Unit 3: Heredity

Lesson 1: DNA Replication, Genes, and Traits

What in the world is going on with the PTC paper?

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What Makes A Good Model?

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Your Favorite Sisters:�https://www.youtube.com/watch?v=8m6hHRlKwxY&list=PLwL0Myd7Dk1F0iQPGrjehze3eDpco1eVz&index=29&t=37s

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What is DNA?

• DNA (or deoxyribonucleic acid) is a long molecule that contains our unique genetic code (what makes us…. well, us)
• DNA makes up our chromosomes
• DNA is the recipe book for proteins
• Specific segments of DNA that make certain proteins are called genes

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DNA Structure: The Double Helix

• DNA exists as a double-HELIX, meaning it is TWO strands of DNA wrapped together
• Each strand has two parts:
1. A sugar-phosphate backbone
2. Nucleotides (A, T, G, C)

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DNA Nucleotides (Purines and Pyrimidines)

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Base-Pairing of Purines and Pyrimidines

• Key Rule (Memorize)

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Adenosine and Thymine

A--T

Cytosine and Guanine

C--G

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“Car in the Garage”��“Apple in the Tree”

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Key Facts About DNA

• DNA exists in the NUCLEUS of the cell
• Remember that DNA comes in the form of a double-helix, with two complementary strands bound together by the nucleotides (A, G, C, T) matching up via hydrogen bonds
• When DNA needs to be replicated (during the S phase of the cell cycle), it undergoes a specific process

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Wednesday 11/14

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BW: What are the three stages of DNA replication? Make sure to include DNA helicase and DNA polymerase in your answer.

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DNA Replication Steps

1. The two strands that make up the double-helix “unwind” because of the enzyme DNA Helicase
2. DNA polymerase comes in and makes new complementary strands for each of these individual strands
3. Once completed, the final product is two exact copies of the original DNA

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DNA Replication in Action

• In the picture, the orange DNA is the _______________ DNA that was created by __________ ______________________
• The NEW DNA is always _______________________ to the strand it is attached to
• The result is TWO exact ____________________ of the original DNA

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Practice Finding the Complementary Strand:

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ATCGCCGT

Practice Finding the Complementary Strand:

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TCAGCTTAC

Practice Finding the Complementary Strand:

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CCATGTAAG

Modeling DNA Replication w Foldables

• Step 1: Fold the paper as shown by Mr. Sheehan

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Modeling DNA Replication w Foldables

• Step 2: Using a marker, write the sequence of nitrogen bases that are displayed on the board down the outer left flap of the paper.

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Modeling DNA Replication w Foldables

• Step 3: Pick a second marker and draw a line from the letter just written on the left side across the paper to the outer edge of the right side of the paper, leaving enough space to write another letter. What do you think this represents?

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Modeling DNA Replication w Foldables

• Step 4: Write the complementary base for each nitrogen base next to the line just drawn across the page, connecting the two nitrogen bases. Remember the Base-Pair Rule.

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Modeling DNA Replication w Foldables

• Step 5: Now model replication by opening the paper to its full 8 ½ x 11 size. What part of replication does this represent?

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Modeling DNA Replication w Foldables

• Step 6: Add a nitrogen base to each of the exposed sides of the original template strand by writing the letter of the complementary  base on both sides of the two original sides.

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Tuesday 11/28

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BW:

1. What is DNA responsible for?
2. What are the 4 nucleotides? Which pairs with which?
3. What is the difference between genotype and phenotype?

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DNA 🡪 Genes 🡪 Proteins

• A DNA molecule isn’t just a long, boring string of nucleotides. Instead, it is divided up into functional units called GENES
• Gene: A segment of DNA that provides instructions to build a protein that will perform a certain task

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Genes 🡪 Traits

• Genes code for TRAITS. Think of genes as “BEHIND THE SCENES” while traits are “ON STAGE”
• TRAIT: a genetically determined characteristic. EX: eye-color or cilantro tasting.

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Chromosomes: The Location of DNA and Genes

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What does pasta sauce have to do with genetics?

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Create your own analogy for genes and traits!

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Same Genes, Different Alleles

• Every chromosome has hundreds or even thousands of genes, and in general, all humans have the SAME GENES IN THE SAME PLACES of the same chromosomes
• However, not everyone has the SAME VERSION of these genes.
• ALLELE: An alternate version of a gene for a trait

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Case Study: Eye Color

• Scientists refer to genes by a COMBINATION OF LETTERS AND NUMBERS
• As an example, OCA2 (found on chromosome 15) is one of the many genes that help determine EYE COLOR
• It instructs a cell to make P protein, the protein that helps regulate the amount of melanin
• EVERYONE has the OCA2 gene, but some have different alleles of this gene:
• Person A has an allele of OCA2 that produces a lot of P protein which creates a lot of melanin which means dark-colored eyes
• Person B has an allele of OCA2 that produces little P protein which creates a little melanin which means light-colored eyes

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Wednesday 11/29��BW: How are genes related to traits?

