Incomplete Dominance is Neither Dominant nor Recessive
Sometimes the two forms of a gene that determine a trait are neither dominant nor recessive. Imagine that you could cross a purebred red snapdragon with a purebred white snapdragon. You might expect the offspring be either red or white, depending on which was the dominant form. However, this is not always the case.
The first Punnett square (Chart 2.4) shows you that all the offspring would be pink, for neither gene is dominant. As you see, the possible genes from the purebred red flower are R or R. The possible genes from the purebred white flower are W or W. The hybrid offspring have the genotype RW. Since neither gene is dominant, the phenotype is pink, a blend of red and white. This is an example of incomplete dominance.
In the 2nd Punnett square (Chart 2.5), you see what happens when 2 hybrid pink snapdragons are crosses. The genotypes of these offspring would be RR, RW, and RW & WW. You knew from looking at the Punnett square that the phenotypes would be 25% red, 25% white and 50% pink.
X Chromosomes & Y Chromosomes
Two chromosomes called the sex chromosomes determine your sex. You receive one sex chromosome from your mother’s egg cell & one sex chromosome from your father’s sperm cell. Sex chromosomes are named the X chromosome & the Y chromosome.
Egg cells contain 1 X chromosome. Males, however, make two kinds of sperm. Half of the sperm contains one X chromosome. The other half of the sperm contains one Y chromosome. If a sperm containing an X chromosome unites with an egg, the genotype of the fertilized egg will be XX. A female will result. Thus the sex of the offspring depends on which kind of sperm unites with the egg.
Since both kinds of sperm have nearly an equal chance of fertilizing an egg, there is nearly an equal chance that offspring will be male or female.
Sex Cells Form by Meiosis
Sperm cells and egg cells are quite different from your body cells. Body cells form by mitosis. All cells produced through mitosis have the same number of chromosomes as their parent cells. As you see, the chromosomes of body cell duplicate and separate. Two new body cells form from one original parent cell. Consider what would happen if two cells formed through mitosis combined in sexual reproduction. The offspring would have twice as many chromosomes as its parents. As this process continued, each succeeding generation would have double the chromosome number of its parents. Sexual reproduction does not increase the number of chromosomes, because gametes have only half the number of chromosomes found in somatic cells.
Sex cells do not form by mitosis. They form by especial kind of cell division called meiosis (two successive nuclear divisions in which the chromosome number is reduced from diploid (2n) to haploid (n) and segregation and reassortment of the genes occur). Meiosis is often called “reduction division”, because it forms sex cells that are reduced to half the number of chromosomes as body cells. Your sex cells have 23 single chromosomes. The chromosomes in somatic cells occur in pairs called homologous chromosomes. The chromosomes in a pair are alike in appearance of in the type of genetic information they carry. The 46 chromosomes in human cells form 23 homologous pairs. Cells that have homologous chromosomes are said to have the diploid number of chromosomes. Gametes have a haploid number of chromosomes. Gametes are formed by a type of nuclear division called meioses. In humans, meiosis occurs in the male testes and the female ovaries.
Meiosis (Fig. 2.5, 2.6) begins with an original parent cell that has 23 pairs of chromosomes. In the first division of meiosis, the DNA has already replicated forming two twin strands. The two twin stranded chromosomes come together in a pair. Often at this point some of the DNA of one chromosome break off and crosses over to another chromosome. Crossing over the DNA helps to explain the difference between parents and their young. The chromosomes then line up, separate, and move to opposite ends of the cell. Two new cells form.
In the second division, the chromosomes line up in the center of each cell. This time, the chromosomes don’t duplicate. The strands merely separate. Each strand moves to opposite sides of the cells and becomes a chromosome. The cells then divide. Four new cells are the sex cells. They have half the number of chromosomes as the original parent cells. When
Fig. 2.5. Process of meiosis
Fig. 2.6. Process of meiosis (continuation)
two sex cells unite, the new organism has the normal number of chromosomes for that species and the especial individual make-up.
Significance of meiosis: prior to the mixing of one individual’s genotype with that of another at fertilization, meiosis provides the opportunity for new combinations of the existing alleles of genes to arise as follows.
Crossing over takes place when breaks occur in chromatids at the beginning of meiosis, when the chromosomes are paired, and the broken end of each chromatid joins with the chromatid of an homologous chromosome. By this means, alleles of linked genes can become separated. This can result in the formation of new combinations of alleles and give four genetically different chromatids each of which ends up in a different gamete. This may lead to the formation of new phenotypes in the next generation. Alleles are exchanged between chromosomes.
