Figure 3 The Genetic Difference Between Males and Females Set (a) shows the chromosome structure of a male, and set (b) shows the chromosome structure
of a female. The last pair of 23 pairs of chromosomes is in the bottom right corner of each set.
Notice that the Y chromosome of the male is smaller than the X chromosome of the female. To
obtain this kind of chromosomal picture, a cell is removed from a person’s body, usually from
the inside of the mouth. The chromosomes are stained by chemical treatment, magnified
extensively, and then photographed.© CMSP/Custom Medical Stock Photo-All rights reserved
Do you notice any obvious differences between the chromosomes of the male and those of the female in Figure 3? The difference lies in the 23rd pair. Ordinarily, in females this pair consists of two chromosomes called X chromosomes; in males the 23rd pair consists of an X chromosome and a Y chromosome. The presence of a Y chromosome is one factor that makes a person male rather than female.
Sources of Variability Combining the genes of two parents in their offspring increases genetic variability in the population, which is valuable for a species because it provides more characteristics on which natural selection can operate (Belk & Borden Maier, 2016; Simon, 2017). In fact, the human genetic process creates several important sources of variability. First, the chromosomes in the zygote are not exact copies of those in the mother’s ovaries and the father’s testes. During the formation of the sperm and egg in meiosis, the members of each pair of chromosomes are separated, but which chromosome in the pair goes to the gamete is a matter of chance. In addition, before the pairs separate, pieces of the two chromosomes in each pair are exchanged, creating a new combination of genes on each chromosome. Thus, when chromosomes from the mother’s egg and the father’s sperm are brought together in the zygote, the result is a truly unique combination of genes.
Another source of variability comes from DNA. Chance events, a mistake by the cellular machinery, or damage caused by an environmental agent such as radiation may produce a mutated gene, a permanently altered segment of DNA (Bauman, 2015; Freeman & others, 2017). Even when their genes are identical, however, as for the identical twins described at the beginning of the chapter, people vary. The difference betweengenotypes and phenotypes helps us understand this source of variability. All of a person’s genetic material makes up his or her genotype. There is increasing interest in studying susceptibility genes, those that make the individual more vulnerable to specific diseases or accelerated aging, and longevity genes, those that make the individual less vulnerable to certain diseases and more likely to live to an older age (Cho & Suh, 2016; Dong & others, 2015; Sutphin & Korstanje, 2016). These are aspects of the individual’s genotype. However, not all of the genetic material is apparent in an individual’s observed and measurable characteristics. A phenotype consists of observable characteristics, including physical characteristics (such as height, weight, and hair color) and psychological characteristics (such as personality and intelligence). For each genotype, a range of phenotypes can be expressed, providing another source of variability (Klug & others, 2016; Solomon & others, 2015). An individual can inherit the genetic potential to grow very large, for example, but good nutrition, among other things, will be essential to achieving that potential.
Genetic Principles What determines how a genotype is expressed to create a particular phenotype? This question has not yet been fully answered (Moore, 2015). However, a number of genetic principles have been discovered, among them those of dominant and recessive genes, sex-linked genes, and polygenically determined characteristics.
Dominant and Recessive Genes In some cases, one gene of a pair always exerts its effects; in other words, it is dominant, overriding the potential influence of the other gene, which is called the recessive gene. This is the dominant-and-recessive genes principle. A recessive gene exerts its influence only if the two genes of a pair are both recessive. If you inherit a recessive gene for a trait from each of your parents, you will show the trait. If you inherit a recessive gene from only one parent, you may never know that you carry the gene. Brown hair, farsightedness, and dimples override blond hair, nearsightedness, and freckles in the world of dominant and recessive genes. Can two brown-haired parents have a blond- haired child? Yes, they can. Suppose that each parent has a dominant gene for brown hair and a recessive gene for blond hair. Since dominant genes override recessive genes, the parents have brown hair, but both are carriers of blondness and pass on their recessive genes for blond hair. With no dominant gene to override them, the recessive genes can make the child’s hair blond.
Sex-Linked Genes Most mutated genes are recessive. When a mutated gene is carried on the X chromosome, the result is called X-linked inheritance. It may have implications for males that differ greatly from those for females (Simon & others, 2016). Remember that males have only one X chromosome. Thus, if there is an absent or altered, disease-relevant gene on the X chromosome, males have no “backup” copy to counter the harmful gene and therefore may develop an X-linked disease. However, females have a second X chromosome, which is likely to be unchanged. As a result, they are not likely to have the X-linked disease. Thus, most individuals who have X-linked diseases are males. Females who have one abnormal copy of the gene on the X chromosome are known as carriers, and they usually do not show any signs of the X-linked disease. Fragile X syndrome, which we will discuss later in the chapter, is an example of X-linked inheritance (Karmiloff-Smith & others, 2016).
Polygenic Inheritance Genetic transmission is usually more complex than the simple examples we have examined thus far (Moore, 2015). Few characteristics reflect the influence of only a single gene or pair of genes. Most are determined by the interaction of many different genes; they are said to be polygenically determined. Even a simple characteristic such as height reflects the interaction of many genes as well as the influence of the environment. Most diseases, such as cancer and diabetes, develop as a consequence of complex gene interactions and environmental factors. The term gene-gene interaction is increasingly used to describe studies that focus on the interdependent process by which two or more genes influence characteristics, behavior, diseases, and development (Cho & Suh, 2016; Hodge, Hager, & Greenberg, 2016). For example, recent studies have documented gene-gene interaction in immune system functioning (Heinonen & others, 2015), asthma (Hua & others, 2016), alcoholism (Yokoyama & others, 2013), cancer (Wu & others, 2016), cardiovascular disease (Musameh & others, 2015), arthritis (Hohman & others, 2016), and Alzheimer disease (Ebbert & others, 2016).
