Exploring genetics and inheritance
The genes in your body’s cells are the reason you have your mother’s brown eyes and your father’s curly hair. Neurons are cells, and, like all cells in the body, they’re controlled by the expression of the DNA within their nuclei. Although the DNA in all non-reproductive cells in the body is the same, how the genes are expressed is what makes the body’s 300-plus cell types different from each other.
This chapter covers the basic genetics common to all cells, such as genes, chromosomes, and inheritance. It also discusses the universal genetic code, the expression of genes, and protein synthesis. And if you ever wondered what makes neurons so special compared to other cells, read on to find out about their unique features and functions. Bringing these ideas together, the final sections talk about what happens when neurons have genetic defects, such as mutations that lead to neurological illness. I also look at how science may be able to fix these problems.
Getting into Genetics
Genetics is the study of genes and how they control inheritance. We can typically see the results of inheritance in the features of offspring, which has received genes from each of its parents. Genes are sequences of nucleotides (see “Greeting chromosomes and genes,” a bit later on for more) located on chromosomes in the cell’s nucleus. Genes specify the production of amino acid sequences. (Amino acids are the constituent units that make up proteins.)
Long before genes were known to be located on chromosomes composed of DNA, science had worked out some basic principles of inheritance, such as dominant and recessive traits. The upcoming sections explore these concepts in more detail.
Introducing inheritance
Mendelian inheritance is one of the cornerstones of genetics, based on the famous pea experiments of the monk Gregor Mendel. Genes determine the features, or traits, that you inherited from your parents. Some traits you can see, such as height or hair color, and others you can’t, such as blood type. Genes are copied and inherited across generations. Different genes cause different traits to present themselves, and each unique form of a single gene is called an allele. So, for example, the gene specifying blue eyes comes from a different allele than the gene specifying brown eyes.
Doubling genes
Organisms typically have two copies of each gene, one inherited from each parent. Each parent also has two copies of each gene, and passes along to its offspring a single copy by a (nearly) random selection of their two genes. This gene is found on a chromosome present in either the mother’s eggs or the father’s sperm. When the egg and sperm join, they form a double set of genes again.
Phenotype and genotype
The set of expressed traits of an organism are called its phenotype, whereas the genes within the organism are called its genotype. Since Mendel’s experiments, we have known that the traits an offspring expresses (phenotype) are not simply a mixing of the traits it got from the two sets of genes it inherited from its parents (genotype).
Determining dominant and recessive traits
One allele (that is, one particular form of a gene) may completely override the expression of another, making this the dominant allele. The non-expressed allele, or the allele that doesn’t show up in the offspring as a trait, is called the recessive allele. For a recessive allele to be expressed in an offspring as a trait, the offspring must receive two copies of the same recessive allele. Otherwise, the traits associated with the dominant allele will be expressed. For example, the O blood type is recessive compared to types A and B. An AO father and BO mother would have A and B blood types, respectively, because the alleles specifying A blood type and B blood type are dominant. However, because these parents both have the recessive O allele, they could potentially produce an OO child. ( This happens about 25 percent of the time.)
Most traits, such as height, depend on several genes. These traits have more complicated, mixed inheritance, because each of a number of genes contributes something to the end result. Mutations, which are random changes in genes, can also occur. Mutations can convert one allele into another, or even create a totally new allele and corresponding trait. (Go to the end of this chapter for more about mutations.)
Greeting chromosomes and genes
Genes are made of DNA (DNA stands for deoxyribonucleic acid), a nucleic acid that is composed of long sequences of nucleotides. DNA exists in the form of a double helix that is tightly coiled within the nucleus of the cell.Only four types of nucleotides make DNA:
Adenine (A)
Cytosine (C)
Guanine (G)
Thymine (T)
Mitotic cell division or mitosis is the process in which chromosomes are duplicated, segregated, and allocated so that each daughter cell has a complete set of chromosomes derived from the parent. Eukaryotic organisms (those whose cells have a nucleus), such as animals and plants, have most of their DNA inside the cell nucleus — although some DNA exists in organelles such as mitochondria. Prokaryotic organisms (those whose cells generally do not have a nucleus), such as bacteria, have DNA in their cytoplasm.
Each organism has a unique sequence of DNA. The only exception: identical twins, although even identical twins may differ slightly because of mutations accrued during their development. Humans have 23 pairs of chromosomes that make up a total of over 3 million nucleotides.
Replicating DNA and the cell life cycle
Cell division (see the preceding section) involves several additional processes.
DNA replication
DNA replication is the process of copying DNA. It’s important for cell division, so that each daughter cell inherits the full genome of its parent cell. The complementary double-stranded DNA molecule splits, and each strand produces a new complement, creating two identical copies of the double-stranded DNA sequence.
Cell division occurs in a cell cycle sequence (which we explore in a moment). A different type of DNA replication occurs