Genetics Lab Exam 2

Meiosis in Drosophila

Cross over only occurs in female flies

Linkage of genes

shown by genes on the same chromosome

when offspring numbers don’t match independent assortment ratio

Independent Assortment of genes

shown by genes on different chromosomes

50% recombination frequency

Linkage Group

group of genes on the same chromosome

number of these linkage groups should correspond to the haploid number of chromosomes

drosophila= 4 linkage groups

Law of Independent Assortment

Each possible gamete is equally likely (1:1:1:1)

Alleles segregate independently of each other when forming gametes

AaBb x aabb→ ¼ AaBb ¼ Aabb ¼ aaBb ¼ aabb

Complete linkage

When two genes that are in close proximity are mated, no independent assortment occurs and all the offspring will have a parental combination of alleles

Only parental (noncrossover) gametes are produced

AaBb x aabb→ ½ AaBb ½ aabb

Crossing over between two linked genes occurs between two nonsister chromatids

Both parental (noncrossover) and recombinant (crossover) gametes are produced

Proportion of parental to nonparental gametes will depend on the distance between the linked genes

% recombination = map unit distance

Interlocus distance or Map Units

distance between two loci on a single chromosome

proportional to the degree of crossing over

Chiasma

point of genetic exchange (crossing over)

created by synapsed chromosomes in meiosis that wrap around each other

less likely to form between two genes that are relatively close to each other

Double crossovers (DCOs)

used to determine the order of 3 genes on the chromosome

Single crossovers

used to determine the distance between 2 linked genes

3 criteria of three-point mapping

• parent must be heterozygous for all three genes under consideration

• phenotypic class must reflect genotype of gametes of parents

• sufficient number of offspring must be produced for representative sample

Noncrossover F₂ Phenotypes

occur in the greatest proportion of offspring

(largest offspring numbers)

Double-crossover F₂ Phenotypes

occur in the smallest proportion of offspring

(lowest offspring numbers)

Reciprocal classes of phenotypes

When F₂ phenotypes complement each other (ex. one is wild type and the other is mutant for all three genes)

Interference (I)

I = 1-C

Positive Interference= fewer DCO events than expected occur

Negative Interference= more DCO events than expected occur

Coefficient of coincidence (C)

the observed number of DCOs divided by the expected number of DCOs

expected frequency= multiply the recombination frequencies

Lod score analysis

relies on probability calculations to demonstrate linkage between two genes in organisms in which linkage analysis relies primarily on pedigrees.

This analysis is limited by the availability of pedigree data

Somatic cell hybridization

involves fusion of two cells in culture to form a single hybrid cell, called a heterokaryon.

Upon continued culturing of the hybrid cell, chromosomes from one of the two parental species are gradually lost until only a few chromosomes of one species remain and most chromosomes are from the other species, creating a synkaryon.

Synteny testing

A panel of cell lines, each containing just a few human chromosomes

the presence or absence of a specific gene product is correlated with the presence or absence of each chromosome

DNA markers

Short segments of DNA with known sequence and location

Map landmarks along the chromosome.

Recombination frequency is the basis for genetic maps.

Actual base-pair distance represents a physical map.

We can correlate inheritance of traits like diseases to inheritance of markers - indicating they are linked (located near each other)

RFLPs: Restriction fragment length polymorphisms

Polymorphic sites

Generated when specific DNA sequences are recognized and cut by restriction enzyme

Microsatellites

Short repetitive sequences

Found throughout genome

SNPs: Single-nucleotide polymorphisms

Found throughout genome

Used by geneticists to identify and locate related genes

Often used in forensics analysis

Used to screen for diseases

Example: Cystic fibrosis

Mapping cross requirements with Drosophila

require female flies that are heterozygous for the genes to be mapped and male flies homozygous for the recessive allele of the genes

Drosophila Experiment Parental Generation

homozygous mutant females y w m / y w m

red, tan, long

x

homozygous wildtype males + + + / — (Y)

white, yellow, mini

Drosophila Experiment F1 Generation

heterozygous females + + + / y w m

x

homozygous recessive males y w m / — (Y)

Recombinant phenotypes for body color and eye color

yellow red & tan white

Recombinant phenotypes for body color and wing length

yellow long & tan mini

Recombinant phenotypes for eye color and wing length

white long & red mini

Population

a group of individuals with a common set of genes that lives in the same geographic area and can or does interbreed

Population’s Gene Pool

all of the alleles present in that population

due to population dynamics, the gene pool can change over time

Hardy-Weinberg Law predictions

1. the frequency of the alleles in the gene pool does not change over time (p+q = 1)

2. after one generation of random mating, the genotype frequencies for two alleles can be calculated as p² + 2pq + q² = 1

p

frequency of dominant allele A

q

frequency of recessive allele a

p²

frequency of AA genotype (homozygous dominant)

q²

frequency of aa genotype (homozygous recessive)

2pq

frequency of Aa genotype (heterozygous)

Key assumptions for Hardy-Weinberg equilibrium

1. no natural selection

2. no mutation

3. no migration

4. population is infinitely large

5. random mating

Predictions based on H-W equilibrium

• Dominant traits do not necessarily increase from one generation to the next

• Genetic variability can be maintained

• By knowing frequency of one genotype, frequencies of other genotypes can be calculated

Natural Selection

• Major force driving allele frequency change

• Chief mechanism for transforming populations

• Principal force that shifts allele frequencies within large populations

Genetic Drift

• Significant random fluctuations in allele frequencies in small populations

• Possible by chance alone

• Degree of fluctuation increases as population size decreases

• Can also occur as result of: Founder effect and Genetic bottleneck