Let's have a look at wonderful Tina Cole and how her appearance as Katie changed over the seasons.
Some hairstyle variation may be attributed to the shooting procedure of M3S, where the actors filmed scenes & close-ups after Fred MacMurray's work was done for the season.

This I believe is how Katie looks when she first meets Rob in California; note the attractive bangs (Season 8):

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As their wedding approaches, we see a variation, with her hair in front combed to the side:

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This is another view from the same photoshoot:

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Wedding photo showing the return of the bangs; she is stunningly beautiful:

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We see a controlled style when the triplets are born, perhaps her shortest (Season 9):

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Here we see her I believe after the time of the triplets' birth; there's relatively subtle curl in her clearly longer hair, an interesting look:

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Finally for now, here is a precursor to her styles of Seasons 10-12.
Note that while not wearing full bangs, her hair is not yet all-one-length;
it seductively comes close to covering her right eye (a sexy look):

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Continuing on, here is a look at Tina's growing out of her bangs; once again looking quite sexy (see also the attached below):

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Among the longest her hair appeared on M3S, a side-swept look in this photo:

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Getting on to the late seasons, we've arrived at an all-one-length style for Season 10:

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In Season 11:

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From the final season, more length has returned:

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Here's a look at Tina from the 1977 reunion special, looking very hot:

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Don Grady was short. Seeing Tina next to him in these shots made me wonder how tall she is. Wiki says 5'1

This is one of my favorite photos of Tina, so I'll add it here.
Really beautiful bust:

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Now we know what Katie did after Rob abandoned her to go to Peru, she became a Playboy Bunny. Ooh la la.:)

Here's a look at Tina from the 1977 reunion special, looking very hot:

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Thank you for answering a question I’ve always pondered: What would Katie Douglas look like if she had Kitty Forman’s hairdo?

Don Grady was short. Seeing Tina next to him in these shots made me wonder how tall she is. Wiki says 5'1

I don’t think he was quite that short - I’d guess 5’7 or so. All the sons were fairly short - Mike was the same height as Jean Pearson, and I don’t remember Ernie being tall.

This little photo from the wedding episode may help with the height question:

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Now let's get down to business with some more SEXY looks. "She's got legs..."

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Cute photos, notice the earring:

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Lovely eyes in this one, from the episode when Katie brings home the donuts. Tasty...

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Finally, a lovely passenger, so very attractive:

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Today I wanted to post some photos from before the M3S years, and after.

Here's something you don't see every day, Tina's high school yearbook entry, circa 1960-61:

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A nice photo from Hawaiian Eye, with Troy Donahue:

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Two images from the 1977 reunion special (M3S/Partridge Family), with Don Grady. A serious look in the first photo -- a very pretty face:

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I like Tina's outfit choice for the special. Very tasteful skirt & blouse, and REALLY nice boots. I also like how her hair comes over her right eyebrow for a seductive look.

1Cor 11:9: Neither was the man created for the woman; but the woman for the man.

Thank God, Katie's maiden name was not Piper, or else Dodie could call her, Katie Diaper.