Lecture 5: Chromosome Mapping Flashcards
In Meiosis, what is the unit of transmission?
Chromosomes! Crossing over takes place on the arms of the chromosomes.
NOT GENES!
Explain Linked Genes
- Can NOT undergo independent assortment
- two or more genes located close together on the same chromosome, which means they are likely to be inherited together due to their proximity
Frequency of crossing over on a single chromosome is proportional to?
The distance between them
Crossing over results in?
RECOMBINATION
Define Chromosome maps
Indicate relative location of genes on chromosome
What are the 3 Meiotic Consequences
- Independent Assortment: No linkage exhibited
- Linkage WITHOUT crossing over: Complete Linkage
- Linkage WITH crossing over: Generates recombinant (crossover) gametes
Define Complete Linkage
- No crossing over between two genes
- Produces parental (non-crossover) gametes
Define Crossing over
- Occurs between two non-sister chromatids
- Both parental and recombinant (crossover) gametes are produced
Define Chiasmata
- Synapsed chromosomes in meiosis wrap around each other
- X shaped intersections with points of overlap
- Points of genetic exchange
Percentage of offspring resulting from recombinant gametes depends on
distance between two genes on same chromosome
T/F: Two genes located close to each other along a chromosome are less likely to have chiasma
TRUE!!!
Explain Single Crossover
- Occurs between two
non-sister chromatids - Recombination is
observed in 50 percent
of gametes - In genes 50 map units
apart, crossing over can
be expected between
100 percent of tetrads[{{type}} Annotation]
What is centimorgans on a gene?
Distance between two genes in map units
What are Single Crossovers used to determine?
Used to determine distance between two linked genes
Explain Double Crossover
- Double exchanges of genetic material
- Used to determine distance between three linked genes
- Genes must be heterozygous for two alleles
Explain Somatic Cell Hybridization
- Made possible the assigning of human genes to their respective chromosomes
- Involves fusing two cells into a single hybrid cell: heterokaryon
Define Synkaryon
Heterokaryons cultured in vivo—nuclei are fused together
What are DNA markers?
- SHORT segments of DNA with KNOWN sequence and location
- Useful landmarks for mapping
- Earliest examples of DNA markers
What are RFLPs: Restriction fragment length polymorphisms?
- Polymorphic sites (different lengths)
- Generated when specific DNA sequences are recognized and cut by restriction enzymes
What are Microsatellites?
- Short repetitive sequences
- Found throughout genome
Explain SNPs: Single-nucleotide polymorphisms (in non-sister chromatids)
- Found throughout genome
- Used by geneticists to identify and locate related genes
- Used to screen for diseases
Ex.) Cystic fibrosis
Ex.) Sickle-cell anemia with a one-base chain
Define Cystic fibrosis
- Gene located by using DNA markers
- Life-shortening autosomal recessive exocrine disorder
- Gene causing disorder found on chromosome 7
Explain Sister chromatid exchanges
- Reciprocal exchanges similar to crossing over
- Between sister chromatids (crossing over is between NON-sisters)
Explain Harlequin chromosomes
- Sister chromatids involved in mitotic exchanges
- Patch-like appearance when stained and viewed under a microscope
Describe some agents that INDUCE chromosome damage
- Viruses
- X-rays
- UV
- Mutagens
- Increase frequency of sister chromatid exchange in Bloom syndrome
Explain Bloom syndrome
- Human disorder
- Caused by mutation in BLM gene
chromosome 15 - Prenatal and postnatal retardation of growth
- Excessive SCEs
- Many translocations
Explain BLM gene
- Encodes enzyme DNA helicase
- DNA helicase’s known role is DNA
replication - Not known how it causes Bloom Syndrome
