Heredity: Cell Division and Inheritance: a full term of science

Ten ready-to-teach lessons for Year 10 Biological sciences. Print this pack and the term is prepared: every lesson comes with a step-by-step plan, the questions to ask, a student worksheet, cut-out cards, a class sheet or exit tickets, an assessment kit and worked answers.

Start here: five minutes to Monday

1. Skim the term at a glance on the next page.
2. Print the lesson you need. Each lesson is three A4 sheets: plan, worksheet, and cards, a class sheet or tickets.
3. Gather the few items under “You need” on the plan. Most come from the science prep room, the classroom or a supermarket.
4. Open the free interactive unit on your board or projector. Every plan tells you which picture to show and when.
5. Teach straight from the plan. Timings, talk prompts, misconceptions and worked answers are all on the one page.

For any Year 10 science teacher

The pack suits a biology specialist and a teacher working outside their own subject alike: each plan explains the idea in plain words, lists the misconceptions students bring, and gives worked answers. The big idea of the term: DNA carries the instructions for making proteins, in sections called genes, and is packed into chromosomes in the nucleus. Mitosis copies a cell exactly, for growth and repair. Meiosis halves the chromosomes and mixes them, so every egg and sperm is different, and fertilisation restores the full set. Because each parent passes on one of its two alleles by chance, a Punnett square can predict the chances of a trait, though not the exact count in a family.

Two ways to run each lesson

Every lesson works as one 60-minute block, or as two 30-minute sessions; the split point is marked in every plan. Look ahead: Lesson 1 needs strawberries and alcohol cold from the freezer, Lesson 3 is richer with microscopes and a prepared root tip slide, and Lesson 7 needs two coins for each pair.

Safety and care in one look

The alcohol in Lesson 1 is flammable: keep every flame and heater off, and pour it yourself. Nothing made in the lab is eaten, including the strawberries. No blood is tested in Lesson 9: the blood groups are examples. Traits that run in families can be personal, so the term uses peas, flies, guinea pigs and flowers rather than students’ own families, and no one is asked about their relatives.

The term at a glance

One lesson a week for a term. Each lesson stands on the ones before it, so run them in order where you can.

How the sequence builds

Lessons 1 and 2 meet the material of heredity: DNA, genes and chromosomes, and how chromosomes come in pairs. Lessons 3 and 4 compare the two kinds of cell division, mitosis for copies and meiosis for sex cells. Lessons 5 to 7 follow Mendel: alleles and traits, the Punnett square, and why real counts only come close to its prediction. Lesson 8 reads inheritance backwards from a family tree, Lesson 9 meets the patterns that are not simply dominant and recessive, and Lesson 10 brings it together.

Curriculum links (Australian Curriculum V9)

The whole term teaches the Science Understanding descriptor AC9S10U01 quoted on the cover. The lessons also work these Science Inquiry and Human Endeavour descriptors:

Assessment in this pack

• Every plan ends with “Answers and look-fors”: what meeting the idea sounds like in a Year 10 voice, with working.
• The assessment sheet near the front has a class observation checklist and a three-level rubric.
• Lesson 10 is the summative pair: the plant breeder task, plus the “Show what we know” check.

Materials for the whole term

One gathering session covers all ten lessons, and most of it is already in the science prep room or the classroom. Two things are worth doing ahead: put the alcohol for Lesson 1 in the freezer the day before, and check whether the prep room has a prepared root tip slide for Lesson 3.

The one-trip list

• From the supermarket: a punnet of strawberries, dishwashing liquid, salt, coffee filters and zip-lock bags.
• From the prep room: ethanol or isopropyl alcohol, test tubes, funnels, safety glasses, microscopes and a root tip slide if there is one.
• From the classroom: scissors, glue sticks, rulers, pipe cleaners or coloured paper, and two coins for each pair.
• The extraction mix: 90 mL of water, 10 mL of dishwashing liquid and half a teaspoon of salt, stirred gently so it does not foam.

Safety and care in one look

• The alcohol is flammable and stings the eyes: no flames or heaters, safety glasses on, and you pour it into each tube.
• Nothing made in the lab is eaten, including the strawberries; wipe the benches and wash hands afterwards.
• No blood is tested: the blood groups in Lesson 9 are examples.
• Inheritance can be personal: the term uses peas, flies, guinea pigs and flowers, and no one is asked about their own family.

Assessment without extra work

The term assesses itself. Every lesson plan ends with worked answers and look-fors, and Lesson 10 is the summative pair: the plant breeder task, in which students explain a real-looking set of results with meiosis, fertilisation and a Punnett square, and the final check. This sheet is the place to note what you see along the way.

The three levels

Class observation checklist

A tick a lesson is plenty; the Lesson 10 task and check fill the gaps.

