Inheritance of Blood Types

If you have AB blood, it means that one of your parents is A or AB & the other is B or AB. If you have A blood, it means that you inherited an A gene from one parent & either an A gene or an O gene from the other. If you have O- type blood, phenotypically, either A or B. The blood groups are examples of multiple alleles (A, B, & O). It is not unusual for genes to have more than two alternative forms.

Variations and Mutations

In the cells of plants and animals that reproduce sexually, at least two genes are present for every trait. In these organisms one set of chromosomes is received from the male parent. The other set of chromosomes is received from the female parent. The offspring may show some of its mother’s traits and some of its father’s traits. It passes on a combination of these traits to its offspring. Each of these offspring then passes on another combination to its own offspring. These different combinations of traits come from different combination of chromosomes and genes, causing a variety within each kind of organism. Another way organisms may become different is by mutations, which are random changes in genes and chromosomes. Mutations occur naturally in the cells of organisms. Usually, mutations produce no noticeable changes in organisms. Some mutations, however, can lower an organism’s chances of surviving and producing the young. A mutation is generally recessive. It may be passed on from generation to generation. Mutations happen all the time. They may have been happening since life first began on the earth. Probably, all of us saw animals (birds) with white fur (feathers) and red eyes. They are albinos. Albinos have a mutation in their genes that prevents them from having normal body colors. This mutation can be harmful to the living organisms since it will not blend in with their surroundings. Therefore, the albino animal (bird) may be more visible to its enemies. Other mutations can be an advantage for an organism. For example, some mosquitoes sprayed by DDT probably had mutations that enabled them to resist the DDT poison. These mosquitoes lived and passed their DDT resistance on to their offspring.

Breeding

The process of selecting individuals with desirable traits to produce the next generation is called controlled breeding. In recent decades breeders have applied the knowledge of genetics to controlled breeding. In this way, they have been able to adapt plants and animals to many human needs in a much shorter period of time than was previously possible.

Modern breeders use combinations of several methods to develop plants and animals with desired traits. The most important methods of controlled breeding are mass selection, hybridization, and inbreeding. Each of these traditional methods has been made more effective through the application of genetic principles.

The process of raising a great many plants and animals and selecting the best in each generation for further breeding is called mass selection.

Frequently breeders want to establish pure lines, or populations of plants or animals made up of genetically similar individuals. Such lines usually breed true for certain traits, which means that offspring are almost identical to their parents in these traits. The pure lines also serve as known quantities in breeding experiments.

Pure lines are established by following mass selection with inbreeding, a method that involves mating genetically similar individuals. With animals, close relatives such as brothers or sisters are mated over several generations. With plants, inbred varieties are produced by self-pollination. After many generations, inbreeding produces individuals that are homozygous for most traits. Some of these traits are desirable. However, others may be undesirable, caused by homozygous recessive genes that are not expressed in a heterozygous individual.

Hybridization is a method of crossing two different species, breeds, varieties, or pure lines. When inbred varieties are crossed, the resulting hybrids may show every possible combination of traits of the parent species. Thus, some hybrid offspring inherit the best traits of each parent and are larger, hardier, and more productive than either parent. When the offspring is superior to both parents for a specific trait, this improvement is known as hybrid vigor.

Hybridization has its drawbacks, however. In some cases offspring inherit the worst traits of each parent. Other hybrids, such as mules, are sterile. Even in hybrids that can reproduce, hybrid vigor may disappear if hybrids are crossed over many generations.

Controlled breeding is just one means of developing organisms adapted to human needs. Biologists have also learned to produce identical copies of desirable organisms. In the last 40 years, biologists have found new ways to change the genetic make up of an organism or its offspring by artificial means.

Non-Nuclear Inheritance

The nucleus is not the only organelle in a eukaryotic cell that carries genetic material. Mitochondria and plastids, which may have arisen from prokaryotes that colonized other cells, contain small numbers of genes. For example, in humans, there are about 30,000 genes in the nuclear genome and 37 in the mitochondrial genome. Plastid genomes are about five times larger than those of mitochondria. In any case, several of the genes of cytoplasmic organelles are important for organelle assembly and function, so it is not surprising that mutations of these genes have profound effects on the organism. The inheritance of organelle genes differs from that of nuclear genes for several reasons:

In most organisms, mitochondria and plastids are inherited from the mother only. As you will see in later chapters, eggs contain abundant cytoplasm and organelles, but the only part of the sperm that survives to take part in the union of haploid gametes is the nucleus. So you have inherited your mother’s mitochondria (with their genes) but not your father’s.

