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4.3: Theoretical Genetics. ★Define genotype, phenotype, dominant allele, recessive allele, codominant alleles, locus, homozygous, heterozygous, carrier and test cross. ★Determine the genotypes and phenotypes of the offspring of a monohybrid cross using a Punnett grid.

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4 3 theoretical genetics
4.3: Theoretical Genetics

★Define genotype, phenotype, dominant allele, recessive allele, codominant alleles, locus, homozygous, heterozygous, carrier and test cross.

★Determine the genotypes and phenotypes of the offspring of a monohybrid cross using a Punnett grid.

★Predict the genotypic and phenotypic ratios of offspring of monohybrid crosses involving any of the above patterns of inheritance.

4 3 theoretical genetics1
4.3: Theoretical Genetics

★State that some genes have more than two alleles (multiple alleles).

★Describe ABO blood groups as an example of codominance and multiple alleles.

★Explain how the sex chromosomes control gender by referring to the inheritance of X and Y chromosomes in humans.

★State that some genes are present on the X chromosome and absent from the shorter Y chromosome in humans.

4 3 theoretical genetics2
4.3: Theoretical Genetics

★Define sex linkage.

★Describe the inheritance of color blindness and hemophilia as examples of sex linkage.

★State that a human female can be homozygous or heterozygous with respect to sex-linked genes.

★Explain that female carriers are heterozygous for X-linked recessive alleles.

★Deduce the genotypes and phenotypes of individuals in pedigree charts.

