Unit 10 genetics
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This unit will review the findings of Gregor Mendel and his pea plants and how Punnett squares and pedigrees can help us study genetics. On the following slides, the red sections highlight the key points . Underlined words = Vocabulary!. Unit 10 - Genetics. Find Someone Who.

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Unit 10 - Genetics

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This unit will review the findings of Gregor Mendel and his pea plants and how Punnett squares and pedigrees can help us study genetics.

On the following slides, the red sections highlight the key points.

Underlined words = Vocabulary!

Unit 10 - Genetics

Find Someone Who

  • Knows the meaning of the term “dominant”?

  • Knows the meaning of the term “homozygous”?

  • Knows the meaning of the term “heterozygous”?

  • Knows the difference between the term “genotype” and the term “phenotype” using the diagram to the left?

  • Knows what a Punnett Square does?

"Punnett squares in genetic crossings." Biology. Ed. Richard Robinson. New York: Macmillan Reference USA, 2010. Science in Context. Web. 14 Jan. 2014.

Heredity is the passing of traits from parents to offspring through their gametes.

Here is the family of Queen Victoria, the British monarch known for carrying the mutation for hemophilia.

What is heredity? Textbook Reference pg. 253-254


  • Remember Queen Victoria? Here is her family pedigree showing the disorder hemophilia. Image

What is a pedigree?Textbook Reference pg. 309-311

  • A pedigree shows the relationships within a family and can track the appearance of a genetic trait, disease or disorder.

    • Boxes represent males; circles represent females.

    • A pedigree always has a key.


Reading a Pedigree

The Smith family pedigree shows four generations of the trait inheritance for neurofibromatosis.

What can we learn about William & Liz’s family?

  • Is this disorder a dominant disorder?

  • Is this a sex-linked disorder?


Extra credit! Choose one genetically linked trait or disease from your family to study.

EX Eye color, Hair type, Freckles, Dimples, Breast Cancer, Etc.

Draw your parents and your parents and your siblings (brothers & sisters); color code the specific trait.

Create a key for the symbols and trait.

My Family Pedigree


Knowledge Check

How many generations does this pedigree show?

How many individuals in this pedigree are affected by a genetic disorder?

Is this disorder a dominant disorder? Explain. Turn and talk to your neighbor.

"pedigree." Experiment Central. U*X*L, 2010. Science in Context. Web. 14 Jan. 2014.

Review Slides

Slides 10 through 22 are review slides from middle school genetics. Our genetics pre-test will give you feedback on your knowledge of these slides; if you pass with 85% accuracy, you would NOT need to copy these slides, only review them.

Mendel: The Father of GeneticsTextbook Reference pg. 253-254

  • An Austrian monk named Gregor Mendel performed experiments during the 1800s to understand the process of traits.

    • Traits are the specific characteristics of an organism (blue eyes, brown hair, etc).

    • Mendel used pea plants from his garden to study.

    • He noticed that sometimes the “parent” pea plants did not match the characteristics in “children.”


Mendel’s Experiments Textbook Reference pg. 254-262

  • Mendel studied the plants and found variations, or differences.

    • EX Purple or white flowers, tall or short plants, and even round versus wrinkled peas.

  • He crossed (or pollinated) plants with different traits and studied the results.


Mendel’s Experiments Cont.

  • He found thatALL purple flowers, round peas, and tall plants were the MOST common (called the F1 generation or first generation).

  • In the second cross, the F2 generation, Mendel observeddifferences (white flowers, wrinkled peas, short plants).

  • He concluded that some traits only appear under certain conditions.


Mendel’s Experiments Cont.Genotype & Phenotype


Genotype & Phenotype Textbook Reference pg. 258

  • The genetic makeup and inherited combination of alleles is known as the offspring’s genotype, or “theletters that you GET.”

    • EX “Tt” or “tt”

  • The physical characteristic and the appearance shown in the organism (purple flowers, blue eyes, etc) is called the phenotype.

  • The phenotype is the “PROOF” of the allele that is there and is what you “SEE”.