There may be hundreds of mitochondria or plastids in a cell. So a cell is not diploid for organelle genes; rather, it is highly polyploid.

Organelle genes tend to mutate at much faster rates than nuclear genes, so there are multiple alleles of organelle genes.

The phenotypes of mutations in the DNA of organelles reflect the organelles’ roles. For example, in plants and some eukaryotic algae, certain plastid mutations affect the proteins that assemble chlorophyll molecules into photosystems and result in a phenotype that is essentially white instead of green. Mitochondrial mutations that affect one of the complexes in the electron transport chain result in less ATP production. They have especially noticeable effects in tissues with a high energy requirement, such as the nervous system, muscles, and kidneys. In 1995, Greg Lemond, a professional cyclist who had won the famous Tour de France three times, was forced to retire because of muscle weakness suspected to be caused by a mitochondrial mutation.

Genetic engineering

Genetic engineering has to do with experiments that change genes so that different traits are inherited. These experiments are also called recombinant DNA experiments. Scientists have discovered enzymes that can change the genetic code in a DNA molecule. Such enzymes can “clip out” a whole section of a DNA molecule. This clipped – out section can then be transplanted into the DNA molecule of another species. Even through genetic engineers work with an ordinary microscope, they can cause changes that are dramatic.

Scientists may also be able to introduce new traits into plants and animals.

Genetic engineering can be useful to people. For example, useful materials such as human growth hormones, insulin, and other materials. As more skill is developed, scientists may be able to transplant a healthy section into a defective gene.

Genetics Problems

1. The sex of fishes is determined by the same X-Y system as in humans. An allele of one locus on the Y chromosome of the fish Lebistes causes a pigmented spot to appear on the dorsal fin. A male fish that has a spotted dorsal fin is mated with a female fish that has an unspotted fin. Describe the phenotypes of the F1 and the F2 generations from this cross.

2. In Drosophila melanogaster, the recessive allele p, when homozygous, determines pink eyes. Pp or PP results in wildtype eye color. Another gene, on another chromosome, has a recessive allele, sw, that produces short wings when homozygous.

Consider a cross between females of genotype PPSwSw and males of genotype ppswsw. Describe the phenotypes and genotypes of the F1 generation and of the F2 generation produced by allowing the F1 progeny to mate with one another.

3. On the same chromosome of Drosophila melanogaster that carries the p (pink eyes) locus, there is another locus that affects the wings. Homozygous recessives, byby, have blistery wings, while the dominant allele By produces wild-type wings. The P and By loci are very close together on the chromosome; that is, the two loci are tightly linked. In answering these questions, assume that no crossing over occurs. a. For the cross PPByBy X ppbyby, give the phenotypes and genotypes of the F1 and of the F2 generations produced by interbreeding of the F1 progeny. b. For the cross PPbyby X ppByBy, give the phenotypes and genotypes of the F1 and of the F2 generations. c. For the cross of Question 7b, what further phenotype(s) would appear in the F2 generation if crossing over occurred? d. Draw a nucleus undergoing meiosis, at the stage in which the crossing over (Question 7c) occurred. In which generation (P, F1, or F2) did this crossing over take place?

4. Consider the following cross of Drosophila melanogaster (allelesas described in Question 6): Males with genotype Ppswsw arecrossed with females of genotype ppSwsw. Describe the phenotypes and genotypes of the F1 generation.

2.2.3. From DNA to Protein: Genotype to Phenotype

This subchapter deals with the mechanisms by which genes are expressed as proteins. We will begin with evidence for the relationship between genes and proteins, and then fill in some of the details of the processes of transcription—the copying of the gene sequence of DNA into a sequence of RNA—and translation—the use of the sequence of RNA to make a polypeptide with a defined order of amino acids. Finally, we will define mutations and their phenotypes in specific molecular terms.