Word wall cards

Cut out the cards and build the wall as the words arrive: DNA, gene and chromosome in Lesson 1; pair of chromosomes and sex chromosomes in Lesson 2; mitosis and cell cycle in Lesson 3; meiosis, gamete, fertilisation and crossing over in Lesson 4; allele, dominant, recessive, genotype, phenotype, homozygous and heterozygous in Lesson 5; Punnett square and probability in Lesson 6; pedigree and carrier in Lesson 8; codominance and incomplete dominance in Lesson 9.

Dear families

This term in science, Year 10 studies heredity: how the instructions for a living thing are stored in DNA, how cells copy themselves by mitosis, how meiosis makes sex cells that each carry half the instructions, and how scientists predict the chances of a trait in the next generation.

Along the way your student will pull strands of DNA out of a strawberry, pair up the chromosomes of a fruit fly, time the stages of cell division from a microscope count, breed forty imaginary pea plants with two coins, and read a family tree of guinea pigs.

Try this at home

• Toss two coins together twenty times. How often do you get two heads, a head and a tail, and two tails? Is it close to 1 : 2 : 1?
• Look at a strawberry plant or a spider plant sending out runners. Every new plant is a copy made by mitosis.
• Read a packet of vegetable seeds. Many say “F1 hybrid”: ask what that means, and why the seeds of the crop may not grow true.
• At a nursery, find one kind of flower in several colours, and talk about what a cross between two of them might give.

What to ask your scientist

• What is the difference between DNA, a gene and a chromosome?
• Why does an egg or a sperm cell carry only half the chromosomes?
• Why are brothers and sisters different, when they have the same parents?
• What can a Punnett square tell you, and what can it not?

A small note on care: inheritance can be a personal topic, so this term uses peas, fruit flies, guinea pigs and flowers as examples, and no student is asked about their own family. One lesson uses alcohol to extract DNA; it is handled with safety glasses and no flames, and nothing made in the lab is eaten.

Warm regards,

The Year 10 science team

Printed from the free seegongsik Heredity: Cell Division and Inheritance teaching pack · seegongsik.com/au/y10/biological/AC9S10U01/pack

From DNA to the cell

Nearly every cell carries a full copy of the instructions for the whole organism, written in DNA. The DNA is a long molecule; a gene is a section of it that carries the instructions for one protein; and each very long DNA molecule, wound tightly around proteins, is a chromosome, kept in the nucleus. Students pull DNA out of a strawberry and see it with their own eyes, then label the levels from a whole cell down to a gene and sort twelve cards into DNA, gene and chromosome.

We are learning to

• describe how DNA, genes and chromosomes are related, and where they are in a cell,
• explain what each step of a DNA extraction does,
• tell apart a molecule, a section of it and a packaged length of it.

Success criteria

• I can put cell, nucleus, chromosome, DNA and gene in order of size and say how each fits inside the next.
• I can explain why the dishwashing liquid, the salt and the cold alcohol are needed.

You need

• for each group: a strawberry in a zip-lock bag, 10 mL of extraction mix, a coffee filter and a funnel, a test tube and a wooden skewer; ice-cold ethanol or isopropyl alcohol, which you pour; safety glasses,
• the scale cards (third sheet), one set per pair, and the worksheet (next sheet), one per student.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the opening, the extraction and the board in Session A, and Parts B to D in Session B.

Watch for these ideas

• “The white strands are a gene” or “are one DNA molecule”. They are DNA from millions of cells, clumped together; one molecule is far too thin to see.
• “Genes, chromosomes and DNA are three different substances.” A gene is a section of DNA, and a chromosome is DNA wound around proteins.
• “Plants do not have DNA.” The strawberry shows they do: every living thing stores its instructions in DNA.

Make it easier, make it bigger

• Easier: give the five level names as a list to choose from in Part B.
• Bigger: plan a fair test of one step of the extraction, such as with and without salt, and predict what will change.

Answers and look-fors

• Part A: mash the strawberry in the bag for two minutes: breaks the fruit into a pulp and opens up the cells; add the extraction mix and mash gently again: the dishwashing liquid breaks down the membranes around each cell and its nucleus; the salt helps the DNA strands clump together; filter the pulp into a test tube: removes the solid bits, so the DNA is left in the clear liquid; pour the cold alcohol gently down the side: DNA does not dissolve in cold alcohol, so it comes out of the liquid where the two layers meet; wind the strands onto the skewer: collects the DNA, which is long and stringy enough to wrap around a stick. The DNA looks like white, cloudy, stringy strands.
• Part B: 1 the cell, 2 the nucleus, 3 a chromosome, 4 DNA, 5 a gene.
• Part C: DNA A, D, F, J, L; Gene B, E, G, K; Chromosome C, H, I.
• Part D: 1. Cell, nucleus, chromosome, gene. 2. It is wound tightly around proteins and coiled up, into 46 chromosomes. 3. DNA from many millions of cells, clumped together. 4. Each strawberry cell holds eight copies of every chromosome, so there is a lot of DNA, and the soft fruit is easy to mash. 5. A gene is a section of a DNA molecule, and that DNA molecule, wound around proteins, is a chromosome.
• Look for students who use “section of” and “made of” correctly, rather than treating the three words as sizes of the same thing.

