Parents often wonder which parent their new baby will look like more. Will the baby have Dad’s eyes and Mom’s nose? Or will he or she be the spitting image of one parent? The characteristics that make up a baby’s appearance are determined by the genetic contributions from each parent. There are a few key factors that influence which parent a baby is more likely to resemble.
Genetics Overview
To understand what determines a baby’s looks, it helps to have a basic grasp of genetics. Human cells contain DNA which acts as an instruction manual for building proteins, the molecules that give our cells structure and enable them to perform specific functions. DNA is packaged into structures called chromosomes. Humans have 23 pairs of chromosomes, with one set of 23 inherited from each parent. When sperm and egg cells are formed, the chromosome pairs separate so that each cell contains only one copy of each chromosome.
At conception, the sperm and egg fuse together, combining 23 chromosomes from the father and 23 from the mother to make a new cell with 46 chromosomes total. This new cell then multiplies rapidly, with each new cell containing identical copies of the same 46 chromosomes. Therefore, a child’s DNA is a blend of the father’s and mother’s DNA. The instructions contained in the DNA determine the child’s genetic makeup and physical characteristics.
Dominant and Recessive Genes
Some traits are controlled by genes that are dominant or recessive. Dominant genes tend to manifest physical characteristics whenever present, while recessive genes only manifest a trait if an individual inherits two copies of the gene (one from each parent). For example, brown eye color (B) is dominant over blue eye color (b). If a child inherits gene B from either parent, they will have brown eyes. Blue eyes will only occur if the child inherits two copies of gene b.
Here are some common dominant and recessive genetic traits:
Dominant Trait | Recessive Trait |
---|---|
Brown eyes | Blue eyes |
Dark hair | Blonde/red hair |
Curly hair | Straight hair |
Attached earlobes | Unattached earlobes |
Dimples | No dimples |
For traits controlled by dominant and recessive genes, the visible trait in a child will usually reflect whichever gene is dominant. For example, if one parent has brown eyes (dominant) and the other has blue eyes (recessive), the baby is much more likely to have brown eyes. However, recessive genes can be carried and passed down unseen for generations before two recessive genes happen to combine in a child and manifest as a physical trait.
Polygenic Inheritance
While some traits are controlled by a single gene pair, more complex physical characteristics are polygenic – meaning they are influenced by multiple genes interacting with each other. Examples of polygenic traits include skin, eye, and hair color, facial structure, height, and body type. Each parent contributes numerous genes that work together to shape these aspects of the child’s appearance. While some genes may be dominant or recessive, most genes influence polygenic traits in an additive or cumulative manner. The effects of all the relevant genes inherited from both parents combine to produce the child’s features.
For instance, at least 8 different genes impact skin color. The combined effect of variations in these numerous genes leads to the child’s skin tone. Rather than categorizing traits like skin color as simply dominant or recessive, polygenic inheritance produces a spectrum of possible outcomes depending on which variations of the involved genes are inherited.
Random Assortment
The inheritance of genes occurs randomly during conception when chromosomes from the egg and sperm come together. While a child always inherits exactly half of the DNA from each parent, it is a matter of chance which copies of each chromosome pair get passed down. Siblings can have markedly different genetic inheritance as a result of this randomness. One child might share more DNA with the father, while another child may share more traits with the mother due to variances in inherited genes.
Epigenetics
While genetics determine the possibilities, a process called epigenetics influences how genes are actually expressed. Environmental factors and lifestyle choices can activate or deactivate certain genes through chemical changes that turn them on or off. A classic example is identical twins, who share the exact same DNA but may develop different physical traits over their lifetimes based on epigenetic changes. Diet, exercise, stress levels and other external conditions create epigenetic tags on DNA that affect gene expression. Therefore, genetics lay the foundation, but the actual manifestation of traits is influenced by forces outside the gene code itself.
Sex Chromosomes
The sex chromosomes (X and Y) also influence what physical characteristics can be inherited. Certain genes are only carried on the X or Y chromosome. For example, genes for facial hair occur on the Y chromosome. A baby boy inherits X chromosome genes from his mother and Y chromosome genes from his father. This means certain traits like facial hair come exclusively from Dad. In contrast, a baby girl inherits X chromosomes from both parents, allowing her to display traits like her mother’s facial structure that are encoded on the X chromosome.
Mitochondrial DNA
Mitochondria are organelles that generate energy in cells and contain their own DNA called mtDNA. Mitochondrial DNA passes directly from a mother to her offspring. Paternal mtDNA is usually destroyed during fertilization. Daughters inherit exact copies of their mother’s mtDNA, while sons inherit only a copy. This means that characteristics influenced by mitochondrial DNA are only inherited maternally.
Genetic Disorders
If either parent carries genetic mutations that cause disorders like cystic fibrosis, sickle cell anemia, or Huntington’s disease, their child may inherit the condition. Diseases caused by abnormal dominant genes have a higher chance of manifesting. But disorders tied to recessive mutations can still occur if both parents, by chance, carry and pass down the same defective gene. Consultation with a genetic counselor allows couples to understand their risks and options when planning children.
Role of Environment
Aside from genetics, a child’s physical environment in the womb also shapes their development and appearance. Nutrition, viruses, toxins, and substances like alcohol or drugs taken by the mother cross the placenta and can impact prenatal growth. Babies of mothers who smoke often have lower birth weight. Poor maternal nutrition or alcohol use may produce facial abnormalities. Stress hormones also cross the placenta and could theoretically influence physical features or temperament. However, the effects of maternal conditions on appearance are complex.
Conclusions
While parents may hope their new baby looks just like Dad or is the spitting image of Mom, the genetic truth is often more blended. Babies inherit a mosaic of genes from both parents. Traits under simple genetic control are more likely to clearly resemble one parent. But for most complex features, the baby represents a combination of inherited genes. The assortment occurs randomly, so appearance and which parent is favored is ultimately unpredictable. Environment also plays a role during gestation. Overall, babies exhibit a unique blend of characteristics from both sides of the family. Certain features may lean more towards one parent, but a variety of genetic factors usually determine who the baby will most resemble.