The somatic (sporophyte) cells of plants commonly have more than two sets of chromosomes and are termed polyploid. They are derived from an egg or sperm that contained more than one set of chromosomes. For example, if a diploid egg cell (2n= 14) unites with a haploid sperm cell (n = 7), the resulting zygote will be triploid (3n) with a total of 21 chromosomes (3n = 21). Gametes that carry more than one set of chromosomes are induced by treating flowers and buds with colchicine. This is especially true of hybrid fruits and vegetables where polyploidy often results in the larger size of vegetables, fruits and flowers. Polyploid plants may be triploid (three sets of chromosomes), tetraploid (four sets), pentaploid (five sets), hexaploid (six sets), and may go all the way to octoploid (eight sets). Generally odd polyploids, such as triploid plants, are sterile and cannot produce viable gametes. Gametes are produced by a special type of cell division known as meiosis. During prophase I of meiosis, the matching (homologous) maternal and paternal chromosomes of the seed and pollen parents must pair up with each other in a process called synapsis. Triploid plants usually exhibit synaptic failure because there is a third set of chromosomes that has no homologous set to pair up with. Since triploid plants cannot produce viable gametes, they are typically seedless. The commercial advantages of seedless watermelons and bananas are readily apparent. Even-numbered polyploids (such as tetraploid plants) are typically fertile because all the maternal and paternal sets of chromosomes have a homologous set of chromosomes to pair up with. Therefore, tetraploid plants can produce seeds. For propagation purposes, fertile tetraploid hybrids are economically desirable.

Sometimes diploid (2n) hybrids are sterile and seedless, even though they have two sets of chromosomes. This is especially true when the seed and pollen parents are two different species (interspecific hybrids) or different genera (bigeneric hybrids). Again, the sterility problem relates to synaptic failure during meiosis. The maternal and paternal sets of chromosomes do not pair up properly because they come from different species and are not truly homologous. A good example of this type of sterility is the diploid (2n) rabbage resulting from a cross between a radish (Raphanus sativus) and a cabbage (Brassica oleracea). The tetraploid (4n) rabbage hybrid is fertile because the two sets of radish chromosomes can pair up with each other, and the two sets of cabbage chromosomes can pair up with each other.

Keeping track of the chromosome numbers in plant hybridization can be a little confusing. For example, the diploid number of the rye plant (sporophyte) is 14. We can represent the two sets of rye chromosomes as DD, where D=7. The gametes of rye are haploid and carry only one set of chromosomes (D). Bread wheat is a hexaploid (6n) composed of 6 sets of chromosomes (AA, BB & CC), each set with 7 chromosomes (A=7, B=7, C=7). Therefore, the number of chromosomes in the wheat hexaploid sporophyte (AABBCC) is 42. The gametes of bread wheat carry three sets of chromosomes (ABC), a total of 21 chromosomes(3n = 21). Triticale (Triticosecale) is a bigeneric hybrid between bread wheat (Triticum aestivum n=21) and rye (Secale cereale n=7). The resulting hybrid (ABCD) contains one set of rye chromosomes (D) and 3 sets of wheat chromosomes (ABC), a total of 28 chromosomes (7 + 21). It is sterile because the rye (D) set has no homologous set to pair up with during synapsis. This sterile hybrid seedling is treated with colchicine to produce a plant with twice as many sets of chromosomes (i.e. 2A's, 2B's, 2C's and 2 D's), a total of 56 chromosomes (8 x 7 = 56). The fertile hybrid is an octoploid (8n) because it contains 8 sets of chromosomes (8n = 56).

Questions 1-12:  See bisexual flower diagram in Flowering Plant Article.