From DNA to the cell

A. Strawberry DNA

Safety glasses on. The alcohol is flammable: no flames. Your teacher pours it. Nothing is eaten.

B. From a cell to a gene

Label the five numbered parts: cell, nucleus, chromosome, DNA, gene.

C. Sort the scale cards

D. Questions

DNA, gene or chromosome?

Cut out the three headings and the twelve cards. Sort each card under the level it describes.

Teacher note: the answer letters are in the plan. The three piles are different sizes.

Counting chromosomes

Chromosomes come in pairs. The two chromosomes of a pair carry the same genes in the same places, and one came from each parent. Laid out in pairs, chromosomes are matched by their length, the place of the pinch called the centromere, and the pattern of bands along them. Students cut out the eight chromosomes of a male fruit fly, the animal geneticists have studied for more than a century, pair them, and find the two that do not match: the X and the Y. Then they count a human’s 46 and compare a body cell with a sex cell.

We are learning to

• explain what a pair of chromosomes is and where each one comes from,
• match chromosomes by length, centromere and bands,
• compare the number of chromosomes in a body cell and a sex cell.

Success criteria

• I can pair the fruit fly’s chromosomes and explain how I matched them.
• I can say how many chromosomes a human body cell and a human sex cell hold, and why they differ.

You need

• the chromosome cut-outs (third sheet), one set per pair, with scissors and glue sticks,
• the worksheet (next sheet), one per student.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the opening, the board and the pairing in Session A, and Parts C and D in Session B.

Watch for these ideas

• “The two chromosomes of a pair are identical.” They carry the same genes in the same places, but often different versions of them: one came from each parent.
• “More chromosomes means a more complex organism.” A potato and a chimpanzee both have 48, and a dog 78.
• “Only males have sex chromosomes.” Everyone has a pair: XX or XY.

Make it easier, make it bigger

• Easier: tell students that three pairs match exactly and two chromosomes do not, and to start with the two smallest.
• Bigger: list every set of four chromosomes a male fly’s sperm cell could carry, one of each pair. How many are there?

Answers and look-fors

• Part A, longest to shortest: A, G, C, E, B, H, D, F.
• Part B, the pairs: A and G; C and E; D and F. The two that do not match: B (the X, with its pinch near the end) and H (the Y).
• Part C: pairs in a body cell 23; pairs that are the same in males and females 22; sex chromosomes in a female XX; sex chromosomes in a male XY; chromosomes in an egg cell, and in a sperm cell 23 and 23.
• Part D: 1. The length, the place of the pinch, and the pattern of bands. 2. Male: it has an X and a Y, which do not match. 3. Four, one of each pair: a sex cell carries half the set, and the full eight return when a sperm joins an egg. 4. No: a potato has 48 and a human 46; the number is about how the DNA is packaged, not how complex the organism is. 5. A fly 8 (4 from each parent), a human 46 (23 from each).
• Bigger: one of 2 for each of the four pairs (counting X or Y as one pair), so 2 × 2 × 2 × 2 = 16 sets.

Counting chromosomes

A. Order by length

Cut out the eight chromosomes. Write their letters from the longest to the shortest.

B. Glue in the pairs

Match the chromosomes by length, the pinch and the bands. Glue each pair side by side, and write its letters.

C. Count them

Use the human count to fill in the table.

D. Questions

A male fruit fly’s eight chromosomes

Cut out the eight chromosomes, each with its letter. They are drawn the way they look just before a cell divides, with the pinch (the centromere) and the dark bands that help geneticists tell them apart.

Teacher note: the pairs are in the plan. The drawings are to scale with each other, so length is a fair clue.

Mitosis: making copies

Before a cell divides, it copies every chromosome, so each one becomes two identical copies joined at the centromere: the X shape seen under a microscope. Mitosis then pulls the copies apart, and each new cell gets one copy of every chromosome. The result is two cells identical to the parent, each with the full set. Mitosis builds the body, repairs it and replaces worn-out cells, and it is how a strawberry runner grows into a new plant. Most of a cell’s life is interphase, growing and copying its DNA; the division itself is short. Students order the stages, then use a count of 200 cells in an onion root tip to work out how long each stage lasts.

We are learning to

• describe what happens to the chromosomes during mitosis,
• explain why the DNA is copied before a cell divides,
• estimate how long each stage lasts from the share of cells counted in it.

Success criteria

• I can put the stages of mitosis in order and sketch the chromosomes at each one.
• I can turn a count of cells into the time a stage lasts, and say what I had to assume.