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Lesson 2: Mendelian Genetics

Monday 12/4��BW: What are alleles? How do they relate to traits?

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Alleles = Genetic Variation in Sexual Reproduction

• Organisms that reproduce sexually must make SEX CELLS, like eggs and sperm
• This means that the offspring has a COMBINATION of genes that is different from either one of its parents
• The inheritance of different alleles from each parent brings about GENETIC VARIATION. Genetic variation is the different combinations of ALLELES of genes found in species

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Review: Independent (Random) Assortment

• A normal body cell has 46 CHROMOSOMES (23 PAIRS)
• When a sex cell forms in meiosis, the egg or sperm only receives one of the two chromosomes in each pair
• Which of the chromosomes in the pair goes into the sex cell is completely RANDOM, also called independent assortment

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Wednesday 2/22 BW: What is independent assortment? Draw an example of independent assortment below.

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Sperm (23) and Egg (23) Create Zygote (46)

• Combination of sperm and egg gives the new offspring 46 chromosomes, 23 from the MOTHER’S EGG and 23 from the FATHER’S SPERM
• As a result, there are TWO COPIES, or one homologous pair, of every kind of chromosome in a person’s cells
• The homologous pairs have the SAME GENES, but might have different ALLELES depending on the parents.

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Friday 1/27��BW: What is an allele? Why is it important?

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Each chromosome in the homologous chromosome pair has the same gene�

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• Each chromosome in the homologous pair has the same gene (one on the father chromosome and one on the mother chromosome)
• This gene can have a DIFFERENT ALLELE (EX: B OR B)

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3 Different Allele Combinations

• Everyone carries TWO COPIES of every gene (one from mother, one from father)
• Homozygous (pure bred): Both alleles of the homologous pair are the same (ex: BB or bb)
• Heterozygous (mix): One dominant and one recessive (ex: Bb)
• Hemizygous: Only one allele in a homologous pair (ex: XY)

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Tuesday 12/5��BW: Describe the difference between homozygous and heterozygous alleles

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What is the end product of meiosis?

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The end product of meiosis is four haploid (n) daughter cells, each genetically unique and containing half the number of chromosomes as the original cell.

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Four GENETICALLY UNIQUE HAPLOID cells

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Only ONE allele present per gene in gamete cell

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Law of Segregation: Only one of two gene copies (or alleles) present in an organism is distributed to each gamete (egg or sperm) that it makes

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Mendel’s Peas

In the mid 1800s, Fr. Gregor Mendel created a model system using pea plants to investigate inheritance. In his experiment, he starts with purebred purple pea plants and purebred white pea plants and breeds them together

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What do you think the result was?

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Why do you think they were all purple?

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Mendel’s Peas Reading

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Wednesday 12/6��BW: Why were Mendel’s first-generation pea plants all purple after breeding a purebred purple pea plant and a purebred white pea plant?

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Dominant vs. Recessive

• When an allele is DOMINANT the characteristic it is connected to (ex: blue eyes) will be EXPRESSED in an individual
• When an allele is RECESSIVE the characteristic it is connected to is LESS LIKELY to be expressed
• A dominant allele is represented by a CAPITAL letter while a recessive allele is represented by a LOWERCASE letter

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Bb

Dominant

Recessive

Dominant vs. Recessive (Definitions)

Dominant Trait: A genetic factor that blocks another genetic factor

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Recessive Trait: A genetic factor that is blocked by the presence of the dominant factor

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Genotype vs. Phenotype

Genotype: Genetic makeup of the cell (what genes/alleles do you have)

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Phenotype: Observable traits (the actual appearance/expressed trait as a result of your specific genotype)

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Bb = Genotype

Blue Eyes = Phenotype

Monday 3/6��BW: The gene that determines nose size is represented by the letter N. A capital N represents big nose while a lowercase n represents small nose. Given these letters, provide an example of heterozygous alleles and tell me what the phenotype will be. Which phenotype is dominant and which phenotype is recessive?

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Law of Segregation (Review)

According to the law of segregation, only one of the two gene copies (alleles) present in an organism is distributed to each gamete (egg or sperm cell) that it makes

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Mendel’s Purebred Purple and White Flowers

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F F

f f

Punnett Squares to Determine Inheritance

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F = dominant allele

f = recessive allele

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Parent 1 (FF) can only produce F gametes

Parent 2 (ff) can only produce f gametes

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Genotypes: 100% Ff

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Phenotypes: 100% show the dominant trait

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When a homozygous dominant (FF) is crossed with a homozygous recessive (ff), all offspring are heterozygous (Ff) and show the dominant trait.

f f

F

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F

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Tuesday 3/7��BW: Mendel breeds 2 first-generation heterozygous (Ff) offspring (phenotype purple) together and expects all offspring to also be purple. Use a Punnett Square to prove why he’s wrong.

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Mendel then breeds his purple plants…

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Return of the Recessive Gene… Later!

