Genetics and Plant Breeding
Genetics and plant breeding in this paper tests inheritance ratios, gene interaction, and the logic of breeding methods used to develop varieties. Questions often ask you to choose a method, interpret a cross, or calculate the recovery of a recurrent parent.
Core rules
- Mendel's law of segregation applies to alleles at one locus; independent assortment applies only to genes on different chromosomes or far apart on the same chromosome.
- Monohybrid F2 is 3:1; dihybrid F2 is 9:3:3:1 when genes assort independently. A test cross (cross to homozygous recessive) gives 1:1 for one gene and 1:1:1:1 for two independent genes.
- Incomplete dominance gives an intermediate phenotype; codominance expresses both alleles. Linkage reduces recombinant frequency; crossing over produces recombinants.
- Breeding methods: introduction; mass selection (heterogeneous population); pure-line selection (self-pollinated crops); pedigree method (record individual progenies); backcross breeding (transfer a simply inherited trait); mutation breeding; heterosis and hybrid seed production; male sterility and self-incompatibility aid hybrid seed.
- Backcross recovery: after n backcrosses to the recurrent parent, donor genome ≈ (1/2)^(n+1), if selection is only for the transferred gene.
Worked example
A breeder makes F1, then five backcrosses to a recurrent parent. What fraction of the recurrent genome is expected? F1 has 1/2 recurrent and 1/2 donor. Each backcross halves the remaining donor fraction. After BC1 donor=1/4; BC2=1/8; BC3=1/16; BC4=1/32; BC5=1/64. Recurrent = 1−1/64 = 63/64 = 98.44%. Reasoning: the exponent is n+1 because F1 is the starting generation before the first backcross.
Traps
Applying independent assortment to linked genes; calling a backcross a test cross when the tester is not homozygous recessive; expecting 9:3:3:1 in a test cross; ignoring that mass selection does not create pure lines; assuming cytoplasmic male sterility is nuclear-inherited.