Questions 13-26:  Some of the matching choices are vegetative plant parts typically described and illustrated in general botany textbooks and dictionaries: Petiole (the stalk of a leaf); blade (the expanded or flattened part of a leaf); stolon (a stem trailing above the ground and rooting where the leaves arise); rhizome (a scaly, underground stem that produces leafy stems above and roots below; tuber (an enlarged, subterranean stem that typically arises at the tip of a rhizome); bulb (an upright subterranean stem surrounded by overlapping, fleshy scales); corm (an underground stem without overlapping scales or with only a few scales); storage roots (enlarged roots that often occur in clusters and without the "eyes" or buds of a tuber); thorn (a modified sharp-pointed stem); spine (a modified sharp-pointed leaf); herbaceous (a plant that lacks woody tissue); perennial (a plant that blooms each year, the herbaceous stems often dying back to a woody rootstock after flowering); annual (a herbaceous plant that lives for one year); biennial (a herbaceous plant that blooms during the second year and then dies). Click on the following link for illustrations of vegetative terminology:

In the flowering plant life cycle, the haploid generation is reduced to a germinated pollen grain containing three nuclei and a 7-celled embryo sac containing eight nuclei. Diploid microspore mother cells inside the anther undergo meiosis (microsporogenesis) forming haploid microspores (each mother cell dividing into four microspores). The microspores develop into binucleate pollen grains, each containing a tube nucleus and a generative nucleus. When the pollen grain lands on a receptive stigma it grows into an elongate pollen tube containing a tube nucleus and a generative nucleus, the latter of which divides into two sperm nuclei.

A diploid megaspore mother cell inside each ovule also undergoes meiosis (megasporogenesis) and forms four haploid megaspores, three of which abort leaving one functional megaspore. The functional megaspore (inside each ovule) undergoes nuclear division into a 7-celled, 8-nucleate embryo sac. At one end of the embryo sac are three antipodal cells. At the opposite end is an egg cell flanked by two synergid cells. A large binucleate cell in the center containing two polar nuclei is called the endosperm mother cell. During pollination, pollen grains land on the stigma where they form pollen tubes that penetrate the style and eventually the ovary of the flower. A separate sperm-bearing pollen tube must reach each ovule in order to fertilize the egg cell inside the embryo sac. During double fertilization two sperm are introduced into the embryo sac from the long pollen tube. One sperm nucleus fuses with the egg nucleus inside the egg cell to form a diploid (2n) zygote which develops into the embryo of the seed. The other sperm nucleus fuses with the two polar nuclei inside the endosperm mother cell to form the tripolid (3n) endosperm of the seed.

After fertilization, the ovule enlarges and develops into a mature seed containing a diploid zygote and triploid endosperm. The seed coat is chromosomally identical to the female parent (ovary tissue) because it was derived from two outer layers of the ovule called the integument (the integument of gymnosperm ovules consists of a single layer). On a mature seed the opening or pore in the seed coat is where the pollen tube once entered a gap in the integument layers called the micropyle. As the ovules develop into seeds, the outer ovary encasing the ovules enlarges and ripens into a fruit. Fruits that develop without double fertilization and without seeds are termed parthenocarpic. Examples of parthenocarpic fruits are navel oranges, bananas, seedless watermelons, and certain varieties of figs. Not all seedless fruits are parthenocarpic. In Thompson seedless grapes, fertilization does occur, but the ovules fail to develop within the fruit. Parthenocarpy can be induced artificially by the application of dilute growth hormone sprays (such as auxins) to the flowers, as in seedless tomatoes. Seedless watermelons come from triploid (sterile) plants; however, to set fruit they must be pollinated by a fertile diploid plant. Some embryos of seeds can develop apomictically without fertilization. A number of angiosperm families contain apomictic species, including figs, blackberries, hawthorns and dandelions. The embryo may develop from a diploid nutritive cell (nucellus tissue) surrounding the embryo sac or from the fusion of hapolid cells within the embryo sac. In general there are two main types of apomixis:

[1] Parthenogenesis (agamogenesis): A haploid or diploid egg within the embryo sac (or diploid cell from 2 fused haploid cells of embryo sac) develops into an embryo. [Formation of haploid cells may involve crossing over during Prophase I of meiosis resulting in some genetic variability.]

[2] Agamospermy: An embryo arises from tissue surrounding the embryo sac. If this involves cells of the nucellus or inner integument layer it is called a nucellar embryo. Nucellar embryos are chromosomally identical to the sporophyte parent. They are essentially clones of the female parent. In varieties of the edible fig (Ficus carica), apomictic seeds allow propagation of choice edible fig cultivars (female trees) without the transmission of viruses through cuttings. Apomixis also enables a pioneer seedling to colonize and become naturalized in a new habitat by reseeding itself without cross pollination.

