Genetics exam 3

microRNAs (miRNAs)

Eukaryotic gene regulaiton

• Naturally occurring regulatory RNA’s encoded in the genome, regulate the expression of other genes (abt 300 target on average)

• Function to regulate target genes by reducing translation of their mRNAs → down regulate by degradation

• Preliminary miRNA transcripts are self-complementary (double stranded)

dsRNAs (double stranded)

Complementary RNA strands, including miRNAs and double stranded RNA used experimentally in RNA interference (RNAi)

Dicer

An enzyme that cuts double-stranded RNAs (dsRNAs) to produce single-stranded guide RNAs

• usually 22 nucleotides long

• will bind to argonaut to form complex RISC

Argonaute (Ago) proteins

Proteins that bind to the single-stranded guide RNAs produced by dicer to form the RNA-induced silencing complex (RISC)

RISC (RNA induced silencing complex)

Complex formed when guide RNAs bind to argonaut proteins, will bind to mRNAs with complimentary to the guide RNA

• mRNA silenced by:

  • digestion of mRNA

  • blocking translation of mRNA

RNA dependent RNA polymerase (RdRP)

Enzyme that creates a new complementary RNA using RISC-bound mRNA as a template

• Results in more dsRNA to amplify gene silencing effect

• RdRP pathway part of of miRNA mechanism and RNAi

RNAi (RNA interference)

Experimental use of double stranded RNA to regulate gene expression

• To silence a target gene

• Used in experiments or therapeutics

Homologous chromosomes

Copies of a chromosome present in a diploid cell (one from each parent)

• similar but not identical

  • may differ in alleles or may have structural variants

Synapsis

The pairing up of homologous chromosomes that occurs during prophase I of meiosis

Bivalent

The structure to describe a pair of homologous chromosomes during meiosis, come together during synapsis

Tetrad

The structure composed of 2 pairs of sister chromosomes

• Homologous pairs within the tetrad separate during anaphase I

• same as bivalent but emphasis on the chromatids (4)

Crossing over

Process that occurs during synapsis, where non sister chromatids exchange genetic material → recombination between linked loci

Chiasma (pl. chiasmata)

Observable regions on non-sister (homologous) chromatids where crossing over occurs 

Polar bodies

Cells produced during gamete maturation in females (ovum maturation)

Ploidy (n)

Indicates number of chromosomes in a single, haploid copy of the genome 

• used with a multiplier

Haploid (1n)

Cells that have only a single copy of each chromosome

• gametes, after meiosis II and cytokinesis

Diploid (2n)

Organisms or cells that have two copies of the genome 

• somatic cells after mitosis, gametes after replication but before meiosis

Gene

A segment of DNA that is responsible for one or more phenotypes, a unit of inheritance

Allelic segregation (Mendel’s first law)

The principle that the two alleles for a trait segregate with equal probability into gametes during meiosis

Independent assortment (Mendel’s second law)

Two or more traits are inherited independently of each other (ex: dihybrid cross)

Test cross

Cross typically used to identify the genotype of an individual expressing a dominant trait by mating it with a homozygous recessive individual

How to generate variation in meiosis

• Independent assortment (2ⁿ, n = haploid number)

• Recombination, crossing over

Mendel

Addressed question of why offspring resemble their parents and how transmission of traits occurs….

To answer:

• Blending inheritance

• Inheritance of acquired characteristics

  • Used pea plants as model organism

    • Could control matings

    • Traits easily recognizable

Mendel’s Postulates

1. Alleles (unit factors) exist in pairs

2. For a pair of unit factors one is dominant and the other recessive

3. The paired unit factors (of the same gene) segregate independently during gamete formation

4. Law of independent assortment

Complete dominance

Phenotype of hetero is identical to phenotype of dominant homozygote (normal)

Partial (incomplete) dominance

Heterozygote exhibits a phenotype that is intermediate (blend) of two homozygotes

Codominance

Both alleles are simultaneously and fully expressed in the phenotype of the heterozygote

• ex: AB blood type

ABO blood group

Example of multiple alleles where three alleles exist at a single locus, ex of com dominance and complete dominance

