Wine geeks get very excited about clonal selection. "Is the clone Calera, Swan, or 777?" they'll ask when tasting a Pinot Noir, a question that means nothing to the typical drinker. These are clones which have been developed over the years by winemakers and grape growers to suit local conditions, and each clone has different attributes: flavour, acidity, tannins, ripening time. Pinot Noir is a variety which has hundreds of clones as it naturally mutates into different versions of itself; this reproduction is taken further by deliberately crossing different clones together to produce versions of the grape which have specific characteristics.
These clones produced through human interference have also naturally been developed over the centuries and some of the most famous grape varieties are crossings of other varieties. The reason vines grow grapes is to attract birds to eat them and spread the seeds elsewhere to propagate more vines. As these seeds are spread, they meet and reproduce with seeds from other varieties of grapes, which is why the exhaustive Wine Grapes book lists over 1,300 different grape varieties.
One of the most famous natural crossings is Cabernet Sauvignon. Originating from Bordeaux, its parents are two other grapes native to the region, Cabernet Franc and Sauvignon Blanc. Given the shared name, it seems obvious that these two grapes together produced Cabernet Sauvignon but its distinct character confused people for some time - Cabernet Sauvignon has thick skins, is late ripening, and produces wines with deep blackcurrant aromas, while Cabernet Franc ripens earlier and its wines have more red fruits and Sauvignon Blanc is a white grape. However, DNA testing in the 1990s definitively proved the heritage of Cabernet Sauvignon, a heritage one can identify through the green, herbaceous aromas sometimes found in Cabernet Sauvignon and often found in both Cabernet Franc and Sauvignon Blanc.
Meanwhile, the other great Bordeaux grape, Merlot, is also a crossing, again of Cabernet Franc and an obscure, barely planted variety called Magdeleine Noire des Charentes which wasn't even given a name until the 1990s when it was found growing outside some villagers' houses.
Along with a now rarely-grown Bordeaux variety called Gros Cabernet, Cabernet Franc is also the parent grape of Carmenère, which is now mostly found in Chile. Cabernet Franc plays a vital if sometimes overlooked part in Bordeaux blends, adding colour, acidity, and aromas of red fruits and pencil shavings. As the parent grape of Cabernet Sauvignon and Merlot, as well as Carmenère, it's perhaps entitled to call itself the most important Bordeaux grape.
Chardonnay, Aligoté, and Gamay
Crossings have been formed from both natural and human interference. Both can be unpredictable, but it's the natural evolution which can be the most surprising yet effective. Pinot Noir is one of the most promiscuous grapes, reproducing to create Pinot Gris and Pinot Blanc. In Burgundy, the grape has mutated, either naturally or through cutting the best vines and planting them in a new vineyard, and this is one of the reasons the villages and vineyards of Burgundy produce such diverse wines.
This has not only led to it creating different versions of itself, but also completely different grape varieties. On three separate occasions, it has procreated with the same variety - Gouais Blanc, an otherwise forgettable white grape - to produce three different off-spring all now grown in Burgundy: Chardonnay, Aligoté, and Gamay. I remember being told this in a class, and a student shouted out, "But that's impossible! How can they produce such different varieties?" The instructor likened it to having children: one couple might have three children that can be hard to tell apart, another couple might have three children remarkably dissimilar, say with dark hair, fair hair, and red hair. So it was with Pinot Noir and Gouais Blanc.
Müller-Thurgau and Scheurebe
That's how nature works, now back to the humans. Riesling is one of the most difficult grapes to grow, a late-ripening variety that only produces complex wines in a cool climate and on difficult stony soils. In the late 19th century and early 20th, German scientists attempted to create crossings that replicated the complexity of Riesling, but ripened much earlier to make life easier on growers. One of the crossings created was by a Dr. Müller, who hailed from the Swiss canton of Thurgau. He thought he was crossing Riesling with another quality German variety, Silvaner, but that other variety was in actual fact Madeleine-Royal, about which there is otherwise little of interest to say. In the 1960s, Müller-Thurgau began to dominate plantings as Germany recovered from the Second World War. It was easy and cheap to grow, its rise was unstoppable, and it so badly damaged Germany's reputation as the heart of once-fashionable wines such as Blue Nun and Black Tower. Since the 1990s, there has been a concerted effort to reduce plantings, but it's still the second-most planted variety in Germany, with 11,000ha of the country's 100,000ha of vines.
Another variety that was once thought to be a Riesling-Silvaner crossing is Scheurebe, created by a Dr. Scheu (Rebe means vine in German). It has grassy, grapefruit aromas, and its high acidity makes it ideal for good-quality sweet wine. Despite its quality, there's not much planted in Germany (just over 2,000ha) as it just doesn't produce wines as classy as Riesling, but there are some dry and sweet wines made from it (try Pfeffingen's dry Scheurebe from Pfalz). In 2012, it was finally discovered what the crossing was: Riesling and a variety I have never heard of called Buckettsrebe.
other notable crossings
One of my favourite crossings is Zweigelt, a fruity, often easy-drinking, yet tannic red that a customer once admitted to me she was addicted to. It's Austria's most planted black grape, and is a crossing of the country's two highest-quality black varieties, Blaufränkisch and St-Laurent. Its name comes from Dr. Zweigelt, who crossed the grapes in 1922. Hans Igler produces an extraordinarily good value Zweigelt from Burgenland.
Another less successful crossing is Pinotage, although South Africans would dispute that claim. The wines are often full of chocolate aromas, with a weird rubberiness. Despite that, it's become South Africa's national grape as it's barely grown elsewhere. Wines are getting much better as winemakers learn to deal with the grape (check out Rijk's). It was developed by South African I. A. Perold in 1925, a random crossing of Pinot Noir and the Rhône grape Cinsault. Both of these varieties are known for their red fruit characteristics, yet Pinotage has big, bold, overripe black fruit aromas. Such is the unpredictability of crossings ....