Dominant and Recessive Alleles at the Population Level

It is a common misconception that dominant alleles must be more common in the gene pool than recessive alleles.

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

From diet (environment)

Phenylalanine-► Tyrosine-► Melanin -

Enzyme 1 Enzyme 2

mRNA 1

Genes (DNA)

Phenylalanine-► Tyrosine-► Melanin -

Enzyme 1 Enzyme 2

mRNA 1

Genes (DNA)

mRNA 2

Phenotype (brown eyes)

Phenotype (brown eyes)

Figure 4.19 The Roles of Environment and Heredity in Producing a Phenotype. Brown eye color requires phenylalanine from the diet (environment) and two genetically coded (hereditary) enzymes to convert phenylalanine to melanin, the eye pigment.

Table 4.4 Basic Terminology of Genetics

Term

Definition

Gene

A segment of DNA that codes for a polypeptide

Genome

All genes possessed by one individual

Gene pool

All alleles present in a population

Homologous chromosomes

Two physically identical chromosomes with the same gene loci but not necessarily the same alleles; one is of maternal origin and the other paternal

Sex chromosomes

Two chromosomes (X and Y) that determine a person's sex

Autosomes

All chromosomes except the sex chromosomes; occur in 22 homologous pairs

Locus

The site on a chromosome where a particular gene is located

Allele

Any of the alternative forms that a particular gene can take

Genotype

The alleles that a person possesses for a particular trait

Phenotype

A detectable trait, such as eye color or blood type

Recessive allele

An allele that is not phenotypically expressed in the presence of a dominant allele; represented with a lowercase letter

Dominant allele

An allele that is phenotypically expressed even in the presence of any other allele; represented with a capital letter

Homozygous

Having identical alleles for a given gene

Heterozygous

Having two different alleles for a given gene

Carrier

A person who carries a recessive allele but does not phenotypically express it

Codominance

A condition in which two alleles are both fully expressed when present in the same individual

Incomplete dominance

A condition in which two alleles are both expressed when present in the same individual, and the phenotype is intermediate between those which each allele would produce alone

Polygenic inheritance

A condition in which a single phenotype results from the combined action of genes at two or more different loci, as in eye color

Pleiotropy

A condition in which a single gene produces multiple phenotypic effects, as in sickle-cell disease

Sex linkage

Inheritance of a gene on the X or Y chromosome, so that the associated phenotype is expressed more in one sex than in the other

Penetrance

The percentage of individuals with a given genotype who actually exhibit the phenotype predicted from it

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

Chapter 4 Genetics and Cellular Function 151

The truth is that dominance and recessiveness have little to do with how common an allele is. For example, type O is the most common ABO blood type in North America, but it is caused by the recessive allele i. Blood type AB, caused by the two dominant ABO alleles, is the rarest. Polydactyly, caused by a dominant allele, also is rare in the population.

Definitions of some basic genetic terms are summarized in table 4.4.

Before You Go On

Answer the following questions to test your understanding of the preceding section:

  1. Why must the carrier of a genetic disease be heterozygous?
  2. State at least three reasons why a person's phenotype can't always be determined from the genotype.
  3. A man can inherit color blindness only from his mother, whereas a woman must inherit it from both her father and mother to show the trait. Explain this apparent paradox.
  4. Cover the left side of table 4.4 with a blank strip of paper, look at the definitions, and fill in the term to which each definition refers. Check your spelling.

Insight 4.4 Clinical Application

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