In this chapter you learned about genetics — the science of heredity. Specifically you learned that:

• Chromosomes are structures made of DNA and proteins that are encoded with genetic instructions for making RNA and proteins. The instructions are organized into units called genes, which are segments of DNA that code for particular pieces of RNA. The RNA molecules can then act as a blueprint for proteins, or directly help regulate various cellular processes.

• Humans normally have 23 pairs of chromosomes. Of these, 22 pairs are autosomes, which contain genes for characteristics unrelated to sex. The other pair consists of sex chromosomes (XX in females, XY in males). Only the Y chromosome contains genes that determine sex.
• Humans have an estimated 20 thousand to 22 thousand genes. The majority of human genes have two or more possible versions, called alleles.
• Genes that are located on the same chromosome are called linked genes. Linkage explains why certain characteristics are frequently inherited together.
• Determining that DNA is the genetic material was an important milestone in biology.

• • In the 1920s, Griffith showed that something in virulent bacteria could be transferred to nonvirulent bacteria, making them virulent, as well.
  • In the 1940s, Avery and colleagues showed that the “something” Griffith found was DNA and not protein. This result was confirmed by Hershey and Chase, who demonstrated that viruses insert DNA into bacterial cells.
  • In the 1950s, Chargaff showed that in DNA, the concentration of adenine is always the same as the concentration of thymine, and the concentration of guanine is always the same as the concentration of cytosine. These observations came to be known as Chargaff's rules.
  • In the 1950s, James Watson and Francis Crick, building on the prior X-ray research of Rosalind Franklin and others, discovered the double helix structure of the DNA molecule.
• Knowledge of DNA’s structure helped scientists understand how DNA replicates, which must occur before cell division. DNA replication is semi-conservative because each daughter molecule contains one strand from the parent molecule and one new strand that is complementary to it.
• The central dogma of molecular biology can be summed up as: DNA → RNA → Protein. This means that the genetic instructions encoded in DNA are transcribed to RNA. From RNA, they are translated into a protein.
• RNA is a nucleic acid. Unlike DNA, RNA consists of just one polynucleotide chain instead of two, contains the base uracil instead of thymine, and contains the sugar ribose instead of deoxyribose.
• The main function of RNA is to help make proteins. There are three main types of RNA: messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA).
• According to the RNA world hypothesis, RNA was the first type of biochemical molecule to evolve, predating both DNA and proteins.
• The genetic code was cracked in the 1960s by Marshall Nirenberg. It consists of the sequence of nitrogen bases in a polynucleotide chain of DNA or RNA. The four bases make up the “letters” of the code. The letters are combined in groups of three to form code “words,” or codons, each of which encodes for one amino acid or a start or stop signal.

• • AUG is the start codon, and it establishes the reading frame of the code. After the start codon, the next three bases are read as the second codon, and so on until a stop codon is reached.
  • The genetic code is universal, unambiguous, and redundant.
• Protein synthesis is the process in which cells make proteins. It occurs in two stages: transcription and translation.

• • Transcription is the transfer of genetic instructions in DNA to mRNA in the nucleus. It includes the steps of initiation, elongation, and termination. After the mRNA is processed, it carries the instructions to a ribosome in the cytoplasm.
  • Translation occurs at the ribosome, which consists of rRNA and proteins. In translation, the instructions in mRNA are read, and tRNA brings the correct sequence of amino acids to the ribosome. Then rRNA helps bonds form between the amino acids, producing a polypeptide chain.
  • After a polypeptide chain is synthesized, it may undergo additional processing to form the finished protein.
• Mutations are random changes in the sequence of bases in DNA or RNA. They are the ultimate source of all new genetic variation in any species.

