Genetics From Mendel to DNA

Gregor Mendel Discovered modern genetics through the study of pea plants. He watched the passing of traits from one generation of plants to the next. Over time, he formed two laws that explained how these traits were being passed on.

Law 1 – Segregation of Alleles He stated that the alleles for a trait separated in equal amounts. He observed that some traits showed up more than other and determined that those were dominant traits. He called the less frequent traits recessive.

Genes and Traits Mendel called the factors that control a trait genes. He observed how the alleles for each gene interacted and devised that some plants were pure-bred (meaning they had two of the same allele) and some were hybrid (meaning they had two different alleles).

Punnett Squares A Punnett square can be used to determine the probable genotype (the set of alleles) and phenotype (the physical appearance) of an offspring based on Mendel’s observations.

Law 2 – Independent Assortment Mendel’s second observation was that the different genes in the pea plants were independent of each other. This meant that alleles for one trait did not affect the inheritance of alleles for another trait.

Independent Assortment

Patterns of Inheritance There are several ways in which traits can be passed down. Complete dominance – One allele dominates another allele. Incomplete dominance – Neither allele is expressed fully (a blending of the traits). Codominance – Both alleles are expressed fully (both are visible independently).

Patterns of Inheritance (cont.) Multiple alleles – more than two alleles for a gene are found within a population. Polygenic traits – many genes contribute to a phenotype.

Human Genetics In humans, the sex of an offspring is determined by special chromosomes. These are called the X and Y chromosomes. Males have XY and females have XX. There are several genetic disorders associated with these chromosomes. Since males only receive one X chromosome, they are more often affected by X linked disorders.

DNA and RNA DNA and RNA are made up of three parts: A phosphate group A sugar (either deoxyribose or ribose) A nitrogenous base There are five types of nitrogenous bases.

Nitrogenous Bases DNA RNA Adenine A Thymine T Uracil U Guanine G Cytosine C

DNA Structure DNA is formed into a Double Helix structure. This structure looks like a twisted ladder. The phosphate groups and sugars act as the legs of the ladder and the nitrogenous bases act as the rungs (steps).

Genes and DNA The hereditary information of genes are found in the sequence of the nitrogenous bases in DNA. Each gene is a long section of DNA, coding for the production of a particular protein, which in turn controls the trait associated with that gene.

DNA Replication DNA is copied from other DNA. The double helix ‘unzips’ and new nitrogenous bases are matched with the original order. The two new double helix of DNA each have one new and one original strand.

Protein Synthesis Protein synthesis is the process of taking the coded messages found on DNA and turning them into proteins. DNA is found inside the nucleus (in eukaryotes) and ribosomes (which make proteins) are found outside of the nucleus. In order to complete the process RNA is used as a messenger between the DNA and the ribosomes.

Transcription and Translation The first step is for mRNA (m = messenger) to copy the sequence of nitrogenous bases of the DNA. mRNA then carries this message out of the nucleus to a ribosome. At the ribosome, mRNA is paired with tRNA (t = transfer) which carries amino acids. The amino acids are put together in the sequence determined by the DNA, thus creating a protein.

Mutations A mutation is a change in the order of the nitrogenous bases of DNA. Some mutations are harmless, some are damaging to the organism, and some are fatal (causing the organism not to develop).