CONCEPTS IN BIOLOGY
PART III. MOLECULAR BIOLOGY, CELL DIVISION, AND GENETICS
10. Patterns of Inheritance
Geneticists Hard at Work
Mendel would be pleased to know what his discovery is revealing
Since Gregor Mendel’s work was accepted as “law” in the early 1900s, geneticists have made many important discoveries. This field has really exploded with new, lifechanging or just plain interesting information since the era of molecular genetics came about during the 1950s and 1960s. Some of these discoveries revealed the existence of actual genes responsible for specific characteristics or conditions. Others help to explain the factors that control whether a gene is expressed or how its expression is modified.
Here just a few recent revelations from scientists working in the field of genetics:
ü Certain soil bacteria have been discovered that have genes that allow them to feed exclusively on antibiotics. This is of concern because these bacteria live in close association with human and livestock pathogens.
ü Charles Darwin proposed that human facial expressions are universal. Recent and continuing research is lending support to this hypothesis. Researchers found that, in fact, facial expressions are genetically determined.
ü While the genetic abnormality causing Huntington’s disease causes neurons in the brain to be destroyed, it also plays a role in destroying cancer cells. People with Huntington’s are less likely than others to suffer from cancer. It appears that the huntingtin gene has more than one effect.
ü The inheritance of “dominant black” coat color in domestic dogs involves a gene that is distinct from, but interacts with, the genes responsible for conventional coat pigmentation. Variations in this gene are responsible for the color differences in yellow, black, and brindle-colored dog breeds. This same gene is responsible for the production of a protein (P-defensin) that in other species is able to aid in the destruction of microbes. The presence of black coat color in wolves is the result of occasional interbreeding of dogs with black coat color and grey wolves.
ü The gene DISCI (Disrupted-in-Schizophrenia 1) has been strongly implicated in cases of schizophrenia, major depression, bipolar disorder, and autism.
• Who was Mendel? What role did he play in the field of genetics?
• How might these discoveries influence your understanding of life?
• In order to make the discoveries noted in the article, what basic ideas do you need to understand?
ü Background Check
Concepts you should already know to get the most out of this chapter:
• The connection between genes, DNA, and chromosomes (chapter 8)
• The patterns of chromosome movement during meiosis (chapter 9)
• The concepts of segregation and independent assortment (chapter 9)
10.1. Meiosis, Genes, and Alleles
Genetics is the branch of science that studies how the characteristics of living organisms are inherited. Classical genetics uses an understanding of meiosis to make predictions about the kinds of genes that will be inherited by the offspring of a sexually reproducing pair of organisms. Offspring are the descendants of a set of parents.
Various Ways to Study Genes
The previous chapters of this text used the term gene. In chapter 8, a gene was described as a piece of DNA with the necessary information to code for a protein and regulate its expression. In chapter 9, on cell division, genes were described as locations on chromosomes. Both of these views are correct, because the DNA with the necessary information to make a protein is packaged into a chromosome. When a cell divides, the DNA is passed on to the daughter cells in chromosomes.
This chapter introduces another way to think about a gene. A gene is related to a characteristic of an organism, such as a color, a shape, or even the ability to break down a chemical. The characteristics usually result from the actions of proteins in the cell.
What Is an Allele?
Recall from chapter 9 that an allele is a specific version of a gene. Consider a characteristic such as earlobe shape. Some earlobes are free and some are attached (figure 10.1). These types of earlobes are two versions, or alleles, of the “earlobe-shape” gene. The two different alleles of this gene produce different versions of the same type of protein. The effect of these different proteins results in different earlobe shapes. Thus, there is an allele for free earlobes and a different allele for attached earlobes.
FIGURE 10.1. Genes Control Structural Features
Whether your earlobe is (a) free or (b) attached depends on the alleles you have inherited. As genes express themselves, their actions affect the development of various tissues and organs. In some people the expression results in the earlobe being separated from the side of the face during fetal development, forming a “free” lobe. In others, the lobe remains “attached.”
Genomes and Meiosis
A genome is a set of all the genes necessary to code for all of an organism’s characteristics. In sexually reproducing organisms, a genome is diploid (2n) when it has two copies of each gene. When two copies of a gene are present, the two copies need not be identical. The copies may be the same alleles, or they may be different alleles of the same gene.
The genome of a haploid (n) cell has only one copy of each gene. Sex cells, such as eggs and sperm, are haploid. Because sperm and eggs are haploid, they have only one allele of a gene (review meiosis in chapter 9). If the parent has two different alleles of a gene, the parent’s sperm or eggs can have either version of the alleles, but not both at the same time. When a haploid sperm (n) from a male and a haploid egg (n) from a female combine, they form a diploid (2n) cell, called a zygote. The alleles in the sperm and the alleles in the egg combine to form a new genome that is different from either of the parents. This means that each new zygote is a unique combination of genetic information.
Meiosis is a cell’s process of making haploid cells, such as eggs or sperm. Understanding the process of meiosis is extremely important to making genetic predictions. If you don’t understand the cellular process of meiosis, your predictions will be less accurate. Figure 10.2 shows a pair of homologous chromosomes that have undergone DNA replication. After DNA replication, each homologous chromosome has two, exact copies of each allele, one on each chromatid.
FIGURE 10.2. Homologous Chromosomes—Human Chromosome 1
Homologous chromosomes contain genes for the same characteristics at the same place. Different versions, or alleles, of the genes may be present on different chromosomes. This set of homologous chromosomes represents chromosome 1 in humans. Chromosome 1 is known to contain genes that play a role in glaucoma, prostate cancer, and Alzheimer’s disease. The three genes shown here may be present in their normal form or in their altered, mutant form. Here, different genes are shown as specific shapes. The alleles for each gene are shown as different colors.
When the cell undergoes meiosis I, the two homologous chromosomes go to different cells. This reduces the cell’s genome from diploid to haploid. In meiosis II, the chromatids of each chromosome are separated into different daughter cells. The cells resulting from meiosis II mature to become sperm or eggs. The probability that an allele will be passed to a sperm or an egg is related to the number of times that allele is present in the cell before meiosis begins. These probabilities are used in making predictions in genetic crosses.
10.1. CONCEPT REVIEW
1. How does the term gene relate to the term allele?
2. Define the term genome.
3. What is meant by the symbols n and 2n?