Questions 27-34:  If the diploid pollen parent is aabbcc, then the haploid sperm would be [abc]. If the diploid seed parent is AABBCC, then the haploid egg would be [ABC]. In double fertilization, one [abc] sperm unites with one [ABC] egg to form a diploid [AaBbCc] zygote. Another haploid sperm [abc] unites with two haploid polar nuclei [ABC] + [ABC] to form a triploid [AAaBBbCCc] endosperm within the seed. Since the seed coat originates from the outer wall of the ovule (called the integument), which was part of the original maternal seed parent, it is chromosomally identical with the original diploid seed parent. The mature pollen grain contains a tube nucleus and generative nucleus, the latter of which divides into two sperm nuclei within the pollen tube. All of these pollen nuclei are chromosomally identical with each other and with the sperm nuclei. The following diagram summarizes double fertilization in this question: [Botany 115 students: Remember that in your question the diploid pollen parent is aabbccdd and the sperm would be abcd; the diploid seed parent is AABBCCDD and the egg would be ABCD.]

Sperm #1 (abc) fuses with a haploid egg (ABC) resulting in a diploid zygote (AaBbCc) that grows into a diploid embryo (AaBbCc) within the seed. Sperm #2 (abc) fuses with the two haploid polar nuclei (ABC and ABC) within the endosperm mother cell resulting in a triploid endosperm cell (AAaBBbCCc) that develops into the nutritive endosperm tissue (AAaBBbCCc) surrounding the embryo. The following remarkable Wayne's Word image shows a minute diploid coconut embryo embedded in the triploid, meaty endosperm within the seed of a coconut palm.

A Note For Biology 100 Students: In exalbuminous seeds, such as lima beans and walnuts, the endosperm has been completely absorbed by the embryo. The embryo of these seeds consists of two prominent halves called cotyledons. Attached between the cotyledons is a minute, primordial, leaf-bearing shoot called the plumule and an elongate primordial root called the radicle. See following photo: The embryo of a lima bean seed showing the embryonic shoot or plumule (A), the embryonic root or radicle (B) and two cotyledons (C). The two fleshy halves called cotyledons are actually part of the embryo. The seed coat (D) has been partially removed from the cotyledons. Since the seed coat originates from the outer wall of the ovule (called the integument), which was part of the original maternal seed parent, it is chromosomally identical with the original diploid seed parent.

Questions 35-40:  A mature angiosperm pollen grain contains a tube nucleus and a generative nucleus, the latter of which divides into 2 sperm within the pollen tube. For this question, a hypothetical sperm-bearing angiosperm pollen tube contains a total of 12 chromosomes. Since the pollen tube contains 3 nuclei (a tube nucleus and 2 sperm nuclei), you must divide the 12 chromosomes by 3 in order to calculate the number of chromosomes per nucleus. Each nucleus within the pollen tube contains one haploid set of chromosomes. The egg also contains a haploid set of chromosomes, while the zygote is diploid (with 2 sets of chromosomes) and the endosperm is triploid (with 3 sets of chromosomes).

Questions 41-47:  This seven-celled embryo sac contains eight haploid nuclei, each with one set of chromosomes. The seven cells include three antipodal cells, one endosperm mother cell, two synergid cells and one egg. All of these seven cells contain a single nucleus except for the endosperm mother cell which contains two polar nuclei. The haploid number of chromosomes per nucleus can be found by dividing the total number of 32 chromosomes in this embryo sac by 8.

Questions 48-51:  The chromosomal answers to these questions can be found in the Wayne's Word article about vegetables of the mustard family. The diploid chromosome number (two sets of chromosomes) for a radish is 18 and the diploid chromosome number for cabbage is also 18. The haploid number (one set of chromosomes) for these species is nine. Therfore, the gametes of the radish and cabbage contain nine chromosomes. Since the radish and cabbage belong to different genera, the offspring resulting from this cross is called a bigeneric hybrid. The diploid (2n) hybrid is sterile because the chromosomes of the parents are not truly homologous; therefore, the hybrid does not have viable gametes and cannot produce viable seeds. The tetraploid (4n = 36) rabbage is fertile and can produce viable seeds. The two sets of nine homologous radish chromosomes can pair up with each other, and the two sets of nine homologous cabbage chromosomes can also pair up with each other.

