*Adapted  from  Laboratory  Topics  in  Botany,  8th  ed.  

Lab 9: Meiosis Review Questions Compare and contrast mitosis and meiosis by placing a check in the appropriate space if the feature applies.

*Adapted  from  Laboratory  Topics  in  Botany,  8th  ed.  

Genetics     Gregor Johann Mendel, an Austrian monk, was the first person to provide a logical explanation for the way genes behave during a cross. Our present knowledge of heredity rests directly on certain principles discovered by Mendel 150 years ago. Although Mendel conducted his research in the 1860s, his ideas were not fully appreciated until 1900, when they were independently rediscovered by four of the world’s first geneticists. In the 1890’s Dutch botanist Hugo DeVries, unaware of Mendel’s work, devised the laws of independent assortment, dominance, and segregation to explain the 3:1 ratio of phenotypes he observed in his plant species hybridization experiments. He is also the originator of mutation theory, and was the first to recognize the occurrence of recombination, or crossing over, between homologous chromosomes. DeVries came across Mendel’s now famous 1866 paper, titled “Experiments in Plant Hybridization.” during a literature search, and adopted some of Mendel’s terminology.

Carl Erich Correns, a German botanist, also rediscovered the basic principals of inheritance while continuing Mendel’s work on hawkweed; he was unaware of the results of Mendel’s experiments with peas that were so revealing and on which Mendel had based his major conclusions. Correns was instrumental in convincing DeVries to accept and publicize the priority of Mendel’s discoveries. Correns went on to discover a type of non-Mendelian inheritance – cytoplasmic or maternal inheritance – in which the results of a cross differ depending on which parent, the pollen donor or the egg donor, carries a trait. The modern understanding of cytoplasmic inheritance is that it applies to characteristics controlled by genes in the chloroplast and mitochondrial genomes, which are inherited (usually), along with the cytoplasm, from the maternal parent (egg donor).

Austrian agronomist Erich von Tschermak used the principles of inheritance and hybridization that he developed to create the world’s first new disease resistant crops; his grandfather had taught Mendel botany in Vienna. Finally, in 1901, William Spillman rediscovered Mendel’s laws during wheat breeding experiments aimed at providing farmers with wheat cultivars that would be better adapted to local growing conditions. Spillman is the only American to have duplicated Mendel’s work.

It is important to note Mendel knew nothing about the physical basis of heredity, nothing about the nature of the genetic material, and nothing about the behavior of chromosomes during meiosis; Mendel never even saw a chromosome. Despite his incomplete knowledge, through careful execution and analysis of numerous test crosses, and logical deduction based on the results, Mendel elucidated principles of heredity that pointed to the existence of genes, chromosomes, and the process of meiosis. For example, he concluded ���that characteristics are inherited as discrete units, and that these units (which he referred to as ��� factors and which we now know to be alleles—one of two or more alternative forms of the same gene) are present in pairs in the body cells of an individual, and that some factors are dominant over others (recessives). He further demonstrated that his experimental results could be explained only if a single factor for a given characteristic is contributed by each parent. The hypothesis that the pairs of alleles are separated during formation of gametes is known as Mendel’s principle of segregation. (We now know that meiosis is the mechanism by which segregation occurs.)

From the results of crosses in which two pairs of factors were studied simultaneously, Mendel concluded that factors for different characteristics were usually

*Adapted  from  Laboratory  Topics  in  Botany,  8th  ed.  

inherited independently of one another. This is known as the principle of independent assortment. We now understand that this is due to the fact that the alleles for different characteristics are often carried on different pairs of homologous chromosomes. During metaphase I, the homologous pairs align in random orientations – the paternal allele for one characteristic may be oriented toward one of the daughter cells, while the paternal allele for another characteristic is oriented toward the other daughter cell. During anaphase I, the alleles segregate independently of one another as the homologs carrying them are distributed to the daughter cells. Although, as mentioned previously, Mendel had no knowledge of either genes, chromosomes, or meiosis, his law of independent assortment is still fundamentally correct for pairs of alleles that are not carried on the same pair of chromosomes.