Chromosome and Gene-Linked Abnormalities In some (relatively rare) cases, genetic inheritance involves an abnormality. Some of these abnormalities come from whole chromosomes that do not separate properly during meiosis. Others are produced by defective genes.
Chromosome Abnormalities Sometimes a gamete is formed in which the combined sperm and ovum do not have their normal set of 23 chromosomes. The most notable examples involve Down syndrome and abnormalities of the sex chromosomes. Figure 4 describes some chromosome abnormalities, along with their treatment and incidence.
Name Description Treatment Incidence
An extra chromosome causes mild
to severe intellectual disabilities
and physical abnormalities.
stimulation, and special
1 in 1,900 births at age 20
1 in 300 births at age 35
1 in 30 births at age 45
An extra X chromosome causes
Hormone therapy can
be effective 1 in 1,000 male births
An abnormality in the X
chromosome can cause intellectual
disabilities, learning disabilities, or
short attention span.
speech and language
More common in males
than in females
A missing X chromosome in females
can cause intellectual disabilities
and sexual underdevelopment.
Hormone therapy in
childhood and puberty 1 in 2,500 female births
An extra Y chromosome can cause
No special treatment
required 1 in 1,000 male births
Figure 4 Some Chromosome Abnormalities The treatments for these abnormalities do not necessarily erase the problem but may improve
the individual’s adaptive behavior and quality of life.
Down Syndrome Down syndrome is one of the most common genetically linked causes of intellectual disability; it is also characterized by certain physical features (Lewanda & others, 2016). An individual with Down syndrome has a round face, a flattened skull, an extra fold of skin over the eyelids, a thickened tongue, short limbs, and retardation of motor and mental abilities. The syndrome is caused by the presence of an extra copy of chromosome 21. It is not known why the extra chromosome is present, but the health of the male sperm or female ovum may be involved. Down syndrome appears approximately once in every 700 live births. Women between the ages of 16 and 34 are less likely to give birth to a child with Down syndrome than are younger or older women. African American children are rarely born with Down syndrome.
These athletes, several of whom have Down syndrome, are participating in a Special Olympics
competition. Notice the distinctive facial features of the individuals with Down syndrome, such as a
round face and a flattened skull. What causes Down syndrome?© James Shaffer/PhotoEdit
Sex-Linked Chromosome Abnormalities Recall that a newborn normally has either an X and a Y chromosome, or two X chromosomes. Human embryos must possess at least one X chromosome to be viable. The most common sex-linked chromosome abnormalities involve the presence of an extra chromosome (either an X or a Y) or the absence of one X chromosome in females.
How Would You…?
As a social worker, how would you respond to a 33-year-old pregnant woman who is concerned
about the risk of giving birth to a baby with Down syndrome?
Klinefelter syndrome is a chromosomal disorder in which males have an extra X chromosome, making them XXY instead of XY. Males with this disorder have undeveloped testes, and they usually have enlarged breasts and become tall (Lunenfeld & others, 2015). Klinefelter syndrome occurs approximately once in every 1,000 live male births. Only 10 percent of individuals with Klinefelter syndrome are diagnosed before puberty, with the majority not identified until adulthood (Aksglaede & others, 2013). Page 44 Fragile X syndrome is a genetic disorder that results from an abnormality in the X chromosome, which becomes constricted and often breaks (Karmiloff-Smith & others, 2016). The outcome frequently takes the form of an intellectual disability, autism, a learning disability, or a short attention span (Hall & others, 2014). This disorder occurs more frequently in males than in females, possibly because the second X chromosome in females negates the effects of the other, abnormal X chromosome (McDuffie & others, 2015; Rocca & others, 2016). Turner syndrome is a chromosomal disorder in females in which either an X chromosome is missing, making the person XO instead of XX, or part of one X chromosome is deleted. Females with Turner syndrome are short in stature and have a webbed neck (Miguel-Neto & others, 2016; Vlatkovic & others, 2014). In some cases, they are infertile. They have difficulty in mathematics, but their verbal ability is often quite good. Turner syndrome occurs in approximately 1 of every 2,500 live female births. XYY syndrome is a chromosomal disorder in which the male has an extra Y chromosome (Lepage & others, 2014). Early interest in this syndrome focused on the belief that the extra Y chromosome found in some males contributed to aggression and violence. However, researchers subsequently found that XYY males are no more likely to commit crimes than are XY males (Witkin & others, 1976).
Gene-Linked Abnormalities Abnormalities can be produced not only by an abnormal number of chromosomes, but also by defective genes. Figure 5 describes some gene-linked abnormalities and outlines their treatment and incidence.
Name Description Treatment Incidence
Glandular dysfunction that
interferes with mucus
production; breathing and
digestion are hampered,
resulting in a shortened life
Physical and oxygen therapy,
synthetic enzymes, and
antibiotics; most individuals live
to middle age. 1 in 2,000 births