lec 10-1 Flashcards
Principles of segregation
Alleles of a gene (R,r) separate independently
Principle of independent assortment
Independent separation of non-homologous chromosomes
Crossing over during meiosis
During late prophase, after replication, homologous chromosomes come very close to each other to make crossing over possible
Crossing over
Exchange of DNA between non-sister chromatids
Result of crossing over
Gamete chromosomes have different combinations of alleles than parental chromosomes, thus mixing of alleles on same chromosome
Recombinant chromosomes/gametes/progeny
Is not the same as the parental
non-recombinant chromosomes/gametes/progeny
same as the parental
High frequency of crossing over means
Alleles of one gene would be inherited independently from many other genes
Not all genes are inherited
Independently
Some are linked
Recombination frequency depends on
How far away from each other two genes are on a chromosome
Recombination frequency is higher
The farther apart two genes are
Linkage group
Set of genes located closely together on the same chromosome and likely inherited together
Notation for linked genes
A B a b
——– X ——–
A B a b
Line separates two homologous chromosomes
alleles of the same gene aligned vertically
To test for linkage
Use a testcross with a plant that is heterozygous for both characteristics crossed to a plant that is homozygous recessive for both
Testing for linkage using a testcross
Two extreme possibilities
Genes are completely unlinked (different ends of the chromosome)
Genes are completely linked very close to each other physically
If alleles are completely un-linked
Crossing over will occur
Some gametes will be non-recombinant and recombinant (50-50 split theoretically)
Completely un-linked results are
The same if two genes are on different chromosomes (Mendel’s dihybrid experiments)
Testcross can not tell
If two genes are far apart on the same chromosome or on two different chromosomes
If alleles are completely linked
Crossing over will not occur between genes and so alleles for two genes stay together
All gametes will be non-recombinant
Completely linked means all progeny will be
Non-recombinant with 1/2 having geno/phenotype from each parent
Partially linked genes
Genes are spread all along the chromosome so many would be partially linked
Over many gametes the majority will be
Non-recombinant
Due to no crossing over making 100 percent non-recombinant while crossing over only makes 50 percent recombinant
Majority of progeny will be what when some crossing over happens
Non-recombinant
Max recombinant percentage is
50 percent as this would occur if genes were far enough apart on the chromosome or on completely different chromosomes
Recombination frequency
recombinant progeny / #total progeny
X100
Steps to predicting cross outcomes with linked genes when recombination frequency is given
Perform a testcross
Predict the frequency of the different progeny
Halve the recombination frequency and assign that to each gamete type of the recombinants
Halve the non-recombinant and assign that to each gamete type of the non-recombinants
Finally, homozygous parents will produce 100 percent non-recombinant gametes (meaning it will be 1)
Use multiplication rule to determine cross outcomes
