GENETICS

The Origin of Genetics Mendel’s Hypothesis: The passing of traits from parent to offspring is called heredity. The scientific study of heredity began with the work of an Austrian monk named Gregor Mendel. Mendel carried out experiments in which he bred different varieties of the garden pea. Mendel was the first to develop rules that accurately predict patterns of heredity. The patterns he discovered form the basis of genetics, the branch of biology that focuses on heredity. Mendel’s Hypothesis: There are four hypothesis that make up the Mendelian theory of heredity—the foundation of genetics. For each inherited trait, an individual has two copies of the gene—one from each parent. There are alternative versions of genes. Different versions of a gene are called alleles. Each allele can be passed on when the individual reproduces. When two different alleles occur together, one of them may be completely expressed, while the other may have no observable effect on the organism’s appearance.

Mendel’s Hypothesis: The expressed form of a trait is called dominant. The trait that was not expressed when the dominant form of the trait was present is called recessive. Mendel’s initial experiment were monohybrid crosses. A monohybrid cross is a cross that involves one pair of contrasting traits. 4. When gametes are formed, the alleles for each gene in an individual separate independently of one another. Thus, gametes carry only one allele for each inherited trait.

Terminology Dominant alleles are indicated by writing the 1st letter of the trait as a capital letter (P). Recessive alleles are indicated by writing the 1st letter of the dominant trait, but is lower case (p). Homozygous – If two alleles of a particular gene present in an individual are the same. Example: The alleles for yellow peas, Y, is dominant to the allele for green peas, y. A plant with two yellow-pea alleles, YY, is homozygous for seed color. Heterozygous – If the alleles of a particular gene present in an individual are different. Example: a pea with one “yellow allele (Y)” and one “green allele (y)” is heterozygous for seed color (Yy). Genotype – The set of alleles that an individual has. Example: YY-homozygous dominant; Yy-heterozygous; yy-recessive Phenotype – The physical appearance of a trait.

The Laws of Heredity The Law of Segregation – the two alleles for a trait segregate (separate) when gametes are formed. The Law of Independent Assortment – The alleles of different genes separate independently of another during gamete formation.

Studying Heredity Punnett Squares: A Punnett square is a diagram that predicts the outcome of a genetic cross by considering all possible combinations of gametes in a cross.

The Genetic Code Different types of RNA are made during transcription, depending on the gene being expressed. Messenger RNA (mRNA) – is a form of RNA that carries the instructions for making a protein from a gene and delivers it to the site of translation. The information is translated from the language of RNA—nucleotides—to the language of proteins—amino acids. The RNA instructions are written as a series of three nucleotide sequences on the RNA called codons. Each codon along the mRNA strand corresponds to an amino acid or signifies a start or stop signal for translation. The nitrogen-bases of RNA are the same as DNA, except there is no thymine. Thymine is replaced with uracil in a RNA molecule. Code (AUG) starts the process. Code (UAA, UAG, UGA) stops the process.

Transcription and Translation Messenger RNA (mRNA) – is the temporary copy of a gene that encodes a protein. The process of making an (m) RNA molecule is called transcription. The mRNA molecule provides the pattern that determines the order in which amino acids are added to the protein being made, a process known as translation. Translation takes place in the cytoplasm. Transfer RNA (tRNA) molecules are single strands of RNA that temporarily carry a specific amino acid on one end. Each tRNA is folded into a compact shape and has an anticodon. Anticodon is a 3-nucleotide sequence on a tRNA that is complimentary to an mRNA codon. Ribosomal RNA – molecules are RNA molecules that are part of the structure of ribosomes.

Mutations Mutations in gametes can be passed on to offspring of the affected individual, but mutations in body cells affect only the individual. Mutations that move an entire gene to a new locations are called gene rearrangement. Changes in a gene’s position often disrupt the gene’s function because the gene is exposed to a new regulatory controls in its new location. Mutations that change a gene are called gene alterations. Gene alterations usually result in the placement of the wrong amino acid during protein assembly.