• • Mutations may happen spontaneously during DNA replication or transcription. Other mutations are caused by environmental factors called mutagens.
  • Germline mutations occur in gametes and may be passed on to offspring. Somatic mutations occur in cells other than gametes and cannot be passed on to offspring.
  • Chromosomal alterations are mutations that change chromosome structure and usually affect the organism in multiple ways. Charcot-Marie-Tooth disease type 1 is an example of a chromosomal alteration.
  • Point mutations are changes in a single nucleotide. The effects of point mutations depend on how they change the genetic code, and may range from no effects to very serious effects.
  • Frameshift mutations change the reading frame of the genetic code and are likely to have a drastic effect on the encoded protein.
  • Many mutations are neutral and have no effects on the organism in which they occur. Some mutations are beneficial and improve fitness, while others are harmful and decrease fitness.
• Using a gene to make a protein is called gene expression. Gene expression is regulated to ensure that the correct proteins are made when and where they are needed. Regulation may occur at any stage of protein synthesis or processing.

• • The regulation of transcription is controlled by regulatory proteins that bind to regions of DNA called regulatory elements, which are usually located near promoters. Most regulatory proteins are either activators that promote transcription or repressors that impede transcription.
  • A regulatory element common to almost all eukaryotic genes is the TATA box. A number of regulatory proteins must bind to the TATA box in the promoter before transcription can proceed.
  • The regulation of gene expression is extremely important during the early development of an organism. Homeobox genes, which encode for chains of amino acids called homeodomains, are important genes that regulate development.
  • Some types of cancer occur because of mutations in genes that control the cell cycle. Cancer-causing mutations most often occur in two types of regulatory genes, called tumor-suppressor genes and proto-oncogenes.
• Mendel experimented with the inheritance of traits in pea plants, which have two different forms of several visible characteristics. Mendel crossed pea plants with different forms of traits.

• • In Mendel’s first set of experiments, he crossed plants that only differed in one characteristic. The results led to Mendel’s first law of inheritance, called the law of segregation. This law states that there are two factors controlling a given characteristic, one of which dominates the other, and these factors separate and go to different gametes when a parent reproduces.
  • In Mendel’s second set of experiments, he experimented with two characteristics at a time. The results led to Mendel’s second law of inheritance, called the law of independent assortment. This law states that the factors controlling different characteristics are inherited independently of each other.
• Mendel’s laws of inheritance, now expressed in terms of genes, form the basis of genetics, the science of heredity. Mendel is often called the father of genetics.
• The position of a gene on a chromosome is its locus. A given gene may have different versions called alleles. Paired chromosomes of the same type are called homologous chromosomes and they have the same genes at the same loci.
• The alleles an individual inherits for a given gene make up the individual’s genotype. An organism with two of the same alleles is called a homozygote, and an individual with two different alleles is called a heterozygote.
• The expression of an organism’s genotype is referred to as its phenotype. A dominant allele is always expressed in the phenotype, even when just one dominant allele has been inherited. A recessive allele is expressed in the phenotype only when two recessive alleles have been inherited.
• In sexual reproduction, two parents produce gametes that unite in the process of fertilization to form a single-celled zygote. Gametes are haploid cells with only one of each pair of homologous chromosomes, and the zygote is a diploid cell with two of each pair of chromosomes.
• Meiosis is the type of cell division that produces four haploid daughter cells that may become gametes. Meiosis occurs in two stages, called meiosis I and meiosis II, each of which occurs in four phases (prophase, metaphase, anaphase, and telophase).
• Meiosis is followed by gametogenesis, the process in which the haploid daughter cells change into mature gametes. Males produce gametes called sperm through spermatogenesis, and females produce gametes called eggs through oogenesis.
• Sexual reproduction produces offspring that are genetically unique. Crossing-over, independent alignment, and the random union of gametes result in a high degree of genetic variation.
• Mendelian inheritance refers to the inheritance of traits controlled by a single gene with two alleles, one of which may be completely dominant to the other. The pattern of inheritance of Mendelian traits depends on whether the traits are controlled by genes on autosomes or by genes on sex chromosomes.

