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9th grade Traits controlled by many genes, and their bell-shaped spread worksheet

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

Grade 9 Science Worksheet

NameDate

Circle the best answer.

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

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

  2. Zee reads a histogram of corn heights from a second field. What is the range of the heights shown?

    • 27 plants
    • 170 cm
    • 70 cm
    • 60 cm

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

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

  4. Read this first

    Classes for a trait with additive alleles (model data)

    genesdark alleles possibleclasses
    10 to 23
    20 to 45
    30 to 67
    4??

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

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

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

    genechance of two light alleles
    A1/4
    B1/4
    C1/4

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

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

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

    genescounts by dark allelesclasses
    11, 2, 13
    21, 4, 6, 4, 15
    31, 6, 15, 20, 15, 6, 17

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

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

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

    • 90 offspring
    • 60 offspring
    • 6 offspring
    • 40 offspring

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

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

  12. 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)
    12060
    140246
    1604024
    1802440
    200624
    22006

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

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

    • 20 cm
    • 40 cm
    • 80 cm
    • 180 cm
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9th grade Traits controlled by many genes, and their bell-shaped spread worksheet – printable
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About this worksheet

Polygenic traits: many genes, bell-shaped spread and the environment. Every version has 10 questions, and the answer key prints on the second page.

What your child will learn

  • I can say that two or more genes shape a polygenic trait, and that the environment also plays a part in many of them.
  • I can say that continuous variation is a spread of many small steps with no clear groups, and that a trait with two clear groups (violet or white pea flowers) fits one gene.
  • I can read the 4 by 4 square for AaBb × AaBb in the additive model (each dark allele, a capital letter, adds 1 unit; each light allele adds 0) and count the boxes with 0, 1, 2, 3 and 4 dark alleles: 1, 4, 6, 4, 1 out of 16.
  • I can fill a blank count in a table so the counts add up to the total (16 for two genes, 64 for three genes), and know that the counts for three genes are 1, 6, 15, 20, 15, 6, 1.
  • I can say that the middle class is the largest in a bell shape and the two ends are the smallest, and that more genes give more classes (2 genes make 5, 3 genes make 7, n genes make 2n + 1) and a smoother bell.
  • I can read the mode, the range and the mean from a histogram, and turn a count of boxes into an expected count out of a model total (6 of 16 of 160 is 60).

Standard: HS-LS3-3

Words to know

gene: A stretch of DNA that carries the plan for one trait

allele: One form of a gene, such as a dark form or a light form

polygenic trait: A trait influenced by two or more genes

continuous variation: Many small steps between the smallest and largest, with no clear groups

additive alleles: Alleles that each add a little to the trait, such as 1 unit per dark allele

class: One step of the trait, such as the group with 2 dark alleles

histogram: A bar graph where each bar counts the plants in one class

mean, mode, range: The average, the most common class, and the tallest class minus the shortest

Common mistakes

Your child said one gene sets the whole trait. Many traits, such as plant height or kernel color, are influenced by two or more genes. Ask your child how many classes 1 gene makes and how many 2 genes make.

Your child thought the middle class is the rarest. In a bell shape the middle class is the largest and the two ends are the smallest. Ask your child which bar is tallest and where it sits.

Your child said genes alone decide a trait. The surroundings also influence a polygenic trait, so the same seeds grow taller in richer soil. Ask your child what stayed the same and what changed.

Questions parents ask

What is the most common mistake with traits controlled by many genes, and their bell-shaped spread?

Your child said one gene sets the whole trait. Many traits, such as plant height or kernel color, are influenced by two or more genes. Ask your child how many classes 1 gene makes and how many 2 genes make.

Are these 9th grade science worksheets free to print?

Yes. All 6 printable versions at each level are free to print, with the answer key, and no account is needed.

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