You need

• microscopes and a prepared onion or garlic root tip slide, if you have them; the worksheet works without them,
• the worksheet (next sheet), one per student, and the exit tickets (third sheet), cut into four.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the graze, the board and Part A in Session A, and Parts B and C and the ticket in Session B.

Watch for these ideas

• “Each new cell gets half the DNA.” The DNA is copied first, so each new cell gets a full set.
• “Interphase is a resting stage.” The cell is busy growing and copying its DNA.
• “The X shape is a pair of chromosomes.” It is one chromosome after copying: two identical copies joined at the centromere.

Make it easier, make it bigger

• Easier: work the first row of Part B together, showing the fraction of 200 and then the share of 1440 minutes.
• Bigger: a root tip cell divides about once a day. If every cell kept dividing, how many cells would one cell become in a week?

Answers and look-fors

• Part A, the order: A 4, B 1, C 5, D 2, E 3. Sketches: four single chromosomes, then four X shapes, lined up across the middle, the copies pulled to each end, and two nuclei of four.
• Part B: interphase 180 ÷ 200 = 0.9, about 1296 minutes; prophase 10 ÷ 200 = 0.05, about 72 minutes; metaphase 4 ÷ 200 = 0.02, about 29 minutes; anaphase 3 ÷ 200 = 0.015, about 22 minutes; telophase 3 ÷ 200 = 0.015, about 22 minutes. The times add to about 1440 minutes.
• Part C: 1. Interphase is by far the longest part of the cycle, so at any moment most cells are in it. 2. That the cells are dividing at different times, not in step, so the share in each stage matches the share of time. 3. Each chromosome is copied, so that each new cell gets a full set. 4. Growth, repair (healing a graze) and replacing worn-out cells such as skin. 5. Yes: the new plant’s cells all come from the parent by mitosis, so it has the same DNA, a clone.
• Exit ticket: 1. 46. 2. So that each new cell gets a full copy of every chromosome. 3. About 18 hours (90 of 100 is 0.9 of 20 hours).
• Bigger: one division a day for seven days doubles seven times: 2 × 2 × 2 × 2 × 2 × 2 × 2 = 128 cells.

Mitosis: making copies

A. Order the stages

Number the stages 1 to 5 in order. Then sketch a cell with two pairs of chromosomes at each stage, in order, and name it.

B. How long is each stage?

A class counted 200 cells in a photo of an onion root tip. The whole cycle takes about 24 hours, which is 1440 minutes. Time of a stage = (cells in the stage ÷ 200) × 1440 minutes.

C. Questions

Exit tickets

Meiosis: halving and mixing

Meiosis makes the sex cells, or gametes. The DNA is copied once and then the cell divides twice: the first division separates the two chromosomes of each pair, and the second separates the copies. The result is four cells, each with one chromosome of every pair, so half the full set; fertilisation restores it. Two things make every gamete different. Each pair lines up and separates independently of the others, so with n pairs there are 2ⁿ mixes, and before they separate, the chromosomes of a pair swap sections, which is called crossing over. Students model meiosis with two pairs, count the mixes and compare the two kinds of division.

We are learning to

• describe the steps of meiosis and what the four cells carry,
• explain how meiosis makes every gamete a different mix,
• compare mitosis and meiosis.

Success criteria

• I can model meiosis with two pairs of chromosomes and show the four kinds of gamete.
• I can compare mitosis and meiosis: the divisions, the cells made and what they are for.

You need

• for each pair: eight pipe cleaners or paper strips, four long and four short, two of each length in each of two colours; the division cards (third sheet), with scissors,
• the worksheet (next sheet), one per student.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the litter, the board and the model in Session A, and Parts B and C with the cards in Session B.

Watch for these ideas

• “Meiosis is mitosis done twice.” The DNA is copied only once, and the first division separates partners, not copies.
• “A gamete gets any half of the chromosomes.” It gets exactly one of each pair, never both of one pair.
• “The chromosomes from one parent stay together.” Each pair lines up independently, so a gamete carries a mix from both.

Make it easier, make it bigger

• Easier: model the steps together at the front, with each pair copying your moves one at a time.
• Bigger: before the first division, swap the tips of two partner chromosomes. Explain why crossing over makes even more kinds of gamete than 2ⁿ.

Answers and look-fors

• Part A: each line-up gives four gametes of two kinds, two of each. One line-up gives each gamete both chromosomes of one colour; the other gives one of each colour. Together there are four kinds: 2 × 2 = 4.
• Part B: 1 pair, 2; 2 pairs, 4; 3 pairs, 8; 4 pairs, 16; 10 pairs, 1,024; 23 pairs, 8,388,608.
• Part C table: number of divisions: mitosis one, meiosis two; cells made: mitosis two, meiosis four; chromosomes in each new human cell: mitosis 46, the full set, meiosis 23, one of each pair; identical to the parent cell?: mitosis yes, meiosis no, every one is a different mix; where it happens: mitosis all over the body, such as skin and root tips, meiosis the ovaries and testes (in a flower, the ovules and anthers); what it is for: mitosis growth, repair, replacing cells, and runners and cuttings, meiosis making gametes for sexual reproduction.
• Cards: Meiosis A, D, F, J, L; Mitosis B, E, G, K; Both C, H, I.
• Questions: 1. So that a sperm and an egg together restore the full set: 23 + 23 = 46, not 92. 2. The pairs line up and separate independently, and partner chromosomes swap sections in crossing over. 3. Each pup came from a different sperm and a different egg, each a different mix of that parent’s chromosomes.