4 3 theoretical genetics3
4.3: Theoretical Genetics
  • Theoretical Genetics is a fancy term for what you have already done before - Punnett squares and pedigrees!
    • Predicting traits in future generations based on parental traits.
  • Who is known as the father of genetics?
    • Gregor Mendel
4 3 theoretical genetics4
4.3: Theoretical Genetics
  • In 1865 Gregor Mendel, an Austrian monk, published results of his experiments on how garden pea plants passed on their characteristics.
    • Why did he chose to work with pea plants?
      • Cheap, fast reproduction, no incest
  • At the time, the term "gene or DNA" did not exist (discovered 100 years later), so he worked with physical characteristics of pea plants - what he could see with his own 2 eyes.
4 3 theoretical genetics5
4.3: Theoretical Genetics
  • Mendel used artificial pollination in his experiments where he carefully chose the pollen of various plants to fertilize other plants.
    • Pollen = gametes (sex cells)
    • Insects do this naturally - that is why many flowers are brightly colored, to attract insects, like bees, to pollinate, or transfer gametes from flower to flower.
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4.3: Theoretical Genetics
  • In one cross, he fertilized short plants with tall plants and the resulting offspring were all tall.
  • When he took those tall offspring, and crossed them with each other, most were tall, but the short characteristic reappeared.
    • Mendel determined that inheritance is based on factors that can be passed on from generation to generation = genes.
    • Different forms of a gene = alleles
4 3 theoretical genetics7
4.3: Theoretical Genetics
  • Genotype: symbolic representation of alleles possessed by an organism, its genetic makeup, represented by a pair of letters.
    • EX: AA or Aa
  • Phenotype: physical characteristics of an organism, can be external like flower color, or internal like sickle-cell anemia.
    • EX: Tall, red, color-blind
4 3 theoretical genetics8
4.3: Theoretical Genetics
  • Dominant allele: an allele that has the same effect on the phenotype whether it's paired with the same allele or a different one. Dominant alleles are always expressed in the phenotype. Represented by a capital letter.
    • EX: genotype Aa (allele A is dominant)
  • Recessive allele: an allele that has an effect on the phenotype only when present as homozygous. Represented by a lowercase letter.
    • EX: genotype aa (allele a is recessive)
4 3 theoretical genetics9
4.3: Theoretical Genetics
  • Codominant alleles: pairs of alleles that both affect the phenotype when in a heterozygous state.
    • EX: a blood type A parent and a blood type B parent will have a child who is type AB
  • Locus: the specific position of a gene on a homologous chromosome. Each gene is found at a specific place on a specific pair of chromosomes.
    • EX: Insulin gene always found at same position on chromosome 11 in humans
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4.3: Theoretical Genetics
  • Homozygous: 2 identical alleles of a gene at the same locus. Alleles can be both dominant or both recessive.
    • EX: AA is homozygous dominant, aa is homozygous recessive.
  • Heterozygous: 2 different alleles of a gene at the same locus. This is because one is the parental allele, other is maternal allele.
    • EX: Aa is a heterozygous genotype
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4.3: Theoretical Genetics
  • Carrier: An individual who has a recessive allele of a gene that does not have an effect on their phenotype (phenotype version of heterozygote)
    • EX: Aa carries the albino gene, but this person wouldn't be albino.
  • Test Cross: Testing a suspected heterozygote by crossing it with a known homozygous recessive. Since a recessive allele can be masked, it is often impossible to tell if an organism is AA or Aa until they produce offspring which have the recessive trait.
    • EX: Aa crossed with aa
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4.3: Theoretical Genetics
  • Let's test out some of these vocab words:
  • Is RR the genotype or phenotype?
  • Is Yellow the genotype or phenotype?
    • If red R is dominant to yellow r, what is the phenotype of each of the following genotypes?
      • RR:
      • Rr:
      • rr:
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4.3: Theoretical Genetics
  • What would be the gametes produced by a parent with each of the following genotypes?
    • Rr:
    • rr:
    • RR:
  • Label the following as either homozygous dominant, homozygous recessive, or heterozygous.
    • Rr:
    • rr:
    • RR:
4 3 theoretical genetics14
4.3: Theoretical Genetics
  • Constructing a Punnett Grid:
    • IB calls them Punnett Grids, we are used to calling them Punnett Squares. Same thing!
    • A Punnett grid shows all possible combos of genetic information for a particular trait.
    • All the Punnett grids in IB are for monohybrid crosses, which means they show the results for one trait only.
      • This means we will only be doing grids with 4 squares.
4 3 theoretical genetics15
4.3: Theoretical Genetics
  • Practice Punnett Grid 1:
    • Trait is for height
    • allele key: T=tall, t=short
      • Parents: TT x tt
