Patterns of Inheritance

Patterns of Inheritance

Most human traits are based on multiple alleles or by interaction between several gene pairs. A small num-ber of human phenotypes are related to a single gene pair, yet most of these traits are extremely limited in nature. They may also reflect variations in single enzymes.

Dominant-Recessive Inheritance

The interaction of dominant and recessive alleles is known as dominant-recessive inheritance. By using a diagram known as a Punnett square, the pos-sible gene combinations in a single trait, which would occur from the mating of parents of known genotypes, can be determined (FIGURE 27 -3). A ­dominant allele is written as a capital “T,” while a recessive allele is written as a lowercase “t.” Therefore, the possible ­combinations in offspring, based on the fact that each heterozygous parent has the “Tt” genotype, is:

■■ Homozygous dominant child (“TT”): 25% possibility

■■ Heterozygous child (“Tt” or “tT”): 50% possibility

■■ Homozygous recessive child (“tt”): 25% possibility

The Punnett square only predicts the probability of a particular genotype and phenotype. When there are more offspring, the likelihood that the ratios will conform to predicted values increases. Often, parents of only two offspring will find that both of them are heterozygous (“Tt”), since this possibility is of higher probability.

To understand the probability of two events hap-pening successively, it is necessary to multiply the probabilities of the separately occurring events. Each child’s development is an independent event that does not influence the development of any other offspring by the same child. The three major modes of inher-itance are autosomal recessive, autosomal dominant, and X-linked recessive.

Dominant Traits

An autosomal dominant condition requires only one disease-causing allele for inheritance. Exam-ples of dominant traits in humans include dimples, freckles, and the development of a widow’s peak in the center of the hairline. There are few disorders that are caused by dominant genes. This is because lethal dominant genes are usually expressed and cause the death of an embryo, fetus, or child. In cer-tain dominant disorders, the individual may be less impaired and survived longer, even into his or her reproductive years. An example of such a dominant disorder is ­Huntington’s disease, which fatally affects the nervous system. The basal nuclei degen-erate because of a delayed action gene expressed by the individual at about age 40. The offspring of a parent with this disease has a 50% chance of inherit-ing the lethal gene. The parent is heterozygous, since the dominant homozygous condition would be lethal to the fetus. With today’s advancements­ in genetics, many ­children of parents with ­Huntington’s disease choose not to have children.

Recessive Traits

In autosomal recessive inheritance, two recessive alleles, one from each parent, transmit a trait. Recessive inheritance is often more desirable, such as recessive alleles that result in normal vision while dominant alleles result in astigmatism (abnormal spherical curve of the cornea or lens that is not equal in all meridi-ans). This condition causes blurred distance vision as well as blurred near vision. Many genetic disorders are inherited as simple recessive traits. For example, cystic fibrosis, which causes excessively thick mucus production that impairs the function of the lungs and pancreas. It sometimes causes a gastrointestinal and respiratory obstruction. If half of a heterozygous male’s sperm have the trait causing this disease and half of the female’s secondary oocytes also have the trait, the random combination of sperm and oocytes can cause:

■■ A 25% chance of each offspring inheriting two “wild-type” alleles, which are those that are not mutations but normal alleles with observable characteristics different from the standard form.

■■ A 50% chance of inheriting a disease-causing allele from either parent, and therefore being a carrier.

■■ A 25% chance of inheriting a disease-causing allele from each parent.

Another serious recessive genetic disorder is Tay­-Sachs disease, in which brain lipid metabolism is affected, due to a deficit of the enzyme hexosaminidase­ A, which is discovered a few months after birth. ­Recessive genetic disorders are more common than dominant genetic disorders. This is because carriers of a single recessive allele for a recessive disorder do not have the disease themselves. They can pass the gene on to their offspring, however. TABLE 27-1 lists traits determined by simple dominant-recessive inheritance.

Incomplete Dominance

When incomplete dominance occurs, a heterozy-gote has a phenotype that is intermediate between the homozygous dominant and the homozygous

recessive­. In this event, one allele variant masks the other. The most well- known example is sickle cell anemia, in which a sickling gene or genes cause substitution of one amino acid in the beta chain of hemoglobin. The molecules of hemoglobin that con-tain abnormal beta chains crystallize when blood oxygen levels are low. This causes the erythrocytes to assume a sickle shape. An individual who has two sickling alleles (ss) has the condition. Anything­that lowers their blood oxygen level, such as exces-sive exercise or respiratory problems, can lead to a sickle cell crisis. The deformed erythrocytes clump and fragment in small capillaries, resulting in intense pain.

People who are heterozygous for the sickling gene (Ss) have sickle cell trait. Their bodies manufacture normal as well as sickling hemoglobin. They are usu-ally healthy, but can suffer a sickle cell crisis if their blood oxygen levels are reduced for a long time, such as when traveling in high altitudes. These individuals may transmit the sickling gene to their offspring.

Multiple Allele Inheritance

Some genes have more than two allele forms. This leads to multiple allele inheritance. While three alleles determine human ABO blood types, every per-son receives two of these. The alleles are:

■■ IA: Codominant, expressed when present, resulting in AB blood type

■■ IB: Also codominant, expressed when present, resulting in AB blood type

■■ I: Recessive to the other type alleles TABLE 27-2 explains the effects of multiple allele inher-itance, using the examples of the ABO blood groups.

Sex-Linked Inheritance

Sex-linked inheritance is determined by genes on the sex chromosome. The X and Y chromosomes are not truly homologous. The Y chromosome contains the gene that determines the male gender. It is much smaller than the X chromosome. The X chromosome has over 1,400 genes, with most of them coding for proteins needed for brain function. However, the Y chromosome only carries about 200 genes. It does not have many of the genes that are present on the X chromosome, including genes that code for some clotting factors, testosterone receptors, and cone pig-ments. A gene that is only found on the X chromo-some is called X-linked. Relatively short areas at the ends of the Y chromosome code for nonsexual char-acteristics that correspond to those on the X chromo-some. These are the only areas that can participate in crossovers with the X chromosome.

X- linked recessive inheritance differs in how it affects females and males. For a female, it is similar to autosomal­ recessive inheritance because she has two X chromosomes—she can be a heterozygote or a homozygote. Males, with only one X chromosome,­ express recessive alleles on that chromosome, inheriting an X-linked recessive condition from a mother who is a carrier or is affected. Examples are Duchenne’s muscular dystrophy, color blindness, and ­hemophilia. X- linked traits are usually passed from mother to son, not father to son, since males receive no X chromosome from their fathers. Moth-ers may also pass recessive alleles to their daugh-ters. However, unless the daughter also receives a recessive allele from her father, the trait will not be expressed.

Polygene Inheritance

Most phenotypes require several gene pairs at dif-ferent locations, which act together. This polygene inheritance causes continuous, quantitative various in phenotypes between two extremes. In polygene inheritance, phenotypic characteristics are deter-mined by ­multiple alleles. It explains a large amount of human characteristics, such as height, intelligence, metabolic rate, and skin color. The color of your skin is controlled by three or more separately inherited genes (FIGURE 27-4). Other examples of polygene inheritance include obesity, cleft palate, diabetes mel-litus, autism, baldness, hypertension, schizophrenia, and alcoholism.

1. Explain the use of the Punnett square in calculating the genotype and phenotype probabilities that may result from the union of two heterozygous parents.

2. Give examples of common dominant and recessive traits.

3. Differentiate between the terms “sex-linked inheritance” and “X-linked”.