AP Biology

Sexual Reproduction and Inheritance

Reading Guide – Chapter 8

Chapter 8 – Genetics

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OBJECTIVES

  • Describe the favored model of heredity in the 19th century prior to Mendel and explain how this model was inconsistent with observations.
  • Explain how Mendel’s hypothesis of inheritance differed from the blending theory of inheritance.
  • List several features of Mendel’s methods that contributed to his success.
  • List four components of Mendel’s hypothesis that led him to deduce the law of segregation.
  • Use a Punnett square to predict the results of a monohybrid or dihybrid cross and state the phenotypic and genotypic ratios of the F2 generation.
  • Distinguish between genotype and phenotype; heterozygous and homozygous; dominant and recessive.
  • Explain how a testcross can be used to determine if a dominant phenotype is homozygous or heterozygous.
  • Understand the laws of probability and how to use them to predict genotype and phenotypes of offspring:
  • Use the rule of multiplication to calculate the probability that a particular F2 individual will be homozygous recessive or dominant.
  • Use the rule of addition or calculate the probability that a particular F2 individual will be heterozygous.
  • Understand the two components of Mendel’s law of independent assortment.
  • Relate the laws of independent assortment of segregation to the process of meiosis.
  • Give an example of incomplete dominance and explain why it is not evidence for the blending theory of inheritance.
  • Explain how the phenotypic expression of the heterozygote is affected by complete dominance, incomplete dominance and codominance.
  • Describe the inheritance of the ABO blood system and explain why the IA and IB alleles are said to be codominant.
  • Explain what is meant by ‘one gene is epistatic to another’.
  • Describe a simple model for polygenic inheritance and explain why most polygenic characters are described in quantitative terms.
  • Describe how environmental conditions can influence the phenotypic expression of a character.
  • Given a simple family pedigree, deduce the genotypes for some of the family members and determine the pattern of inheritance for a disease.
  • Describe the inheritance pattern and expression of cystic fibrosis, Tay-Sachs disease, PKU, and sickle cell disease.
  • Explain why Drosophila melanogaster is a good experimental organism.
  • Define linkage and explain why linkage interferes with independent assortment.
  • Distinguish between parental and recombinant phenotypes.
  • Explain how crossing over can unlink genes.
  • Map a linear sequence of genes on a chromosome using given recombination frequencies from experimental crosses.
  • Describe sex determination in humans.
  • Describe the inheritance of a sex-linked gene such as color-blindness, muscular dystrophy, and hemophilia.
  • Explain why a recessive sex-linked gene is always expressed in human males.
  • Explain how an organism compensates for the fact that some individual have a double dosage of sex-linked genes while other have only one.

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For additional help, try the website Here you will find an excellent series of tutorials for Chapters 10-18 of our book. Explore panels 1-11 to guide you through this chapter. Be sure to click on the animation and problem links to get the complete info.

QUESTIONS – Section 8.1: Mendelelian Laws of Inheritance

  1. Why was the blending theory of inheritance inconsistent with Darwin’s theory of evolution by natural selection?
  1. Describe several features of Mendel’s experiments that allowed him to develop his theories.
  1. What does it mean to say a strain of organisms is ‘true breeding’?

State Mendel’s Law of Segregation.
  1. Explain how he arrived at the Law of Segregation from conducting the monohybrid crosses.
  1. Define and give an example of each of the following:

Gene locus
Homozygous
Heterozygous
Genotype
Phenotype
Allele
Dominant allele
Recessive allele
  1. When we predict the expected genotype of an offspring, why do we consider the alleles they inherit as two separate, independent events? And what probability law applies?
  1. When predicting phenotype, what probability law applies?
  1. Demonstrate that the expected phenotype and genotype probabilities of offspring from two heterozygous parents is the same whether you use the two probability laws above or a Punnett square.
  1. What is the genotypic ratio then of a heterozygous cross? What is the phenotypic ratio?
  1. What does it mean when we say ‘Chance has no memory’?
  1. How did Mendel use a testcross to test the law of segregation?
  1. How can a testcross differentiate between a heterozygous dominant and a homozygous dominant?