    • EX “Tall plant” or “short plant”

What are alleles? Textbook Reference pg. 256

  • Genesare sequences of DNA that codes for a protein and determines a trait (EX the protein for brown fur; blue eyes).

  • The different forms (options) of a gene are known asalleles.

    • EX widows peak versus no widows peak

"Widow's Peak." Experiment Central. U*X*L, 2010. Science in Context. Web. 8 Jan. 2014.

Rule of Dominance Textbook Reference pg. 256

  • The stronger trait that will always appear is the dominant allele(shown with a capital letter).

  • The allele that did not appear, sometimes called the hidden trait, is called recessive(shown with a lowercase letter).

Detached earlobes are a dominant allele; attached earlobes are recessive. Illustration by Temah Nelson.

"earlobes." Experiment Central. U*X*L, 2010. Science in Context. Web. 8 Jan. 2014.


Sketch the following!

HomozygousTextbook Reference pg. 258

  • An organism can be homozygous dominant or homozygous recessive if they have two identical alleles for a trait.

    • From Greek, homos  - the same

    • Homozygous dominant (AA)

    • Homozygous recessive (aa)

      • Therecessive allelecan ONLY be seen in the homozygous form (normally HIDDEN).



Heterozygous Textbook Reference pg. 259

  • An organism with a mixed allele pair is heterozygous (Aa) and often called a hybrid has the opportunity of having offspring with both traits.

    • If two heterozygous organisms cross, you can have offspring that exhibit (or show) the recessive trait.


Reginald Punnett is one of the most “unknown” well-known scientists because he developed the Punnett square to model genetic probability.

He published Mendelism in 1905 to explain the concepts of Mendelian genetics.

A Punnett square is a chart used to predict and compare the genetic combinations that will result from sexual reproduction.

Review: Punnett SquaresTextbook Reference pg. 260-261

"Punnett Squares." Genetics. Ed. Richard Robinson. New York: Macmillan Reference USA, 2010. Science in Context. Web. 8 Jan. 2014.

Punnett Squares Cont.

  • In the Punnett square here, you can visualize the genes the parents are passing to offspring.

    • Each square of the box represents 1 offspring (or child).

    • Eachparent donates 1 allele.

    • The dominant (capital) allele will always go first, and will always be seen in the organism.

"Punnett squares in genetic crossings." Biology. Ed. Richard Robinson. New York: Macmillan Reference USA, 2010. Science in Context. Web. 8 Jan. 2014.

Knowledge Check

Why is the Punnett Square to the left called a “monohybrid cross”? How many offspring will display the dominant trait versus the recessive trait? What genotypic ratio will be seen in the offspring? Turn and talk to your neighbor.

"Punnett Square with Monohybrid Cross." Science Online. Facts On File, Inc. Web. 14 Jan. 2014.

Incomplete Dominance Textbook Reference pg. 315-316

  • We now know that there are also instances of incomplete dominance, where there is none that is dominant or recessive but a mixture (blending) of both alleles.

  • EX in certain flowers; a white crossed with a red can produce a pink flower.


CodominanceTextbook Reference pg. 316-317

  • There are also instances of codominance, where both dominant alleles are expressed.

  • EX in erminitte chickens; a white crossed with a black will produce a chicken with both black and white feathers.


Codominance Cont:

  • A chestnut horse (CBCB) crossed with a cremello horse (CwCw) will produce a palomino horse (CBCW)

  • More on horse genetics!







Multiple Alleles Textbook Reference pg. 317

  • Not all genes have just two alleles for the trait; traits with more than two alleles are described as having multiple alleles.

    • EX Hair color, eye color, and blood types are allexamples of human traits with multiple alleles.


Multiple Alleles Cont.

  • 3 Alleles for Blood Type: A, B, O

  • “A” and “B” are codominant over “O” which is recessive

Polygenic Traits Textbook Reference pg. 320

  • Not every trait is controlled by one gene. Traits controlled by two or more genesare said to be polygenic traits, which means “having many genes”.

  • Human stature (or height) is controlled by multiple genes.