Meiosis: halving and mixing

A. Model meiosis

B. How many mixes?

Each pair can send either of its two chromosomes into a gamete, so every extra pair doubles the number of mixes.

C. Compare the two divisions

Meiosis, mitosis or both?

Cut out the three headings and the twelve cards. Sort each card under the division it describes.

Teacher note: the answer letters are in the plan. The three piles are different sizes.

Genes, alleles and traits

A gene can come in different versions, called alleles. A body cell carries two alleles of every gene, one on each chromosome of the pair, so one from each parent. In the 1850s and 1860s Gregor Mendel, a monk in Brno, bred thousands of pea plants and found that traits are passed on as separate units, and that one form can hide another. A dominant allele shows its trait whenever it is present; a recessive allele shows only when there are two. The alleles are the genotype, such as Tt; the trait that shows is the phenotype, such as tall. Students meet Mendel’s seven traits, write genotypes for them, and match six genotypes with what the plant looks like.

We are learning to

• explain the difference between a gene and an allele,
• use dominant, recessive, genotype and phenotype correctly,
• work out the genotypes that give a trait.

Success criteria

• I can write every genotype for a trait and say which phenotype each one gives.
• I can explain why a plant that shows the dominant trait may still carry the recessive allele.

You need

• the genotype cards (third sheet), one set per pair, with scissors,
• the worksheet (next sheet), one per student.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the story, the board and Part A in Session A, and Parts B and C in Session B.

Watch for these ideas

• “Dominant means more common.” A dominant allele can be rare; dominant only means it shows when one copy is present.
• “The dominant allele is stronger or better.” It is neither: it simply masks the recessive one in the phenotype.
• “Tongue rolling and earlobe shape are simple dominant traits.” Studies of families and twins show they are not, which is one reason geneticists, and this term, use peas.

Make it easier, make it bigger

• Easier: do the first two rows of Part A together, saying each genotype aloud with what it looks like.
• Bigger: Mendel needed pure-breeding plants to start with. How could he be sure a tall plant was TT and not Tt?

Answers and look-fors

• Part A: seed shape: round RR or Rr, wrinkled rr; seed colour: yellow YY or Yy, green yy; flower colour: purple PP or Pp, white pp; pod shape: inflated II or Ii, pinched ii; pod colour: green GG or Gg, yellow gg; flower position: along the stem AA or Aa, at the tip aa; stem length: tall TT or Tt, short tt.
• Part B: G with A (Tt); D with J (tt); E with L (PP); B with H (pp); K with F (Rr); I with C (yy).
• Part C: 1. A gene is a section of DNA for one protein; an allele is one version of that gene. 2. No: it could be TT or Tt, because one T is enough to make it tall. 3. tt: short is recessive, so it shows only with two short alleles. 4. Every offspring was Tt: the short allele was there but hidden. 5. Homozygous plants have two identical alleles (TT or tt); heterozygous plants have two different ones (Tt).
• Bigger: let the plant pollinate itself for several generations; if only tall offspring ever appear, it is TT. A Tt plant would give some short ones.

Genes, alleles and traits

A. Mendel’s seven traits

A capital letter is the dominant allele and a small letter the recessive one. Write every genotype that gives each form.

B. Match the genotypes

Match each genotype card with the card that describes the plant. Write the pairs of letters.

C. Questions

Genotype and phenotype

Cut out the twelve cards. Six give a genotype; six describe what a plant with that genotype is like. Match each pair.

Teacher note: the pairs are in the plan. Every card has exactly one partner.

Punnett squares

Each parent passes on one of its two alleles, and meiosis makes either one equally likely. A Punnett square writes one parent’s two kinds of gamete across the top and the other parent’s down the side; each of the four boxes is then one equally likely offspring, a 1 in 4 chance. Counting the boxes gives the probability of each genotype and phenotype, as a fraction, a percentage or a ratio. Students complete four crosses of tall and short pea plants and learn what a square can and cannot predict.

We are learning to

• draw a Punnett square for a cross of two parents,
• give the chance of each genotype and phenotype as a fraction, a percentage and a ratio,
• explain why a square predicts chances, not exact numbers.

Success criteria

• I can complete a Punnett square and read the chance of each offspring from it.
• I can choose a cross that tells a breeder whether a tall plant is TT or Tt.