      • 4 gametes: T, T, t, t
  • Genotype ratio: 4 Tt
  • Phenotype ratio: 4 tall
4 3 theoretical genetics16
4.3: Theoretical Genetics
  • Practice Punnett Grid 2:
    • Trait is for color
    • allele key: R=red, t=white
      • Parents: Rr x rr
      • 4 gametes: R, r, r, r
  • Genotype ratio:
  • Phenotype ratio:
4 3 theoretical genetics17
4.3: Theoretical Genetics
  • Practice Punnett Grid 3:
    • Trait is for albinism
    • allele key: A=normal, a=albino
      • Parents: Aa x Aa
      • 4 gametes:
  • Genotype ratio:
  • Phenotype ratio:
  • What are the chances of this couple having an albino child?
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4.3: Theoretical Genetics
  • Practice Punnett Grid 4:
  • Trait is for albinism
  • allele key: A=normal, a=albino
  • Cross a heterozygote with a homozygous dominant individual.
  • Parents: ____ x ____
  • Genotype ratio:
  • Phenotype ratio:
  • What are the chances of this couple having a normal child?
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4.3: Theoretical Genetics
  • Practice Punnett Grid 5:
  • Trait is flower color
  • allele key: G=green, g=orange
  • Cross a orange plant with a homozygous green plant.
  • In the F2 generation, what percent chance will there be orange offspring?
4 3 theoretical genetics20
4.3: Theoretical Genetics
  • Codominance and Multiple Alleles:
  • So far, we have only dealt with 2 possibilities for a gene: either dominant A, or recessive a.
  • With 2 alleles, 3 genotypes are possible (AA, Aa, aa) which produce 2 phenotypes.
  • In real life, it is not always this simple, sometimes there are 3 or more alleles for the same gene.
      • EX: blood type, height, skin color
4 3 theoretical genetics21
4.3: Theoretical Genetics
  • ABO human blood type system in humans has 4 possible phenotypes:
    • A, B, AB, O
  • To create these 4 blood types there are 3 different alleles of the gene.
  • These 3 alleles can produce 6 different genotypes.
  • The gene for the ABO blood type is represented by the letter I.
    • To represent more than just 2 alleles, I and i are both used.
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4.3: Theoretical Genetics
  • The 3 alleles for blood type are written as:
    • IA = allele for type A blood
    • IB = allele for type B blood
    • i = recessive allele for type O blood
  • Crossing these together in all possible combos creates 6 genotypes for 4 phenotypes.
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4.3: Theoretical Genetics
  • Notice how the genotype IA IB clearly shows codominance.
    • Neither allele is masked or hidden, both show expression in the phenotype at the same time.
    • Rather than being either A or B blood, both are dominant together to produce AB blood.
  • A person's blood type depends on which combination of alleles he/she receives. With blood type, only 2 alleles can be inherited, which means one from the mom, one from the dad.
    • Remember, blood type is an example of a trait that is both codominant and has multiple alleles!
4 3 theoretical genetics24
4.3: Theoretical Genetics
  • Practice Punnett Grid 6:
  • Cross a homozygous blood type A parent with a type O parent.
    • Parent cross: ____ x ____
    • 4 gametes:
  • Genotype ratio:
  • Phenotype ratio:
  • What are the chances of this couple having a type O child?
4 3 theoretical genetics25
4.3: Theoretical Genetics
  • Practice Punnett Grid 7:
  • Cross a heterozygous blood type B parent with a type AB parent.
    • Parent cross: ____ x ____
    • 4 gametes:
  • Genotype ratio:
  • Phenotype ratio:
  • What are the chances of this couple having a type AB child?
4 3 theoretical genetics26
4.3: Theoretical Genetics
  • Sex Chromosome Inheritance:
    • The 23rd pair of chromosomes are called sex chromosomes because they determine a person's gender.
    • XX = Female Xy = Male
    • There is a 50/50 shot of having a girl or boy baby.
    • WHY? Do a punnett grid for the answer:
    • Parent cross: XX x Xy
4 3 theoretical genetics27
4.3: Theoretical Genetics
  • Who determines the gender of the child, the mother or father?
  • Explain:
4 3 theoretical genetics28
4.3: Theoretical Genetics
  • The X chromosome is significantly larger, and thus carry more genes than the y chromosome.
    • For our future Punnett grids, the X chromosome will be the ONLY chromosome to carry the alleles.
      • The Y chromosome will carry nothing.
  • Any genetic trait whose allele has its locus on the X chromosome is said to be sex-linked.
    • Often genetic traits which show sex linkage affect one gender more than the other.
4 3 theoretical genetics29
4.3: Theoretical Genetics
  • 2 examples of genetic traits which are this particular and we will use at length are color-blindness and hemophilia.
    • Colorblindness: inability to distinguish between certain colors, often green and red. To people who are color blind, these colors look the same.
    • Hemophilia: disorder where blood does not clot properly. These people risk bleeding to death from what most people would consider a minor injury.
4 3 theoretical genetics30
4.3: Theoretical Genetics
  • Since the alleles for both color blindness and hemophilia are found only on the X chromosome, the letter X is used in representing them