• • Examples of human autosomal Mendelian traits include albinism and Huntington’s disease. Examples of human X-linked traits include red-green colour blindness and hemophilia.
• Two tools for studying inheritance are pedigrees and Punnett squares. A pedigree is a chart that shows how a trait is passed from generation to generation. A Punnett square is a chart that shows the expected ratios of possible genotypes in the offspring of two parents.
• Non-Mendelian inheritance refers to the inheritance of traits that have a more complex genetic basis than one gene with two alleles and complete dominance.

• • Multiple allele traits are controlled by a single gene with more than two alleles. An example of a human multiple allele trait is ABO blood type.
  • Codominance occurs when two alleles for a gene are expressed equally in the phenotype of heterozygotes. A human example of codominance occurs in the AB blood type, in which the A and B alleles are codominant.
  • Incomplete dominance is the case in which the dominant allele for a gene is not completely dominant to a recessive allele, so an intermediate phenotype occurs in heterozygotes who inherit both alleles. A human example of incomplete dominance is Tay Sachs disease, in which heterozygotes produce half as much functional enzyme as normal homozygotes.
  • Polygenic traits are controlled by more than one gene, each of which has a minor additive effect on the phenotype. This results in a continuum of phenotypes. Examples of human polygenic traits include skin colour and adult height. Many of these types of traits, as well as others, are affected by the environment, as well as by genes.
  • Pleiotropy refers to the situation in which a gene affects more than one phenotypic trait. A human example of pleiotropy occurs with sickle cell anemia, which has multiple effects on the body.
  • Epistasis is when one gene affects the expression of other genes. An example of epistasis is albinism, in which the albinism mutation negates the expression of skin colour genes.
• Genetic disorders are diseases, syndromes, or other abnormal conditions that are caused by mutations in one or more genes or by chromosomal alterations.

• • Examples of genetic disorders caused by single-gene mutations include Marfan syndrome (autosomal dominant), sickle cell anemia (autosomal recessive), vitamin D-resistant rickets (X-linked dominant), and hemophilia A (X-linked recessive). Very few genetic disorders are caused by dominant mutations because these alleles are less likely to be passed on to successive generations.
  • Nondisjunction is the failure of replicated chromosomes to separate properly during meiosis. This may result in genetic disorders caused by abnormal numbers of chromosomes. An example is Down syndrome, in which the individual inherits an extra copy of chromosome 21. Most chromosomal disorders involve the X chromosome. An example is Klinefelter’s syndrome (XXY, XXXY).
  • Prenatal genetic testing (by amniocentesis, for example) can detect chromosomal alterations in utero. The symptoms of some genetic disorders can be treated or prevented. For example, symptoms of phenylketonuria (PKU) can be prevented by following a low-phenylalanine diet throughout life.
  • Cures for genetic disorders are still in the early stages of development. One potential cure is gene therapy, in which normal genes are introduced into cells by a vector such as a virus to compensate for mutated genes.
• Genetic engineering is the use of technology to change the genetic makeup of living things for human purposes.

• • Genetic engineering methods include gene cloning and the polymerase chain reaction. Gene cloning is the process of isolating and making copies of a DNA segment, such as a gene. The polymerase chain reaction makes many copies of a gene or other DNA segment.
  • Genetic engineering can be used to transform bacteria so they are able to make human proteins, such as insulin. It can also be used to create transgenic crops, such as crops that yield more food or resist insect pests.
  • Genetic engineering has raised a number of ethical, legal, and social issues including health, environmental, and privacy concerns.
• The human genome refers to all of the DNA of the human species. It consists of more than 3.3 billion base pairs divided into 20,500 genes on 23 pairs of chromosomes.
• The Human Genome Project (HGP) was a multi-billion dollar international research project that began in 1990. By 2003, it had sequenced and mapped the location of all of the DNA base pairs in the human genome. It published the results as a human reference genome that is available to anyone on the Internet.
• Sequencing of the human genome is helping researchers better understand cancer and genetic diseases. It is also helping them tailor medications to individual patients, which is the focus of the new field of pharmacogenomics. In addition, it is helping researchers better understand human evolution.