You need

• the worksheet (next sheet), one per student,
• the exit tickets (third sheet), cut into four.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the hands, the board and the first two crosses in Session A, and the rest in Session B.

Watch for these ideas

• “Two Tt parents will have three tall and one short offspring.” The square gives the chance for each offspring, not a count; a family of four can be all tall.
• “The parents’ alleles go in the boxes.” Each parent’s gametes go outside the square, one allele each; the boxes are the offspring.
• “3 : 1 means three quarters are TT.” Three quarters are tall, but only one of those three boxes is TT.

Make it easier, make it bigger

• Easier: complete the first cross together on the board, saying each box aloud as one allele from each parent.
• Bigger: a Tt plant is crossed with a Tt plant and a tall offspring is picked. What is the chance that it is Tt? (Careful: only the tall boxes count.)

Answers and look-fors

• Part A: TT × tt: 4 Tt; tall 4/4 (100%), short 0/4 (0%); all tall. Tt × tt: 2 Tt, 2 tt; tall 2/4 (50%), short 2/4 (50%); 1 : 1. Tt × Tt: 1 TT, 2 Tt, 1 tt; tall 3/4 (75%), short 1/4 (25%); 3 : 1. TT × Tt: 2 TT, 2 Tt; tall 4/4 (100%), short 0/4 (0%); all tall.
• Part B: 1. Each parent’s two alleles are equally likely to be in a gamete, and any sperm can meet any egg. 2. No: each offspring has a 3 in 4 chance of being tall, but the chances do not have to come out evenly in a small family. 3. Tt: Tt × tt gives half Tt and half tt. 4. Cross it with a short tt plant: if any offspring are short, it is Tt; if a large number are all tall, it is almost certainly TT.
• Exit ticket: Pp × pp gives Pp, Pp, pp, pp, so the chance of white flowers is 2 in 4, a half or 50%.
• Bigger: 2 of the 3 tall boxes are Tt, so two thirds.

Punnett squares

A. Four crosses

T is the allele for tall and t for short. Fill in each square, then the chances below.

B. Questions

Exit tickets

Chance

A Punnett square predicts chances, and real counts only come close to them. Students model a cross of two Tt pea plants with two coins, one for each parent: heads passes on T, tails passes on t. Each pair breeds 40 offspring, the class pools its results, and then everyone checks three of the counts Mendel himself published. Small samples wander; large ones settle near 3 : 1. That is why geneticists count large numbers, and why the prediction for a single family is only ever a chance.

We are learning to

• model a cross with a chance device, and say what each part of the model stands for,
• compare a ratio from real counts with the predicted ratio,
• explain why a larger sample usually comes closer to the prediction.

Success criteria

• I can turn counts into a ratio and compare it with 3 : 1.
• I can explain why the class total came closer to 3 : 1 than most pairs did.

You need

• two coins for each pair; the class results sheet (third sheet), on the board or a bench,
• the worksheet (next sheet), one per student; calculators.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the board, the tossing and the class total in Session A, and Parts B and C in Session B.

Watch for these ideas

• “After three tall offspring, the next must be short.” Each offspring is a fresh chance; the coins do not remember.
• “Our results are wrong because they are not exactly 3 : 1.” Chance makes small samples wander; that is the result, not a mistake.
• “Mendel’s counts prove the ratio is exactly 3 : 1.” They come close, as chance predicts, and closest for the largest totals.

Make it easier, make it bigger

• Easier: toss 20 times instead of 40, and work out the ratio with the class after pooling.
• Bigger: model Tt × tt with the coins. What must the second coin do differently, and what ratio do you expect?

Answers and look-fors

• Part A: about 10 TT, 20 Tt and 10 tt are expected, so 30 tall and 10 short; most pairs will be a few away from that, and that is fine.
• Part B: tall 787 and short 277, total 1064, ratio 2.84; round seeds 5474 and wrinkled seeds 1850, total 7324, ratio 2.96; yellow seeds 6022 and green seeds 2001, total 8023, ratio 3.01.
• Part C: 1. Depends on the results; the class ratio is usually nearer 3 than most pairs’ ratios. 2. yellow seeds and green seeds, 3.01, is closest to 3; yellow seeds and green seeds used the most plants (8023) and tall and short the fewest (1064), with the ratio furthest from 3: the bigger the sample, the closer the ratio. 3. In a small sample a few chance results shift the ratio a lot; in a large one they even out. 4. Each coin is a parent’s gamete: heads and tails are equally likely, like T and t in a Tt parent, and the two coins do not affect each other, like a sperm and an egg.
• Bigger: the second coin must always give t (or be replaced by a card saying t); expect 1 : 1.

Chance

A. Breed 40 offspring

Each coin is one Tt parent: heads passes on T, tails passes on t. Toss both coins together. Two heads is TT, a head and a tail is Tt, two tails is tt. Tally 40 offspring.

