Have you ever wondered what traits a baby gets from its mom? Or dad? Expecting parents often daydream about what their baby will be like when it’s born –  what kind of personality will they have? Who will they look like? Will they have their dad’s height or their mom’s eye color? Genetically, these questions can be hard to answer!

Babies receive half of their genetic instructions (DNA) from their mom and half of their DNA from their dad. DNA is stored in structures called chromosomes, and each parent typically passes on 23 chromosomes, so babies end up with 46 chromosomes total, or 23 pairs of chromosomes, with one chromosome in each pair coming from each parent. This means that approximately 50% of a baby’s DNA comes from each parent, but it can be hard to predict how this 50/50 split will translate into how a baby looks or acts or which parent they resemble. 

Today, we’re going to examine some myths about genetics and inheritance and look into why these common misconceptions aren’t quite right. 

A hispanic family celebrating a new pregnancy

True or False - If mom and dad both have blue eyes, the baby will have to have blue eyes, too - it’s impossible for the baby to have brown eyes?

False - this is a myth! Scientists used to believe eye color was a simple trait determined by just one or two genes, with blue eye color being recessive and brown eye color being dominant. This would mean that if a parent had even one copy of the gene for brown eyes, they would have brown eyes, so it would be impossible for two blue-eyed parents to have a brown-eyed child because blue-eyed parents wouldn’t have any brown-eye genes to pass on (otherwise they would have had brown eyes themselves!). However, it turns out eye color is a bit more complicated than that. These days, scientists think it’s a combination of several genes that determine eye color, rather than a simple blue eye gene or brown eye gene. So, even though it’s likely that two parents with blue eyes will have a blue-eyed child, there are factors and genes we don’t quite understand yet, so a brown-eyed, green, or hazel-eyed baby is still possible.

True or False - Hair loss in men (male pattern baldness) is inherited from the mother’s side of the family?

False! This is a myth - although there is a little bit of truth behind it. The AR gene on the X chromosome is strongly linked to male pattern baldness, and men inherit their X chromosome from their mother. So, if a mother’s X chromosome has a version of the AR gene associated with hair loss, her son is more likely to develop hair loss. But, the AR gene isn’t the only factor that determines baldness - recent studies say that at least 60 different genes contribute to male pattern baldness, and only a handful of these genes are located on the X chromosome. This means that many genetic factors contributing to hair loss are inherited from the father in addition to the mother, and research shows that more than 80 percent of people experiencing balding had a father who also lost their hair. So mom is not entirely to blame if hair loss runs in the family! 

You may be wondering, if eye color and hair loss can’t be attributed to just mom or just dad, then what traits can we say come from one parent or another? 

For one, we know that a baby’s sex is determined by the father’s chromosome contribution. Remember, each parent passes on one copy of each chromosome, or 23 chromosomes total. The last pair of chromosomes are called sex chromosomes because they determine the baby’s sex. Males have XY sex chromosomes, while females have XX chromosomes. Since women have two X chromosomes and no Y chromosomes, they will always pass on a copy of the X chromosome to a baby. Since men have one X chromosome and one Y chromosome, they will either pass on an X chromosome OR a Y chromosome. If dad passes on the X chromosome, the baby will be a girl, and if he passes on the Y chromosome, the baby will be a boy! 

We also know that certain genetic conditions are carried on the X chromosome and can be passed from a healthy mother to an affected son since boys receive their X chromosome from their mom. For example, a woman could be an X-linked carrier for colorblindness and have normal vision herself, but if she has a boy and passes on her X chromosome with the colorblindness gene, her son will be colorblind. Other conditions that are carried on the X chromosome include hemophilia, Duchenne muscular dystrophy, and Fragile X syndrome. Even for X-linked conditions, it’s not always as straightforward as females being healthy and males being affected, so speaking with an expert like a genetic counselor is always a good idea if there is any concern for one of these conditions in your family. 

Overall, genetic knowledge has grown rapidly in the past few decades, and genetic analysis can be a good way to predict certain conditions or features in a baby, but we still have a lot more to discover about the way our genes interact and code for how we develop. So, when you think about questions like – how tall will my baby be? Will they be athletic like their dad? Will they have their mom’s sense of humor? Will they have their grandmother’s smile? – keep in mind that most traits are determined by many different genetic markers from both mom and dad as well as external factors like environment. Every baby is a unique combination of both parents, so your little one will keep you guessing all the way up to the day they’re born and beyond!

- Authored by Kelly Miller, MS, LCGC | Genetic Counselor


JunoDx.com and the materials and information it contains are not intended to be and do not constitute medical advice, other health advice, or diagnosis.  Do not use JunoDx.com or the materials and information published at JunoDx.com as a substitute for medical care and treatment. You should always consult with a qualified physician or healthcare provider about your specific circumstances.  


November 15, 2022 — Stephanie McClintock