D. Recombination-reassortment of genes into different combinations from those of the parents
Crossing over of homologous chromosomes Chiasmata formed during a synapsis
Crossover Results A genotype HE B genotype He C genotype hE D genotype he Start A genotype HE B genotype HE C genotype he D genotype he Crossover B and C become recombinants A B C D H E h e A B C D H E h e A B C D H E h e
Punnett square-shows possible outcomes from a test cross
Mendel studied characteristics of pea plants
Wrinkled and round peas (round peas are dominant)
Gregor Mendel’s Findings Dwarf Tall Length of stem 7. Terminal Axial Position of flowers 6. Yellow Green Color of unripe pods 5. Constricted Inflated Form of ripe pods 4. White Grey Color of seed coat 3. Green Yellow Color of seed albumen 2. Wrinkled Smooth Form of ripe seed 1. Recessive Expression Dominant Expression Trait
States: The separation of the pair of parental factors, so that one factor is present in each gamete. (This is how it is written in the IB book.)
The two alleles for each characteristic segregate during gamete production. This means that each gamete will contain only one allele for each gene. This allows the maternal and paternal alleles to be combined in the offspring, ensuring variation. (This is from wikipedia.)
Any combination of chromosomes is possible in metaphase I (there is no ‘master plan’ for the order that they line up on the metaphase plate before separation)
Mendel thought all genes were inherited separately and had no relationship
-Ex: Pea plants could be green or yellow and wrinkled or round. Shape and color had nothing to do with each other, because the genes are on separate chromosomes. Any combination could have been produced (wrinkled/green, wrinkled/yellow, round/green, round yellow)
C. This is demonstrated in Dihybrid crosses.
D. Today we realize that there are many exceptions to this law.
A. Any one of a pair of characteristics may combine with either one of another pair
gene=shape of pea
alleles=round (R) or wrinkled (r)
gene=color of pea
alleles = yellow (Y) or green (y)
*When crossing two plants that are heterozygous for both traits the offspring will show all combinations. This shows that genes for shape and color are independent (unlinked).
Dihybrid crosses Parent genotypes: SsYy x SsYy -S=smooth s=wrinkled -Y=Yellow y=green Possible allele combos from one parent ssyy ssYy Ssyy SsYy sy ssYy ssYY SsYy SsYY sY Ssyy SsYy SSyy SSYy Sy SsYy SsYY SSYy SSYY SY sy sY Sy SY
Possible ratios for SsYy x SsYy F1 Genotypic ratios 1:SSYY 2:SSYy 1:SSyy 2:SsYY 4:SsYy 2:Ssyy 1:ssYY 2:ssYy 1:ssyy F1 Phenotypic ratios 9: smooth-yellow 3:smooth-green 3: wrinkled-yellow 1: wrinkled-green ssyy ssYy Ssyy SsYy sy ssYy ssYY SsYy SsYY sY Ssyy SsYy SSyy SSYy Sy SsYy SsYY SSYy SSYY SY sy sY Sy SY
Perform a test cross for P generation: SSYY x ssyy
All F1 generation offspring are heterozygous (SsYy). What will the outcome be if you cross two individuals from F2? SsYy SsYy SsYy SsYy Sy SsYy SsYy SsYy SsYy Sy SsYy SsYy SsYy SsYy sy SsYy SsYy SsYy SsYy sy SY SY SY SY
Figure 3. Comparison of known gel results for normal hemoglobin (AA), sickle cell disease (SS) and sickle cell trait (AS). S represents the molecular size marker. What results are present in the lanes marked with a question mark?