Genes and Alleles

Each chromosome carries many genes. The location of a particular gene on a chromosome is called its locus. Homologous chromosomes have the same gene at the same locus, although they may carry different forms of that gene, called alleles12 (ah-LEELS), which produce alternative forms of a particular trait. Frequently, one allele is dominant and the other one recessive. If at least one chromosome carries the dominant allele, the corresponding trait is usually detectable in the individual. A dominant allele masks the effect of any recessive allele that may be present. Recessive alleles are therefore expressed only when present on both of the homologous chromosomes— that is, when the individual has no dominant allele at that locus. Typically, but not always, dominant alleles code for a normal, functional protein and recessive alleles for a nonfunctional variant of the protein.

The shape of the outer ear presents an example of dominant and recessive genetic effects. When the ears are developing in a fetus, a "death signal" is often activated in cells that attach the earlobe to the side of the head. These cells die, causing the earlobe to separate from the head. A person will then have "detached earlobes." This occurs in people who have either one or two copies of a dominant allele which we will denote D. If both homologous chromosomes have the recessive version of this gene, d, the cell suicide program is not activated, and the earlobes remain attached (fig. 4.16a). (It is customary to represent a dominant allele with a capital letter and a recessive allele with its lowercase equivalent.)

Individuals with two identical alleles, such as DD or dd, are said to be homozygous13 (HO-mo-ZY-gus) for that trait. If the homologous chromosomes have different alleles for that gene (Dd), the individual is heterozygous14 (HET-er-oh-ZY-gus). The alleles that an individual possesses for a particular trait constitute the genotype (JEE-no-type). A detectable trait such as attached or detached earlobes, resulting either from the genotype or from environmental influences, is called the phenotype15 (FEE-no-type).

We say that an allele is expressed if it shows in the phe-notype of an individual. Earlobe allele d is expressed only

12allo = different

14hetero = different

15pheno = showing, evident

Chapter 4 Genetics and Cellular Function 147

Phenotypic Traits Humans
(b)

D 1

D

DD

Dd

Homozygous,

Heterozygous,

detached

detached

earlobe

earlobe

Dd

dd

Heterozygous,

Homozygous,

detached

attached

earlobe

earlobe

Figure 4.16 Genetics of Attached and Detached Earlobes.

(a) Detached earlobes occur if even one allele of the pair is dominant (D). Attached earlobes occur only when both alleles are recessive (d). (b) A Punnett square shows why such a trait can "skip a generation." Both parents in this case have heterozygous genotypes (Dd) and detached earlobes, but there is a one in four chance that their offspring could have attached earlobes. Each parent is a carrier for attached earlobes.

Figure 4.16 Genetics of Attached and Detached Earlobes.

(a) Detached earlobes occur if even one allele of the pair is dominant (D). Attached earlobes occur only when both alleles are recessive (d). (b) A Punnett square shows why such a trait can "skip a generation." Both parents in this case have heterozygous genotypes (Dd) and detached earlobes, but there is a one in four chance that their offspring could have attached earlobes. Each parent is a carrier for attached earlobes.

when it is present in a homozygous state (dd); allele D is expressed whether it is homozygous (DD) or heterozygous (Dd). The only way most recessive alleles can be expressed is for an individual to inherit them from both parents.

Recessive traits can "skip" one or more generations. A diagram called a Punnett square (fig. 4.16fc) shows how two heterozygous parents with detached earlobes can produce a child with attached lobes. Across the top are the two genetically possible types of eggs the mother could produce, and on the left side are the possible types of sperm from the father. The four cells of the square show the genotypes and phenotypes that would result from each possible combination of sperm and egg. You can see that three of the

Saladin: Anatomy & I 4. Genetics and Cellular I Text I © The McGraw-Hill

Physiology: The Unity of Function Companies, 2003 Form and Function, Third Edition

148 Part One Organization of the Body possible combinations would produce a child with detached lobes (genotypes DD and Dd), but one combination (dd) would produce a child with attached lobes. Therefore, the attached-lobe trait skipped the parental generation in this case but could be expressed in their child.

This phenomenon becomes more significant when parents are heterozygous carriers of hereditary diseases such as cystic fibrosis—individuals who carry a recessive allele and may pass it on, but do not phenotypically express it in themselves. For some hereditary diseases, tests are available to detect carriers and allow couples to weigh their risk of having children with genetic disorders. Genetic counselors perform genetic testing or refer clients for tests, advise couples on the probability of transmitting genetic diseases, and assist people in coping with genetic disease.

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Responses

  • Fesahaye
    Why must people with fixed earlobes always be homozygous?
    3 years ago
  • Tuula M
    What chromosome has the allele for ear lobe cshape?
    1 year ago
  • DOMINIK BAUMGARTNER
    Are detached earlobes dominant or recessive?
    9 months ago

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