10. Patterns of Inheritance


10.8. Other Influences on Phenotype


You might assume that the dominant allele is always expressed in a heterozygous individual; however, it is not that simple. As in other areas of biology, there are exceptions. For example, the allele for six fingers (polydactylism) is dominant over the allele for five fingers in humans. Some people who have received the allele for six fingers have a fairly complete sixth finger; in others, it may appear as a little stub. In some cases, this dominant characteristic is not expressed or perhaps shows on only one hand. Thus, there may be variation in the degree to which an allele expresses itself in an individual. Geneticists refer to this as variable expressivity.




FIGURE 10.11. Baldness and the Expression of Genes

It is a common misconception that males have genes for baldness and females do not. Male-pattern baldness is a sex-influenced trait, in which both males and females possess alleles coding for baldness. These genes are turned on by high levels of the hormone testosterone. This is an example of an internal gene-regulating mechanism.


Both internal and external environmental factors can influence the expression of genes. A characteristic whose expression is influenced by internal gene-regulating mechanisms is that of male-pattern baldness (figure 10.11). In males with a genetic disposition to balding, the enzyme 5-alpha-reductase is produced in high levels. 5-alpha-reductase uses testosterone in males to produce dihydrotestosterone (DHT). DHT slows down blood supply to the hair follicle and causes baldness. In nonbalding males, 5-alpha-reductase is produced at lower levels, DHT is not produced, and baldness does not occur. The internal environment in females has lower levels of testosterone, so DHT is not produced at high levels even if the 5-alpha-reductase is expressed. Differences in the internal environment of males and females alter the phenotype. An example of external environmental factors that affect gene expression is sunlight. Genes for freckles do not show themselves fully unless a person’s skin is exposed to sunlight (figure 10.12).




FIGURE 10.12. The Environment and Gene Expression

The expression of many genes is influenced by the environment. The allele for dark hair in the cat is sensitive to temperature and expresses itself only in the parts of the body that stay cool. The allele for freckles expresses itself more fully when a person is exposed to sunlight.


Diet is an external environmental factor that can influence the phenotype of an individual. Diabetes mellitus, a metabolic disorder in which glucose in the blood is not properly metabolized and is passed out of the body in the urine, has a genetic basis. Some people who have a family history of diabetes are thought to have inherited the trait for this disease. Evidence indicates that they can delay the onset of the disease by reducing the amount of sugar in their diet. This change in the external environment influences gene expression in much the same way that sunlight affects the expression of freckles in humans. Similarly, diet is known to affect how the genes for intelligence, pigment production, and body height are expressed. Children who are deprived of protein during their growing years are likely to have reduced intelligence, lighter skin, and shorter overall height than children with adequate protein in their diet.

Whether a honeybee larva will become a worker or a queen is largely determined by its diet. Only larvae that are fed “royal jelly” mature into queen bees. Recent evidence indicates that royal jelly has the epigenetic effect of decreasing the expression of the gene that controls the transformation of larvae into workers.



23. What type of factor can cause a dominant allele to not be expressed?

24. Give two examples of environmentally influenced genetic traits.



Genes are units of heredity composed of specific lengths of DNA that determine the characteristics an organism displays. Specific genes are at specific loci on specific chromosomes. Mendel described the general patterns of inheritance in his Law of Dominance, his Law of Segregation, and his Law of Independent Assortment. Punnett squares help us predict graphically the results of a genetic cross. The phenotype displayed by an organism is determined by the alleles present and the ways the environment influences their expression. The alternative forms of genes for a characteristic are called alleles. There can be many different alleles for a particular characteristic. Diploid organisms have two alleles for each characteristic. Organisms with two identical alleles for a characteristic are homozygous; those with different alleles are heterozygous. Some alleles are dominant over other alleles, which are recessive. Sometimes, two alleles do not show dominance and recessiveness but, rather, both express themselves. Codominance and lack of dominance are examples. Often, a gene has more than one recognizable effect on the phenotype of the organism. This situation is called pleiotropy. Some characteristics are polygenic and are determined by several pairs of alleles acting together to determine one recognizable characteristic. In humans and some other animals, males have an X chromosome with a normal number of genes and a Y chromosome with fewer genes. Although the X and Y chromosomes are not identical, they behave as a pair of homologous chromosomes. Because the Y chromosome is shorter than the X chromosome and has fewer genes, many of the recessive characteristics present on the X chromosome appear more frequently in males than in females, who have two X chromosomes. The degree of expression of many genetically determined characteristics is modified by the internal or external environment of the organism.


Basic Review

1. Homologous chromosomes

a. have the same genes in the same places.

b. are identical.

c. have the same alleles.

d. All of the above are correct.

2. Phenotype is the combination of alleles that an organism has, whereas genotype is its appearance. (T/F)

3. A homozygous organism

a. has the same alleles at a locus.

b. has the same alleles at a gene.

c. produces gametes that all carry the same allele.

d. All of the above are correct.

4. Segregation happens during meiosis. (T/F)

5. The sex of an organism is determined by the number of chromosomes it possesses. (T/F)

6. Genes that are found only on the X chromosome in humans most consistently illustrate

a. pleiotropy.

b. the concept of diploid organisms.

c. sex-linkage.

d. All of the above are correct.

7. Double-factor crosses

a. follow 2 alleles for 1 gene.

b. follow the alleles for 2 genes.

c. look at up to 4 alleles for 1 gene.

d. None of the above are correct.

8. Mendelian principles apply when genes are found close to each other on the same chromosome. (T/F)

9. _____ occur when there are more than 2 alleles for a given gene.

10. Dominant alleles mask _____ alleles in heterozygous organisms.

11. The place where a gene is located on a chromosome is known as its _____.

12. The term _____ describes the multiple effects a gene has on a phenotype.

13. When a heterozygote appears to be a “blend” of the two parental phenotypes, the trait is considered to be exhibiting _____.

14. In the ABO system, A and B show _____ when they are together in an individual, but both alleles are dominant over the O allele.

15. What is the probability that parents heterozygous for a trait will have a homozygous offspring?



1. a 2. F 3. d 4. T 5. F 6. c 7. b 8. F 9. Multiple alleles 10. recessive 11. locus 12. pleiotropy 13. incomplete dominance 14. codominance 15. 50%


Thinking Critically

Nature vs. Nurture

The breeding of dogs, horses, cats, and many other domesticated animals is done with purposes in mind—that is, producing offspring that have specific body types, colors, behaviors, and athletic abilities. Cows are bred to produce more meat or milk. Many grain crops are bred to produce more grain per plant. Similarly, some people have the muscle development to be great baseball players, whereas others cannot hit the ball. Some have great mathematical skills, whereas others have a tough time adding 2 + 2. How do you think you have been genetically programmed? What are your strengths? As a parent or child, what frustrations have you experienced in teaching or learning? What are the difficulties in determining which of your traits are genetic and which are not?