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The Chapter in Review
Mendel's Laws

In 1860, Gregor Mendel, an Austrian monk, developed two laws of heredity based on crosses utilizing the garden pea.

Law of Segregation

Mendel's law of segregation states the following:

  • Each individual has two factors for each trait.

  • The factors segregate (separate) during the formation of the gametes.

  • Each gamete contains only one factor from each pair of factors.

  • Fertilization gives each new individual two factors for each trait.

In the context of genetics today,

  • Gene pairs are on the chromosomes, one allele on each homologue.

  • Alternative forms of a gene are called alleles. An individual may have different alleles of a gene on each homologue.

  • Alleles are assigned uppercase letters if they are dominant, or lowercase letters if they are recessive. An individual's genotype may be homozygous dominant (AA), heterozygous (Aa), or homozygous recessive (aa).

  • Homologues separate during meiosis, and the gametes have only one allele for each trait—either an A or an a.

  • Fertilization gives each new individual two alleles for each trait.

Law of Independent Assortment

Mendel's law of independent assortment states the following:

  • Each pair of factors segregates (assorts) independently of the other pairs.

  • All possible combinations of factors can occur in the gametes.

In the context of genetics today,

  • Each pair of homologues separate independently of the other pairs.

  • All possible combinations of chromosomes and their alleles occur in the gametes.

  • Mendel's laws are consistent with the manner in which homologues and their alleles separate during meiosis.

Common Autosomal Genetic Crosses

A Punnett square allows all types of sperm to fertilize all types of eggs and gives these results:

Beyond Mendel's Laws

In some patterns of inheritance, the alleles are not just dominant or recessive.

Incomplete Dominance

In incomplete dominance, the heterozygote is intermediate between the two homozygotes. For example, the offspring of red and white four-o'clocks produce pink flowers. The red and white phenotypes reappear when pink four-o'clocks are crossed.

Multiple-Allele Traits

The multiple-allele inheritance pattern is exemplified in humans by blood type inheritance. Every individual has two out of three possible alleles: I A , I B , i. Both I A and I B are expressed; therefore, this is also a case of codominance.

Polygenic Inheritance

In polygenic inheritance, a trait is controlled by more than one set of alleles. The dominant alleles have an additive effect on the phenotype.

Environment and the Phenotype

Many phenotypes, especially those governed by polygenes, are modified by the environment.


In pleiotropy, one gene (consisting of two alleles) has multiple effects on the body. For example, all the disorders common to Marfan syndrome are due to a mutation that leads to a defect in the composition of connective tissue.

Sex-Linked Inheritance

Males produce two different types of gametes—those that contain an X and those that contain a Y. The contribution of the male determines the sex and gender of the new individual. XX = female, XY = male.

Certain alleles are carried on the X chromosome, but the Y has very few genes. Therefore, a male only needs to inherit one recessive allele on the X chromosome to have a recessive genetic disorder.

Common X-Linked Genetic Crosses
  • X B X b x X B Y All daughters will be normal, even though they have 50% chance of being carriers, but sons have a 50% chance of being color blind.

  • X B X B x X b Y All children are normal (daughters will be carriers).

Inheritance of Linked Genes

Alleles on the same chromosome are linked, and they are generally inherited together, but crossing-over can shuffle them. Crossing-over data can be used to determine the distance between gene loci and to construct a chromosome map, which shows the sequence of gene loci along the chromosome.

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Key Terms
Testing Yourself

Choose the best answer for each question.

  1. In Mendel's particulate theory of inheritance, the “particles” are now known to be

    1. chromosomes.

    2. genes.

    3. plants.

    4. pollen grains.

  2. The offspring ratio from a testcross (F1 x homozygous recessive) should be

    1. all dominant.

    2. ¾ dominant, ¼ recessive.

    3. ½ dominant, ½ recessive.

    4. all recessive.

  3. Which of the following is not a component of the law of segregation?

    1. Each gamete contains one factor from each pair of factors in the parent.

    2. Factors segregate during gamete formation.

    3. Following fertilization, the new individual carries two factors for each trait.

    4. Each individual has one factor for each trait.

  4. When using a Punnett square to predict offspring ratios, we assume that

    1. each gamete contains one allele of each gene.

    2. the gametes have all possible combinations of alleles.

    3. male and female gametes combine at random.

    4. All of these are correct.

  5. If you cross a black spaniel with a red spaniel and get a litter of 8 black and 1 red, what is the genotype of the black parent?

    1. BB

    2. Bb

    3. bb

    4. impossible to determine

  6. Cystic fibrosis (CF) is an autosomal recessive disorder in humans. If two unaffected individuals have a child with CF, what is the chance that their second child will have CF?

    1. 25%

    2. 50%

    3. 75%

    4. impossible to determine

    5. 100%

  7. When one physical trait is affected by two or more pairs of alleles, the condition is called

    1. incomplete dominance.

    2. codominance.

    3. homozygous dominant.

    4. multiple allele.

    5. polygenic inheritance.

  8. Fill in the blank spaces in the following Punnett square.

    For questions 9–11, consider that coat color and spotting pattern in cocker spaniels depend on two genes. Black (B) is dominant to red (b), and solid color (S) is dominant to spotted (s).

