Relationship Between Gene Expression and Traits

Section 2: Relationship Between Gene Expression and Traits

We know that an organism's traits are controlled by genes. But how exactly do genes exert control over traits? The discussion above prompts us to consider that genes and traits do not always have a one-to-one correspondence. What does this mean?

Relationship Between Gene Expression Products and Traits

Over a century ago, Mendel studied the contrasting traits of round and wrinkled peas, offering a brilliant explanation through his theory of genetic factors. Today, how can we provide a deeper explanation from the perspective of gene expression? It turns out that the DNA of wrinkled peas contains an inserted foreign DNA sequence, disrupting the gene encoding starch branching enzyme. This disruption leads to an abnormality in starch branching enzyme activity, significantly reducing starch content within cells. Starch plays a role in retaining water within cells. When peas mature, those with high starch content effectively retain moisture, appearing plump, whereas those with low starch content shrink due to dehydration (Figure 4-9).

From the above example, it's evident that genes control metabolic processes by regulating enzyme synthesis, thereby influencing an organism's traits. For instance, albinism in humans results from an abnormal gene encoding tyrosinase. Tyrosinase, present in normal human skin and hair, converts tyrosine into melanin. If a person lacks tyrosinase due to a genetic anomaly, melanin cannot be synthesized, resulting in symptoms of albinism.

Furthermore, genes can directly control traits by influencing protein structure. For example, in approximately 70% of cystic fibrosis patients, there is a deletion of three nucleotides in the gene encoding CFTR protein, a transporter protein. This deletion causes a phenylalanine deficiency at position 508 of the CFTR protein, altering its spatial structure. As a consequence, CFTR's ability to transport chloride ions is compromised, leading to excessive mucus production in the bronchi, obstructing airways, and promoting bacterial growth in the lungs, ultimately severely impairing lung function.

Selective Gene Expression and Cellular Differentiation

The formation of various traits in an organism is based on cellular differentiation. Different types of cells within the same organism have identical genes, yet they exhibit different morphologies, structures, and functions. Why is this so?

Scientists have found that genes in cells are selectively expressed; in different types of cells, expressed genes can be broadly categorized into two types. One type includes genes expressed in all cells, guiding the synthesis of proteins essential for maintaining basic cellular activities, such as ribosomal protein genes and ATP synthase genes. The other type includes genes specifically expressed in certain types of cells, such as ovalbumin genes and insulin genes. The essence of cellular differentiation lies in selective gene expression, which is regulated by gene expression control mechanisms.

Epigenetics

When and where genes are expressed, as well as the level of their expression, are all regulated, directly influencing traits.

In the examples above, even though the nucleotide sequences of the Lcyc gene in the morning glory and the A" gene in mice remain unchanged, some nucleotides undergo methylation modifications (Figure 4-10), suppressing gene expression and thereby affecting phenotype. This DNA methylation modification can be inherited, causing offspring to exhibit the same phenotype. This phenomenon, where genetic sequences remain unchanged but heritable changes occur in gene expression and phenotype, is called epigenetic inheritance.

Epigenetic phenomena are widespread throughout the entire life cycle of organisms, influencing growth, development, and aging processes. For example, slight differences between genetically identical monozygotic twins are related to epigenetic inheritance. In a bee colony, both the queen bee and worker bees develop from fertilized eggs, but they differ significantly in morphology, structure, physiology, and behavior due to epigenetic inheritance playing a crucial role. Interested students can refer to additional literature to learn more about examples of epigenetic inheritance.

In summary, genes control traits through their expression products—proteins. Whether genes are expressed in cells and the level of their expression are tightly regulated. The essence of cellular differentiation results from selective gene expression. Epigenetic inheritance allows organisms to exhibit heritable changes in traits without changes in the genetic sequence.

In most cases, the relationship between genes and traits is not straightforward. Multiple genes can influence a single trait, such as human height, where each gene has a certain effect on height. Conversely, a single gene can influence multiple traits. For instance, researchers in China discovered that the Ghd7 gene in rice not only regulates flowering but also plays a crucial role in the growth, development, and yield of rice. Additionally, traits are not solely determined by genes; environmental factors also play a significant role. For example, nutrition and physical exercise postnatally significantly affect human height, and the formation of two types of leaves in "problem exploration" is also related to environmental factors.

Genes interact with other genes, gene expression products, and the environment in a complex network, finely regulating the traits of organisms.