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Using a linkage analysis, geneticists discovered male pattern baldness was associated with
SNPs on the p-arm of the X chromosome
What are linkage groups?
Groups of genes that are physically connected, They can have different alleles and crossing over breaks up linkage groups
Linked genes far apart vs linked genes close together
Genes far apart appear to segregate independently whereas genes close to each other will not assort independently
Who discovered two genes that did not assort independently?
William Bateson and Reginald Punnett
Who were the scientists involved in the race to prove recombinations?
Harriet Creighton and Barbara McClintock vs Curt Stern
What is the purpose of genetic mapping?
To determine the linear order of linked genes along chromosomes. Each gene has its own unique locus
What is the purpose of genetic maps?
Estimates the relative distance between linked genes.
Why is the distance between genes important?
Recombination frequencies are correlated with the distance between genes.
Far apart genes=Many recombinant offspring
Close Genes=Very few recombinant offspring
One map unit is equivalent to
1% recombination frequency
Genetic mapping experiments are typically accomplished by a test cross. What is a test cross?
Mating between an individual that is heterozygous for multiple genes and one that is homozygous recessive for the same genes
A test cross is expected to yield a maximum of only 50% recombinant offspring, Why?
1.If genes exhibit 50% recombination, the most that can be said is they belong to different linkage groups
2.Distance between genes far apart on the same chromosome tend to be underestimated because of double cross over events
Construct a genetic map based on the following data
Answer
What is coupling?
Where one chromosomes contains both wild-type alleles or both mutant alleles
What is repulsion?
Wild type alleles are found on opposite chromosomes
Double recombinant only have which gene altered?
The middle gene
What is interference?
Crossovers are not independent events because one crossover inhibits the other
What is positive interference?
The first crossover decreases the possibility that a double crossover event will happen nearby
Levels of recombination among species
There is 2x more recombination in humans than rodents
Levels of recombination among chromosomes do a single species
Chromosomes 21 and 22 have highest recombination rates; 2 and 4 have lowest
Recombination hot spots
Appear to be associated with trinucleotide repeats
In humans, tend to be near, but not in, active genes
Areas near centromeres often have reduced rates
Levels of recombination among sexes
Female tend to have higher recombination than males
What is synteny?
Refers to genes that are physically located on the same chromosome
What are syntenic blocks?
Refers to the evolutionary conservation of syntenic genes (genes that have remained together during evolution)
When recombination rate is high, linkage blocks will be
Short
When recombination rate is low, linkage blocks will be
Long
What are deletions?
The loss of a chromosomal segment
What are duplications?
The repetition of a chromosomal segment compared to the normal parent chromosome
What are inversions?
A change in the direction of part of the genetic material along a single chromosome
What are translocations?
A segment of one chromosome becomes attached to a different chromosome
What are effects of deletions?
1.If deletion includes centromere, chromosomes will not attach to microtubules and will be lost.
2.deletions are lethal in the homozygous state
3.individual heterozygous for a deletion causes an imbalance in gene dosage (pseudodominance can occur)
4.Haploinsufficiency
What is pseudodominance?
Expression of a normally recessive gene
What is haploinsufficiency
When the single copy of a gene is not enough to produce wild-type phenotype
How do duplications form?
Abnormal events during recombination, by repetitive sequences or misaligned crossovers
What is a pericentric inversion?
Inversion involving the centromere
What is a paracentric inversion?
Inversion that does not involve the centromere
What is the break point effect?
An inversion break point occurs in a vital gene
What is the position effect
A gene is repositioned in a way that alters its gene expression
What is a reciprocal translocation?
A two-way exchange of segments
What is simple translocation?
Genetic material moves from one chromosome to another without trading
What is a robertsonian translocation?
Long arm of two acrocentirc chromosomes become joined, creating chromosome with two long arms and one with two short arms.
How often does a robertsonian translocation occur?
1 in 900 births
Note
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