science-2018:8.LS.3
science-2018:8.LS.3 The characteristics of an organism are a result of inherited traits received from parent(s).
Learning goal: Explain that an organism's characteristics result from inherited traits received from its parent or parents through genetic information (DNA), use the terms gene, allele, dominant and recessive correctly, predict outcomes for a one-gene trait with a Punnett square, and state the limits of such predictions.
From parents to offspring. The characteristics of an organism are the result of inherited traits received from its parent or parents. What is passed on is not the trait itself but genetic information, carried in DNA, that influences how the trait develops. In asexual reproduction, the lone parent contributes the DNA. In sexual reproduction, both parents contribute DNA, so an offspring carries information from each. This lesson stays with what genetic information does, the link between DNA and traits; how DNA is built and how it works inside a cell are left for later courses.
Gene and allele. A gene is a unit of genetic information that influences a trait. For the genes this lesson follows, the ordinary body cells of an organism produced by sexual reproduction carry two copies, one received from each parent (the reproductive cells, as the next lesson explains, carry one). That two-copy model fits the traits used here; a few genes in some organisms do not follow it, sex-linked traits among them, and those are left for high school. The copies need not be identical; different versions of a gene are called alleles. Write alleles with letters. Using the flower-color example that Gregor Mendel studied in pea plants, P stands for the allele associated with purple flowers and p for the allele associated with white flowers. A plant's pair of alleles is one of three combinations: PP, Pp or pp.
Dominant and recessive, defined carefully. In pea flower color, plants with PP or Pp have purple flowers and only pp plants have white flowers. The allele whose effect shows when the two copies differ (Pp) is called dominant; the allele whose effect shows only when both copies match (pp) is called recessive. That is the entire meaning. Dominant does not mean stronger, healthier, better or more common. A dominant allele can be rare: some inherited forms of extra fingers or toes are passed on in a dominant pattern yet are uncommon. A recessive allele can be very common in a population. The words describe how a pair of alleles shows up in one individual, nothing more.
Why Pp can pass on white. A Pp plant has purple flowers, but it still carries p. When it reproduces, each offspring receives exactly one of the plant's two alleles, chosen with equal chance, so half of the genetic contributions it makes carry p. (This is Mendel's first law, segregation: the two alleles separate, and each reproductive cell carries one of them.) A recessive allele is not lost when it is hidden; it is simply not visible in that individual. This is why two purple-flowered parents can have a white-flowered offspring.
Codominance: both alleles show. Dominance is not the only pattern. For some traits neither allele hides the other, and an individual with two different alleles shows both. Roan cattle are the standard example: a red-coated parent and a white-coated parent produce roan calves whose coats mix red hairs and white hairs, each hair fully one color or the other. The calf is not a blended pink; both traits are expressed side by side, which is why the pattern is called codominance. A Punnett square works the same way, but three appearances are possible (red, roan, white) instead of two, and crossing two roan animals is expected to give one quarter red, one half roan and one quarter white. Crossing two roan animals and getting fully red and fully white calves again is good evidence that alleles are not diluted or mixed when they are passed on; each is transmitted intact.
The Punnett square, worked. A Punnett square lists each parent's possible contributions and combines them. For the cross Pp × Pp, one parent's alleles label the columns (P, p) and the other's label the rows (P, p); in the lesson diagram the column letters are printed along the top of the grid and the row letters at its left. The four cells are PP, Pp, Pp and pp, each equally likely. Expected results: one quarter PP, one half Pp, one quarter pp. In terms of visible traits, three quarters purple (PP and Pp) and one quarter white (pp). The diagram shows this square. For Pp × pp the cells are Pp, Pp, pp, pp: half purple and half white. For PP × pp every cell is Pp: all purple, all carrying a hidden p.
Probability is not a count. The square gives the chance for each offspring, not a guaranteed tally. Four seeds from a Pp × Pp cross are expected to give three purple and one white, but two white or zero white is entirely possible, just as four coin flips need not give exactly two heads. Across hundreds of seeds the observed shares approach the predicted ones. Mendel's counts across large numbers of plants came close to three to one, which is what made the pattern visible.
Limits of the model. The simple single-gene Punnett squares in this lesson apply to a trait controlled mainly by one gene with two alleles and a clear dominance or codominance pattern. Most traits are not like that. Many traits are influenced by more than one gene, human height and skin color among them, and many genes influence more than one trait, so a single square cannot predict them. Mendel's second law, independent assortment, says that in simple cases the allele passed on for one gene does not affect which allele is passed on for another; the next lesson uses it to count combinations, and crosses that follow two genes at once are left for high school. And the expression of all traits is shaped by both genes and environmental factors to varying degrees: two pea plants with identical alleles can differ in height because of soil, water and light. Genetic information sets possibilities; conditions during growth affect which possibility appears. A prediction from a square is a statement about inherited information in a simple case, not a complete forecast of an organism.
Misconceptions. "The dominant trait is the common one" confuses how a pair shows up in one individual with how often an allele occurs in a population. "A Pp plant is a little bit white" is wrong for this trait; in pea flower color the Pp plant is fully purple, and the p allele is present but not visible. "Each child gets the traits of the parent they look most like" is wrong; each parent contributes one allele of each gene in the pair, and resemblance depends on which combinations result and on the environment.
Investigation you can do. Model a Pp × Pp cross with two coins. Label each coin's heads side P and tails side p; one coin is each parent. Flip both coins together and record the pair (PP, Pp or pp) 40 times, then tally. The expected counts are 10 PP, 20 Pp and 10 pp, and 30 purple to 10 white by appearance. Compare your tally with the expectation and with another group's; record how far each is from 30 to 10. A result near three to one, with groups scattered on either side, shows what the model predicts when each parent passes one of its two alleles at random, including how much chance scatter to expect. The coins were made to behave that way, so they cannot show that real plants do; the biological evidence is counts of real offspring, such as Mendel's pea counts close to three to one. The coins cannot show what a gene is, how alleles produce a color, whether other genes or the environment change the outcome or what happens when a trait involves many genes.
