Genetics Exam 2

Tetrad stage

occurs during prophase I of meiosis, when homologous chromosomes pair up and form a structure called a tetrad (consisting of four chromatids)

Tetrad

group of 4 chromatids formed when 2 homologous chromosomes align closely

Synapsis

the alignment essential for genetic recombination

linkage

The condition in which genes are present on the same chromosome, causing them to be inherited as a unit, provided that they are not separated by crossing over during meiosis

True

True or False: The chromosome is the unit of transmission during meiosis, NOT the gene

Recombination

crossing over or reshuffling of alleles (always occurs in the tetrad stage)

Frequency of Crossing Over between any 2 loci

= the distance between them (map units)

Recombinant/Crossover gametes

new allele combinations created from crossing over

Non-crossover gametes

2 chromatids not involved in crossing over

Complete linkage produces only…

Parental gametes

Complete linkage Ratio

1:2:1

(Usually observed only when genes are very close together and the number of progeny is relatively small)

Test Cross with F1 produces…

1:1 ration of thin brown and heavy, red flies

Linkage Group

each chromosome represents 1 linkage group

Thomas Hunt Morgan

discovered genetic linkage and recombination

Most offspring showed parental traits when…

When crossing females with mutant yellow body (y) and white eye (w) alleles with wild-type males

Discovery of Crossing Over

Morgan proposed that homologous chromosomes form chiasmata during meiosis, where genetic exchange (crossing over) occurs

Morgan concluded…

• Genes are arranged linearly on chromosomes.

• The likelihood of crossing over depends on the distance between genes: closer genes recombine less frequently.

Sturtevant

• Sturtevant defined 1 map unit (mu) = 1% recombination.

• He created the first genetic map, later expanded to include five X-linked genes.

• He and Calvin Bridges also showed that autosomal genes exhibit linkage and crossing over.

Close loci

less likely to recombine

Distant loci 

more likely to recombine 

Morgan and Sturtevant’s work confirmed…

the chromosomal theory of inheritance

Theoretically… when two linked genes are more than 50 mu apart…

a crossover is expected to occur between them in 100 percent of tetrads

-each tetrad would yield equal proportions of the four gametes

To calculate the crossover percent…

product rule is used

The gene in the middle

found by comparing DCO to parental types, it’s the only gene that switches

Finding map distance between genes

Requires you to add up SCOs and DCOs that include two genes to find map unit between those 2

Determining Gene Order

• Assuming any one of the three orders, first determine the arrangement of alleles along each homolog of the heterozygous parent giving rise to noncrossover and crossover gametes (the female in our example).

• Determine whether a double-crossover event occurring within that arrangement will produce the observed double-crossover phenotypes. Remember that these phenotypes occur least frequently and are easily identified.

• If this order does not produce the predicted phenotypes, try each of the other two orders. One must work!

The farther apart two genes are…

the greater the probability that undetected crossovers will occur

Most accurate maps are constructed when…

Genes are close together

Interference

the phenomenon through which a crossover event in one region of the chromosome inhibits a second event in nearby regions, causes this reduction.

-describes the extent to which a crossover in one region of a chromosome influences the occurrence of a crossover in an adjacent region of the chromosome and is quantified by calculating the coefficient of coincidence (C).

Coefficient of Coincidence

Observed DCO/Expected DCO

Restriction Fragment Length Polymorphisms (RFLPs) and Microsatellites

the earliest examples of DNA markers

Bacteriophages

viruses that use bacteria as their hosts

Minimal medium

organic carbon source, variety of ions/organic salts

-bacteria that synthesize all essential organic compounds (amino acids, purines, pyrimidines, vitamins, and fatty acids) grow on these mediums=prototroph

Auxotroph

bacteria that loses the ability to synthesize one or more organic compounds through mutation

Prototroph

bacteria that synthesizes all needed organic compounds

3 processes of genetic recombination in Bacteria

conjugation, transformation, transduction

Vertical gene transfer

when genetic information occurs between generations of the same species

Horizontal Gene Transfer

transfer between unrelated cells

Conjugation

a process by which genetic information from one bacterium is transferred to and recombined with that of another bacterium

F+ Cells (F+ = fertility)

donate parts of chromosome

F- cells

recipient bacteria that receive the donor chromosome material and recombine it with part of their own chromosome

Hfr cells

-special class of F+ cells

-exhibit high frequency of recombination

Interrupted Mating Technique

allows you to determine the order of genes transferred

-used for mapping in time units

Plasmids (ex. F. factor)

autonomously replicating DNA molecules found in the bacterial cytoplasm, sometimes containing unique genes conferring antibiotic resistance as well as the genes necessary for plasmid transfer during conjugation.

Transformation

small pieces of extracellular DNA are taken up by a living bacterium, potentially leading to a stable genetic change in the recipient cell

-another mechanism for recombining genetic information in some bacteria

competence

the transient state or condition during which the cell can bind and internalize exogenous DNA molecules, making transformation possible.

Bacteriophage T4

-E. coli host

-intricate structure

-DNA encased in icosahedral protein coat

-head connected to a complex tail structure consisting of a collar

-base plate of 15 proteins

Plaque Assay

way to study how bacteriophages reproduce in host cell

Transduction

Virally mediated bacterial recombination. Also used to describe the transfer of eukaryotic genes mediated by a retrovirus.

For a molecule to serve as genetic material it must possess 4 main characteristics:

-replication

-storage of information

-expression of information (central dogma)

-variation by mutation

Proteins

initially favored to serve as the genetic material

Griffith Experiment

-concluded that heat-killed IIIS bacteria somehow converted live avirulent IIRcells into virulent IIIS cells

-called the process transformation

The Hershey-Chase Experiment

-E. coli bacteria

-T2 bacteriophage

-composed of protein coat and DNA core

-goal was to determine whether DNA or protein carries the genetic instructions for viral replication

-DNA carried the genetic info

Retroviruses

replicate by using RNA as a template for the synthesis of a complementary DNA molecule (reverse transcriptase)

Pyrimidine Ring

Purine Ring

Cytosine

Uracil

Thymine

Guanine

Adenine

Ribose

Deoxyribose

Nucleoside

nucleotide without phosphate group

Phosphodiester bonds

covalent bonds

create the backbone of nucleic acid molecules

The phosphate group connects two sugar molecules in adjacent nucleotides

DNA consists of one ___ strand and one ____ strand

old;new (semiconservative)

Indirect evidence that DNA is the genetic material

-DNA is sequestered in the nucleus, protein is found everywhere in the cell

-DNA content of haploid cells is consistently approximately half that of diploid

-UV light is most mutagenic at of wavelength f 260nm at which nucleic acids, DNA and RNA, strongly absorb

Direct evidence that DNA is the genetic material

-Recombinant DNA molecules can confer heritable information responsible for a product of a gene

-Genomics has shown that differences in DNA sequences can explain specific traits and heritable disorders

When calculating number of double crossovers…

Use product law for gene map units then multiply by number of progeny

Calculating DCO with interference

Interference=1-C

C= observed DCO/expected DCP