The following pages contain a number of genetics problems for you to solve. On the final page you will find a glossary of terms commonly used in genetics that you may find useful. For additional information, see Chapter 8 of the text, pp 159-173. Exercise I: Understanding the Principles of Inheritance through Genetics Problems Through chromosomal replication and meiosis, heritable information of an individual is passed on to the next generation. When fertilization follows meiosis, the alleles carried by one gamete are recombined with the alleles for the same characteristics, or traits, of the second gamete.

In simple types of inheritance, it is possible to follow both the phenotypes and genotypes of parents and progeny (offspring) in regard to specific characteristics by using the Punnett square. In this device, all the possible gametic allele contributions from the female parent are listed on one side of a square; those of the male parent are listed along the other. The potential genotypes of the progeny are obtained by combining allele contributions from one parent with those from the other and listing the combinations in the boxes within the square. For example, in tomato fruit, red-flesh color is dominant over yellow-flesh color. Therefore, we use the capital letter R to represent the dominant red allele; the lower case r represents the recessive yellow allele. The upper and lower case forms of the same letter are used to signify that the two conditions are alternative forms— alleles—for the same characteristic. The dominant and recessive alleles for the same characteristic are present at corresponding locations, or loci (singular: locus), on homologous chromosomes.

The Punnett square works as follows: In a tomato that is homozygous for red-fleshed fruit, the genotype is represented by RR, denoting that two dominant alleles are present. In a tomato that has yellow-fleshed fruit, the genotype is represented by rr.

1) Why can we be sure that a plant with yellow-fleshed fruits has genotype rr?

*Adapted  from  Laboratory  Topics  in  Botany,  8th  ed.  

The red-fleshed tomato can produce gametes of only one type: gametes carrying the R allele. The yellow-fleshed tomato can produce only gametes carrying the r allele. As diagramed in Example 1, all of the offspring of a pairing of RR and rr tomatoes would be Rr genotype. 2) What is their (the Rr individuals) phenotype? ___________________

The offspring produced by a cross are called the first filial generation (∫). ���Crossing two F1 individuals produces the second filial generation (F2). To obtain the F2 generation in this example, the Punnett square would be set up as follows:

3. What are the genotypes of the potential gametes for each of these parents?

Egg ���__________________________ Sperm ___________________________

4. Why is each gamete indicated as carrying a single allele for fruit flesh color? ���

________________________________________________________________

5. What are the possible phenotypes of the F2 generation? ____________________

_________________________________________________________________ In what proportion (ratio) would the occur? _____________________________ 6. What is the genotypic ratio? ___________________ The example of the tomato fruit outlines the simple method to be followed in working out all subsequent genetics problems. If you have difficulties, review the discussions here and above for meiosis. You have probably forgotten to use some simple fact or device, or you do not understand the reasons for certain steps in arriving at a solution or explanation.

*Adapted  from  Laboratory  Topics  in  Botany,  8th  ed.  

Remember: In these examples, you are dealing with diploid parents undergoing meiosis to give rise to microspores and megaspores and eventually to haploid gametes. Part A: Monohybrid problems concerning tomato. Given:

• Tall stature is dominant over dwarf stature (D, d). • Smooth-skinned fruit is dominant over peach (hairy) fruit (P, p). • Oblong fruit shape is dominant over normal fruit shape (O, o). • Simple inflorescence is dominant over compound inflorescence (S, s). • Purple stem is dominant over green stem (A, a). • Fasciated fruit (consisting of may chambers) is dominant over nonfacciated fruit

(F, f) • Red-fleshed fruit is dominant over yellow-fleshed fruit (R, r). • Opaque skin, as in bright fruit, is dominant over transparent skin, as in dull fruit

(Y, y). Note: The genes for stature, skin type, fruit shape, and inflorescence belong to one linkage group (that is, they are all inherited together); genes for stem color and fasciation belong to another linkage group. Genes R and Y are independent of these linkage groups and of each other. 7) Why are certain genes usually inherited together? (that is, what is a linkage group?)

a) A homozygous tall tomato plant (genotype: ___________) is pollinated by a homozygous dwarf plant (genotype: ______________).

1) What allele(s) will be carried by the pollen grain nuclei? _____________

By the egg nuclei? ______________

2) What will be the genotype of the first filial generation (F1)? ______________.

