Adding specific column value according to row value

I have a dataframe H

Genotype Preference
Rice     1
Rice     2
Lr       3
Lr       3
th       4
th       7

I want the dataframe to look like

Genotype Preference Haplotype
Rice     1          1
Rice     2          1
Lr       3          2
Lr       3          2
th       4          0.5
th       7          0.5

That is I want to add a numerical variable to be added to the each type of genotype. I have around 100 observations for each type of genotype. I want to be able to add the numberical variable into a new column in a single line of code and ensure that 1 is added corresponding to rice, 2 to Lr and 0.5 to th.

I tried constructing the code with the mutate/ifelse ` h3 %>% select(Genotype) %>% mutate(Type = ifelse (Genotype = c("Rice"), 1, Genotype)) Other results which I looked up, provide solutions for adding a column with a calculated value from the previous columns but not specific values.

I have found this dplyr mutate with conditional values and Apply R-function to rows depending on value in other column but dont know how to modify it for my code.

Any help with this will be greatly appreciated.