laboratory

Activities:

ACTIVITY 1. CHROMOSOME STRUCTURE AND KARYOTYPES
ACTIVITY 2. MEIOSIS AND GENETICS
Variation among offspring
Meiosis in a flowering plant
ACTIVITY 3. MONOHYBRID CROSSES
  Punnett square analysis
  Relationships among alleles

Meiosis in a flowering plant

Meiosis is a crucial step in the life cycle of almost all multicellular organisms. It produces gametes (eggs and sperm) in animals, but forms spores in plants. The haploid male and female spores develop into tiny organisms that then produce gametes. Meiosis is easier to view in plants, so we will use a plant example to visualize the meiotic process.

flower

Many flowering plants have both male and female organs. This photograph of a lily flower illustrates the male and female parts.

The anthers produce male spores (called microspores) by meiosis. They will develop into pollen grains. The pistil contains an ovary at its base in which female spores (called megaspores) will be produced by meiosis.

ovules

The ovary is usually deep within the flower. It is shown in this cut-away view of a flower model. Within the ovary, many tiny ovules are present which produce female megaspores by meiosis. If the resulting gametes are fertilized by gametes from the pollen, the ovules will develop into diploid seeds.

Note the cross section view of the ovary and ovules at the bottom of the ovary. This is the view you will see on microscope slides.

Now you are ready to view meiosis within the anthers of a lily. Go to the web site below and examine each of the images under the first heading "Microsporogenesis in Lilium". A cross section of the anther is shown in the left column at low power and the dividing cells are shown in the right column at higher power. Click on the images to enlarge them. You should be able to distinguish prophase, metaphase, anaphase and telophase since these stages look the same as in mitosis. It is more difficult to tell whether the cells are in a stage of meiosis I or meiosis II. Compare the thickness of the chromosomes at metaphase/anaphase in the 1st vs. 2nd meiotic division. The chromosomes appear thicker if the chromatids have not separated. Also note that the daughter cells remain together until the end of meiosis. They finally separate as the haploid microspores mature into pollen grains.

web site - Pollen formation in Lily

When you understand meiosis in Lily anthers, view these 4 images of meiosis from meiosis I and/or meiosis II of pollen formation. Place the images in their proper order for question 5. View this image of a lily anther and name the stage of meiosis to answer question 6.

Meiosis within an ovule is similar to that within the anther, except that only one cell per ovule undergoes meiosis and female megaspores are produced.

ovary This slide is a cross section of a lily ovary. Arrows point to 3 of the ovules within the section.

Examine the following slide of a lily ovary viewed by virtual microscopy. Study each of the 6 ovules to determine whether a meiotic stage is present. Locate a prophase, capture the image at high magnification, and label the ovule and chromosomes. Submit your labeled image to WebAssign for question 7.

Slide- Lily ovary with ovules, viewed by virtual microscopy

ACTIVITY 3. MONOHYBRID CROSSES (crosses involving one gene locus)

Punnett square analysis

The easiest way to examine the transmission of information between generations is to translate what happens in meiosis into a Punnett square analysis. The Punnett square is a summary of every possible combination of one maternal allele with one paternal allele for each gene being studied in the cross. This allows the investigator to determine the probability of an offspring having a particular genotype. Remember that alleles are alternate forms of a gene. Thus there are two alleles for each gene in a diploid cell. If the alleles are the same, the individual is homozygous for the trait. If the alleles are different, the individual is heterozygous. If you are unsure about the meaning of these terms, review these links from the Patterns of Inheritance topic: terminology and alleles.

At this point, you should fill out diagram 1 on your help sheet. Add the alleles "A" and "a" to your chromosome drawings and determine the types of gametes formed at the end of the meiotic process. The completed diagram will help you relate meiosis to construction of the following Punnett square.

Starting with a parent who is heterozygous for a given gene results in the most variation in gametes at the end of meiosis, with 1/2 carrying the "A" allele and 1/2 carrying the "a" allele.  Since we will be following the inheritance of one gene in this exercise, you should draw a 2 by 2 punnett square. If your square is drawn so that possible maternal gametes are represented in columns, then possible paternal gametes will be represented in the rows. Let us start with the most complex mating, one involving two Aa parents. Red indicates the types of eggs produced by the mother, while blue represents the types of sperm produced by the father.

 
Maternal
A
a
Paternal
A
AA
Aa
a
Aa
aa

Note that the purple boxes represent the possible combinations resulting from the union of maternal and paternal gametes. From this data, we can calculate the ratio of different genotypes in the offspring when sperm fertilize eggs at random. Note also that the offspring are diploid whereas the gametes that gave rise to the offspring are haploid. Some of the offspring carry two alleles that are the same and thus considered homozygous, while others carry two different alleles and are considered heterozygous.