3) What will be its stature (phenotype)? _____________________. 4) If the F1 genetation is self-pollinated, what will be the appearance (phenotype) of the

second filial generation (F2). ________________________.

5) What alleles were carried by the pollen grains of the F1 parents? ___________

*Adapted  from  Laboratory  Topics  in  Botany,  8th  ed.  

By the eggs of the F1 parents? ___________________? For the F2 generation, the genotypic ratio is _____________ and the phenotypic ratio is ________________. b. A tall tomato plant is pollinated by a dwarf tomato plant. About half the progeny are tall plants and half are dwarf plants.

13. What are the genotypes of the parents? Tall ___________________. Dwarf _________________. Of the progeny? _______________. PART B. DIHYBRID PROBLEMS CONCERNING TOMATO Dthybrid crosses involve the inheritance of two sets of alleles, that is, two genes. We can readily determine the genotypes of the offspring of a dihybrid cross by using a Punnett square. The first step in solving this kind of genetics problem is to determine the number of different kinds of gametes produced by each parent. Mendel’s principles of segregation and independent assortment are both invoked in the identification of the genetic composition of the gametes.

The results of the Punnett square can be most easily interpreted if the alleles are recorded in a consistent order, if both alleles governing a given characteristic are always written side by side, and if the dominant allele in heterozygous individuals is always recorded in the first position. The example given here will illustrate a procedure for you to follow in working subsequent problems.

A homozygous tall, red-fruited tomato is pollinated by a homozygous dwarf yellow-fruited plant. The parental genotypes are DDRR (female) and ddrr (male). 16. What alleles will be carried by the eggs? _______________ By the pollen grains? ________________ 17. What are the possible genotypes of the progeny?

The phenotypes of the progeny? ________________

*Adapted  from  Laboratory  Topics  in  Botany,  8th  ed.  

The F1 plants are self-pollinated. 18. What possible combinations of alleles will be carried by the pollen grains? ________ By the eggs? _________________ Complete the Punnett square below and write in the genotypes of the parents in the spaces provided.

19. What will be the possible phenotypes of the F2 population? __________________ _______________________________________________________. The genotypes of the F2 population? ______________________________________________________ 20. How could you most easily determine the genotype of a tall, red-fruited tomato plant? 21. What possible genotypes might such a have? ________________________________ A tomato plant homozygous for tall stature and oblong fruit is pollinated by a plant that is homozygous for dwarf stature and normal-shaped fruit. Note that these genes belong to one linkage group. (See exercise I, Part A). 22. What are the genotypes of the parents?

tall/oblong : __________________ dwarf/normal: ____________________

*Adapted  from  Laboratory  Topics  in  Botany,  8th  ed.  

23. What alleles will their pollen grains carry? _____________________ What alleles will the eggs carry?���____________________ 24. What are the possible genotypes of the progeny? ________________________ The phenotypes of the progeny? ________________________________ The F1 plants of problem b above are self-pollinated. (For this problem, assume no crossing-over has taken place.) 25. What alleles will their gametes carry? eggs ______________, sperm _____________. 26. What are the genotypes of the F2 population?���_______________________________. The phenotypes of the F2 population? ____________________________________ A tomato plant heterozygous for stem color and fasciation (the dominant alleles on the same chromosome) is pollinated by a plant heterozygous for stem color and homozygous recessive for fasciation (that is, it is nonfasciated). 27. What alleles wilt the pollen grains carry? _________________________ 28. What alleles wilt the eggs cariy?���________________________________ 29. What are the genotypes of the progeny? _______________________________ The phenotypes of the progeny? __________________________________________ _______________________________________________________________________ 30. If the dominant alleles for stem color and fasciation are commonly located on the same chromosome, how could the dominant allele for one of these characteristics and the recessive allele for the other be on the same chromosome? _______________________________________________________________________________________    _______________________________________________________________________________________   A tomato plant homozygous for tall stature and smooth-skinned fruit is crossed with a dwarf plant bearing peach fruit. The F1 plants are then testcrossed with a dwarf peach-fruited plant. The results of the testcross are as follows: 196 tall, smooth; 6 tall, peach; 8 dwarf smooth; and 195 dwarf peach.