Leave the shaded tally boxes empty: work out the tall and short counts from the first three.

B. Mendel’s own counts

Mendel crossed two plants that were each heterozygous for one trait. Work out the total and the ratio for each.

C. Questions

Class results: Tt × Tt

Each pair adds its counts. When the table is full, total each column and work out the class ratio of tall to short.

Reading a pedigree

A pedigree is a family tree for one trait: squares are males, circles are females, a line between two animals joins a pair of parents, and their offspring hang below. Shaded symbols have the trait. In guinea pigs, long hair is recessive to short hair, so a long-haired animal must be ss, and two short-haired parents with a long-haired pup must both be carriers, Ss. Students work out the genotypes in three generations of a breeder’s records, and predict the chance of a long-haired pup in the next litter. The same reasoning is used by genetic counsellors and by breeders of plants and animals.

We are learning to

• read the symbols of a pedigree,
• work out genotypes from the traits of parents and offspring,
• use a Punnett square to predict the next generation from a pedigree.

Success criteria

• I can give the genotype of every animal in the pedigree, or say why it can be either of two.
• I can explain how a recessive trait can appear in a pup when neither parent shows it.

You need

• the worksheet (next sheet), one per student,
• the exit tickets (third sheet), cut into four.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the pup, the board and Part B in Session A, and Parts C and D and the ticket in Session B.

Watch for these ideas

• “A recessive trait skips a generation.” It can appear in any generation where two carriers have offspring, or stay hidden for many.
• “A short-haired animal is SS.” It can be SS or Ss; only its parents or its offspring can tell you which.
• “Long hair is rare, so it must be recessive.” How common a trait is does not tell you; the pattern in the pedigree does.

Make it easier, make it bigger

• Easier: give the three possible genotypes, SS, Ss and ss, and work out the first generation together.
• Bigger: II-2 is mated with a long-haired female. What could the pups show that would tell you II-2’s genotype?

Answers and look-fors

• Part A: squares are males, circles females, shaded animals have long hair, and the line between two animals joins parents.
• Part B: I-1 Ss, I-2 Ss, II-1 ss, II-2 SS or Ss, II-3 Ss, III-1 Ss, III-2 ss, III-3 Ss.
• Part C: ss × Ss gives Ss, Ss, ss, ss, so the chance of a long-haired pup is 2 in 4, a half or 50%.
• Part D: 1. II-1 has long hair, so it is ss and got an s from each parent; both parents are short-haired, so each must be Ss. 2. Its parents are both Ss, so it could be SS or Ss; nothing in the pedigree decides. 3. III-2 has long hair, so it got an s from each parent, including II-3, which is short-haired and so must be Ss. 4. A carrier has one recessive allele hidden by a dominant one: I-1, I-2, II-3, III-1 and III-3 are carriers.
• Exit ticket: 1. dd. 2. Both are Dd, carriers. 3. A quarter, or 25%.
• Bigger: if any pup has long hair, II-2 must be Ss; if a large litter is all short-haired, II-2 is very likely SS.

Reading a pedigree

A breeder kept records of three generations of guinea pigs. Short hair (S) is dominant to long hair (s). The animals are numbered by generation: I at the top, then II, then III.

A. The symbols

B. The genotypes

Write each animal’s genotype. If it could be either of two, write both.

C. The next litter

D. Questions

Exit tickets

Beyond dominant and recessive

Not every gene follows complete dominance. In incomplete dominance the heterozygote is in between: a red snapdragon crossed with a white one gives pink flowers, and two pinks give red, pink and white again, so the alleles never blend away. In codominance both alleles show in full, as in roan cattle, with red and white hairs side by side. Human blood groups show both codominance and more than two alleles: A and B are codominant, and both are dominant over O. Sex is decided by a pair of chromosomes, XX or XY, and a Punnett square shows why the chance of each is a half. Students cross pink snapdragons, work out blood groups, sort twelve examples and decide a baby’s sex.

We are learning to

• recognise complete dominance, codominance and incomplete dominance from what the offspring look like,
• predict the offspring of a cross in each pattern,
• explain how the X and Y chromosomes decide sex.

Success criteria

• I can predict the offspring of two pink snapdragons, and of two parents with blood groups A and B.
• I can show with a Punnett square why the chance of a girl or a boy is a half each time.

You need

• the pattern cards (third sheet), one set per pair, with scissors,
• the worksheet (next sheet), one per student. No blood is tested: the blood groups are examples.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the snapdragons, the board and Parts A and B in Session A, and Parts C and D in Session B.

Watch for these ideas

• “Pink means the colours have blended for good.” Two pinks give red and white again: the alleles stay separate, and only the heterozygote looks in between.
• “Codominance and incomplete dominance are the same.” Codominance shows both traits in full, side by side; incomplete dominance shows a blend.
• “The mother decides the baby’s sex.” Every egg carries an X; the sperm carries either an X or a Y.

