SEXUAL REPRODUCTION, MEIOSIS, GENETICS
Objectives:
1. Know three sources of genetic diversity.
2. Understand how meiosis increases genetic diversity.
3. Know the stages and events of meiosis.
4. Know the differences between meiosis and mitosis.
5. Be able to do simple monohybrid and dihybrid crosses.
Since many plants, and some simple animals, can reproduce asexually, what is the advantage of sexual reproduction? Why did it evolve? The answer seems to be that sexual reproduction increases the genetic diversity within a species, and allows new species to develop. As a result, in an environment where physical factors, such as availability of water and light, temperature, and / or biological factors, such as competition and predation, are subject to change, the populations of a particular species are able to change as well. This type of change is due to differential reproduction, i.e. -some individuals produce more surviving offspring than others. The result is the tremendous variety of adaptations found in the plant and animal world.
What is the source of the genetic variation that makes this process possible? The ultimate source of genetic variation is the mutation of genes resulting in different alleles, or forms of a gene. Mutations provide the raw material for three other additional sources of variation, which are unique to sexual reproduction.
Sexual reproduction universally requires two events: meiosis and fertilization.
Meiosis is a special type of nuclear division which reduces a diploid number of chromosomes to a haploid number. In a diploid nucleus, each chromosome has a partner which carries the same sequence of genes. These partner chromosomes, , called homologous chromosomes are not necessarily identical because the alleles, or variations of the genes they carry, may be different. In human body cells, we have 23 pairs of chromosomes. Therefore our diploid number (2n) is 46. In meiosis, the diploid number of chromosomes is reduced by half, in order to form a gamete, or sex cell in animals, that can fuse with another haploid cell in the process of fertilization. In plants, meiosis usually results in haploid spores, which germinate and grow into haploid plants, which then produce gametes. The haploid sex cells must receive one chromosome from each pair in the parent nucleus, so that fertilization results in a new diploid cell, the zygote, which has the complete set of homologous chromosomes. What is the haploid (n) number of chromosomes in a human gamete?
Meiosis itself provides the second source of genetic variation. Genetic variation is increased through the processes of segregation and independent assortment, and crossing over. Together, these are referred to as recombination, because they contribute to unique combinations of alleles in the gametes. The third avenue of genetic variation in a population comes from cross fertilization, in which the gametes from one individual are fertilized by gametes from another individual.
Figure 1
MEIOSIS MITOSIS
Interphase: Doubling of each chromosome Doubling of each chromosome
Prophase I: Homologous pairs attach No homologous pairing
Metaphase I Paired chromosomes line up Chromosomes line up singly
Anaphase I (not shown)Homologous pairs separated Sister chromatids separated
Telophase I: Daughter nuclei form Daughter nuclei form, cytokinesis occurs
Meiosis II Second nuclear division Two identical daughter cells (2n)
Prophase II ( Not shown ) Chromosomes recondense
Metaphase II unpaired chromosomes line up
Anaphase II (not shown) Sister chromatids separated
Telophase II and Cytokinesis Four haploid daughter cells or gametes are produced
Activity 1: Comparison of mitosis and meiosis.
Follow along with your instructor and draw the changes on Figure 1 that occur in the nuclei of two cells which illustrate the basics of mitotic and meiotic division. Not all the stages of both events will be drawn. In addition to the terms you learned for mitosis, learn the following:
Bivalents- a pair of synapsed homologous chromosomes connected to each other during prophase I
Crossing over- the exchange of equivalent parts of chromatids during prophase I by a pair of homologues (an exchange of alleles).
After filling in figure 1:
Compare the daughter cells produced by Mitosis. Are they the same as the parent cell?
Compare the daughter cells produced by Meiosis. Are they identical to each other?
How do they differ from the parent cell?
What are the two critical differences between Mitosis and Meiosis?
