Traits, Genes, and Genetic Engineering Study Guide

Mendel's Hereditary Factors: Traits, Genes, and Alleles

While Gregor Mendel lacked modern knowledge regarding DNA and genes, he correctly hypothesized the existence of a hereditary factor responsible for carrying genetic information. These factors are known as genes. A gene is defined as a specific piece of DNA that provides a set of instructions to a cell for the production of a particular protein. Every gene occupies a specific physical location on a pair of homologous chromosomes, referred to as its locus. This locus serves as an "address" to identify the gene's position. Human cells contains a total of $46$ chromosomes arranged in $23$ pairs of homologous chromosomes. Genes located on these chromosomes are transferred to offspring during reproduction, forming the basis of heredity. Mendel's work demonstrated that it is not the traits themselves that are passed down, but the genes responsible for those traits.

Genes contain genetic information that varies significantly between organisms because of different alleles. An allele is defined as any of the alternative forms or versions of a gene that may occur at a specific locus. In human cells, there are two alleles for every gene, located on the homologous chromosomes. One allele is inherited from each parent. This pattern of inheritance is common to almost all sexually reproducing organisms, such as pea plants. The variations Mendel observed in traits, such as plant height or flower color, were the result of these varying alleles.

Genotype and Phenotype

When discussing combinations of alleles, the term homozygous describes a condition where an organism has two of the same alleles at a specific locus. Conversely, heterozygous describes a condition where an organism has two different alleles at the same locus. For instance, an individual might inherit an allele for freckles from one parent and an allele for no freckles from the other. Similarly, a pea plant might possess one allele for purple flowers and one for white flowers, making it heterozygous for that specific trait.

The term genotype refers to the actual genetic makeup of an organism. If a pea plant possesses one allele for round seeds and one allele for wrinkled seeds, its genotype is heterozygous. Phenotype refers to the actual physical characteristics or traits expressed by an individual. Even if a plant has a genotype containing an allele for wrinkled seeds, its phenotype may express round seeds if the round allele is dominant.

Allele Dominance and Recessivity

In many instances, one allele in a pair determines the expressed trait over the other. A dominant allele is the allele that is expressed when two different alleles or two dominant alleles are present. A recessive allele is only expressed when two recessive copies are present together. Allele combinations are represented by letters, with two letters needed for each gene in a body cell. Uppercase letters signify dominant alleles, and lowercase letters signify recessive alleles.

In pea plants, the dominant allele for round peas is represented by the letter $R$, while the recessive allele for wrinkled peas is represented by $r$. The round phenotype occurs in plants with the homozygous dominant ($RR$) or heterozygous ($Rr$) genotypes. The wrinkled phenotype only occurs when the plant has the homozygous recessive ($rr$) genotype.

Meiosis I: Process and Genetic Diversity

Meiosis involves two stages of division. Before meiosis I begins, DNA is copied during the S phase of the cell cycle. Meiosis I is characterized by the separation of homologous chromosomes, resulting in two haploid cells with duplicated chromosomes.

1. Prophase I: The nuclear membrane breaks down, centrosomes and centrioles migrate to opposite cell sides, and spindle fibers assemble. Duplicated chromosomes condense, and homologous chromosomes pair up precisely, gene for gene, along their entire length. Sex chromosomes also pair and align.

2. Metaphase I: Homologous chromosome pairs line up randomly along the cell equator (middle) while attached to spindle fibers. This random arrangement results in $23$ chromosomes—a mix from the mother and father—lining up on each side. This process mixes chromosomal combinations to maintain genetic diversity.

3. Anaphase I: The paired homologous chromosomes separate and move toward opposite poles of the cell. Critically, sister chromatids remain together during this phase.

4. Telophase I: The cell undergoes cytokinesis. In some species, the nuclear membrane reforms and spindle fibers disassemble during a period between meiosis I and meiosis II.

Meiosis II: Separation of Sister Chromatids

Meiosis II follows meiosis I without any intervening DNA replication. This stage separates sister chromatids, resulting in chromosomes that are not doubled.

