MS-LS3-2: Why Offspring Are Copies or Mixes
Also called NGSS MS-LS3-2 · Topic: Growth, Development, and Reproduction of Organisms · Life Science
The Official Wording
Develop and use a model to describe why asexual reproduction results in offspring with identical genetic information and sexual reproduction results in offspring with genetic variation.
Clarification statement: Emphasis is on using models such as Punnett squares, diagrams, and simulations to describe the cause and effect relationship of gene transmission from parent(s) to offspring and resulting genetic variation.
NGSS Lead States, 2013 · Read it on nextgenscience.org · checked October 15, 2026
What MS-LS3-2 Means in Plain English
Middle schoolers use Punnett squares, diagrams and simulations to explain two ways of making offspring. With one parent, the young get an exact copy of that parent's genes. With two parents, each passes on half, so the young get new mixes and vary.
Students work with simple traits controlled by one gene with two versions. This builds on inherited traits in 3-LS3-1 and links to mutations as another source of variation (MS-LS3-1). High school adds how traits spread in populations (HS-LS3-3).
MS-LS3-2 I Can Statements
- I can explain why asexual reproduction makes identical offspring.
- I can use a Punnett square to predict offspring from two parents.
- I can explain why sexual reproduction creates genetic variation.
MS-LS3-2 Worked Example
An allele card draw that compares a plant spreading by runners with the same plant making seeds.
Why do runner plants all look like their parent while seed plants can differ?
Set It Up: A model plant with flower color alleles R (red) and r (white), allele cards, two paper bags and a Punnett square sheet.
- Runners: copy the parent's card pair, Rr, for each of 10 new plants.10 of 10 are Rr, all red
- Seeds: put one R and one r card in each bag, one bag per Rr parent.Each parent gives one card
- Draw one card from each bag, record the pair and return the cards. Do it 40 times.RR 9, Rr 21, rr 10
- Fill in a Punnett square for Rr × Rr and compare.Predicted 1 RR : 2 Rr : 1 rr
- Count how many seed plants have white flowers.10 of 40 white, none from runners
Every runner plant matched its parent, but a quarter of the seed plants had white flowers neither parent showed.
Why it works: With one parent the genes are copied, but with two parents each passes a random half, so new combinations appear.
Common Mistakes with MS-LS3-2
Reading a Punnett square as exact
“Four seeds means exactly one white flower.”
Try this: A Punnett square gives chances for each offspring. With few offspring the real counts can differ, so test many draws.
Thinking asexual young vary
“Each runner plant gets a random mix of genes.”
Try this: Runners, cuttings and budding copy one parent's genes, so the young match unless a rare mutation occurs.
Blending the traits
“Red and white parents make pink flowers.”
Try this: In this model each offspring gets whole alleles, so flowers come out red or white, with red showing whenever an R is present.
Where MS-LS3-2 Fits
Practice MS-LS3-2
Quick Check
- A strawberry plant sends out runners. Will the new plants be identical to the parent? Why?
- Two Rr parents have offspring. What fraction would you expect to be rr?
- Why is variation useful to a species?