyme glycosyl transferase has 3 common alleles in human
(i, I^A, I^B)
Codominance
ABO locus has 3 main allelic forms: A, B, O
Allele i encodes a nonfunctional enzyme
Alleles I^A & I^B are codominant
Pleiotropy and why it occurs and example
multiple effects of a single gene on the phenotype of an organism
Occur:
The gene product may affect cell function in multiple ways
The gene may be expressed in different cell types
The gene may be expressed at different stages of development
Example: cystic fibrosis→ a disease with MANY SYMPTOMS caused by ONE gene
Epistasis
one gene affects the expression of a different gene
Example: baldness gene covers up the genes for hair color)
Epistatic interactions often arise because two different proteins participate in a common cellular function
