CHAPTER SUMMARY
16.1 THE EARLIEST THEORIES OF HEREDITY INCORRECTLY ASSUMED THE INHERITANCE OF ACQUIRED CHARACTERISTICS AND BLENDING OF PARENTAL TRAITS IN THE OFFSPRING.
- The inheritance of acquired characteristics suggests that traits that develop during the lifetime of an individual can be passed on to offspring. With rare exceptions, this mode of inheritance does not occur.
- Blending inheritance is the incorrect hypothesis that characteristics in the parents are averaged in the offspring. This model predicts the blending of genetic material, which does not occur. Different forms of a gene maintain their separate identities even when present together in the same individual.
16.2 MODERN TRANSMISSION GENETICS BEGAN WITH GREGOR MENDEL, WHO USED THE GARDEN PEA AS HIS EXPERIMENTAL ORGANISM AND STUDIED TRAITS WITH CONTRASTING CHARACTERISTICS.
- Mendel started his experiments with true-breeding plants, ones whose progeny are identical to their parents. He followed just one or two traits at a time, allowing him to discern simple patterns, and he counted all the progeny of his crosses.
- In crosses of one true-breeding plant with a particular trait and another true-breeding plant with a contrasting trait, just one of the two characteristics appeared in the offspring. The trait that appeared in this generation is dominant, and the trait that is not seen is recessive.
- Mendel explained this result by hypothesizing that there is a hereditary factor for each trait (now called a gene), that each pea plant carries two copies of the gene for each trait, and that one of two different forms of the gene (alleles) is dominant to the other one.
16.3 MENDEL’S FIRST KEY DISCOVERY WAS THE PRINCIPLE OF SEGREGATION, WHICH STATES THAT MEMBERS OF A GENE PAIR SEPARATE EQUALLY INTO GAMETES.
- When Mendel allowed the progeny of the first cross to self-fertilize, he observed a 3:1 ratio of the dominant and recessive traits among the progeny.
- Mendel reasoned that the parent in this generation must have two different forms of the same gene (A and a) and that these alleles segregate from each other to form gametes with each gamete getting A or a but not both. When the gametes combine at random, they produce progeny in the genotypic ratio 1 AA:2 Aa:1 aa, which yields a phenotypic ratio of 3:1 because A is dominant to a. This idea became known as the principle of segregation.
- The principle of segregation reflects the separation of homologous chromosomes that occurs in anaphase I of meiosis.
- Some traits show incomplete dominance, in which the phenotype of the heterozygous genotype is intermediate between those of the two homozygous genotypes.
- The expected frequencies of progeny of crosses can be predicted using the addition rule, which states that when two possibilities are mutually exclusive, the probability of either event occurring is the sum of their individual probabilities, and the multiplication rule, which states that when two possibilities occur independently, the probability of both events occurring is the product of the probabilities of each of the two events.
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16.4 MENDEL’S SECOND KEY FINDING WAS THE PRINCIPLE OF INDEPENDENT ASSORTMENT, WHICH STATES THAT DIFFERENT GENE PAIRS SEGREGATE INDEPENDENTLY OF ONE ANOTHER.
- In a cross with two traits, each with two contrasting characteristics, the two traits behave independently of each other. This idea became known as the principle of independent assortment. For example, in a self-cross of a double heterozygote, the phenotypic ratio of the progeny is 9:3:3:1, reflecting the independent assortment of two 3:1 ratios.
- Independent assortment results from chromosome behavior during meiosis, specifically from the random orientation of different chromosomes on the meiotic spindle.
- In some cases, genes interact with each other, modifying the expected ratios in crosses. Epistasis is a form of gene interaction in which one gene affects the expression of another.
16.5 THE PATTERNS OF INHERITANCE THAT MENDEL OBSERVED IN PEAS CAN ALSO BE SEEN IN HUMANS.
- In a human pedigree, females are represented by circles and males as squares; affected individuals are shown as filled symbols and unaffected individuals as open symbols; and horizontal lines denote matings.
- Dominant traits appear in every generation and affect males and females equally.
- Recessive traits skip one or more generations and affect males and females equally. Affected individuals often result from matings between close relatives.
- Although a given individual has only two alleles of each gene, there can be many alleles of a particular gene in the population as a whole.
