Genetics Final

Linkage, Gene Mapping, Mendelian Genetics

Linkage

Genes located on the same chromosome

Linked Genes and Independent Assortment

linked genes DO NOT assort independently

Complete Linkage

NO crossing over; ONLY parental gametes produced

Incomplete Linkage

crossing over DOES occur; parental > recombinant

Parental Non-Recombinant

same allele combination as the parents

Recombinant

allele combinations produced by crossing over

Recombination Frequency

proportion of recombinant offspring

Recombination Frequency Formula

RF = # of recombinants/total progeny

RF = 0.3602

• 36.02% of offspring are recombinant

• the genes are 36.02 cM apart

• crossing over occurs 36% of the time

Maximum Observable RF

Max RF = 50%; if RF > 50%, assortment as if unlinked

Two-Point Mapping

1. perform a testcross (het x hom. recessive)

2. identify parental and recombinant classes

3. calculate RF

4. convert RF to map distance in cM

Three-Point Mapping

• determine gene order

• determine map distance

• detect double crosses (DCOs)

• detect single crosses (SCOs)

1. identify parental and DCOs

2. parental vs DCO → middle gene flips

3. calculate distances

• distance AB = (SCAB + DCO)/total

• distance BC = (SCBC + DCO)/total

1. draw gene map

Incomplete Dominance

intermediate heterozygote phenotype (1:2:1)

Codominance

both alleles are expressed (e.g. AB blood type)

Multiple Alleles

ABO Blood System

A: IA, AO

B: IB, BO

AB: IA, IB

O: OO

Haplosufficient

one functional allele provides enough gene product for a normal (WT) phenotype

Haploinsufficient

one functional allele is NOT ENOUGH; you need BOTH COPIES for normal function (recessive)

Pleiotropy

one gene has many effects

• a single gene can affect multiple phenotypes because the product is used on several tissues, pathways, or developmental stages

Silent Mutation

DNA change that DOES NOT affect the amino acid sequence due to genetic redundancy

Missense Mutation

DNA change that DOES CHANGE the amino acid sequence

EX: GAG → GTG changes from Glutamate to Valine

Nonsense Mutation

DNA change that causes a premature STOP codon

• proteins become dysfunctional because they are too short

Insertions and Deletions

small addition or loss of nucleotides (NEVER MULTIPLES OF 3)

Frameshift Insertion

• insert q nucleotide

NORMAL: ATG-AAA-GGC-…

NEW: ATG-GAA-AGG-C…

Frameshift Deletion

• remove 2 nucleotides

NORMAL: ATG-CCC-ACT…

NEW: ATC-CAC-…

In-Frame Indel

adds or removes 3 NUCLEOTIDES → adds or removes ONE AMINO ACID (no shift)

Conditions of Hardy-Weinberg

1. no mutation

2. no gene flow (migration)

3. large population size

4. random mating

5. no natural selection

HWE Formulas

p + q = 1

p² + 2pq + q² = 1

p = A frequency

q = a frequency

p² = AA frequency

q² = aa frequency

2pq = Aa frequency

HWE Calculation Steps

1. find q

2. find p

3. calculate genotype percentages