One Gene, One Polypeptide

There are many steps between genotype and phenotype. Genes cannot, all by themselves, directly produce a phenotypic result, such as a particular eye color, a specific seed shape or a cleft chin, any more than a compact disk can play a symphony without the help of a CD player. The first historical step in relating genes to phenotypes was to define phenotypes in molecular terms. The molecular basis of phenotypes was actually discovered before the discovery that DNA was the genetic material. Scientists had studied the chemical differences between individuals carrying wild-type and mutant alleles in organisms as diverse as humans and bread molds. They found that the major phenotypic differences were the result of differences in specific proteins. In the 1940s, a series of experiments by George W. Beadle and Edward L. Tatum at Stanford University showed that when an altered gene resulted in an altered phenotype, that altered phenotype was always associated with an altered enzyme. This finding was critically important in defining the phenotype in chemical terms. The roles of enzymes in biochemistry were being described at this time, and it occurred to Beadle and Tatum that the expression of a gene as phenotype could occur through an enzyme. They experimented with the bread mold Neurospora crassa. The nuclei in the body of this mold are haploid (n), as are its reproductive spores. (This fact is important because it means that even recessive mutant alleles are easy to detect in experiments.) Beadle and Tatum grew Neurospora on a minimal nutritional medium containing sucrose, minerals and a vitamin. Using this medium, the enzymes of wild-type Neurospora could catalyze the metabolic reactions needed to make all the chemical constituents of their cells, including proteins. These wild-type strains are called prototrophs (“original eaters”). Beadle and Tatum treated wild-type Neurospora with X rays, which act as a mutagen (something known to cause mutations). When they examined the treated molds, they found some mutant strains could no longer grow on the minimal medium, but needed to be supplied with additional nutrients. The scientists hypothesized that these auxotrophs (“increased eaters”) must have suffered mutations in genes that code for the enzymes used to synthesize the nutrients they now needed to obtain from their environment. For each auxotrophic strain, Beadle and Tatum were able to find a single compound that, when added to the minimal medium, supported the growth of that strain. This result suggested that mutations have simple effects, and that each mutation causes a defect in only one enzyme in a metabolic pathway described as the one-gene, one-enzyme hypothesis. One group of auxotrophs, for example, could grow only if the minimal medium was supplemented with the amino acid arginine. (Wild-type Neurospora makes its own arginine.). These mutant strains were designated arg mutants. Beadle and Tatum found several different arg mutant strains. They proposed two alternative hypotheses to explain why these different genetic strains had the same phenotype:

The different arg mutants could have mutations in the same gene, as in the case of the different eye color alleles of fruit flies. In this case, the gene might code for an enzyme involved in arginine synthesis.

The different arg mutants could have mutations in different genes, each coding for a separate function that leads to arginine production. These independent functions might be different enzymes along the same biochemical pathway.

Some of the arg mutant strains fell into each of the two categories. Genetic crosses showed that some of the mutations were at the same chromosomal locus, and so were different alleles of the same gene. Other mutations were at different loci or on different chromosomes and so were not alleles of the same gene. Beadle and Tatum concluded that these different genes participated in governing a single biosynthetic pathway—in this case, the pathway leading to arginine synthesis. By growing different arg mutants in the presence of various compounds suspected to be intermediates in the synthetic metabolic pathway for arginine, Beadle and Tatum were able to classify each mutation as affecting one enzyme or another, and to order the compounds along the pathway. Then they broke open the wild-type and mutant cells and examined them for enzyme activities. The results confirmed their hypothesis: Each mutant strain was indeed missing a single active enzyme in the pathway. The gene–enzyme connection had been proposed 40 years earlier in 1908 by the Scottish physician Archibald Garrod, who studied the inherited human disease alkaptonuria. He linked the biochemical phenotype of the disease to an abnormal gene and a missing enzyme. Today we know of hundreds of examples of such hereditary diseases. The gene–enzyme relationship has undergone several modifications in light of our current knowledge of molecular biology. Many enzymes are composed of more than one polypeptide chain, or subunit (that is, they have a quaternary structure). In this case, each polypeptide chain is specified by its own separate gene. Thus, it is more correct to speak of a one-gene, one-polypeptide relationship: the function of a gene is to control the production of a single, specific polypeptide. Much later, it was discovered that some genes code for forms of RNA that do not become translated into polypeptides, and that still other genes are involved in controlling which other DNA sequences are expressed. While these discoveries have supplanted the idea that all genes code for proteins, they did not invalidate the relationship between genes and polypeptides. But how does this relationship work—that is, how is the information encoded in DNA used to specify a particular polypeptide?

DNA, RNA, and the Flow of Information

The expression of a gene to form a polypeptide occurs in two major steps:

Transcription copies the information of a DNA sequence (the gene) into corresponding information in an RNA sequence.

Translation converts this RNA sequence into the amino acid sequence of a polypeptide.

RNA differs from DNA

RNA is a key intermediary between DNA and polypeptide. RNA (ribonucleic acid) is a polynucleotide similar to DNA, but it differs from DNA in three ways:

RNA generally consists of only one polynucleotide strand.

The sugar molecule found in RNA is ribose, rather than the deoxyribose found in DNA.