*Adapted  from  Laboratory  Topics  in  Botany,  8th  ed.  

31. What is the genotype of the tall, smooth-skinned parent? _____________________ The dwarf, peach-fruited parent? ______________-- The F1 plants? ________________ The testcrossed parent?���_________________ The tall, smooth progeny?______________ The tall peach progeny? _____________ The dwarf, smooth progeny? ___________ The dwarf, peach progeny? ________________. 32. What is the explanation for the results obtained? ____________________________ ______________________________________________________________________   33. What percentage of testcrossed offspring are genotypically like the parents? ______ Phenotypically like the parents? _________________ 34. What percentage are different from the parents? ________________ 35. If the genes responsible for these two characteristics (that is, stature and fruit texture) were not in the same linkage group, what phenotypes—and in what ratio—would you expect from the testcross? _______________________________________________________________________   _______________________________________________________________________  PART C. MONOHYBRID PROBLEMS HAVING MODIFIED RATIOS Not all characteristics, or traits, exhibit complete dominance. That is, one of a pair of alleles may not necessarily be dominant over the other, so that the heterozygous condition is phenotypically expressed in traits that are intermediate between the dominant and recessive characteristics.

A representative case of incomplete dominance is the inheritance of hair on tomato stems. The homozygous condition HH results in a completely hairless stem, and the homozygous condition hh results in a very hairy stem. However, the heterozygous condition Hh produces stems with relatively few large hairs. A very hairy plant (genotype ______________) is crossed with a hairless plant (genotype___________). 36. If the F1 generation is self-pollinated, what will be the appearance of the F2 generation?

*Adapted  from  Laboratory  Topics  in  Botany,  8th  ed.  

_________________________________________________________________   37. The genotypic ratio of the F2 generation is ______________________ The phenotypic ratio of the F2 generation is _________________________ 38. How do hese ratios compare with the genotypic and phenotypic ratios of the F2 generation of problem a in Exercise I Part A? ____________________________________________________________________ ____________________________________________________________________

*Adapted  from  Laboratory  Topics  in  Botany,  8th  ed.  

*Adapted  from  Laboratory  Topics  in  Botany,  8th  ed.  

Some Terms Commonly Used in Genetics Allele: One of two or more alternative forms of the same gene. Alleles occupy the same site (locus) on homologous chromosomes. Cross: The bringing together of genetic material from different individuals in order to achieve genetic recombination. a. Backcross: The crossing of a hybrid with one of its parents or with an individual that is

genetically equivalent to one of its parents.

b. Testcross: The crossing of an individual expressing a dominant trait with an mdi vidual that is homozygous recessive for that trait; used to determine whether the individual expressing the dominant trait is homozygous or heterozygous.

c. Monohybrid cross: A cross involving the alleles affecting one characteristic.

d. Dihybrid cross: A cross involving the alleles affecting two characteristics.

Crossing-over: The reciprocal exchange of corresponding segments of genetic material between chromatids of homologous chromosomes during prophase I of meiosis. Dominant allele: The allele that produces its full effect even though its contrasting allele is present. Epistasis: A form of genetic interaction whereby one gene interferes with the phenotypic expression of another gene so that the phenotype is determined effectively by the epistatic gene and not the latter gene when both occur in the genotype. Gene: A unit of hereditary information located on a chromosome. Genotype: The genetic makeup of the individual. Heterozygous: Having different alleles at the same locus on homologous chromosomes. The individual is called a heterozygote. Homozygous: Having identical alleles at the same locus on homologous chromosomes. The individual is called a homozygote. Hybrid: An individual that is heterozygous for one or more (usually many) pairs of genes. Incomplete dominance: The condition that results when two different alleles produce an effect intermediate between the effects of these same alleles in the homozygous condition. Linkage: The tendency for certain genes to be inherited together because they are located on the same chromosome. Locus (plural: loci): The site of a gene on a chromosome. ��� Phenotype: The physical appearance of the individual resulting from its genetic makeup. Recessive allele: The allele that is masked or will not produce its effect in the presence of the dominant, or contrasting, allele.