(CNN) -- Whenever a new baby is on the way, the family can't help but speculate on what the little one might look like. Will he or she have straight hair like Dad or curly hair like Mom? Be tall like Aunt Susan or short and stubby like Uncle Leroy?
When the new arrival is the future king or queen of England, more than just the family wants to know.
Catherine, Duchess of Cambridge, is reportedly due to give birth July 13, and the world will watch as the baby grows up.
Will he or she turn out to have blue eyes like Prince William or green ones like Catherine? Be light-haired like Dad or dark-haired like Mom? Will Prince Charles' ears be passed on to a future generation? Online bets are already being placed on the child's hair color.
Whatever happens, this baby might stand out genetically among royals because his mother is the first commoner to marry into the royal family since the 17th century.
Marriage within a relatively small gene pool, like the royals, means a bad gene has a pretty good chance of rearing its ugly head. The blood clotting disorder hemophilia, for example, has run in the British royal family.
"It's very good that they're bringing in new genes," said Dr. Anand Saggar, a consultant in the South West Thames Regional Genetics Department at St. George's Hospital Medical School in London. "It freshens up the gene pool."
Catherine's commoner genes might lead to a somewhat darker-skinned baby, Saggar said.
The royals, he explained, are pretty pale. Catherine's skin has a considerably more olive tone, and the baby will likely be somewhere between the two -- but more like Catherine because her genes are dominant over lighter ones.
"The odds are the child will have darker skin color than the royals might be used to," Saggar said.
The same is true for eye color. The chances that the baby will have true blue eyes like William's are relatively unlikely. Most likely, Saggar said, the baby's eyes will be a shade closer to Catherine's.
With hair color, there are basically four options: light like Dad, dark like Mom, a shade in between the two or red like the baby's Uncle Harry.
Saggar said it's possible to calculate the odds of a redhead.
Based on the fact that William's brother, Prince Harry, is a ginger, there's a 66% chance that William carries a gene for red hair, he said. Less is known about Catherine's heritage, but assuming it's similar to the genes of other British people, he calculates there's a 6% chance the baby will have red hair.
Apparently Britons are more confident the baby will have red hair. At William Hill online betting, there are 4-1 odds the baby will be a ginger.
"If you're going to put some money on it, you might want to think about it," Saggar said.
While hair color might be debatable, one physical trait seems pretty clear: William and Catherine's child will likely be quite tall.
William is reportedly around 6 foot 3 inches tall, and his wife about 5 feet 10 inches. A son would likely be between 5 feet 11 inches and 6 feet 7 inches, and a daughter between 5 feet 6 inches and 6 feet 2 inches, according to Barry Starr, director of The Tech at Stanford School of Medicine.
Of course, it's impossible to use genetics to predict completely what any baby will look like, royal or not.
"That's the fun of having babies," Saggar said. "You don't know what you're going to get."
CNN's John Bonifield and William Hudson contributed to this report.