Cells and Heredity, Chapter 3 Genetics
Understanding Heredity and Traits
Heredity: The passing of traits from parents to offspring. It is the fundamental concept in genetics, explaining why offspring resemble their parents but are not identical copies.
Trait: A specific characteristic that an organism can pass to its offspring. Traits can be physical characteristics, such as eye color or height, or behavioral characteristics. Examples from Mendel's work include pea plant height, seed color, and flower position.
Genetics: The scientific study of heredity, focusing on how traits are passed down from one generation to the next and the molecular mechanisms that govern their expression.
Fertilization: The process in sexual reproduction where male and female reproductive cells (gametes) join to form a new cell (zygote), initiating the development of a new organism. This is a crucial step for combining genetic material from two parents.
Purebred: An organism that is the offspring of many generations that have the same trait. When self-pollinated or bred with another purebred for the same trait, they consistently produce offspring with that same trait. Genetically, purebred organisms are homozygous for the trait in question.
Hybrid: An organism that has two different alleles for a specific trait. Hybrids are typically the result of crossing two purebred parents with contrasting traits (e.g., a purebred tall pea plant with a purebred short pea plant). Genetically, hybrids are heterozygous.
Genes and Alleles: The Basis of Inheritance
Gene: A segment of DNA on a chromosome that codes for a specific trait. Genes are the basic units of heredity and are passed from parents to offspring.
Alleles: The different forms or variations of a gene. For example, the gene for pea plant height can have two alleles: one for tallness and one for shortness.
Dominant Allele: An allele whose trait always shows up in the organism when the allele is present. It masks the effect of a recessive allele if both are present. Represented by a capital letter (e.g., T for tall).
Recessive Allele: An allele whose trait is masked, or hidden, when a dominant allele is present. This trait only shows up if the organism inherits two copies of the recessive allele. Represented by a lowercase letter (e.g., t for short).
Mendel's Experiments and the Control of Traits
Results of Mendel’s experiments: Gregor Mendel, the "father of genetics," conducted experiments with pea plants. He cross-pollinated purebred pea plants with contrasting traits (e.g., purebred tall with purebred short). His key findings include:
First Filial Generation (F1): All offspring in the F1 generation displayed only one of the two parental traits (e.g., all tall plants resulted from crossing tall and short purebreds). The other trait seemed to disappear.
Second Filial Generation (F2): When F1 plants were allowed to self-pollinate, the "lost" trait reappeared in the F2 generation. This reappearance consistently occurred in a specific ratio, often approximately 3:1 (e.g., 3 tall plants for every 1 short plant).
These results led to the understanding that traits are controlled by discrete units (now called genes/alleles) and that some alleles are dominant while others are recessive.
What controls the inheritance of traits in organisms: The inheritance of traits is controlled by genes, which are specific segments of DNA located on chromosomes. Each organism inherits two copies of each gene (alleles), one from each parent. The combination of these alleles determines the expression of the trait.
Probability and Genetic Crosses
Probability: A measure of how likely it is that some event will occur. In genetics, probability is used to predict the chances of offspring inheriting specific traits from their parents. It is expressed as a number between 0 and 1 (or 0\% and 100\%), where 0 means an event is impossible and 1 means it is certain.
The formula for probability is: P = \frac{\text{Number of favorable outcomes}}{\text{Total number of possible outcomes}}
For example, the probability of flipping a coin and getting heads is 1/2. In genetics, if there are two equally likely alleles an offspring can inherit, the probability of inheriting one specific allele is also 1/2.
Punnett Square: A chart that shows all the possible combinations of alleles that can result from a genetic cross. It is a visual tool used to determine the probability of an offspring having a particular genotype and phenotype.
To use a Punnett square, the alleles from one parent's gametes are placed along the top, and the alleles from the other parent's gametes are placed along the side. The boxes within the square are then filled in with the combinations of alleles from a potential offspring.
Genotype and Phenotype
Phenotype: An organism's physical appearance, or its visible traits. This is what you can observe about an organism. Examples include a pea plant being "tall" or having "round seeds."
Genotype: An organism's genetic makeup, or allele combinations. This refers to the actual set of alleles an individual possesses for a particular gene. Rather than observing, it is often inferred from the phenotype or known through genetic testing.
Homozygous: Having two identical alleles for a particular gene. An organism can be homozygous dominant (e.g., TT for tallness) or homozygous recessive (e.g., tt for shortness).
Heterozygous: Having two different alleles for a particular gene. For example, a pea plant with the genotype Tt is heterozygous for height. In cases of simple dominance, a heterozygous individual will express the dominant phenotype.