Activity 2: Independent Assortment
Use Figure 2 to illustrate the effects of independent assortment with a simplified nucleus that has two homologous pairs of chromosomes. In addition, each pair of chromosomes has 2 different alleles. Use a solid line to represent one pair, and a dashed line to represent the other pair. Use A and a as the alleles on the solid pair, and B and b as the alleles on the dashed pair. Therefore, the parent cell in each case has a genotype of AaBb.
At step 3, representing metaphase 1, be sure to have the homologous pairs line up differently in cells 1 and 2. For example, if in cell 1, the AA and BB chromosomes are on the same side of the equator, flip one of them to the other side in cell 2. Note the genotypes of the gametes produced by cell 1. Are they different from the gametes produced by cell 2?
How many different gamete genotypes are possible from these two parent cells?
In what proportions are the different gametes produced?
Figure 2
Cell # 1 Cell # 2
Step 1 Interphase: Doubling of each chromosome
Step 2 Prophase I: Homologous chromosomes pair up
Step 3 Metaphase I Chromosomes line up in homologous pairs
Anaphase I not shown Homologous pairs separated
Step 4 Telophase I Nuclei reform
Meiosis II Second nuclear division
Step 5 Metaphase II
Anaphase II not shown Sister chromatids separated
Step 6 Telophase II Four haploid daughter cells or gametes
Activity 3: Meiosis and Gamete Formation
Instead of drawing the entire process of meiosis every time, learn to determine the genotypes of the possible gametes by shorthand.
If the parent cell is AA, what are the possible Gametes?
If the parent cell is aa, what are the possible gametes?
If the parent cell is Aa, what are the possible gametes?
List the possible gamete genotypes for the following parental cells:
AABB: AaBB:
AaBb: aaBb:
Activity 4: Meiosis in a Lily Anther
Obtain a slide of a lily flower sectioned in the bud stage. Meiosis takes place in many cells in the anther in order to produce haploid cells that will then divide by mitosis to form a miniature haploid male plant (the pollen grain). Can you see any evidence of meiosis taking place? What phases can you identify?
Figure
3: Lily bud x.s. Locate ovary with ovules, anthers.
Draw one anther at high magnification, showing meiosis during spore formation.
Now that you understand the processes of meiosis, and can determine the gamete genotypes possible, you can learn to determine the possible genotypes and phenotypes of the offspring resulting from a cross between two individuals. The phenotype is the outward expression of the genotype of the individual. We will use the offspring's outward appearance as its phenotype, but in real life, the phenotype relates to the production of proteins and enzymes that affect all aspects of the physiology, behavior, and traits of the individual. Learn the following terms:
Homozygous- having a pair of the same alleles for any one gene; i.e. RR, rr, TT, tt
Heterozygous- having different alleles for any one gene; i.e. Rr,Tt
Dominant allele- an allele that masks the expression of another allele of the same gene, usually
represented by a capital letter. Dominant alleles are expressed in both the homozygous or
heterozygous conditions; i.e. RR or Rr
Recessive allele- is masked by a dominant allele: usually given a lower case letter.
Recessive alleles are only expressed in the homozygous condition; i.e. tt, rr
Purebred- Having specified homozygous alleles, implies the genotype of parents and
offspring have been and will continue to be homozygous.
Monohybrid- A cross between parents differing with respect to one set of alleles only;
i.e. RR versus Rr, Tt versus tt
Dihybrid- A cross between parents differing with respect to 2 pairs of alleles; i.e.
RRTT versus rrtt
Activity 5: Monohybrid Cross with Simple, Complete Dominance
A plant that is homozygous for tall height is crossed with a plant that is a purebred dwarf. Let T represent the allele for tall height and t represent dwarf height.
Parental genotypes are :
gamete genotypes are:
F1 offspring genotypes are:
F1 phenotypes are:
If you allow 2 of the F1 offspring to cross:
gamete genotypes are:
F2 offspring genotypes are:
F2 phenotypes are:
A Punnett Square is often used to illustrate a
cross:
Activity 6: Monohybrid Backcross or Testcross
In order to determine the genotype of an individual with the dominant phenotype, it may be necessary to do a testcross with a purebred homozygous individual. Suppose you cross a tall plant with an unknown genotype with a dwarf plant. What are the two possible outcomes of the cross? How does the outcome tell you the genotype of the dominant individual?