Mendel was surprised once again when after breeding all purple plants from the first generation, he ended up with some white offspring in the second generation

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Let’s help Mendel out with a Punnett square

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Step 1: Define the alleles

• P = purple flowers (dominant)�
• p = white flowers (recessive)�

Step 2: First generation (what Mendel crossed)

• Parent plants: PP (purple) × pp (white)�
• All offspring in the first generation (F₁) are Pp�
• All F₁ plants appear purple because purple is dominant

Step 3: Second generation cross

Mendel crossed the F₁ plants:

• Pp × Pp

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Step 4: Punnett Square

P p

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P | PP | Pp |

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p | Pp | pp |

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Step 5: Results

• Genotypes:�-1 PP�-2 Pp�-1 pp�
• Phenotypes:�-3 purple plants (75%)�-1 white plant (25%)

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Thursday 12/7

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BW: What is the difference between a dominant allele and recessive allele? What happens if a dominant allele is paired with a recessive allele?

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Key Takeaways from Mendel’s Study

• Characteristics such as flower color, plant height, and seed shape were controlled by pairs of heritable factors that came in different versions.
• One version of a factor (the dominant form) could mask the presence of another version (the recessive form).
• The two paired factors separated during gamete production, such that each gamete (sperm or egg) randomly received just one factor.
• The factors controlling different characteristics were inherited independently of one another.

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Punnett Square Practice

If black hair (B) is dominant in humans over brown hair (b), what would be the probability that an offspring from a mother who is BB and a father who is Bb would have black hair?

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Punnett Square Practice

If round pea pods (R) is dominant in pea plants over wrinkled pea pods (r), what would be the probability that an offspring from a mother who is RR and a father who is rr would have wrinkled pea pods?

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Amoeba Sisters Monohybrid Video:�https://www.youtube.com/watch?v=i-0rSv6oxSY

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Spongebob Inheritance WS (Finish for HW)

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Wednesday 3/8��BW: Chili’s Man Syndrome runs in Mr. Sheehan’s family. Chili’s Man Syndrome results in green skin and is the recessive phenotype. The letter for this gene is the letter C. If Mr. Sheehan’s dad is green and Mr. Sheehan’s mom has a regular skin tone, but is heterozygous, what is the chance of Mr. Sheehan’s siblings being green?�

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Monday 12/11

�BW: Everyone in Squidward’s family has light blue skin, which is the dominant trait for body color in his�hometown of Squid Valley. His family brags that they are a “purebred” line. He recently married a nice girl�who has light green skin, which is a recessive trait. Create a Punnett square to show the possibilities that would result if Squidward and his new bride had children. Use B to represent the dominant gene and b to represent the recessive gene.

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Things can get more complicated…

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Dihybrid Crosses With Mendelian Traits

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Amoeba Sisters Dihybrid Cross Video and Handout: https://www.youtube.com/watch?v=qIGXTJLrLf8

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Wednesday 12/13�BW: Round peas are represented by the letter r. The dominant allele is round, while the recessive is wrinkled. Flower color is represented by F. The dominant allele is purple, while the recessive is white. If a pea plant that is heterozygous purple with wrinkled pods is bred with a pea plant that is white with wrinkled pods, what’s the probability that the offspring with be purple with wrinkled pods?

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Thursday 12/14

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BW: Rudolph and Clarice have a reindeer baby. Having a black nose (R) is dominant over red nose (r) and being fuzzy (B) is dominant over short-haired (b). If Rudolph has a red nose and is homozygous fuzzy and Clarice is heterozygous black nose, heterozygous fuzzy, what’s the probability that their offspring will have a red nose and be fuzzy?

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Lesson 3: �Non-Mendelian Genetics

Wednesday 3/29��BW: What is the difference between homozygous (XX or xx) and heterozygous (Xx) phenotypes?

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In snapdragons, red flowers are represented by the genotype RR and white flowers are represented by the genotype rr. If I bred a red snapdragon and a white snapdragon together, what would the offspring look like? Use a Punnett Square

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https://www.youtube.com/watch?v=YJHGfbW55l0

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Thursday 3/30��BW: Share your example of incomplete dominance with your table partner

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Friday 3/31��BW: What is the difference between incomplete dominance and codominance?

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Complete Dominance: An individual has a heterozygous genotype (Rr), the dominant allele (R) shows up in the offspring and the recessive allele (r) does NOT show up

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Incomplete Dominance: Some heterozygous genotypes allow both alleles to partially show up by blending together how they are expressed

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Monday 4/3��BW: If I bred a pink snapdragon with a white snapdragon, what is the phenotypic % ratio of the offspring? R is the letter that represents this gene

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Codominance: Heterozygous genotypes allow both alleles to be completely expressed at the same time like spots or stripes

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Tuesday 4/4��BW: Share your example of codominance with your table partner

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Wednesday 4/5��BW: Check your answers with your table partner. Circle any you got different answers for and discuss.

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HW Answers

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Tuesday 4/11��BW: The gene(s) that determine height is an example of incomplete dominance and represented by the letter h. What are all the possible genotypic crosses for this gene (hint: there are five)?

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