Using a drop down to add a relationship between has_many :through models.

select_has_many_through.rb

# models

class Genotype < ActiveRecord::Base
  has_many :genotypes_colors
  has_many :colors, through: :genotypes_colors
  
  # to post nested attributes in a form using fields_for
  accepts_nested_attributes_for :genotypes_colors
  # if you're using Rails 3 attr_accessible
  attr_accessible :genotypes_colors_attributes, ...rest of accessible attribs 
end

class GenotypesColor < ActiveRecord::Base
  belongs_to :genotype
  belongs_to :color
end

class Color < ActiveRecord::Base
  has_many :genotypes_colors
  has_many :genotypes, through: :genotypes_colors
end

# controller

class GenotypesController < ApplicationController
  def edit
    @genotype = Genotype.find params[:id]
    @colors = Color.all
  end
  
  def update
    @genotype = Genotype.find params[:id]
    
    if @genotype.update! genotype_params # or params[:genotype] if you're not using Rails 4 Strong Parameters
      # success, the genotypes_colors aassociation should now be created and now you can access @genotype.colors
    else
      # oh noes
    end
  end
  
  private
  
  # if you're using Rails 4 Strong Parameters
  def genotype_params
    params.require(:genotype).permit(genotypes_colors_attributes: [:color_id], ...rest of genotype attribs)
  end
end

# edit view

<%= form_for @genotype do |f| %>
  <%= select_tag 'genotype[genotypes_colors_attributes][][color_id]', options_from_collection_from_select(@colors, 'id', 'angora_color') %>
  .. rest of code ...
<% end %>

The idea is to generate a drop down for all colors you want to assign to a genotype. Then assign the color's id to the color_id attribute of the genotypes_colors association by simply selecting it from the dropdown. Then assign those params in the controller to the genotype.