Codominance
Codominance: A genetic situation in which the alleles of a gene are neither dominant nor recessive towards each other. Instead, both alleles are fully expressed in the phenotype of a heterozygous individual. This results in offspring with a phenotype that clearly shows both parental traits simultaneously, not a blend. An example is a chicken with an allele for black feathers and an allele for white feathers having both black and white feathers (barred).
The Role of Chromosomes in Inheritance and Meiosis
The role chromosomes play in inheritance: Chromosomes are thread-like structures located inside the nucleus of eukaryotic cells. They are made of DNA tightly coiled many times around proteins. Genes are linearly arranged on chromosomes. Chromosomes are the vehicles that carry genetic information (genes) from parents to offspring. During reproduction, chromosomes are passed from parent to offspring, ensuring the continuity of genetic traits.
Events that occur during meiosis: Meiosis is a type of cell division that reduces the number of chromosomes in the parent cell by half and produces four gamete cells. It is essential for sexual reproduction. It involves two rounds of division, Meiosis I and Meiosis II, each with prophase, metaphase, anaphase, and telophase stages.
Meiosis I (Reductional Division): Homologous chromosomes separate.
Prophase I: Chromosomes condense, homologous chromosomes pair up (forming bivalents), and crossing over (exchange of genetic material) occurs, leading to genetic recombination. The nuclear envelope breaks down.
Metaphase I: Homologous pairs align along the metaphase plate.
Anaphase I: Homologous chromosomes separate and move to opposite poles of the cell. Sister chromatids remain attached.
Telophase I & Cytokinesis I: Chromosomes decondense, nuclear envelopes may reform, and the cell divides into two haploid cells (each with duplicated chromosomes).
Meiosis II (Equational Division): Sister chromatids separate.
Prophase II: Chromosomes recondense, and nuclear envelopes break down again.
Metaphase II: Chromosomes align individually along the metaphase plate in each of the two cells.
Anaphase II: Sister chromatids separate and move to opposite poles.
Telophase II & Cytokinesis II: Chromosomes decondense, nuclear envelopes reform, and each of the two cells divides, resulting in a total of four unique haploid (n) daughter cells, each with unduplicated chromosomes (gametes). These gametes contain half the number of chromosomes as the original parent cell.
Chromosomes, Genes, and the Genetic Code
Relationship between chromosomes and genes: Genes are specific segments of DNA found at particular locations (loci) on chromosomes. Each chromosome contains hundreds to thousands of genes. Chromosomes are the physical structures that carry and organize genes within the cell nucleus, ensuring their proper transmission during cell division and reproduction.
What forms the genetic code: The genetic code is formed by the sequence of nitrogenous bases in DNA (Adenine, Thymine, Guanine, Cytosine). These bases are read in groups of three, called codons. Each codon specifies a particular amino acid or signals for the termination of protein synthesis. This universal code dictates the sequence of amino acids in a protein.
Protein Production: From Gene to Function
How a cell produces proteins: Protein production (protein synthesis) involves two main stages: transcription and translation.
Transcription: The process where the genetic information from a gene (DNA) is copied into a molecule of messenger RNA (mRNA). This occurs in the nucleus. The mRNA molecule carries the genetic code from the DNA to the ribosomes in the cytoplasm.
Translation: The process where the information in mRNA is used to assemble a protein. This occurs at the ribosomes.
Transfer RNA (tRNA) molecules play a crucial role. Each tRNA molecule has an anticodon (a three-base sequence complementary to a specific mRNA codon) and carries a specific amino acid. As the ribosome moves along the mRNA, tRNA molecules bring the corresponding amino acids based on the mRNA codons.
The amino acids are linked together in a specific sequence, forming a polypeptide chain, which then folds into a functional protein.
Mutations
How mutations can affect an organism: A mutation is any change in the DNA sequence of an organism. Mutations can range from changes in a single DNA base pair (point mutations) to large-scale changes involving entire chromosomes. Mutations can affect an organism in several ways:
Beneficial Effects: Some mutations can be advantageous, leading to new traits that help an organism adapt to its environment, contributing to evolution.
Harmful Effects: Many mutations can be detrimental, leading to genetic disorders, diseases (like cancer), or reduced fitness, especially if they occur in critical genes or lead to non-functional proteins.
Neutral Effects: A significant number of mutations may have no noticeable effect on an organism's phenotype or survival. These are often called silent mutations or occur in non-coding regions of DNA.
Mutations can alter the genetic code, leading to changes in the amino acid sequence of a protein, which in turn can affect the protein's structure and function, ultimately impacting the organism's traits and overall health.