Trend test for comparing brain size

Hello all! I am interested in performing a trend test to compare brain size between two different genotypes. In my experiment, I measure the size of a given brain region at several different points along the rostral-caudal axis in a single mouse - I am making the measurements on mouse brain slices. In the graph below, the size of the brain region is along the y-axis, the brain slice measured (along the rostral-caudal axis) is along the x-axis, and each data point represents the the mean brain region area of all of the mice in a given genotype. I have used repeated measures two-way ANOVA to compare the two genotypes, however I am also interested in using a trend test to compare the two lines. However, I do not think a standard parametric test for linear regression fits my needs since the slopes of my two lines are similar, it is the difference between them that I am interested in comparing.
I am also thinking about simply comparing the brain slices at each individual point along the rostral caudal axis - for instance, at "brain slice 1" I would simply compare the striatum area between the two genotypes using an unpaired t test. I do not think there is a trend test that can be applied to a two-group comparison - at least not one that I have found - but please correct me if I am wrong!

Thank you very much in advance for you input.

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