  • In both cases, there is no allele on the Y chromosome, so Y is written alone without any superscript.
4 3 theoretical genetics32
4.3: Theoretical Genetics
  • Practice Punnett Grid 8:
  • For colorblindness, cross a normal male and a heterozygous female.
    • Parent cross: _____ x _____
    • 4 gametes:
  • Genotype ratio:
  • Phenotype ratio:
  • What are the chances of this couple having a normal child?
4 3 theoretical genetics33
4.3: Theoretical Genetics
  • Practice Punnett Grid 9:
  • For hemophilia, cross a carrier woman with a hemophiliac male
    • Parent cross: _____ x _____
    • 4 gametes:
  • Genotype ratio:
  • Phenotype ratio:
  • What are the chances of this couple having a hemophiliac male?
4 3 theoretical genetics34
4.3: Theoretical Genetics
  • Pedigrees:
    • The term 'pedigree' refers to the record of an organism's ancestry.
    • Pedigree charts are diagrams which are constructed to show biological relationships - how a trait can pass from generation to generation.
    • From a pedigree, you can determine if a trait is dominant or recessive, if it is sex-linked, and probability of it affecting future generations.
    • To build such a chart to see the flow of genotypes, symbols and rules must be followed.
4 3 theoretical genetics36
4.3: Theoretical Genetics
  • Practice Pedigree 1:
  • This is for color-blindness, which is sex-linked and recessive.
  • Color-blindness is represented with an allele b.
  • Determine all possible genotypes.
4 3 theoretical genetics37
4.3: Theoretical Genetics
  • Practice Pedigree 2:
  • This is for hemophilia, which is sex-linked and recessive.
  • Hemophilia is represented with an allele h.
  • Determine all possible genotypes.
topic 4 genetics
Topic 4: Genetics
  • 1. Which of the following is an inherited disease that is due to a base substitution mutation in a gene?
    • A. Trisomy 21
    • B. Sickle cell anemia
    • C. AIDS
    • D. Type II Diabetes
      • Answer: A
topic 4 genetics1
Topic 4: Genetics
  • 2. Which of the following is the cause of sickle-cell anemia?
    • A. Tryptophan is replaced by leucine.
    • B. Leucine is replaced by valine.
    • C. Glutamic acid is replaced by valine.
    • D. Lysine is replaced by glutamic acid.
      • Answer: C
topic 4 genetics2
Topic 4: Genetics
  • 3. What is the cause of sickle-cell anemia?
    • A. A change to the base sequence of a hemoglobin gene.
    • B. Mosquitos acting as the vector for malaria.
    • C. Iron deficiency due to the malaria parasite.
    • D. Production of more white blood cells than red blood cells by bone marrow
      • Answer: A
topic 4 genetics3
Topic 4: Genetics
  • 4. A human cell has between 20,000-25,000 genes whereas an E. coli cell has approximately 4,000 genes. Which of the following statements is true?
    • A. The human genome is larger than the E. coli genome.
    • B. There are more genes on each human chromosome than on the E. coli chromosome.
    • C. The human cell and the E. coli cell produce approximately the same variety of proteins.
    • D. The DNA in both organisms is associated with histones (proteins).
      • Answer: A
topic 4 genetics4
Topic 4: Genetics
  • 5. Which of the following statements about homologous chromosomes is correct?
    • A. Each gene is at the same locus on both chromosomes.
    • B. They are two identical copies of a parent chromosome which are attached to one another at the centromere.
    • C. They always produce identical phenotypes.
    • D. They are chromosomes that have identical genes and alleles.
      • Answer: A
topic 4 genetics5
Topic 4: Genetics
  • 6. What happens in crossing over?
    • A. Exchange of genetic material between homologous chromosomes.
    • B. Exchange of genes during metaphase of mitosis.
    • C. Random distribution of chromosomes during meiosis.
    • D. Homologous chromosomes fail to separate during meiosis.
      • Answer: A
topic 4 genetics6
Topic 4: Genetics
  • 7. A cell in the testis of a male chimpanzee contains 48 chromosomes. It is about to undergo meiosis. How many molecules of DNA will be present in the nucleus of the sperm cells just after meiosis?
    • A. 96
    • B. 48
    • C. 24
    • D. 12
      • Answer: C
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Topic 4: Genetics
  • 8. If the haploid number of a species is 14, how many chromatids will there be in metaphase I in a dividing diploid cell?
    • A. 7
    • B. 14
    • C. 28
    • D. 56
      • Answer: D
topic 4 genetics8
Topic 4: Genetics
  • 9. How many autosomes are there in a human sperm?
    • A. 1
    • B. 22
    • C. 23
    • D. 46
      • Answer: B
topic 4 genetics9
Topic 4: Genetics
  • 10. If the amount of DNA in a haploid gamete is represented by X, what is the net quantity of DNA in a cell from the same organism at the start of meiosis?
    • A. 0.5x
    • B. x
    • C. 2x
    • D. 4x
      • Answer: D
topic 4 genetics10
Topic 4: Genetics
  • 11. In the following diagram, which pair represents homologous chromosomes?
  • A. 1 and 2
  • B. 3 and 4
  • C. 2 and 5
  • D. 4 and 6
      • Answer: D
topic 4 genetics11
Topic 4: Genetics
  • 12. Which of the following types of information are needed to construct a karyotype?