Activity 7: Incomplete Dominance
In some cases, neither allele is completely dominant over the other. This is called incomplete dominance or co-dominance. In these cases, the heterozygous individual shows a phenotype intermediate between the two homozygous conditions. Let R stand for an allele for red flower color, r stand for an allele for white flower color; that is, an RR plant has red flowers and an rr plant has white flowers. What color would you expect an Rr plant to be? Show a cross between two homozygous individuals below. What are the genotypic and phenotypic ratios in the F1 offspring?
Now show a cross between two F1 offspring. What are the genotypic and phenotypic ratios in the F2?
Activity 8: Green versus Albino Alleles in Corn
Obtain a tray of corn planted about two weeks ago. Count the number of green and white seedlings present. Include dead plants if they are identifiable as to color.
Put your data on the board and combine it with the data from other groups. What can you infer about the genotypes of the parents from these results?
What are the genotypes of the green offspring? the albinos?
Will the albinos survive to reproduce?
In a cross between two heterozygous plants, what would the expected ratio of green/white offspring be?
How close is our ratio to the expected?
Activity 9: Grain Color in Corn
Obtain an ear of corn with yellow and purple colored grains. These represent the F2 generation of a certain cross.
Which color is dominant?
Count the number of yellow and purple grains in 5 rows on an ear. Put your data on the board. What is the expected ratio?
What is the observed ratio?
Which is closer to the expected ratio, the class data or an individual group's data?
Obtain another ear of corn from the instructor. Does this look different from the first?
What type of cross does this represent?
Activity 10a: Dihybrid Crosses
Let T stand for the dominant tall allele, t stand for short height, and P stand for the dominant purple flower allele, p stand for white flowers. Show a cross between a purebred tall, purple flowered plant with a purebred short white.
Parental genotypes:
gamete genotypes:
F1 genotypes and phenotypes:
Now, cross two F1 plants:
First, figure out the possible gamete types, you should have four. Then, use a Punnett square (next page) to determine the phenotypes of the F2 generation.
List phenotypes:
What is the phenotypic ratio in the offspring?
10b. Obtain a tray of dihybrid corn seedlings planted about two weeks ago.
What phenotypes are present?
Count the number of individuals of each phenotype and combine with the class data for analysis. What is the observed ratio?
If the parents were TtGg and TtGg, what would the expected ratio be?
10c. Obtain an ear of corn that represents the result of a dihybrid cross. You should see four types of grains:
purple full, purple wrinkled, yellow full and yellow wrinkled.
The endosperm on the full grains is starchy, and in the wrinkled grains it is sugary.
Count the number of each phenotype in five rows. Combine your data with the class data for analysis.
What are the dominant alleles?
Questions:
1. What are the three sources of genetic variation in sexually reproducing species?
2. What two events are always a part of sexual reproduction?
3. What are the two most important differences between mitosis and meiosis?
4a. List the gametes that would be produced by an individual with the genotype Ss.
b. List the gametes that would be produced by an individual with the genotype SsTt.
5. What does homozygous mean? What does purebred mean?
6a. What is meant by a monohybrid cross?
b. What is meant by the term dihybrid cross?
7. If half the offspring of a cross show the dominant phenotype, and half show the recessive phenotype,
what were the probable genotypes and phenotypes of the parents?
8. What are the probable parental genotypes and phenotypes that result in a phenotypic ratio in the offspring
of 3 dominant :1 recessive?
9. What are the probable genotypes and phenotypes of the parents if the offspring have a 9:3:3:1 phenotypic ratio?
10. Make up an example in which there is co-dominance or incomplete dominance, and show a cross through
two generations.