- Is the inheritance of two genes controlling two different features. The alleles may be on the same chromosome (homologous chromosome) so are linked or they are on different chromosomes, so are not linked.
Dihybrid inheritance - no linkage
- Typical form of inheritance eg pea plants tall stem (T) dominant to short (t) and yellow seed (Y) dominant to green (y). The two genes are on different homologous chromosomes.
- When crossed (bred) TTYY tall yellow with ttyy short green, each gamete gets an allele for height + seed colour, but since they are homozygous it will be TY and ty.
- Cross to produce heterozygous TtYy.
- F1 cross heterozygous produces four different gametes TY Ty tY ty.
- When crossed (bred) the ratio is 9:3:3:1. This ratio is always the probability of getting each phenotype when two heterozygous dihybrids with complete dominance and non linked alleles are crossed.
Dihybrid inheritance - linked genes
- Linked genes are on the same chromosome but code for different characteristics.
- When two linked genes are far apart on the chromosome, crossing over with recombinantion is likely to occur.
- Linkage and crossing over have no effect on inheritance if genotypes are homzygous .
- It will affect inheritance if genotypes are heterozygous.
- The standard method to see the effect of crossing over is to compare the results of a test cross without linkage to a test cross with linkage.