Worked cross Pp × Pp: Columns P, p from one parent; rows P, p from the other. Cells: PP, Pp, Pp, pp. Expected fractions: 1/4 PP, 2/4 = 1/2 Pp, 1/4 pp. Visible traits: PP and Pp are purple, so 3/4 purple and 1/4 white. Among 200 seeds the expected counts are 150 purple and 50 white, but an actual sample of 200 could reasonably give, say, 143 purple and 57 white.
Worked cross Pp × pp: Columns P, p; rows p, p. Cells: Pp, pp, Pp, pp. Expected 1/2 Pp (purple) and 1/2 pp (white). This cross is useful because every purple offspring must be Pp, so an unknown purple-flowered plant can be tested: cross it with a white pp plant, and if any white offspring appear, the unknown plant was Pp rather than PP.
Worked cross PP × pp: Columns P, P; rows p, p. Every cell is Pp, so all offspring are purple, yet each carries one p. A cross of two of these Pp offspring returns to the first square and can produce white flowers again in the following generation, which shows that a recessive allele hidden in one generation is still transmitted.
Worked limit, a polygenic trait: A student tries to draw a Punnett square for human height using T and t. The square cannot be built honestly, because height is influenced by many genes, each with small effects, and by nutrition and health during growth. The correct response is to say that the trait is not a single-gene, two-allele trait and to describe height as the result of both inherited information from both parents and environmental factors, with no simple three-to-one prediction available.
Supported representation: Draw Punnett squares for Pp × Pp, Pp × pp and PP × pp. Under each, write the expected fractions for allele pairs and for visible traits, then color the cells by appearance (purple or white). Add one sentence under each square stating what the square predicts and one stating what it cannot predict. Then draw a simple three-generation family chart (a pedigree) for a PP × pp cross followed by a Pp × Pp cross among the offspring, marking each plant purple or white, and circle the generation in which white reappears after skipping one.
Independent performance: Complete the two-coin model of a Pp × Pp cross with 40 flips and write a report. Success criteria: (a) all 40 outcomes are recorded and tallied as PP, Pp and pp; (b) expected counts 10, 20, 10 and expected appearance 30 purple to 10 white are stated and compared with the tally; (c) the report explains, using the terms allele, dominant and recessive correctly, why two purple parents can produce a white offspring; (d) it states that the square predicts probabilities rather than counts and uses the tally to show this; (e) it names one trait that a Punnett square cannot predict and gives two reasons, one about multiple genes and one about environmental factors.
An adult may read the prompts aloud. Answer independently with words, objects, drawings or a demonstration before opening the answer guide. Award 1 point for a correct result or claim and 1 point for a matching explanation or demonstration; where a question asks only for a result (a corrected sentence, a spelling, a count), a fully correct result earns both points, and the answer guide shows what a complete response contains. Record prompting separately. The total (two points per question) is local practice feedback, not an Ohio proficiency level.
Explain the difference between a gene and an allele, using the pea flower-color example.
A gene is a unit of genetic information that influences a trait, here flower color. Alleles are different versions of that gene: P is associated with purple and p with white. Each pea plant carries two alleles for the gene, one from each parent, giving PP, Pp or pp.
What does "dominant" mean, and why is it wrong to say a dominant allele is stronger or more common?
Dominant means the allele whose effect appears when an individual carries two different alleles, as purple appears in Pp plants; recessive means the effect appears only when both copies match, as in pp. The terms describe how one pair shows up in one individual, not strength or population frequency; a dominant allele can be rare, as with some inherited forms of extra fingers, and a recessive allele can be common.
Two purple-flowered pea plants produce a white-flowered offspring. What must be true of the parents, and what fraction of their offspring is expected to be white?
Both parents must be Pp, because a white plant is pp and needs one p from each parent, and a PP parent has no p to give. The Pp × Pp square gives cells PP, Pp, Pp, pp, so 1 in 4 of the offspring is expected to be white and 3 in 4 purple.
Draw or describe the Punnett square for Pp × pp and give the expected fractions of purple and white flowers.
Columns P and p from the Pp parent; rows p and p from the pp parent. Cells: Pp, pp, Pp, pp. Half the offspring are expected to be Pp (purple) and half pp (white). The square also shows that every purple offspring of this cross carries a hidden p.
A Pp × Pp cross produces 8 seeds, and 4 of the plants have white flowers. Does this show the Punnett square was wrong? Explain.
No. The square predicts that each seed has a 1 in 4 chance of being pp; it does not promise exactly 2 white plants in 8. Small samples often differ from the expected share, just as 8 coin flips need not give exactly 4 heads. Over hundreds of seeds the observed share is expected to approach 1 in 4.
Why can a simple single-gene Punnett square like the ones in this lesson not predict a person's height, even though height is inherited?
Height is influenced by many genes, each contributing a small effect, so there is no single gene with two alleles to place in a square. It is also shaped by environmental factors such as nutrition during growth. Inherited information from both parents sets possibilities, and conditions affect which appear, so no simple fraction like 3 to 1 applies.
A PP plant is crossed with a pp plant, and all the offspring are purple. A student concludes the p allele has disappeared. Correct the conclusion with a second cross.
Every offspring is Pp: purple in appearance but carrying one p. If two of these Pp offspring are crossed, the square gives PP, Pp, Pp, pp, so about 1 in 4 of the next generation is expected to be white. The recessive allele was transmitted but not visible, not lost.