Genes

Genes

Children inherit traits from their parents and rel-atives as determined by DNA. Individual segments of DNA are known as genes. When a gene’s DNA sequence changes or mutates, an illness or other condition may result. Spontaneous mutations occur because of random DNA replication errors. The field of genetics investigates how genes result in certain characteristics affecting health or contributing to natural variation. It also focuses on how genes are passed from generation to generation. Today, fetal chromosome checks provide clues to a child’s future health.

All human cells, except gametes, have a diploid number of chromosomes (46). This number consists of 23 pairs of homologous chromosomes. One pair is from the father (via the sperm) and the other pair is from the mother (via the egg). Homologous chro-mosomes appear similar, carrying genes for the same traits. However, they do not always bring about the same expressions of the traits.

Autosomes are pairs 1 through 22 and do not carry genes that determine sex, but function to guide the expression of most other traits. Only pair 23, the X and Y chromosomes, is sex chromosomes. Females have two X chromosomes and males have one X and one Y chromosome. A sex chromo-some pair may be very different in size. Karyotypes are charts that display the 23 chromosome pairs of human somatic cells (FIGURE 27 -1). The diploid genome is actually an individual’s genetic (DNA) makeup. It represents the two sets of genetic instruc-tions, from the egg and from the sperm.

Alleles

Somatic cells have two copies of each autosome, and therefore two copies of each gene. Gene copies can be identical or slightly different in a DNA sequence. Such varying forms of a gene are called alleles. A gene’s position on a chromosome is called a locus. An allele may code for identical or different forms of a given trait. An individual with two identical alleles of a gene is homozygous for that gene. A person with two different alleles is heterozygous for that gene. A heterozygote is also called a carrier. A person with only one copy (allele) of a gene is referred to as hemizygous.

Patterns of inheritance through families are known as modes of inheritance. A dominant allele masks expression of a recessive allele. Genes cannot be expressed unless they are present in a homozy-gous condition. Capital letters are used to designate dominant alleles. The extent to which a certain allele is expressed when it is present is called its expressiv-ity. The following generalizations describe modes of inheritance:

■■ Autosomal conditions that affect both sexes: X-linked characteristics affect males much more often than females. Y-linked characteristics are only passed from father to son.

■■ Autosomal recessive conditions from two healthy carrier parents: Recessive conditions can “skip” generations.

■■ Dominant condition with at least one affected par-ent: Generations are not skipped in these cases.

Genotype and Phenotype

The alleles situated at one or more sites on homologous chromosomes make up an individual’s ­genotype (actual genetic makeup). A pair of alleles is usually designated by letters or symbols, such as “AA” when the alleles are identical and “Aa” when they are differ-ent. Penetrance is the regularity with which an allele is expressed in an individual who carries it. If the allele always produces its effect on the phenotype, it is called fully penetrant. The condition called achondroplasia is caused by a fully penetrant allele. When the allele is present, achondroplasia results.

A phenotype consists of the complete observable characteristics of an organism or group. It includes anatomic, physiologic, biochemical, and behavior traits that are determined by the interaction of genetic makeup and environmental factors. A certain inher-ited trait may be anticipated to occur in the offspring of two individuals when their genes and chromosomes are considered.

1. Explain the modes of inheritance.

2. How do the sex chromosomes differ between males and females?

3. Explain dominant and recessive alleles.

4. How does X-linked recessive inheritance affect males and females?

5. What are the differences between a genotype and a phenotype?