Questions 63-74:  Answers to these questions can be found in the following articles, particularly the articles about the common fig (Ficus carica) that is grown commercially in the San Joaquin Valley of California. [See Calimyrna Figs Figs and Sex Determination in figs.

A Genetic Cross Between Watermelons

Questions 75-80:  Questions about a cross between two hypothetical watermelons.

In watermelons the gene for green rind (G) is dominant over the gene for striped rind (g), and the gene for short fruit (S) is dominant over the gene for long fruit (s). The alleles for rind color and fruit length occur on two different pairs of homologous chromosomes. For this question, assume that a gene for large melons (L) and and gene for many seeds (F) occur at opposite ends of another chromosome (linkage). The alleles for size and seed number, i.e. the genes for small melons (l) and few seeds (f), occur on a third homologous chromosome. A watermelon plant bearing large, green, short fruits containing many seeds was crossed with a plant bearing large, striped, long fruits containing many seeds. Some of the offspring from this cross produced small, striped, long fruits with few seeds.

Assuming no crossing over between homologous chromosomes, what is the fractional chance of producing the following offspring? Remember that there are three pairs of homologous chromosomes in this problem, and one of the homologous pairs exhibits autosomal linkage. The chromosomes of each parent are shown in the following illustration:

There are several ways to solve this problem, but one way is to construct a 16 square checkerboard with eight rows and two columns. To the left of each row, put the eight gametes of the parental plant bearing large, green, short fruits containing many seeds. At the top of each column, put the two gametes of the parental plant bearing large, striped, long fruits containing many seeds. The most difficult part of this problem is to figure out the exact gene combinations of the gametes. Once this is known, you can simply fill in the squares of the checkerboard with the correct gene combinations (genotypes) for each offspring. Remember that each genotype must contain eight letters: An LF or lf, plus two G's (GG, Gg or gg) and two S's (SS, Ss or ss). For example, one of the 16 squares contains the genotype LLFFGgSs; one of the 16 squares contains the genotype LLFFGgss; two of the 16 squares contains the genotype LlFfGgss; and one of the 16 squares contains the genotype llffGgSs. There a total of 12 different genotypes in the checkerboard.

The gene combinations of the gametes are shown in the above Table 1. The plant bearing large, striped, long fruits containing many seeds can produce only two different kinds of gametes (shown in red in Table 1) . The gametes must contain one of the LF or lf chromosomes, one of the g chromosomes, and one of the s chromosomes. Therefore, the two possible gametes are: LFgs and lfgs. The LF and lf genes always appear together because they occur on the same chromosomes. Without crossing over, you could never have Lf together or lF together.

The plant bearing large, green, short fruits containing many seeds can produce eight different kinds of gametes (shown in green in Table 1). The gametes must contain one of the LF or lf chromosomes, one of the G or g chromosomes, and one of the S or s chromosomes. Since there are two possibilities for each of the three kinds of chromosomes, there are eight different possible gametes (2 x 2 x 2 = 8). Four of the eight gametes will contain LF plus GS, Gs, gS or gs. Four of the eight gametes will contain lf plus GS, Gs, gS or gs.

When all the 16 squares of the checkerboard are filled in, simply find the genotypes in the squares that are described in questions 77-80. The correct answers are expressed as a fractional ratio, such as 1/16. Remember that L = large fruit and l = small fruit; F = many seeds and f = few seeds; G = green rind and g = striped rind; S = short fruit and s = long fruit. The capital letters represent dominant genes (alleles) while the small case letters represent recessive genes (alleles). Therefore, a plant with a genotype of LlFfGgss would produce large (L), green (G), long (s), fruits containing many (F) seeds. A genotype of LlFfggSs would produce large (L), striped (g), short (S) fruits containing many (F) seeds

Part V, Part VI, Part VII, Part VIII and Part XI contain matching questions 81 - 216. Most of the answers to these questions can be found in the Reading List for Exam #3, the Wayne's Word Index, and Economic Plant Families. Answers to many of the fruit type questions are in the Fruit Identification Outline. Useful hyperlinks for questions about plant genetics, figs, watermelons, etc. are found in the above sections for questions 1 - 80.