109 LAB 9 – Principles of Genetic Inheritance Overview In this laboratory you will learn about the basic principles of genetic inheritance, or what is commonly referred to as “genetics”. A true appreciation of the nature of genetic inheritance will require solving of a variety of genetics problems, and to do so you will need to understand several related concepts, some which should be familiar and others which may be new to you. Thus you will begin this lab by examining the concepts of genes, gamete production by meiosis, and probability. You will then use these concepts to work through a series of genetics problems addressing various aspects of genetic inheritance in plants and animals. Part 1: KEY GENETIC CONCEPTS We all know that when living organisms reproduce, their offspring are much like their parents. Chickens don’t give birth to lizards and apple trees don’t give rise to pine trees. So what is the biological basis for this obvious reality? You probably already know this has to do with genes, genes one inherits from one’s parents. However the process of passing on genes from one generation to the next is more complex than it may appear. The simplest form of genetic inheritance involves asexual reproduction. This is the case when a single parent organism passes its genes to offspring which are basically clones of the parent (i.e., genetically, and for the most part, physically identical). Although this mode of reproduction is quite convenient (imagine if you could simply have children identical to yourself, no partner necessary!), it has one extremely significant shortcoming: NO genetic diversity! For some species asexual reproduction works quite well, however for most plants and animals (including humans) this just won’t cut it, genetic diversity is too important. So how is genetic diversity produced? The answer is sexual reproduction: the production of gametes (sperm and eggs) by meiosis followed by the fusion of sperm and egg (fertilization) to form a new, genetically unique individual. Although a lot more work, sexual reproduction essentially “shuffles” the genes of each parent producing a unique combination of parental genes in each and every offspring. This is the sort of genetic inheritance we will focus on, genetic inheritance based on sexual reproduction. Through sexual reproduction, each offspring inherits a complete set of genes from each parent, however the study of genetic inheritance is generally limited to one or two genes at a time. Thus when you begin to work with genetics problems you will focus initially on a single gene at time, and then learn how to follow the inheritance of more than one gene. To focus on large numbers of genes would be rather complicated and is not necessary for our purposes. Before you begin to examine genetic inheritance via genetics problems, you will need to understand some important concepts that are central to the process: the nature of chromosomes, genes and genetic alleles; the process of gamete production by meiosis; and the concept of probability. Once these concepts and their associated terminology are clear, you will then be ready to immerse yourself into the world of genetic inheritance.110 Chromosomes, Genes and Alleles As you learned in the previous lab, chromosomes are extremely long pieces of DNA in the nuclei of cells that contain up to a thousand or more genes each. Each species of organism has a characteristic number of chromosome types: distinct chromosomes each having a unique set of genes and a unique length. For example, the fruit fly Drosophila, an organism used in many genetic studies, has only 4 types of chromosomes – 3 autosomes (non-sex chromosomes) and the X and Y sex chromosomes. Human beings (Homo sapiens) on the other hand have 23 types of chromosomes – 22 autosomes and the sex chromosomes (X and Y) as illustrated in the human male karyotype shown below (notice the X and Y sex chromosomes): Notice one more thing about this human karyotype: there are two of each autosome as well as two sex chromosomes. This is because human beings are diploid, which means having two of each chromosome type. Most plants and animals are in fact diploid, and as we investigate the process of genetic inheritance we will only concern ourselves with diploid species. However you should be aware that not all organisms are diploid. Some are normally haploid (one of each chromosome) such as the fungi, and some may have more than two of each chromosome (e.g., four of each = tetraploid, eight of each = octoploid) as seen in a fair number of plant species as well as a few animal species. As shown in the diagram to the left, genes are discrete sections of chromosomal DNA responsible for producing a specific protein or RNA molecule. The process of gene expression, the production of protein or RNA from a gene, will be addressed in next week’s lab. The functional protein or RNA molecule produced from a particular gene is its gene product. It is important to realize that the DNA sequence of a gene, and hence its gene product, can vary within a species. In other words, a particular gene in a species such as Homo sapiens can have different versions, what are referred to in genetics as alleles. The gene products produced from an organism’s genetic alleles account for its physical and behavioral characteristics, what we collectively call an organism’s traits. Specific traits an individual exhibits, whether physical or behavioral, are referred to as the individual’s phenotype. The specific genetic alleles an individual has for a particular gene is the individual’s genotype. As you shall soon see, an individual’s phenotype is largely determined by its genotype.111 Since diploid organisms have two alleles for each gene, an individual can have two copies of the same allele for a gene or two different alleles. If the alleles are the same, the individual is said to be homozygous for that gene. If the alleles are different, the individual is said to be heterozygous for that gene. When an individual is heterozygous for a gene, one allele may override or “mask” the other allele by determining the phenotype regardless of the other allele. In this situation, the allele that