Haploinsufficient 

Describes a gene where a single functional copy of the allele is insufficient to produce the wild-type phenotype

• often results in a dominant mutant phenotype

Loss of function mutation

A mutation that reduces or eliminates the normal function of a gene product due to two alleles encoding non-functional proteins

• often recessive 

Gain of function mutation

A mutation that causes a gene to do something extra/new, causes the protein to be active when it should be inactive

• These mutations often dominant

Hemizygous

A genotype in which only one copy of a gene is present

• Ex: genes located on the single X chromosome in human males

Compound heterozygotes

Individuals who have two different mutant alleles (different base pair changes) at a given locus

Autosomal dominant

Mode of inheritance where the trait is controlled by a gene on an autosome, only 1 copy of allele is required for the phenotype to be expressed

Autosomal recessive

Mode of inheritance where trait is controlled by a gene on an autosome, two copies of allele are required for the phenotype to be expressed

Incomplete penetrance

When an individual has the genotype for a trait but does not express the corresponding phenotype

Variable expressivity

When individuals with same genotype exhibit a range in the severity or expression of a phenotype

Chi-square test

Stat test used to compare observed genetic data (offspring ratios) to expected results from hypothetical model

Product rule 

Used to calculate probability of two or more independent events occurring together 

• ex: segregation of alleles at two diff loci

• AND rule

Sum rule

Used to calculate probability of any one of two or more mutually exclusive events occurring 

• OR rule

p < 0.05

Reject the null hypothesis, difference between observed and expected results is substantial

• 0.05 bc value is at least two standard deviations from the expected mean

Recessive lethal alleles

Mutations where lethality requires two (homozygous) copies of the allele

• can still have a dominant phenotype in heterozygotes

Dominant lethal alleles

Alleles that cause death even in heterozygotes

• Only maintained in a population if they impact survival after reproductive maturity

• ex: Huntington’s disease

Pleiotropy

Influence of one gene on multiple traits

• ex: frizzle mutation in chickens effects feather shape, body temp, blood flow, etc.

Antagonistic pleiotropy

When some phenotypic effects increase fitness, while others decrease it

Gene interaction

When genes at multiple loci collectively determine a single phenotype 

• ex: A- and B- blood antigens are produced from substance H, made by enzyme FUT1

Epistasis

Phenotype of one locus (earlier) masks/prevents the phenotypic expression of a second locus (acting later)

• How to recognize: 

  • Focused one trait with 2 to 4 character states

  • F2 phenotypic ratios are fractions of 16

Epistasis in squash example

Duplicate recessive epistasis

Homozygous condition of either recessive allele masks the expression of the dominant allele at the other locus

Epistasis combines categories from the og 9:3:3:1 ratio

Genetic background

Influence of numerous weak epistatic interactions (between multiple genes) that affect a single phenotype

Genetic suppression

mutation that makes the phenotype look more wild-type (less mutant) by overriding another mutation

Genetic enhancement

Mutation that makes the phenotype more mutant by increasing the severity of another mutation, complementation

Complementation analysis (test)

Crosses used to determine if two independent mutations causing a similar phenotype are alleles of the same gene

• only works for recessive alleles

• inbred (homozygous) stocks of each mutation are crossed

Complementation

Occurs when crossing two homozygous mutants yields a wild-type phenotype in the F1 descendants, indicating mutations are in different genes

Complementation group

A group of mutant strains that fail to complement one another, all members of the groups are understood to have mutations in the same gene

Allelic mutations

Two mutations that cause a similar phenotype and reside in the same gene

Maternal effect inheritance

An inheritance pattern where the offspring’s phenotype is determined by the genotype of the mother, regardless of the offspring’s own genotype

Sex-influenced trait

Trait where the phenotype is modulated/influenced by the sex of the individual

• loci responsible may be linked to autosomes or sex chromosomes

Reciprocal cross

Pair of crosses used to test the influence of parental sex on offspring phenotypes, often shows different outcomes for sex-linked genes

Isogametic

Gametes with multiple mating types that are biochemically compatible or not

• similar in size and form, can’t be distinguished as male (smaller) or female (larger)