Millerandage - is the affect of grape bunches producing berries that differ greatly in size - and the result possibly have a negative impact and affect on flavour, character development and quality of the wine - (a.k.a. ‘hen and chicken’ - large and small berries).
Millerandage is also an irregular fruit set - in which the berries on grape bunches are not uniform in size, with some growing to full size while others remain small and seedless. Large berries contain seeds - while the smaller berries are seedless, and will have ongoing difficulty in ripening. Yield is inevitably lower than normal when these conditions arise. But if subsequent conditions are favourable, fruit may be concentrated. As a small portion of millerandage is not always a bad thing for the quality of the final harvest.

 

The most common cause is cold, wet weather during the key stage of the vines flowering, during the growth cycle. While millerandage causes a reduction in vine yield, its potential impact on wine quality can vary, particularly by grape. Some varietals are prone to uneven ripeness within a bunch, such as Sangiovese and Gewürztraminer. The development of millerandage can result in ‘green flavours’ within the unripe grapes growing on the bunches. Other varietals, such as Pinot Noir - the wine quality can potentially be improved due to the reduced overall berry size - due to the higher skin to juice ratio, but colour, aromas and flavour extraction is not always even.
Millerandage is the poor fertilization of the grape flowers, and while most often attributed to poor weather, other factors such as nutritional deficiencies (particularly mineral boron which is needed to synthesis the growth hormone auxin and facilitate the movement of sugars in the vine) or viral infections. For grapevines, flowering occurs usually 8 weeks after the beginning of bud-burst. Ideally the temperature and weather conditions during this period should be warm, sunny and dry (and with little wind) to insure optimal flowering. For some varietals, such as Merlot, flowering may be more staggered which poses a greater risk for inclement weather disrupting the process and encouraging millerandage. 
Following flowering, the flowers of the grape vine go through pollination and fertilization over the next 2 to 3 days. Here is another opportunity where poor weather can influence the outcome - example if temperature drops below 10°C potentially damaging the ovules of the flowers before they can be fertilized. Since wine grapevines are hermaphroditic (containing both male and female parts) and usually rely on self-pollination, the presence of wind to circulate pollen or insects usually doesn't influence the success or failure of this pollination stage. While not as influential as temperature, the presence of rain can wash off the pollen from the stigma or greatly dilute the stigmatic fluid, causing the pollen to absorb too much water, swelling and bursting before it reaches the ovules.
Grapes that develop millerandage will not have seeds, making them smaller and with potentially a higher juice to skin ratio which may be manageable. However, these smaller berries may not fully ripen and could potentially add high acidity and green/astringent flavours to the wine.
Even in the most ideal conditions, usually only 20-30% of flowers develop into mature fruit with fully developed seeds. While millerandage will always have an economic impact in reduced harvest yields, it may not always have a negative impact on the resulting quality of the wine. In some areas, wine regions the presence of millerandage in the vineyard can be a positive quality for a vintage due to the reduced average berry-size. Some growers can even use sprays to deliberately encourage millerandage.
However, the small, seedless berries may never fully ripen and stay hard and green (with harsh acidity) throughout the growing season. Some grape growers may choose to remove bunches with high occurrence of millerandage through a green harvest. Or harvest the crop later at higher ripeness levels to balance the high acidity and potential ‘green flavours’ of the small underdeveloped berries. Other winemakers will remove the small (seedless) grapes post-harvest on a sorting table along with other unripe or damaged grapes.