As most people know from their own experience, height has a strong genetic component. Short parents tend to have short children, and tall parents tend to have tall children. Some people wonder if a boy's height is determined primarily by his father's height whereas a girl's height is determined primarily by her mother's. This turns out not to be true. The height of a child is influenced by the heights of both parents.
You may recall from school that some genetic traits are dominant while others are recessive, and so you might wonder whether tall stature is dominant over short stature or vice versa. In other words, if one parent is tall and the other is short, will the children tend to be short or tall? The answer turns out to be more complicated. Stature is determined by many different genes, and therefore it is neither dominant nor recessive. Each child gets a unique, complex combination of stature-determining genes from his or her parents. If one parent is tall and the other short, the child's stature can be similar to the mother's or the father's, but usually is in between the two. Occasionally, the random assortment of genes can even produce a child who is substantially shorter or taller than both parents.
Child Target Height
If we know the heights of both parents, we can make an educated guess about the height that their child will attain as an adult. This educated guess is called the target height or adjusted midparental height. Target height can be calculated as follows:
For a boy,
1. Start with his mother's height.
2. Add 5 inches (or 13 cm).
3. Then add his father's height.
4. Divide by 2.
For a girl,
1. Start with her father's height.
2. Subtract 5 inches (or 13 cm).
3. Then add her mother's height.
4. Divide by 2.
In case you're curious, the 5 inches represents the difference in height between the average man (5 feet, 9 1/2 inches) and the average woman (5 feet, 4 1/2 inches). For a boy, we add 5 inches to his mother's height to take into account the fact that she is female. Adding 5 inches gives us the male equivalent of her height. Then we take that gender-adjusted height and average it with the father's height.
So if a 5-foot, 8-inch man marries a 5-foot, 1-inch woman, we would estimate that their sons would be, on average, about 5 feet, 7 inches (61 inches + 5 inches + 68 inches = 134 inches. 134 2 = 67 inches), and we would estimate that their daughters would be, on average, about 5 feet, 2 inches (68 inches - 5 inches + 61 inches = 124 inches. 124 2 = 62 inches).
If you are not fond of mathematical formulas, you can use table 2 to convert the parents' height from feet and inches into inches.
Then to determine your child's target height, use table 3 for boys or table 4 for girls.
Look along the left-hand margin of the table to find the father's height. Then look along the top margin of the table to find the mother's height. Follow the column corresponding to the mother's height until it intersects the row corresponding to the father's height. The intersection represents the target height of the child.
Actually, the target height exaggerates the impact of parental height a bit. If the parents are tall, the children tend to be not quite as tall as their target height would predict, and if the parents are short, their children tend to be not quite as short as their target height would predict.
Again, remember that the target height is just a rough estimate. If it were exact, then all the boys in a family would be the same height, as would all the girls, and we know that this is not true. In reality, each child gets a different, random assortment of genes from the parents and so can end up taller or shorter than the parents. The majority of children end up within 2 inches of their target height, but it is not unusual for individual children to end up as much as 4 inches taller or shorter than their target height.
Also, nongenetic factors, such as nutrition, play a role. If a child's nutrition is better than the nutrition her parents received in childhood, then she will probably exceed her target height. On the other hand, if she has a significant illness that interferes with growth, she may not achieve her target height.
Certain ethnic groups tend to be tall or short. A striking example is the African Pygmy; adult men are approximately 4 feet, 11 inches, and women approximately 4 feet, 8 inches. For most ethnic groups, the tendency to be tall or short is partly genetic and partly nongenetic. For instance, in the Netherlands the average height for young men is approximately 6 feet, and the average height for young women is approximately 5 feet, 7 inches. This tall stature presumably reflects a favorable environment, including good nutrition and good medical care, and a tall gene pool. In the United States, Caucasian and African Americans have similar heights. Caucasian and African American young men average approximately 5 feet, 10 inches, and young women average approximately 5 feet, 4 1/2 inches. Mexican Americans tend to be shorter. Mexican American young men average approximately 5 feet, 7 inches, and young women approximately 5 feet, 2 inches.
Single Genes That Can Affect Growth
As we discussed, many genes normally act in combination to determine height. Occasionally, however, a mutation in one single gene can have a powerful influence on growth. Mutations are changes in the DNA sequence. For example, a mutation in one gene (called FGF receptor-3) causes a condition called achondroplasia, in which the growth of the arms and legs is markedly reduced, and the adult height is decreased by about 16 inches. Mutations are often inherited from one or both parents. Specific genetic disorders that cause short stature will be discussed in chapter 7.
Chromosomes and Growth
In addition to mutations in specific growth-determining genes, we know that missing an entire chromosome, as occurs in girls with Turner syndrome, or having an extra chromosome, as occurs in Down syndrome, can have a negative influence on growth.
Key Points to Keep in Mind
Height is influenced by multiple genes. We can use the parents' heights to calculate a child's target height-the height that we would expect the child to attain-but it is just an educated guess. Different ethnic groups tend to be taller or shorter. Occasionally, a mutation in a single gene can strongly affect growth. Although genetics has a powerful effect on growth, it is just half the story. The other half is the environment, that is, the circumstances in which the child is raised.