Unit 9: Mendelian Genetics and Chromosomes
Do you ever wonder why you look like your brother or sister or where you got your freckles from? Are you concerned about developing a disease another family member struggles with? These are the types of questions that we can answer with an understanding of genetics. In this unit, we learn about the basic principles of inheritance and how likely it is to pass on certain traits from one generation to another.
Completing this unit should take you approximately 5 hours.
Upon successful completion of this unit, you will be able to:
- explain how information flows from genotype to phenotype at the molecular level;
- explain how the terms genotype, phenotype, homozygous, heterozygous, dominant, recessive, co-dominant, and sex-linkage are used;
- given a pedigree, infer whether the trait in the diagram is dominant or recessive, and indicate individual genotypes when possible;
- solve genetic problems involving monohybrid crosses with dominant and recessive traits, codominant traits, and sex-linked traits;
- solve genetic problems involving dihybrid crosses with dominant and recessive traits, codominant traits, and sex-linked traits; and
- define mutation and explain how a mutation, such as that in sickle cell, can result in a changed phenotype.
9.1: Introduction to Genetics
Genetics is the study of heredity and how different characteristics are passed from generation to generation. It is also the study of variation of characteristics that is important for the diversity of life. Scientists have studied the laws of inheritance and the many exceptions that occur. We often say that our genes give us our traits. That's true, but only indirectly.
Many consider Gregor Mendel to be the father of genetics because his experiments in simple genetics provided a foundation for modern genetics. Read this section to review Mendel's laws. Be sure to spend some time reviewing the figures. Read this brief introduction to genetics to learn about one of the pioneers of the genetic laws of inheritance.
Genes are sequences of DNA. These genes serve as instructions for how to make corresponding sequences of RNA. The sequences of RNA, in turn, serve as instructions for how to build proteins. Proteins are the final product of gene expression, and the particular proteins that are built in the cells of an organism are what give that organism its traits. The traits are directly determined by the proteins, but the traits are indirectly determined by the genes because the genes indirectly instruct the cells how to build the proteins. For any given characteristic (like hair color, for example), there might be multiple possible traits (such as black hair and brown hair).
The particular trait exhibited by an individual for a particular characteristic (like the black hair trait for the hair-color characteristic) is known as that individual's phenotype. The phenotype depends on the proteins produced, which depends on the version of the corresponding gene (DNA) that an individual possesses.
We use the term "genotype" to refer to that individual's particular DNA sequence for that particular gene. Therefore, genotype indirectly determines phenotype.
The laws of inheritance can be used to predict traits in offspring. Read this section to review these laws, and take time to review the figures to review the movement of traits to offspring.
Read this section to explore the laws of inheritance in more detail. After you read, you should be able to describe the exceptions of Mendelian genetics, define incomplete dominance and codominance, and explain the inheritance of the ABO blood system.
A mutation is an accidental change in the DNA sequence of an organism. As you know, your DNA is arranged in chromosomes, which is a long sequence of DNA nucleotides. A gene is a subsequence of DNA nucleotides within the longer sequence making up the chromosome. A mutation arises when the sequence gets changed either by an error during DNA replication or by some other accident (including exposure to certain chemicals or forms of radiation, for instance). The reason a gene is able to serve as a code (stored information) is because of its particular sequence of DNA nucleotides.
Mutation changes that sequence. Therefore the code is changed, and when that altered code is used to make RNA, the RNA will also be altered. Similarly, when the code in that altered (mutated) version of RNA is used to build a protein, that protein might be different from the unmutated version.
Since proteins directly determine phenotypes, a mutant protein (resulting from a mutant form of DNA for the corresponding gene) might result in an altered phenotype. Mutation is the original source of genetic variation, and it is the reason why there are different species and why there are differences between individuals of the same species. As you continue to study genetics, try to keep in mind that differences in genotypes and phenotypes are ultimately due to mutations that have accidentally occurred.
After you watch these videos, make sure you can define mutation and explain how mutations, such as that in sickle cell, can result in a changed phenotype.
9.2: Heredity
Heredity is the study of how characteristics are passed from parent to offspring. Genetic information is physically expressed through the production of proteins. We call this physical expression the phenotype. Scientists study the probability of certain phenotypes being passed to future generations to help demonstrate the laws of heredity.
Watch these lectures for an explanation of inheritance and genetics. Punnett squares are named after the geneticist Reginald Punnett, who developed the method for predicting probabilities. Work the examples to practice Punnett squares.
After you watch, you should be able to distinguish between dominant and recessive, distinguish between heterozygous and homozygous, distinguish between genotype and phenotype, use a Punnett square to predict the results of a monohybrid cross, use a Punnett square to predict the results of a dihybrid cross, and state the phenotypic and genotypic ratios of Punnett crosses.
Watch this brief video on pedigrees, which are how we learn more about genetics in human populations. Pedigrees are useful in populations like these, where we cannot test in a controlled environment.
Watch this video to review the unique inheritance patterns linked to sex-determining chromosomes.