Traits controlled by many genes, and their bell-shaped spread

Grade 9 Science Worksheet

Circle the best answer.

1. 1.

   Luz reads a histogram of corn plant heights. What is the mode, the height with the most plants?

   • 28 plants
   • 60 cm
   • 150 cm, the shortest class
   • 180 cm
2. 2.

   Ari reads a small histogram of corn heights. What is the mean height of the 10 plants, using the class heights?

   • 170 cm
   • 5 plants
   • 175 cm
   • 172 cm

3. Read this first

   Classes for a trait with additive alleles (model data)

   genes	dark alleles possible	classes
   1	0 to 2	3
   2	0 to 4	5
   3	0 to 6	7
   4	?	?

4. 3.

   Ren reads a table for traits with additive alleles. How many classes would a trait with 4 genes have?

   • 2 classes, since more genes fold the classes together
   • 3 classes, since one gene sets the trait
   • 9 classes
   • 6 classes, since each gene adds one class

5. Chance of two light alleles in a cross with 3 genes (model data)

   gene	chance of two light alleles
   A	1/4
   B	1/4
   C	1/4

6. 4.

   Esme crosses model wheat that is AaBbCc with a second plant that is AaBbCc. What is the chance an offspring has no dark alleles?

   • 3/4
   • 1/4, since one gene sets the extreme class
   • 1/16, the chance for 2 genes
   • 1/64

7. Counts for traits with 1, 2 and 3 genes (model data)

   genes	counts by dark alleles	classes
   1	1, 2, 1	3
   2	1, 4, 6, 4, 1	5
   3	1, 6, 15, 20, 15, 6, 1	7

8. 5.

   Milo compares the count rows for traits with 1, 2 and 3 genes. Which trait has the smoothest bell shape, and why?

   • The 2-gene trait, since 5 classes are fine enough
   • The 1-gene trait, since fewer genes give a smoother bell
   • The three look alike, since each peaks in the middle
   • The 3-gene trait, since 7 small steps make the finest bell
9. 6.

   Pia grows 160 model offspring from AaBb × AaBb. About how many are expected to have exactly 2 dark alleles?

   • 6 offspring
   • 90 offspring
   • 40 offspring
   • 60 offspring
10. 7.

    Jun grows 320 model offspring from AaBbCc × AaBbCc. About how many are expected to have exactly 3 dark alleles?

    • 5 offspring, one box like the ends
    • 100 offspring
    • 75 offspring
    • 20 offspring

11. Corn plant height from one seed lot, 100 plants in each soil, rounded to the nearest 20 cm (model data)

    Height (cm)	Poor soil (plants)	Rich soil (plants)
    120	6	0
    140	24	6
    160	40	24
    180	24	40
    200	6	24
    220	0	6

12. 8.

    Noah grows one corn seed lot in two plots, one with poor soil and one with rich soil. What best explains the taller plants in rich soil?

    • The rich-soil seeds carry different alleles, since genes alone set height
    • The soil shifted growth up, and the seeds carry the same alleles
    • Chance alone, since any two plots differ
    • Rich soil changed the alleles in the seeds, so the plants grew taller
13. 9.

    Kira compares the middle of each bell in the table. By how many cm did rich soil shift the middle?

    • 80 cm
    • 40 cm
    • 180 cm
    • 20 cm
14. 10.

    Bo grows the same seed lot in a third plot with poor soil. What does the table predict for that plot?

    • Any center, since soil has no effect on height
    • A bell centered near 180 cm, since rich soil changed the lot's alleles
    • A bell centered near 160 cm, like the first poor-soil plot
    • Two clear groups, one near 120 cm and one near 200 cm