  9. What phenotypic ratio do you expect for a dihybrid cross?

    1. 1 black solid : 1 red solid : 1 black spotted : 1 red spotted

    2. 9 black solid : 3 red solid : 3 black spotted : 1 red spotted

    3. 1 black solid : 3 red solid : 3 black spotted : 9 red spotted

    4. all black solid

  10. If you cross a black spotted dog with a black solid dog and get a ratio of 3 black solid : 3 black spotted : 1 red solid : 1 red spotted, what is the genotype of black spotted parent?

    1. BBss

    2. BbSs

    3. Bbss

    4. bbSs

    5. bbss

  11. What is the genotype of the black solid parent in question 10?

    1. BBSS

    2. BbSS

    3. BbSs

    4. Bbss

    5. BbSs

    For questions 12–15, match the cross with the predicted phenotypic ratios in the key.


    1. 3:1

    2. 9:3:3:1

    3. 1:1

    4. 1:1:1:1

    5. none of these

  12. AaBb x aabb

  13. Aa x Aa

  14. AAbb x aaBB

  15. Aa x aa

  16. AaBb x aabb

  17. AABB x AaBb

  18. Page 160
  19. AaBb x AaBb

  20. When two monohybrid round squashes are crossed, the offspring ratio is ¼ flat: ½ oblong: ¼ round. Squash shape, therefore, is controlled by incomplete dominance. What offspring ratio would you expect from a cross between a plant with oblong fruit and one with round fruit?

    1. all oblong

    2. all round

    3. ¾ oblong: ¼ round

    4. ¾ round: ¼ oblong

    5. ½ oblong: ½ round

  21. If a man of blood group AB marries a woman of blood group B whose father was type O, what phenotypes could their children be?

    1. A only

    2. A, AB, B, and O

    3. AB only

    4. A, AB, and B

    5. O only

  22. Anemia sometimes results from a mutation in a single gene, causing the blood's oxygen supply to be inadequate. A homozygous recessive person has a number of problems, including lack of energy, fatigue, rapid pulse, pounding heart, and swollen ankles. This is an example of

    1. pleiotropy.

    2. sex-linked inheritance.

    3. incomplete dominance.

    4. polygenic inheritance.

    5. codominance.

    For questions 22–25, list the progeny phenotypes from the following key that would result from each of the crosses in Drosophila. Red eye color is dominant over white. The gene for eye color is on the X chromosome. Answers can be used more than once.


    1. red-eyed female

    2. red-eyed male

    3. white-eyed female

    4. white-eyed male

  23. homozygous red-eyed female x white-eyed male

  24. heterozygous female x white-eyed male

  25. white-eyed female x white-eyed male

  26. heterozygous female x red-eyed male

  27. Alice and Henry are at the opposite extremes for height, a polygenic trait. Their children will

    1. exhibit the middle height between their two parents.

    2. exhibit a 9:3:3:1 ratio of heights between that of their parents.

    3. exhibit a 3:1 ratio of tall to short heights.

    4. be the same height as Alice or Henry.

  28. Mary's son is color-blind, but Mary is not color-blind. Which of these is a more likely genotype for a relative?

    1. mother XcXc

    2. father XcY

    3. brother XcXc

    4. Both b and c are likely.

Thinking Scientifically
  1. Multiple gene pairs may also interact to produce a single phenotype. In peas, genes C and P are required for pigment production in flowers. Gene C codes for an enzyme that converts a compound into a colorless intermediate product. Gene P codes for an enzyme that converts the colorless intermediate product into anthocyanin, a purple pigment. A flower, therefore, will be purple only if it contains at least one dominant allele for each of the two genes (C__ P__). What phenotypic ratio would you expect in the F2 generation following a cross between two double heterozygotes (CcPp)?

  2. Geneticists often look for unusual events to provide insight into genetic mechanisms. In one such instance, researchers studied XX men and XY women. They found that the XX men contained a chromosomal segment normally found in men but not in women, while XY women were missing that region. What gene do you suppose is on that chromosome piece?

  3. What would cause a particular polygenic trait to become more frequent in a population over time?

Bioethical Issue
Selecting the Sex of Your Child

As you know, the sex of a child depends upon whether an X-bearing sperm or a Y-bearing sperm enters the egg. But a new technology that can separate X-bearing and Y-bearing sperm offers prospective parents the opportunity to choose the sex of their child. First, the sperm are dosed with a DNA-staining chemical. Because the X chromosome has slightly more DNA than the Y chromosome, it takes up more dye. When a laser beam shines on the sperm, the X-bearing sperm shine more brightly than the Y-bearing sperm. A machine sorts the sperm into two groups on this basis. The results are not perfect. Following artificial insemination, there's about an 85% success rate for a girl and about a 65% success rate for a boy. But is it morally acceptable to select the sex of your child?

Those who find this practice unethical contend that using such technology is akin to “playing God.” They are greatly concerned that this new ability may lead to a society with far more members of one sex than another, which could lead to serious problems. Furthermore, they contend that allowing parents to select the sex of their children could lead to other ethical concerns, such as selecting for specific traits in children.

Proponents of sex-selection technology argue that there are many instances in which the ability to choose the sex of the child may benefit society. For instance, if a mother is a carrier of an X-linked genetic disorder, such as hemophilia or Duchenne muscular dystrophy, this would be the simplest way to ensure a healthy child. Previously, a pregnant woman with these concerns had to wait for the results of an amniocentesis test and then decide whether to abort the pregnancy if it were a boy. In such cases, is it better for all involved to ensure that a child does not have a specific genetic disorder than to take the risk?

Regardless of which side of this issue you fall on, most agree that once you separate reproduction from the sex act, many ethical issues soon follow.

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