The human Y chromosome is unusual in that it carries very few genes. The X chromosome, however, carries many genes. A trait controlled by genes on the sex chromosomes is called a sex-linked trait.
One sex-linked gene determines red-green color-blindness and has trouble telling red colors from green colors.
Females are seldom color-blind. Each of their two X chromosomes has one gene that influences the trait of color vision. The gene for normal color vision is dominant over the gene for color-blindness.
More males than females tend to be color-blind. The Y chromosome has no genes for color vision. If the male’s only X chromosome has the gene for color-blindness, then here will be color-blind. This is because his Y chromosome has no color vision to mask the color-blind gene.
Another sex-linked gene causes the disease hemophilia. A person with hemophilia has blood that does not clot. Hemophilia is caused by a recessive gene on the X chromosome, so it occurs generally in males. Females, however, may be carriers. These females have one normal gene that dominates the recessive gene for the trait. Carriers do not have hemophilia, but they can pass the recessive gene to their offspring.
Mendelian inheritance in man
A number of man’s many inherited characteristics have been shown to follow the simple patterns of inheritance first observed by Mendel. One such characteristic is brachydactylism; this and other characteristics involving fingers and toes, including possession of extra ones, seem to involve simple dominants. Another simple dominant trait is tongue rolling. Can you roll your tongue? Can your parents? What is your genotype for tongue rolling? If you cannot roll your tongue, does this mean that neither of your parents can? This may seem to be a very trivial sort of characteristic, one that neither natural nor society would favor, but oddly enough, very small differences such as this are often the reflection of more fundamental differences which may have considerable importance over an evolutionary time span.
Of more immediate consequence are a number of congenital diseases which are the result of the coming together of recessive genes. One such disease is sickle cell anemia. In persons homozygous for the stickling gene, a large proportion of the red blood cells “sickle”- that is, form a sickle shape-and then clog the small capillaries, causing blood clots and depriving vital organs of their full supply of blood. This produces continuous, painful illness and, usually, death at an early age. About 4 percents of the population in certain tropical regions in Africa are born with sickle cell anemia, and almost half of the members of some African tribes are known to carry the recessive gene. In this country, it is found almost exclusively among blacks.
Mental deficient in infants, is also the result of a “double dose” of a recessive gene; so is Tay-Sachs disease, which appears almost exclusively among Jews of Central European ancestry.
Human Genetics Disorders
As in Drosophila, the Y chromosome of a man carries much less genetic information than the chromosome. Genes for color vision, for example, are carried on the X chromosome in humans but not on the Y chromosome. Color-blindness is produced by a recessive allele of the normal gene. The normal allele is dominant; a woman with the normal allele and one X chromosome with the allele for color blindness will have normal color vision. If she transmits the X chromosome with the recessive allele to a daughter, the daughter also will have color vision if she receives a normal X chromosome from her father (that is, if he is not color-blind). If, however, the X chromosome with the recessive allele is transmitted to a son, he will be color-blind since, lacking a second X chromosome, he has only the recessive allele.
Probably the most familiar characteristic in human beings that is determined by a single group of alleles is the ABO blood series. The existence of blood groups was discovered in 1900 by Karl Landsteiner. Mixing samples of blood taken from members of his laboratory staff, Landsteiner found that sometimes the red blood cells would clump together, or agglutinate, & sometimes they would not. He worked out four major groups of blood: A, B, AB, & O. Before long, it was established that these blood types are inherited according to Mendelian laws.
If your blood type is A, this means that on the surface of your red blood cells is a specific polysaccharide, A, that is not found on the surface of blood cells of persons with type O or B. Persons with type B have polysaccharide B on their red blood cells; persons with type AB have two types of polysaccharides; & persons with type O have neither A nor B polysaccharides. People of blood type A have in their blood antibodies to B. Similarly, type B has antibodies to A. Type O individuals have antibodies to both A & B, while type AB ones have neither. As a consequence, if you – still hypothetically blood type A- are given a transfusion of blood type B or blood type AB, your body’s antibodies against the B or AB cells will agglutinate the donor B-type cells in their bloodstream. This reaction can be so violent that it is sometimes fatal. You can receive O cells safely, however, since they contain no polysaccharide that your body will recognize as foreign.
The agglutination phenomenon is caused by antibodies, globular proteins that react against foreign substances in the blood.
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.
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.
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 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.
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.