5. Prophase II: The nuclear membrane breaks down again, centrosomes and centrioles move to opposite sides, and spindle fibers assemble.

6. Metaphase II: Spindle fibers align the $23$ chromosomes at the cell equator. Each chromosome at this stage still consists of two sister chromatids.

7. Anaphase II: Sister chromatids are pulled apart and migrate toward opposite sides of the cell.

8. Telophase II: Nuclear membranes form around the chromosomes at opposite ends, spindle fibers break apart, and the cell undergoes cytokinesis.

Cloning Organisms and Agricultural Engineering

Cloning is the process of producing genetically identical offspring. Many plants naturally produce clones through asexual reproduction. Humans have utilized this for thousands of years by taking cuttings from plants with desirable traits, such as larger or more flavorful fruit, to produce identical plants. In bacteria, clones are produced via binary fission, where the bacterial chromosome is replicated and the cell splits into two genetically identical daughter cells. This allows beneficial traits, such as antibiotic resistance, to spread rapidly.

In complex organisms like vertebrates, cloning has a lower success rate. In 1903, Hans Spemann demonstrated embryo twinning by separating the cells of two-celled salamander embryos. Each cell developed into a complete, normal salamander, proving that embryonic cells contain a full set of genetic material.

As the global population rises, there is a higher demand for food. Scientists have engineered drought-resistant crops to maintain food production in dry climates where traditional commercial crops are not adapted. Other strategies include water conservation, sustainable farming, and improved fertilizers.

Milestones in Nuclear Transfer and Mammalian Cloning

Cloning mammals typically involves nuclear transfer: replacing the nucleus of an unfertilized egg with the nucleus of a cell from the animal being cloned. The egg is then implanted into a surrogate mother.

1. 1952: Robert Briggs and Thomas King performed the first successful nuclear transfer using an embryonic frog cell nucleus and an enucleated egg cell, which developed into a tadpole.

2. 1996: Dolly the sheep became the first mammal cloned from an adult somatic (body) cell. Somatic cells are differentiated, meaning many genes are deactivated; these must be reactivated for cloning to work. This was a low-efficiency process; Dolly was the only survivor out of $277$ attempts.

3. Post-Dolly: Milestones include the cloning of primates, generating sheep from genetically engineered cells, cloning endangered species, and creating stem cells through somatic cell nuclear transfer. Pet cloning services now exist to produce genetic copies of pets, though environmental factors mean these clones may behave or look different from the original.

Cloning Ethics and Immortal Cells

In 1951, Henrietta Lacks died of cervical cancer. Before her death, a researcher took a sample of her tumor without her knowledge, leading to the creation of the HeLa cell line. HeLa cells are "immortal" because they do not die when cultured in a lab and instead divide indefinitely. These cells have been fundamental to scientific research for over half a century, contributing to the development of the polio vaccine, AIDS research, cloning, and space experiments. However, new advancements in cloning, particularly the prospect of human cloning, continue to raise significant ethical concerns.

Polymerase Chain Reaction (PCR)

Invented by Kary Mullis in 1983 (who shared the 1993 Nobel Prize in Chemistry), the Polymerase Chain Reaction (PCR) solved the problem of amplifying small segments of DNA for testing. PCR uses oligonucleotides (short DNA segments) as primers and a thermocycler to regulate temperature. The three-step cycle includes:

1. Separating: The sample is heated to approximately $95\,^{\circ}\text{C}$ ($203\,^{\circ}\text{F}$) to separate complementary DNA strands.

2. Binding: The sample cools to approximately $55\,^{\circ}\text{C}$ ($131\,^{\circ}\text{F}$). Two primers, which are short nucleotide segments, bind to the beginning of the target segment on each separate strand to allow DNA polymerase to attach.

3. Copying: The temperature is raised to $72\,^{\circ}\text{C}$ ($162\,^{\circ}\text{F}$). DNA polymerase adds free nucleotides to the primers to synthesize new complementary strands.

