Allele

Alleles are variant forms of the same gene that give rise to different versions of a particular trait. At conception, when the male gamete (sperm) and female gamete (egg or ovum) fuse to form a single fertilized egg called a zygote, the newly formed organism receives one complete set of chromosomes from each parent, contained in the deoxyribonucleic acid, or DNA, of each gamete cell. These two sets of chromosomes carry the genes that code for the proteins that will make up the new organism. The two sets of chromosomes are similar, in that chromosomes in the same position code for roughly the same genetic information, but they contain small differences in the sequence of the DNA bases. These two slightly different versions of the same gene are alleles. Differences between alleles allow the new organism to resemble its parents without looking identical to either one.

Alleles can either be dominant or recessive, which affects the appearance of the new organism as well as its susceptibility to various diseases and disorders. An organism can carry a dominant allele and a recessive allele for the same trait, with the recessive allele present but not expressed within its genome. Despite the lack of expression, the recessive allele can still be passed on to the organism’s offspring; if the other parent also passes on a recessive allele for the same trait, the offspring will manifest that trait.

Background

The groundbreaking work that laid the foundations for the modern understanding of genetic inheritance started with the field notes of an Austrian gardener. Gregor Mendel (1822–84) was an Augustinian friar with little background in biology, but while tending pea plants in the friary gardens, he became curious about how they passed on characteristics such as petal color, seed shape, and plant height to the next generation. His subsequent experiments, detailed in an 1866 paper on plant hybridization, remained unnoticed by the scientific community until after his death, but the eventual rediscovery of his work paved the way for the future study of genetics.

By closely observing how basic traits were passed from one generation of pea plants to the next, Mendel posited that the evident links between generations of plants were far from random and that there must have been physical constructs within the plants themselves that determined which traits would be inherited. He sought to discover why some traits are passed down while others are not expressed in the first generation of offspring but can be passed to subsequent generations. Based on his observations, Mendel proposed what he called the law of segregation, the law of independent assortment, and the law of dominance, which together are known as Mendel’s laws of heredity.

The law of segregation states that each gamete contains only one hereditary particle for each trait (an allele), which has been segregated out of a pair of two chromosomes belonging to each parent during the formation of gametes, a process now known as meiosis, and then joined with the other parent gamete’s segregated alleles at fertilization. The law of independent assortment holds that separate alleles determine each trait and that each trait is expressed independently of other traits. The law of dominance states that alleles may be either recessive or dominant and that a dominant allele will always mask the expression of a recessive allele.

Although Mendel lacked the sophisticated technology to document his theoretical work, subsequent research by geneticists such as William Bateson (1861–1926), Reginald Crundall Punnett (1875–1967), and E. R. Saunders (1865–1945) refined Mendel’s ideas, leading them to observe the effects of gene linkage on heritability, which challenged Mendel’s proposed law of independent assortment. Punnett also developed a grid, now known as the Punnett square, that accounts for the occurrences of fundamental characteristics and the heritability of various traits. Swedish geneticist Herman Nilsson-Ehle (1873–1949) later described the principles of polygenic inheritance to account for traits that are determined by more than one gene pair.

Overview

The allele is critical to the heritability of each trait; it is fundamentally a sequence of coded information that carries traits across generations. Each gene within a chromosome has two alleles, which may be either dominant or recessive. Most organisms are diploid, meaning they have two sets of chromosomes, one from each parent, and thus two alleles for each trait. Gamete cells are haploid, meaning they contain only a single set of chromosomes.

In humans, all cells in the body, with the exception of the gametes, contain twenty-three pairs of chromosomes, or forty-six total. To form a gamete (sperm or ovum), a diploid cell undergoes a process called meiosis, a form of cell division in which each of the two resulting daughter cells is haploid, containing half of the chromosomes of the parent cell. When the sperm and ovum fuse together at fertilization to form the zygote, the twenty-three chromosomes of the male sperm and the twenty-three chromosomes of the female ovum join together to form the complete genetic blueprint of the new organism.

To understand how the expression of traits in an individual is determined by alleles, consider hair color. (While most traits, including hair color, are determined by more than one gene, a simplified overview of the process can serve as a helpful example.) Each allele may be dominant or recessive; in the case of hair color, the allele for dark hair is dominant, while the allele for light hair is recessive. If both inherited alleles encode for dark hair, that gene is considered homozygous, and the offspring will have dark hair. If the offspring inherits one allele for dark hair and one allele for light hair, the gene is said to be heterozygous, or mixed; the dominant allele will mask the expression of the recessive allele, and that offspring will also have dark hair. However, the recessive characteristic is still encoded in the offspring’s genome and can potentially be passed down to the subsequent generation.

If both parents pass a recessive allele to their child, even if both are heterozygous for that trait and therefore do not express it, that child will present the recessive trait. For example, if both parents carry a heterozygous gene for hair color, they will both have dark hair themselves, but any child of theirs will have a one-in-four chance of inheriting both recessive alleles and thus having light-colored hair. A gene with two identical recessive alleles is also considered to be homozygous.

Although the concept of alleles and their function within the transmission of genetic information is often explained using basic biological characteristics such as hair color (or pea-flower color), within the field of human genetics the allele is key to understanding far more complicated inherited tendencies. While Mendel’s experiments happened to focus on traits whose expressions were not affected by other traits—thus informing his law of independent assortment—scientists today understand that this law is only true for genes that are not linked. Many of the traits Mendel chose to observe in his experiments are now known to be determined by alleles that are spaced distantly from one another on the same chromosome or that occupy different chromosomes entirely. Traits with alleles that are located on the same chromosome are linked, meaning that the expression of one allele may make other traits more likely to be expressed. Scientists are currently trying to determine the genetics of many complex traits and inherited disorders, which are likely determined by multiple genes working in concert with one another and further affected by the prenatal and postnatal environment.

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