Where new gene combinations come from

Grade 9 Science Worksheet

Circle the best answer.

1. 1.

   “Chromosome cards (model, not to scale). One cell with 2 sets of chromosomes; pair 1 is shown. Before the crossover,” Ari follows a pair with genes A and B. The dark chromosome has A B and the light one has a b. One crossover happens between the genes. Which list shows the four chromatids afterward?

   • A B, A b, a B, a b
   • A B, A B, a b, a b
   • A b, A b, a B, a B
   • A B, A b, a b, a b

2. Read this first

   Ten model cells with one crossover each

   item	value
   cells	10
   crossovers in each cell	1
   chromatids in each cell	4

3. 2.

   Bo models 10 cells. In each cell one crossover swaps pieces between one dark and one light chromatid of a pair. How many chromatids in the 10 cells are recombinant?

   • 20 chromatids
   • 10 chromatids
   • 12 chromatids
   • 40 chromatids

4. Use this

   Chromosome cards (model, not to scale). One cell with 2 sets of chromosomes; each chromosome has 2 chromatids. Pair 1: a dark chromosome with alleles A and B and a light chromosome with alleles a and b. Pair 2: a dark chromosome with allele C and a light chromosome with allele c. The cell is early in meiosis, with each pair lying side by side.

5. 3.

   Esme looks at a cell with 2 pairs of chromosomes. Genes A and B are on pair 1, and gene C is on pair 2. Which chromatids can swap pieces in crossing over?

   • A dark and a light chromatid of pair 1, in prophase II
   • The two sister chromatids of one chromosome, in prophase I
   • A dark chromatid of pair 1 and a light chromatid of pair 2, in prophase I
   • A dark and a light chromatid of pair 1, in prophase I

6. Pea plant fact card

   item	value
   pairs of chromosomes	7
   ways each pair can line up	2

7. 4.

   Jun reads that a pea plant has 7 pairs of chromosomes. How many line-ups can the 7 pairs make at the middle, before crossing over?

   • 128 line-ups
   • 2 line-ups
   • 49 line-ups
   • 14 line-ups

8. Human cell fact card

   item	value
   pairs of chromosomes	23
   ways each pair can line up	2

9. 5.

   Luz reads that human cells have 23 pairs of chromosomes. About how many line-ups can the 23 pairs make at the middle, before crossing over?

   • 46 line-ups
   • 2 line-ups
   • About 8.4 million line-ups
   • 529 line-ups

10. Two parents (model data)

    item	value
    pairs in each parent	3
    kinds of gametes from each parent	8

11. 6.

    Milo reads a card for two parents. Each can make 8 kinds of gametes by independent assortment alone. How many offspring combinations can random fertilization give?

    • 24 combinations
    • 16 combinations
    • 8 combinations
    • 64 combinations

12. Offspring combinations from two parents (model data)

    pairs in each parent	kinds from each parent	offspring combinations
    2	4	16
    3	8	64
    4	16	?

13. 7.

    Noah reads how many offspring combinations two parents can make. Both parents have the same number of pairs. What belongs in the blank for 4 pairs?

    • 112
    • 256
    • 128
    • 16

14. Two human parents (arithmetic)

    item	value
    pairs in each parent	23
    line-ups for each parent	8,388,608
    line-ups for the two parents together	?

15. 8.

    Pia reads the arithmetic for two parents with 23 pairs each. Which statement gives the count for combined line-ups after random fertilization?

    • 1, since each parent passes on one fixed set of chromosomes to the offspring
    • 2²³ + 2²³ = 2²⁴, about 16.8 million, since the two parents' line-ups form one longer list
    • 2²³ × 2²³ = 2⁴⁶, about 70 trillion, since any line-up from one parent can meet any from the other
    • 2²³, about 8.4 million, since just one parent's line-ups count for the offspring
16. 9.

    Ren watches model cell X. Its 3 pairs line up at the middle, and each pair faces a random pole. Which source of variation is this?

    • Crossing over, the swap of pieces in prophase I
    • Random fertilization, the joining of two gametes
    • Independent assortment, the random line-up of pairs
    • Mutation, a change in the DNA of a gene

17. Chromatid card (model)

    item	value
    top allele on this chromatid	H
    bottom allele on this chromatid	j
    alleles on the dark chromosome	H and J
    alleles on the light chromosome	h and j

18. 10.

    Tala finds a chromatid with allele H from the dark chromosome and allele j from the light one. Which process made it, and what does that process make?

    • Crossing over: chromatids with brand-new alleles, in prophase I
    • Crossing over: chromatids that mix both homologs, in prophase I
    • Independent assortment: whole chromosomes in new mixes, in metaphase I
    • Crossing over: chromatids that mix both homologs, in prophase II