Make it easier, make it bigger

• Easier: give students the three outcomes of the pink cross (red, pink, white) and ask them to place each in the square.
• Bigger: a red snapdragon is crossed with a pink one. Predict the colours and their chances.

Answers and look-fors

• Part A: RW × RW gives RR, RW, RW, WW: 1 red, 2 pink, 1 white, so 1 : 2 : 1, and the chance of pink is a half.
• Part B: XX × XY gives XX, XX, XY, XY: a half chance of a girl and a half of a boy. 1. The father’s: every egg carries an X, and half the sperm carry an X and half a Y. 2. A half: each child is a fresh chance.
• Part C: AA group A, AO group A, BB group B, BO group B, AB group AB, OO group O. 3. AO × BO gives AB, AO, BO and OO: groups AB, A, B and O, a quarter each. 4. Yes: two parents with groups A and B can have a child with group O, if both carry O.
• Part D: complete dominance A, D, F, J, L; codominance B, E, G, K; incomplete dominance C, H, I.
• Bigger: RR × RW gives RR, RR, RW, RW: half red and half pink, and no white.

Beyond dominant and recessive

C. Blood groups

A and B are codominant; both are dominant over O. Write the blood group for each genotype, then complete the square for a parent with AO and a parent with BO.

D. Sort the pattern cards

Which pattern of inheritance?

Cut out the three headings and the twelve cards. Sort each example under the pattern it shows.

Teacher note: the answer letters are in the plan. The three piles are different sizes.

Show what we know

The term comes together. In the task, a plant breeder crosses two tall pea plants and counts 390 offspring: 291 tall and 99 short. Students work out the ratio, find the parents’ genotypes with a Punnett square, explain the result with meiosis and fertilisation, and decide how the breeder can find the pure-breeding plants. Then they sit the eight-question final check, and the class plays the unit quiz as a game on the board.

We are learning to

• explain a set of results with meiosis, fertilisation and a Punnett square,
• work backwards from offspring to the parents’ genotypes,
• compare real counts with a prediction, and explain the difference.

Success criteria

• I can explain the breeder’s results from the parents’ alleles to the offspring.
• I can answer the final check on my own.

You need

• the task (next sheet) and the final check (third sheet), one each per student; calculators,
• the interactive unit open on the board for the quiz; mini whiteboards if you have them.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the word wall and the task in Session A, and the final check and the quiz in Session B.

Watch for these ideas

• Students who give the parents as TT and tt: that cross gives only tall plants. Ask what the short offspring need from each parent.
• Students who call the breeder’s counts wrong because they are not exactly 3 : 1: return to Lesson 7.
• Students who say the parents’ genes are copied into the offspring: each gamete carries one allele of each gene, by meiosis.

Make it easier, make it bigger

• Easier: give the three crosses two tall plants can make (TT × TT, TT × Tt, Tt × Tt) to test with squares.
• Bigger: one tall offspring crossed with a short plant gives 48 tall and 52 short. What is its genotype, and how do you know?

Answers and look-fors

• Task: 1. 291 ÷ 99 = 2.94, close to 3 : 1. 2. Both parents are Tt: only Tt × Tt gives short offspring from two tall parents, a quarter of them. 3. In meiosis each parent’s T and t separate into different gametes, so half its gametes carry t; at fertilisation a t gamete meets a t gamete in a quarter of the offspring, which are tt and short. 4. Expected 292.5 tall and 97.5 short: offspring come one at a time by chance, so real counts wander a little from the prediction. 5. Two thirds of the tall plants, about 194, are expected to be Tt. 6. Cross each tall plant with a short one (or let it pollinate itself): any short offspring show it is Tt; only plants that give all tall offspring are kept. 7. 7: a gamete carries one of each pair.
• Final check: 1. gene. 2. 23, and 46. 3. Any two: mitosis is one division, meiosis two; mitosis makes two cells, meiosis four; mitosis keeps the full set, meiosis halves it; mitosis gives identical cells, meiosis different ones; mitosis is for growth and repair, meiosis for gametes. 4. heterozygous. 5. Pp × Pp gives PP, Pp, Pp, pp: a 1 in 4 chance (25%) of white. 6. incomplete dominance. 7. Both parents are Ss carriers; each passed an s to the pup, which is ss. 8. Each child comes from a different sperm and a different egg; in meiosis each pair of chromosomes separates independently and partners swap sections, so every gamete is a new mix.
• Bigger: Tt: crossing with tt gives about half tall and half short.
• Mark the task against the rubric in the assessment kit; the check covers DNA and genes, cell division, predicting and patterns.

The plant breeder

A plant breeder crosses two tall pea plants. In pea plants, tall (T) is dominant to short (t). The breeder plants every seed from the cross and counts the offspring when they have grown.

Show what we know