REVIEW PROBLEMS - Laws of Probability

  1. Give the Rule of Multiplication:
  1. You have 2 coins, what is the probability that you will flip two heads?
  1. What is the probability that offspring of an F1 generation cross will be homozygous recessive?
  1. Give the Rule of Addition:
  1. You have 2 coins. What is the probability that you will flip a heads and a tails?
  1. What is the probability that two heterozygous parents will produce heterozygous offspring?
  1. What is the probability that two parents heterozygous for both height and flower color will produce tall offspring with purple flowers?
  1. For the following crosses, indicate the probability of obtaining the indicated genotype in an offspring. Remember it is easiest to treat each gene separately as a monohybrid cross and then combine the probabilities.

Cross / Offspring / Probability
AAbb x AaBb / AAbb
AaBB x AaBb / aaBB
AABbcc x aabbCC / AaBbCc
AaBbCc x AaBbcc / aabbcc

Mendel’s Law of Independent Assortment

  1. What is a dihybrid cross?

State Mendel’s Law of Independent Assortment
  1. Explain how Mendel arrived at the law of independent assortment using a dihybrid cross.
  1. Explain how a two-trait testcross can be used to differentiate between a homozygous and a heterozygous dominant.
  1. Using Figure 12.8 – describe which events in meiosis correlate with Mendel’s Law of Segregation and Law of Independent Assortment.

Human Genetic Disorders and Pedigree Analysis

  1. Why do genetic counselors construct genetic pedigrees? What information can they obtain?
  1. What do each of the following symbols represent in a pedigree:

Square / Circle / Horizontal
Line / Vertical
Line / Shaded Circle or Square
  1. How can a pedigree analysis distinguish between an autosomal dominant pattern of inheritance versus an autosomal recessive pattern of inheritance?

QUESTIONS – Section 8.2: Interactions Between Alleles

  1. Differentiate between a unifactorial and multifactorial trait.
  1. Describe of how each of the following inheritance patterns deviate from expected 3:1 ratio of a heterozygous cross and give an example of each in humans.

Inheritance Pattern / Deviation from 3:1 ratio / Example
Incomplete Dominance
Multiple
Alleles
Codominance
Polygenic Inheritance
  1. A rooster with blue (actually gray) feathers is mated with a hen of the same phenotype. Among their offspring, 15 chicks are blue, 6 are black, and 8 are white.
  1. What is the simplest explanation for the inheritance of these colors in chickens?
  1. What offspring would you predict from the mating of a blue rooster and a black hen?
  1. Explain why a type O person can donate blood to all other blood types but can only receive type O blood.
  1. Blood typing has often been used as evidence in paternity cases. For the following mother and child combination, indicate which blood groups of potential fathers would be exonerated.

Blood Type of Mother / Blood Type of Child / Blood Group that would Exonerate Man
AB / A
O / B
A / AB
O / O
B / A
  1. Color pattern in a species of duck is determined by a single pair of genes with three alleles. Alleles H and I are codominant, and allele i is recessive to both. How many phenotypes are possible in a flock of ducks that contains all the possible combinations of these three alleles?
  1. What is epistasis? Give an example.
  1. In guinea pigs, the gene for production of melanin is epistatic to the gene for the deposition of melanin. The dominant allele M causes melanin to be produced; mm individuals cannot produce the pigment. The dominant allele B causes the deposition of a lot of pigment and produces black guinea pigs, whereas only a small amount of pigment is laid down in bb animals, producing a light-brown color. Without an M allele, no pigment is produced so the allele B has not affect and the guinea pig is white. A homozygous black guinea pig is crossed with a homozygous recessive white: MMBB x mmbb. give the phenotypes of the F1 and F2 generations.
  1. The height of spike weed is a result of polygenic inheritance involving three genes, each of which can contribute 5 cm to the plant. The base height of the weed is 10 cm, and the tallest plant can reach 40 cm.
  1. If a tall plant (AABBCC) is crossed with a base-height plant (aabbcc), what is the height of the F1 plants?
  1. How many phenotypic classes will there be in the F2? List them.

