Where new gene combinations come from

Grade 9 Science Worksheet - Building Confidence

Circle the best answer.

1. 1.

   Chromosome cards (model, not to scale). Nothing has been exchanged yet. Ari looks at a model cell early in meiosis. The two chromosomes of a pair lie side by side, gene for gene. What is this tight pairing called?

   • Synapsis, the pairing of homologous chromosomes
   • Replication, the copying of each chromosome
   • Crossing over, the swap of pieces between chromatids
   • Independent assortment, the random line-up of pairs

2. Read this first

   Use this

   Chromosome cards (model, not to scale). One cell with 2 sets of chromosomes; pair 1 is shown, and each chromosome has 2 chromatids. The dark chromosome from the mother (alleles A and B) and the light chromosome from the father (alleles a and b) touch at an X-shaped point between gene A and gene B. At that point one dark chromatid and one light chromatid have traded pieces.

3. 2.

   Bo sees dark and light chromosomes touching at an X-shaped point. A piece of a dark chromatid and a piece of a light chromatid have traded places. What is happening?

   • Crossing over: pieces swap between non-sister chromatids
   • Crossing over: pieces swap and new alleles form at the break
   • Crossing over: pieces swap between sister chromatids
   • Independent assortment: the pairs line up in a random way

4. Stages of meiosis (model)

   stage	what the chromosomes do
   prophase I	homologous chromosomes pair up tightly
   metaphase I	pairs line up at the middle of the cell
   anaphase I	homologous chromosomes are pulled to opposite poles
   prophase II	each of the 2 new cells gets ready to divide again
   anaphase II	sister chromatids are pulled to opposite poles

5. 3.

   Esme reads a table of meiosis stages. Crossing over swaps pieces between homologous chromosomes that are paired. In which stage does it happen?

   • Anaphase I, when the homologous chromosomes are pulled apart
   • Metaphase I, when the pairs line up at the middle
   • Prophase II, when the 2 new cells get ready to divide again
   • Prophase I, while the homologous chromosomes are paired

6. Chromatids after one crossover between genes D and E (model data)

   chromatid	alleles	pieces
   1	D E	dark, dark
   2	D e	dark, light
   3	d E	light, dark
   4	d e	light, light

7. 4.

   Kira reads a table of four chromatids after one crossover between genes D and E. How many carry pieces from both the dark and the light chromosome?

   • 1 chromatid
   • 4 chromatids
   • 0 chromatids
   • 2 chromatids

8. Sister chromatids of the dark chromosome after one crossover with a light chromatid (model data)

   sister chromatid	alleles	pieces
   1	F G	dark, dark
   2	F g	dark, light

9. 5.

   Luz reads the two sister chromatids of a dark chromosome after a crossover with a light chromatid. Which statement fits the table?

   • Neither is recombinant, since sister chromatids stay identical copies
   • Both are recombinant, since both took part in the swap
   • Chromatid 2 is recombinant, so the sisters are no longer identical
   • Chromatid 1 is recombinant, since it is dark from end to end

10. Word card (model)

    term	hint
    recombinant chromatid	made when crossing over swaps pieces

11. 6.

    Milo reads a word card for recombinant chromatid. Which meaning fits the card?

    • A chromatid with pieces from both homologous chromosomes
    • A chromatid made by a swap with its own sister chromatid
    • An exact copy of a chromatid, joined to it at the middle
    • A chromatid with a brand-new gene made by the swap
12. 7.

    Noah reads a word card for independent assortment. Which meaning fits the card?

    • Each pair lines up at the middle in a random direction
    • Pieces of non-sister chromatids swap places with each other
    • Dark chromosomes stay together on one side of the middle
    • Homologous chromosomes pair up tightly along their length
13. 8.

    Chromosome cards (model, not to scale). Ren deals the chromosome cards of a model cell with 3 pairs. At the middle each pair can face either pole, in 2 ways. How many different line-ups are possible?

    • 6 line-ups
    • 9 line-ups
    • 2 line-ups
    • 8 line-ups

14. Line-ups at the middle (model data)

    pairs	line-ups
    1	2
    2	4
    3	8
    4	?

15. 9.

    Tala reads a table of how many line-ups pairs can make at the middle. What belongs in the blank for 4 pairs?

    • 12
    • 16
    • 14
    • 4
16. 10.

    Chromosome cards (model, not to scale). Each chromosome has 2 chromatids. Each pair lines up on its own. Zee lists a model cell with 5 pairs. Each pair can face either pole at the middle. How many different line-ups are possible?

    • 10 line-ups
    • 2 line-ups
    • 32 line-ups
    • 25 line-ups