After the third cycle, the first fragment of the target DNA sequence is synthesized. After $30$ cycles, more than $1 \times 10^9$ (one billion) fragments are created.

Genetic Testing and Replicating Genes

Genetic testing involves analyzing DNA to determine the risk of genetic disorders. This ranges from examining entire chromosomes to individual genes or measuring proteins that serve as indirect indicators of DNA patterns. Because identifying disease-causing genes among the $20,000$ to $25,000$ genes in the human genome is difficult, PCR is used to amplify target sequences to provide enough material for testing in hours rather than weeks.

DNA microarrays are tools used to study the expression of thousands of genes simultaneously. These are small chips (often one square inch) containing a grid of thousands of gene samples. They allow researchers to see which genes are expressed in specific tissues and under specific conditions.

Types and Effects of Mutations

Mutations are changes in DNA that can be categorized as gene mutations or chromosomal mutations. Whether a mutation is inherited depends on the cell type. Germ cells are involved in gamete formation; mutations here can be passed to offspring and affect their phenotype. Often these are harmful, though they can rarely be beneficial. Somatic cells (body cells) can also develop mutations, and while these are not passed to offspring, they can build up over time, potentially contributing to aging or cancer.

1. Gene Mutations: These are changes in a single gene's DNA sequence, often occurring during replication. While DNA polymerase has a proofreading function, some errors persist.

2. Frameshift Mutations: These occur when an insertion or deletion of a nucleotide shifts the entire reading frame, changing all subsequent codons and the resulting amino acids.

3. Trinucleotide Repeat Expansions: These involve repeats like CAG CAG CAG. DNA polymerase may "slip" during replication, forming a hairpin loop that results in a longer double strand as the cell divides.

4. Chromosomal Mutations: These involve changes to chromosome segments or whole chromosomes, often during mitosis or meiosis.     - Gene Duplication: During crossing over in meiosis, if chromosomes misalign, one chromosome may end up with two copies of a gene while the other undergoes gene deletion.     - Evolutionary Benefit: Gene duplication in the douc langur produced digestive enzymes for leaves and fruits. Duplicated genes allow one copy to maintain function while the other accumulates mutations that may lead to new structures or functions, increasing genetic variation.

Causes and Repair of Mutations

Mutagens are environmental agents that increase mutation frequency. Examples include radiation (X-rays, UV light), chemicals (processed foods, cosmetics, carcinogens like cigarettes), and biological agents (viruses and bacteria).

A specific example is the thymine dimer caused by UV light, where neighboring thymine nucleotides bond to each other instead of adenine. This creates a kink in the DNA that interferes with replication. Correction processes involve enzymes that remove the dimer, replace the segment, and bond it in place. If these repairs fail in genes regulating cell growth, cancer may result.

Questions & Discussion

Question: How is an allele related to a gene? Answer: An allele is an alternative form or version of a gene that occurs at a specific locus.

Question: What is one question you could ask about how traits are expressed when an organism has heterozygous alleles for a trait? Answer: (This prompt encourages student inquiry into dominance, incomplete dominance, or codominance).

Question: Why is a clone not an exact copy of a donor animal? Consider the effect of genetics and environmental conditions. Answer: Although genetically identical, clones are influenced by environmental factors that affect phenotype and behavior, leading to differences between the clone and the donor.

Question: Why can't genetic testing identify all diseases? How does inheriting cystic fibrosis differ from developing cardiovascular disease due to poor diet and exercise? Answer: Genetic testing identifies disorders caused by specific gene abnormalities. Cystic fibrosis is a direct result of inherited genetic material, whereas cardiovascular disease is often a complex result of both genetics and environmental/lifestyle factors like diet and exercise.

Question: Is the sickle cell allele dominant or recessive? Explain how you know. If two carriers have children, what is the probability of one of their children having the disease? Answer: Sickle cell anemia (HbS) is recessive because individuals with only one copy are carriers and do not express the disease; two copies are required for the phenotype. For two carriers ($Aa \times Aa$), there is a $25\%$ (or $\frac{1}{4}$) probability that a child will inherit the disease ($aa$).