QUESTIONS – Section 8.3: The Chromosomal Theory of Inheritance

  1. Explain the crosses conducted by T.H. Morgan in Figure 12.18. with his white-eyed fruit flies. Explain how/why the observed phenotypic ratios deviate from those predicted by a simple Mendelian dihybrid cross involving a heterozygous and homozygous recessive parents.
  1. Why didn’t all the offspring from this cross exhibit only maternal or paternal traits?
  1. What is a recombinant phenotype?
  1. Define the following terms:

Gene linkage
Linkage group
Recombinant Frequencies
  1. What is a linkage (genetic) map? What do the numbers listed in the chromosome mean?
  1. What is the equation for recombination (cross-over) frequency?
  1. If the genes for two traits are located on separate chromosomes, what would be the possible gametes produced from an AaBb individual?
  1. If the following genes are unlinked (on separate chromosomes) what would the expected phenotypic ratio of the following cross: AaBb x aabb
  1. Calculate the recombination frequency for this cross – you need to understand what a recombinant phenotype is first. (I know the genes are not linked –just humor me and do the math – I have a plan here….)
  1. If the genes for these traits are located on the SAME chromosome (linked), what possible gametes could be produced from an AaBb individual?
  1. What would be the expected phenotypic ratio of an AaBb x aabb cross if these genes are linked?
  1. How many recombinant phenotypes were produced here? What is the recombinant frequency then?
  1. Now, let’s say you cross an AaBb with an aabb individual and obtain the following results:

35 Dominant in both A and B

15 Dominant for A, recessive for B

15 Dominant for B, recessive for A

35 Recessive for both

  1. Identify the recombinant phenotypes here. Then calculate the recombinant frequency.
  1. Do you believe the genes for these traits are on the same chromosome or different chromosomes?
  1. If you believe they are on the same chromosome, how many map units apart are they?
  1. Now, you what if you did this cross and got the following results:

25 Dominant for both A and B

25 Dominant for A, recessive for B

25 Dominant for B, recessive for A

25 recessive for both A and B

  1. Identify the recombinants here. Calculate the recombination frequency.
  1. Do you believe the genes for these traits are on the same chromosome or different chromosomes? Can you even tell? (Hint: Compare this number with the number you got in question 18.)
  1. Can you say anything about the upper and lower limits of calculating the recombination frequency?
  1. So if two genes are linked, they should reside on the same chromosome and the recombination frequency will range between….? But, if two genes are unlinked, are they necessarily on separate chromosomes? Why or Why not?
  1. In general, if 2 genes are linked (on the same chromosome) what is the relationship between the cross-over frequency and the distance between these genes?
  1. Why does recombination frequency increase with distance?
  1. Go back to meiosis now and be sure you understand how cross-over works and why it occurs. To test your knowledge, predict the gamete combinations that could be produced from the following diploid cells, and the expected frequency of recombinant gametes given. (Trace thru the process of meiosis and the chromosomal divisions.)

Assume genes are 10 map units apart in both parent cells.

Practice Problems – Gene Linkage

  1. A wild-type fruit fly (heterozygous for gray body color and normal wings) was mated with a black fruit fly with vestigial wings. The offspring gave the following distribution: wild-type, 778; black-vestigial, 785; black-normal, 158, gray-vestigial, 162.
  1. What are the phenotypes of the recombinants?
  1. What is the recombination frequency between the genes for body color and wing type?
  1. In guinea pigs, black (B) is dominant to brown (b), and solid color (S) is dominant to spotted (s). A heterozygous black, solid-colored pig is mated with a brown, spotted pig. The total offspring for several litters is:

16 Black sold5 Black spotted

5 Brown solid14 Brown spotted

  1. Calculate the recombination frequency for the cross.
  1. Are these genes linked or nonlinked?
  1. How do you know?
  1. The following recombination frequencies were found. Determine the order of these genes on the chromosome.

a—c 10%b—c 4%c—d 20%

a—d 30%b—d 16%

a—e 6%b—e 20%

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