Although three of the nitrogenous bases (adenine, guanine, and cytosine) in RNA are identical to those in DNA, the fourth base in RNA is uracil (U), which is similar to thymine but lacks the methyl (—CH₃) group. The central dogma raised two questions:

How does genetic information get from the nucleus to the cytoplasm? (most of the DNA of a eukaryotic cell is confined to the nucleus but proteins are synthesized in the cytoplasm.)

What is the relationship between a specific nucleotide sequence in DNA and a specific amino acid sequence in a protein?

To answer these questions, Crick proposed two hypotheses.

The messenger hypothesis and transcription

To answer the first question, Crick and his colleagues proposed the messenger hypothesis. They proposed that an RNA molecule forms as a complementary copy of one DNA strand of a particular gene. The process by which this RNA forms is called transcription). This messenger RNA or mRNA, then travels from the nucleus to the cytoplasm, where it serves as a template for the synthesis of proteins.

Crick’s hypothesis has been tested repeatedly for genes that code for proteins and the answer is always the same: each gene sequence in DNA that codes for a protein is expressed as a sequence in mRNA.

The adapter hypothesis and translation

To answer the second question, Crick proposed the adapter hypothesis: there must be an adapter molecule that can bind a specific amino acid with one region and recognize a sequence of nucleotides with another region. In due course, these adapters, called transfer RNA or tRNA, were identified. Because they recognize the genetic message of mRNA and simultaneously carry specific amino acids, tRNAs can translate the language of DNA into the language of proteins. The tRNA adapters line up on the mRNA so that the amino acids are in the proper sequence for a growing polypeptide chain — a process called translation. Once again, actual observations of the expression of thousands of genes have confirmed the hypothesis that tRNA acts as the intermediary between the nucleotide sequence information in mRNA and the amino acid sequence in a protein. Summarizing the main features of the central dogma, the messenger hypothesis and the adapter hypothesis, we may say that a given gene is transcribed to produce a messenger RNA (mRNA) complementary to one of the DNA strands, and that transfer RNA (tRNA) molecules translate the sequence of bases in the mRNA into the appropriate sequence of linked amino acids during protein synthesis.

RNA viruses modify the central dogma

Certain viruses are rare exceptions to the central dogma. Viruses are infectious particles that reproduce inside cells.

Many viruses, such as the tobacco mosaic virus, influenza virus, and poliovirus, have RNA rather than DNA as their genetic material. With its nucleotide sequence, RNA could potentially act as an information carrier and be expressed as proteins. But since RNA is usually single-stranded, its replication is a problem. The viruses generally solve this problem by transcribing from RNA to RNA, making an RNA strand that is complementary to their genome. This “opposite” strand is then used to make multiple copies of the viral genome by transcription. The human immunodeficiency virus (HIV) and certain rare tumor viruses also have RNA as their genome, but do not replicate it as RNA-to-RNA. Instead, after infecting a host cell, they make a DNA copy of their genome and use it to make more RNA. This RNA is then used both as genomes for more copies of the virus and as mRNA to produce viral proteins. Synthesis of DNA from RNA is called reverse transcription, and not surprisingly, such viruses are called retroviruses.

Transcription: DNA-Directed RNA Synthesis

Although the RNA viruses present a modification of the central dogma, the fact remains that in normal prokaryotic and eukaryotic cells, RNA synthesis is directed by DNA. Transcription—the formation of a specific RNA from a specific

DNA — requires several components:

A DNA template for complementary base pairing

The appropriate ribonucleoside triphosphates (ATP, GTP, CTP, and UTP) to act as substrates

An enzyme, RNA polymerase. Within each gene, only one of the two strands of DNA — the template strand — is transcribed. The other, complementary DNAstrand, referred to as the non-template strand, remains untranscribed. For different genes in the same DNA molecule, different strands may be transcribed. That is, the strand that is the non-template strand in one gene may be the template strand in another. Not only mRNA is produced by transcription. The same process is responsible for the synthesis of tRNA and ribosomal RNA (rRNA), whose important roles in protein synthesis will be described below. Like polypeptides, these RNAs are encoded by specific genes. In DNA replication, as we know, the two strands of the parent molecule unwind, and each strand serves as the template for a new strand. In transcription, DNA partly unwinds so that it can serve as a template for RNA synthesis. As the RNA transcript is formed, it peels away, allowing the DNA to be rewound into the double helix. Transcription can be divided into three distinct processes: initiation, elongation and termination. Let’s consider each of these in turn.