Question

activity 4.3 modeling crossing over
during prophase i of meiosis, chromosomes line up and an exchange of genetic material between homologous pairs may occur. in this activity, you will model crossing over using strips of clay. each twist tie represents the centromere of the chromosome, holding the sister chromatids (the strips of clay) together. the letters represent three genes on the chromosomes.
materials

• modelling clay (two colours)
• twist ties
• marker
• ruler
• masking tape

procedure

1. roll out two 10 cm lengths of modelling clay.
2. attach twist ties, and use pieces of tape to label each strip of clay as shown in the diagram.
3. perform three or four different crossovers with the chromosome models. draw or take a picture of the resulting chromosomes.

questions

1. in this activity, you performed different crossovers. each crossover had a different outcome in terms of the exchange of genetic material between the chromosomes. explain the differences between the crossovers.
2. scientists have found that there are exceptions to independent assortment. genes that are close together on a chromosome tend to be inherited together. how do your models support this?
3. what do you think would happen if a crossover occurred between homologous pairs within a gene?

learning check

1. how is the outcome of meiosis different from the outcome of mitosis?
2. how many genetically different gametes can be produced from a diploid cell with seven chromosome pairs? explain your reasoning.
3. the gametes in figure 4.18 illustrate the concept of independent assortment. how could the diagram be altered to also demonstrate crossing over?
4. using the concepts you have learned, explain how you can have your grandfathers eyes and your grandmothers nose.
5. although you will inherit a combination of traits from your parents, you may notice that some traits seem to be inherited in combination (for example, hair colour and eye colour). why may some traits be inherited together?
6. a young boy needs a bone - marrow transplant, which requires a closely matching genetic donor. who would be an ideal donor? explain your answer.

errors during meiosis
the two processes that produce genetic variation, independent assortment and crossing over, also provide the potential for chromosomal abnormalities. many of the errors that occur during meiosis produce gametes that cannot survive. however, some gametes do survive. if they are fertilized, they will produce a zygote. since every cell in an offspring is produced from the one zygote cell, all of the cells in the offspring will contain the error. there are two types of chromosomal errors that can occur during meiosis: changes in chromosome structure and changes to chromosome number.

Explanation:

1. Different crossovers lead to varied genetic exchanges due to the unique break - and - rejoin points on homologous chromosomes.
2. The models with genes close together on a chromosome can show that they are more likely to be inherited together as they are less likely to be separated by crossover events, supporting exceptions to independent assortment.
3. A crossover within a gene could disrupt the gene sequence, potentially altering the protein it codes for and leading to new or non - functional gene products.
4. Meiosis results in four genetically different haploid cells, while mitosis produces two genetically identical diploid cells.
5. From a diploid cell with seven chromosome pairs, \(2^7 = 128\) different gametes can be produced due to independent assortment of chromosomes during meiosis.
6. To demonstrate crossing over in the independent assortment diagram, draw additional lines to show the exchange of genetic material between homologous chromosomes.
7. You can have your grandfather's eyes and grandmother's nose because during meiosis in your parents, genetic recombination (crossover and independent assortment) occurs, and they inherit a mix of their parents' (your grandparents') genes which they can pass on to you.
8. Some traits are inherited in combination because the genes for those traits are located close together on the same chromosome (linked genes), and are less likely to be separated during meiosis.
9. An ideal bone - marrow transplant donor for the young boy would be a sibling as they share a significant amount of genetic material due to inheriting genes from the same parents, increasing the likelihood of a close genetic match.

Answer:

1. Different crossovers have different outcomes because of unique break - and - rejoin points on chromosomes.
2. Models with close - located genes support exceptions to independent assortment as they are less likely to be separated by crossovers.
3. Crossover within a gene could disrupt the gene sequence.
4. Meiosis: four genetically different haploid cells; Mitosis: two genetically identical diploid cells.
5. 128 different gametes can be produced (\(2^7\)) due to independent assortment.
6. Add lines to show genetic material exchange in the diagram.
7. Genetic recombination in parents allows inheritance of traits from grandparents.
8. Traits are inherited in combination if genes are close on the same chromosome (linked genes).
9. A sibling would be an ideal donor due to shared genetic material from parents.