- Interpreting pedigrees can be difficult because of incomplete penetrance and variable expressivity. Penetrance is the percentage of individuals with a particular genotype who show the expected phenotype, and expressivity is the degree to which a genotype is exhibited.
- Genetic testing enables the genotype of an individual to be determined for one or more genes. Such tests carry both benefits and risks.
Self-Assessment Question 1
Describe two ways in which Mendel’s experimental approach differed from others of the time.
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Model Answer:
Mendel’s experimental approach differed from others of the time because he focused on easily classified traits with contrasting characteristics (green seeds vs. yellow seeds) that were true breeding (i.e., plants with yellow seeds gave rise to offspring with yellow seeds). Mendel also counted the progeny of his crosses and looked for statistical patterning.
Self-Assessment Question 2
Distinguish among gene, allele, genotype, and phenotype.
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Model Answer:
A gene is a unit of heredity. Alleles are the different forms of the gene. A genotype is the particular combination of alleles present in an individual and the phenotype is the expression of that trait in an individual. For example, in Mendel’s experiments he looked at the gene for seed color. This gene had two alleles, yellow (dominant) and green (recessive). The genotype of a seed could be AA or Aa and the resulting phenotype would be yellow seeds. The genotype of green seeds (phenotype) was aa.
Self-Assessment Question 3
Name and describe Mendel’s two laws.
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Model Answer:
Mendel’s two laws are: (1) the principle of segregation states that alleles will separate equally into different gametes, and (2) the principle of independent assortment, which states that different gene pairs segregate independently of one another.
Self-Assessment Question 4
Explain the relationship between Mendel’s two laws and the mechanics of meiosis.
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Model Answer:
The segregation of alleles reflects the separation of chromosomes in meiosis. Homologous chromosomes separate during anaphase I of meiosis I, leading to the segregation of the alleles. Chromosomes display independent assortment during meiosis when they are sorted into daughter cells randomly.
Self-Assessment Question 5
Explain how you can predict the genotypes and phenotypes of offspring if you know the genotypes of the parents.
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Model Answer:
By knowing the genotypes of the parents, the genotypes and phenotypes of the offspring can be predicted by using a Punnett square. A Punnett square predicts the probability of every possible combination of one maternal allele with one paternal allele for a particular cross. For example, if a homozygous dominant red parent (RR) was crossed with a homozygous recessive white parent (rr) the offspring predicted by the Punnett square would all have the genotype Rr and phenotype of red (R, red, is dominant over r, white).
Self-Assessment Question 6
Describe an instance when a testcross would be useful.
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Model Answer:
A testcross would be useful if you did not know which allele was dominant. For example, Mendel didn’t know if yellow or green seeds were dominant, but by doing a testcross he was able to figure out that the yellow seed trait was dominant over the green seed trait by looking at the progeny when he crossed the two parental plants.
Self-Assessment Question 7
Apply the addition and multiplication rules to determine the probability of the offspring of a given cross.
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Model Answer:
The addition rule applies when the possible outcomes being considered cannot occur simultaneously. The multiplication rule applies when outcomes can occur simultaneously and the occurrence of one has no effect upon the likelihood of the other. Using these two rules, the probability that a particular genotype will occur can be determined from a known parental cross.
Self-Assessment Question 8
Construct a human pedigree for a dominant and a recessive trait and explain the patterns of inheritance.
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Model Answer:
See examples below. The first pedigree displays a recessive trait. It is recessive because it does not occur in every generation, and two non-affected individuals had two affected children. The second pedigree displays a dominant trait. It is dominant because it appears in every generation.
Self-Assessment Question 9
Discuss the benefits and risks of genetic testing and personal genomics.
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Model Answer:
The benefits of genetic testing include screening for potentially fatal or damaging diseases. For example, there are certain mutations in the BRCA2 gene that puts an individual at an extremely high risk for breast cancer. If a patient is screened and found to have this mutation, preventative measures can be taken to hopefully avoid cancer. There are risks of genetic testing, however. Discrimination against individuals based on their genetic information is an issue, especially when it comes to health care. There are also direct-to-consumer companies that give you your genetic information without the consent of your doctor. This information could be misleading if you do not understand how the tests were performed and what the probabilities mean.