I. Size of the chromosomes

II. Gene mutations of the chromosomes

III. Age of the individual

      • A. I only
      • B. II only
      • C. I and II only
      • D. I, II and III
        • Answer: A
topic 4 genetics12
Topic 4: Genetics
  • 13. Which phase of cell division is photographed in order to make a karyotype?
    • A. Anaphase of mitosis
    • B. Anaphase I of meiosis
    • C. Metaphase of mitosis
    • D. Metaphase II of meiosis
      • Answer: C
14. What does the karyotype below correspond to?
  • A. A normal male
  • B. A normal female
  • C. A female with Down Syndrome
  • D. A male with Down Syndrome
    • Answer: A
15. What can be concluded from the karyotype provided below?
  • A. There was non-disjunction during meiosis in the mother.
  • B. There was non-disjunction during meiosis in the father.
  • C. The fetus is male.
  • D. The fetus is female.
    • Answer: D
topic 4 genetics13
Topic 4: Genetics
  • 16. Which is the set of alleles that an individual passes?
    • A. A gene
    • B. A genotype
    • C. A genome
    • D. A genus
      • Answer: B
topic 4 genetics14
Topic 4: Genetics
  • 17. What is a genetic test cross?
    • A. Testing a suspected homozygote by crossing it with a known heterozygote.
    • B. Testing a suspected heterozygote by crossing it with a known heterozygote.
    • C. Testing a suspected homozygote by crossing it with a known homozygous dominant.
    • D. Testing a suspected heterozygote by crossing it with a known homozygous recessive.
      • Answer: D
topic 4 genetics15
Topic 4: Genetics
  • 18. What is a suspected heterozygous individual crossed with in a test cross?
    • A. Homozygous dominant
    • B. Homozygous recessive
    • C. Heterozygous dominant
    • D. Heterozygous recessive
      • Answer: B
topic 4 genetics16
Topic 4: Genetics
  • 19. A new allele that provides herbicide resistance is identified in soybean plants. The allele is dominant. Which of the following would be carried out in a herbicide-resistant plant to find out if it is homozygous or heterozygous for the gene?
    • A. Gel electrophoresis
    • B. Karyotyping
    • C. Test cross
    • D. DNA profiling
      • Answer: C


topic 4 genetics17
Topic 4: Genetics
  • 20. A parent organism of unknown genotype is mated in a test cross. Half of the offspring have the same phenotype as the parent. What can be concluded from this result?
    • A. The parent of unknown genotype is heterozygous
    • B. The parent of unknown genotype is homozygous dominant.
    • C. The parent of unknown genotype is homozygous recessive.
    • D. The parent of known genotype is heterozygous.
      • Answer: A


topic 4 genetics18
Topic 4: Genetics
  • 21. If an organism that is homozygous recessive for a trait is crossed with a heterozygote, what is the chance of getting a homozygous recessive phenotype in the first generation?
    • A. 0%
    • B. 25%
    • C. 50%
    • D. 100%
      • Answer: C


topic 4 genetics19
Topic 4: Genetics
  • 22. Boys can inherit the recessive allele (c) that causes red-green color blindness from their mother, not from their father. The allele for normal red and green vision is C. Which of the following genotypes are possible in men?
    • A. c only
    • B. C or c only
    • C. CC or cc only
    • D. CC, Cc or cc only
      • Answer: B


topic 4 genetics20
Topic 4: Genetics
  • 23. A man of blood group A and a woman of blood group B have a child. If both are heterozygous for the gene, what are the chances of them having a child with blood group B?
    • A. 0%
    • B. 25%
    • C. 50%
    • D. 75%
      • Answer: B
topic 4 genetics21
Topic 4: Genetics
  • 24. If a man has blood group O and a woman has blood group AB, what is the probability that their child will be blood group O?
    • A. 0%
    • B. 25%
    • C. 50%
    • D. 100%
      • Answer: A
topic 4 genetics22
Topic 4: Genetics
  • 25. A woman is a carrier for hemophilia and a man who does not have hemophilia have a child. What is the probability that the child will have hemophilia?
      • Answer: A
topic 4 genetics23
Topic 4: Genetics
  • 26. The pedigree chart below shows the blood types of three members of a family. What could be the blood types of individuals 1 and 2?
      • Answer: A


topic 4 genetics24
Topic 4: Genetics
  • 27. The blood groups of a mother and 4 children are indicated on the pedigree chart below. What are the possible blood groups of the father?
  • A. Group A only
  • B. Group A or B only
  • C. Group AB only
  • D. Group A, B or AB only
    • Answer: C
topic 4 genetics25
Topic 4: Genetics
  • 28. What type of inheritance is shown in this pedigree chart?
  • A. X-linked dominant
  • B. Y-linked dominant
  • C. X-linked recessive
  • D. Y-linked recessive
    • Answer: C
29. What evidence is given in the pedigree chart below to establish that the condition is caused by a dominant allele?
  • A. Two unaffected parents have unaffected children.
  • B. Two affected parents have affected children.
  • C. An affected parent and an unaffected parent have affected children.
  • D. Two affected parents have an unaffected child.
    • Answer: D
topic 4 genetics26
Topic 4: Genetics
  • 30. Which disease is an example of sex-linked (X-linked) inheritance?
  • A. AIDS
  • B. Down syndrome
  • C. Sickle-cell anemia
  • D. Hemophilia
    • Answer: D