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.


★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.


★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.


•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


•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.


•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.


•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


•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


•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)


•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


•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


•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


•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:


•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:


•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.


•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

Gametes from Tall

parent

T T

Gametes from short

parent

t Tt Tt

t Tt Tt



•Trait is for color

•allele key: R=red, t=white

•Parents: Rr x rr

•4 gametes: R, r, r, r

•Genotype ratio:

•Phenotype ratio:

Gametes from white

parent

Gametes from

red parent



•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?

Gametes

Gametes


• 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?

Gametes

Gametes


• 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?


•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


•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.


•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.

Phenotype/blood group Genotype

A IA IA , IAi

B IB IB , IBi

AB IA IB

O ii


•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!



•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?

Gametes from A parent

Gametes from

O parent



•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?

Gametes from B parent

Gametes from

AB parent


•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


•Who determines the gender of the child, the mother or father?

•Explain:


•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.


•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.


•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.

XB dominant allele, sees regular colors/normal

Xb recessive allele for color blindness

XH dominant allele, ability to clot/normal

Xh recessive allele for hemophilia


Genotype Phenotype (for color blindness)normal male for color blindness

color blind malehomozygous normal female for color

blindnessheterozygous female for color blindness

color blind femaleGenotype Phenotype (for hemophilia)

XH Xh

XH XH

Xh Xh

XH y

Xh y



•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?

Gametes from male

Gametes from female



•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?

Gametes from male

Gametes from female


•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.


Symbol Key

normal female

carrier/heterozygous female

diseased/recessive trait female

normal male

carrier/heterozygous male

diseased/recessive trait male

Pedigree Rules

Horizontal lines linking the male and female represent a

marriage or a mating

Vertical lines underneath a man and woman represent

their children

Offspring are shown in the order of their birth (oldest to the left, youngest to the right)

Each row in a pedigree represents a generation


•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.


•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

•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: 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: 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: 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: 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: 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: 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

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: Genetics

•9. How many autosomes are there in a human sperm?

•A. 1

•B. 22

•C. 23

•D. 46

•Answer: B

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: 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: 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: 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: 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: 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: 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: 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

36

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

36

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

36

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

36

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: 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: 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

If it's a girl If it's a boy

A 0% 50%

B 0% 0%

C 50% 50%

D 50% 0%

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

Individual 1

Individual 2

A A AB

B AB B

C O AB

D B A

B

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: 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: 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

4.3: Theoretical Genetics

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