In 1911, the first chromosomal gene mapwas produced and showed the gene locations on the chromosomes of a fruit fly. The banding areas represent a named gene [EX NF1] that controls a specific trait(s).

Gene maps are now widely used in understanding genetic mutations and sickness since the human genome project.

You can actually download the “GATTACA” sequence for humans at the National Center for Biotechnology Information.

What is a gene map? Textbook Reference pg. 349


Gene Maps Cont.

The map is based on the idea of linkage, which means that the closer two genes are to each other on the chromosome, the greater the probability that they will be inherited together. By following inheritance patterns, the relative locations of genes along the chromosome are established.

Adapted fromGenome.Gov

Two Factor Punnett Square Textbook Reference pg. 261

  • We understand how a simple Punnett square works by outlining the possibilities for one specific trait

    • EX eye color = E’s, skin color = G’s

  • The two factor cross shows the possibilitiescomparing TWO unique traits (usingtwo different letters).

    • Male Genotype = Gg & Ee

    • Female Genotype = GG & ee

Step #1 – The Gametes

  • Remember that meiosis creates haploid gametes (sex cells) in an organism with half the number of chromosomes.

  • The first step in a two factor cross is to determine the gametes. What genes will be held in the gametes?

  • Draw a box with 4 boxes.

  • Place the 1st genotype (Gg)on the top of the box.

  • Place 2nd genotype (Ee) on the side of the box.

  • Fill in the four squares. These will be the sex cells represent meiosis and the genes within the (gametes).

Male Gametes

  • Each of these boxes represents a single sperm cell containing these specific genes. Repeat for the sperm.

G g







Female Gametes

  • Each of these boxes represents a single egg cell containing these specific genes.








Step #2 – The Dihybrid Cross

  • Draw a Punnett square with 16 boxes.

  • Place the gametes on the top and sides (red & blue)

  • Always group the same genes together (G’s with g’s, etc).

    EX – in the first box, place a GGEe.

  • Fill in the remaining boxes.

The Dihybrid Cross

  • A dihybrid Punnett square shows 16 possible genotypic combinations.

Ge Ge Ge Ge












Practice at the Boards

  • Male = Heterozygous using “T”; Homo recessive using “P”; Female = Homo dominant using “T”; Homo recessive using “P”

  • Male = Homo recessive using “R”; Heterozygous using “Y”; Female = Heterozygous using “R”; Hetero “Y”

  • Male = Homo dominant using “B”; Homo recessive using “E”; Female = Heterozygous using “B”; Homo dominant using “E”

Genes that are located on the X or Y chromosomes are called sex-linked genes.

Many sex-linked genes are found on the X chromosome (which contains more than 100 genetic disorders).

Colorblindness and hemophilia are just two examples of genes relating to the X chromosome.

The human Y chromosome is much smaller than the X and only contains a few genes.

Sex-Linked Genes Textbook Reference pg. 318-320


Knowledge Check

Duchenne muscular dystrophy is a genetic disease due to the loss-of-function mutation. The bottom diagram shows a typical pedigree for inheritance of an x-linked trait such as Duchenne muscular dystrophy.

Which sex is most affected by this disorder? Why? How does the Punnett square relate to the information displayed in the pedigree? Turn and talk to your neighbor.

"Genetic trait of Duchenne muscular dystrophy." Biology. Ed. Richard Robinson. New York: Macmillan Reference USA, 2010. Science in Context. Web. 14 Jan. 2014.

Practice with an X Linked Cross

  • In red-green colorblindness, individuals cannot perceive the colors red and green in the same way as individuals with full color vision. Full color vision is coded by a dominant allele (B) on the X chromosome. Red-green colorblindness is caused by a recessive allele (b) on the X chromosome.

  • Identify the phenotype of a female with the genotype XBXb.

  • Identify the phenotype of a male with the genotype XBY.

  • Draw a Punnett square for the cross , and identify the following:

    • the percentage of offspring expected to be male and colorblind

    • the percentage of offspring expected to be female and colorblind

    • Explain why red-green colorblindness occurs more frequently in males than in females.

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