MBG 2040 Lecture Notes - Lecture 4: Homologous Recombination, Sister Chromatids, Punnett Square
2 May 2018
School
Department
Course
Professor
Linkage and Recombination
Outline:
• Linkage, recombination and crossing over
• Chromosome mapping
• Cytogenetic mapping
• Linkage analysis in humans
• Recombination and evolution
Learning Outcomes:
1. Define, recognize and apply Mendel's second principle
2. Be familiar with the calculation of genetic distance of linked genes
3. Use the gene location on a chromosome to understand and predict cross results
4. Understand and identify whether alleles are in a cis or trans configuration
5. Track lined genes through pedigrees
6. Use our probability rules to assign genotypes and gene configurations of linked genes in pedigrees
Linkage revealed the extremely important process of recombination
• Linkage and recombination made possible the mapping of genes in practically all eukaryotic
organisms
Linkage, Recombination, and Crossing Over:
• Genes that are on the same chromosome travel through mitosis/meiosis together
• However, alleles on the same chromosome can be recombined between the chromatids by crossing
over
• The concept of linked genes: T.H. Morgan & A.H. Sturtevant
o Observations: some genes do not segregate randomly (as per Mendel's principle)
o Morgan:
• Genes are "linked" if present on the same chromosome
• The divergence from linkage is called recombination
o Strutevant:
• The frequency of recombination reflects distance between genes
Mesemerising Meiosis and Co-segregation:
• Option 1
o Meiosis I:
• Separate the homologous chromosomes: AA & aa; BB & bb
• Following two chromosome pairs, now you can see how independent assortment
produces many variations in gametes
o Meiosis II:
• Just like mitosis
• Separate the sister chromatids
• Gametes: A&B; A&B; a&b; a&b
• Option 2
o Meiosis I:
• Separate the homologous chromosomes: AA & aa; BB &bb
o Meiosis II:
• Gametes: A&b; A&b; a&B; a&B
• *a 4x4 Punnett square will determine probabilities
• Linkage: co-segregation of genes on the same chromosome
o Ex. Homologous chromosomes (AaBb):
find more resources at oneclass.com
find more resources at oneclass.com
• One has AA & bb, other has aa & BB
• Meiosis I:
▪ Paternal - AA & bb
• Gametes: Ab, Ab
▪ Maternal - aa & BB
• Gametes: aB, aB
• Possibilities:
▪ Person A: Ab, Ab, aB, aB
▪ Person B: Ab, An, aB, aB
Recall: Mendel's Law of Independent Assortment
• The inheritance pattern of one trait does not affect the inheritance pattern of another trait (9:3:3:1)
• The classical experiment of Bateson and Punnett in peas:
o Some pairs of genes DO NOT segregate independently --> 24:1:1:7
o Ex. Red flowers with long pollen grains X white flowers with short
• If R and L are linked, we expect at 3(RL):1(rl)
• If they are not linked, 9(RL): 3(rl): 3(Rl): 1(rl)
• --> we get 24:8 (3:1)
▪ Possible explanations:
• the R and L genes are linked on the same unit of heredity (chromosomes),
therefore RL and rl alleles segregate as linked genes are give close to a 3
RL: 1 rl ratio
• Infrequent exchange between chromosomes that harbor RL and rl produces
the deviations from this ratio
• = exchange recombination
• We can calculate the frequency of recombination by scoring the Rl and rL phenotypes
▪ 920 parental (RL or rl) ; 80 recombinant (rL or Rl)
▪ Frequency of recombinants = 80/(80+920) = 0.08
▪ Note: this frequency will be different for different pairs of genes
Homologous Recombination
• Homologous recombination occurs during prophase I of meiosis
• Similar to mitosis, a tetrad of two bivalents (4 homologous chromatids) is formed
• In Meiosis I, a chiasma (physical junction of the DNA of the homologous chromatids) is formed
o The chiasma is the site where recombination occurs
• *see slide: four products of meiosis…ABAB & abab
o Non-recombinant chromosome: AB
o Recombinant chromosome: Ab
o Recombinant chromosome: aB
o Non-recombinant chromosome: ab
• Multiple crossovers can be simultaneously taking place over the same tetrad
o *see slide
• Double crossover
• Triple crossover
• Quadruple crossover
• A cross over between sister chromatids
• Early Evidence for Crossing over
o *see slide
o The first experiment involved two chromosomes 9 in maize
o The "abnormal" one contains distinguishable features: a "knob" and "tail"
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Outline: linkage, recombination and crossing over, chromosome mapping, cytogenetic mapping, linkage analysis in humans, recombination and evolution. Linkage revealed the extremely important process of recombination: linkage and recombination made possible the mapping of genes in practically all eukaryotic organisms. Linkage, recombination, and crossing over: genes that are on the same chromosome travel through mitosis/meiosis together, however, alleles on the same chromosome can be recombined between the chromatids by crossing over, the concept of linked genes: t. h. Mesemerising meiosis and co-segregation: option 1, meiosis i, separate the homologous chromosomes: aa & aa; bb & bb, following two chromosome pairs, now you can see how independent assortment produces many variations in gametes, meiosis ii: Red flowers with long pollen grains x white flowers with short. If r and l are linked, we expect at 3(rl):1(rl) If they are not linked, 9(rl): 3(rl): 3(rl): 1(rl)
