Gen 4100 Exam 1 - Iowa State University

Mendel’s Law of Segregation

during the formation of gametes (sperm and egg cells), the two alleles for a given trait separate from each other, so that each gamete only receives one allele

Mendel’s Law of Independent Assortment

different genes are sorted into gametes independently. This means that the allele a gamete receives for one gene does not affect the allele it receives for another gene. 

Pedigree

a chart that shows the relationships between family members or animals

Testcross

a breeding experiment that determines an organism's genotype

Carriers

an individual who carries and is capable of passing on a genetic mutation associated with a disease and may or may not display disease symptoms

Complete Penetrance

an affected individual carries at least one dominant allele of a dominant trait or two recessive alleles of a recessive trait

Rare-In-Population

individuals who marry into the pedigree in the second and third generations; are not carriers

Not Y-Linked

the causative genes in these problems may be autosomal or X-linked but are not Y-linked

Recessive Trait

• a trait that is only expressed when both inherited copies of a gene are recessive

• any affected individual has two unaffected parents

• example: a → aa

Dominant Trait

• a characteristic that is passed down from a parent to their child, and is expressed when at least one dominant allele is inherited

• every affected child has an affected parent

• two affected parents can produce unaffected children

• example: Aa or AA

Autosomal Recessive

• a pattern of genetic transmission where a mutated gene is located on an autosome (non-sex chromosome)

• if any affected daughter has two unaffected parents

• no obvious bias between males and females

X-Linked Recessive

• a pattern of passing down genetic traits or disorders that are caused by mutations on the X chromosome

• when an affected son has two unaffected parents (daughter normally unaffected)

• traits appear more in males (mother carrier to son transmission)

Complete Dominance

• a genetic phenomenon where one allele of a gene completely masks the other allele

• hybrid resembles one of the two parents

Incomplete Dominance

• a genetic phenomenon where the phenotype of a heterozygote is a blend of the phenotypes of its parents

• hybrid resembles neither parent (intermediate phenotype)

Codominance

• a genetic pattern where two different versions of a gene (alleles) are both expressed in an individual

• hybrid shows traits from both parents

Pleiotropy

the phenomenon of a single gene/allele determining several distinct and seemingly unrelated characteristics. Occurs when one gene influences two or more seemingly unrelated phenotypic traits

Pleiotropic Genes - Recessive Lethal Alleles

• pairs of identical alleles that cause death when present in both copies in an organism

• can code for either dominant or recessive traits

• do not always cause death

Penetrance

the percentage of individuals with a particular genotype that show the expected phenotype

Novel Phenotypes

a unique visual characteristic of an organism that differs from its parents

Complementary Gene Action

occurs when two or more genes must work together to produce a specific phenotype

 Epistasis

an allele at one gene masks the phenotype of alleles at another gene

Hypostatic

one whose phenotype is altered by the expression of an allele at a separate locus in an epistasis event

Complementary Gene Action

a genetic interaction where two separate genes must both have a dominant allele present in order to produce a specific phenotype

Redundant Genes

multiple genes in an organism that perform the same function

How Do Mutations Effect Gene Expression and Function?

Alters the DNA sequence, potentially leading to changes in the produced protein, including: no protein production, a non-functional protein, or a protein with altered activity, which can result in various phenotypic effects depending on the gene involved, ranging from no noticeable change to severe disease states; the effects can include complete loss-of-function, gain-of-function, or altered regulation of gene expression depending on the nature of the mutation and its location within the gene

Loss-of-Function

• the function of a gene is reduced or eliminated

• this can be due to decreased expression or impaired protein function

• usually recessive

Gain-of-Function

• a gene acquires increased or new function

• this can be due to altered expression or altered protein function

• usually dominant

Types of Mutations

• insertional mutagens: fragment of DNA is inserted into a chromosome or near a gene

• substitution: the base is replaced by one of the other bases

• deletion: block of one or more DNA pairs is lost

• insertion: block of one or more DNA pairs is added

• chromosomal rearrangements: affect many genes at one time (inversion/translocation)

Loss-of-Function Mutations

• usually recessive to wild-type

• null (amorphic mutations): completely block the function of a gene product

• hypomorphic mutations: gene product has weak but detectable activity

Gain-of-Function Mutations

• usually dominant

• hypermorphic mutations: generate more gene product or the same amount of a more efficient gene product

• neomorphic mutations: generate gene product with a new function or that is expressed at an inappropriate time or place

Complementation Test

• a genetic procedure that determines if two mutations are in the same gene or different genes. It's used to understand how genetic strains interact

• a cross between two mutations in the same gene (usually) fails to complement (ex., mutant phenotype)

• a cross between two mutations in separate genes (usually) do complement (ex., wild-type progeny)

Types of Genetic Approaches

• CRISPR/Cas9

• transgenic reporter

• gene/Enhancer trap

• conditional mutants

• gene drive

CRISPR/Cas9

• gene-editing technology that uses RNA and a protein to modify DNA 

1. A guide RNA binds to a specific DNA sequence

2. The Cas9 protein follows the guide RNA to the DNA

3. The Cas9 cuts both strands of the DNA

4. The cell repairs the damaged DNA

• scientists use this process to change genes 

• it can be used in gene activation

• Can cause unwanted mutations and it’s not super efficient

Transgenic Reporter

a gene that produces a protein that can be used to identify a cell type, protein, or circuit

Gene/Enhancer Trap

• An insertion element (ex., transposon) contains a splice acceptor (SA) site and a reporter gene (GFP, LacZ, etc.) using CRISPR/Cas9

• Upon insertion into a gene, the reporter becomes spliced to an exon in the mRNA of the trapped gene

• The GFP reporter now shows the expression pattern of the trapped gene

• In addition, the trapped gene is also mutated

Gene Drive

a genetic system that promotes the inheritance of a particular gene to increase its prevalence in a population

Conditional Mutants

an organism with a genetic change that only results in a mutant phenotype under certain environmental conditions

Forward Genetics

•  the approach of determining the genetic basis responsible for a phenotype

• natural and induced mutations → phenotypic selection → mapping of candidate genes

• to identify mutants and genes that control a trait of interest through genetic screening

Reverse Genetics

• the approach to study the function of a gene by targeted mutation and phenotypic characterization

• mutations in a gene of interest → phenotypic analysis 

• to study the function of a gene of interest through mutational analysis