Learning Objectives
16.1 Mendel’s Laws of Inheritance
- Explain the advantages of Mendel’s experimental system.
- Evaluate the outcome of a monohybrid cross.
- Explain Mendel’s principle of segregation.
- Predict the outcome of genetic crosses by using Punnett squares.
- Interpret data from test crosses to infer genotypes.
- Evaluate the outcome of a dihybrid cross.
- Explain Mendel’s Principle of Independent Assortment.
16.2 The Chromosome Theory of Inheritance
- Outline the tenets of the chromosome theory of inheritance.
- Understand the physical basis of independent assortment.
- Compare the segregation of alleles with the behavior of homologues in meiosis.
16.3 Pedigree Analysis of Human traits
- Describe how a pedigree is used to analyze the transmission of an inherited trait over the courses of several generations.
- Determine whether a particular trait is caused by the presence of a dominant allele or a recessive allele based on pedigree analysis.
16.4 Sex Chromosomes and X-linked Inheritance Patterns
- Differentiate between the major categories of sex determination in animals.
- Describe sex-linked inheritance in fruit flies.
- Explain the evidence for genes being on chromosomes.
16.5 Variations in Inheritance Patterns and Their Molecular Basis
- Describe how assumptions in Mendel’s model result in oversimplification.
- Discuss a genetic explanation for continuous variation.
- Explain the genetic basis for observed alterations to Mendel’s ratios.
- Compare and contrast the molecular basis for variations in inheritance patterns, including relevant examples from the medical community.
16.6 Genetics and Probability
- Understand the rule of addition and the rule of multiplication.
- Apply the rules of probability to genetic crosses.
Chapter Summary
Early geneticists believed that genetic material from each parent blended in the offspring and that variability was not introduced from outside the species. Blending and lack of variability, though, should result in individuals that greatly resemble rather than differ from one another. This paradox was partly solved by early plant breeders who found that hybrids differed greatly from their parents and often from one another. They reported that certain physical traits disappeared for a generation and reappeared in the next. Gregor Mendel cross-bred seven well-documented varieties of a pea. Most importantly, he quantified his experiments, meticulously counted seeds of hundreds of crosses and grouped them by apparent physical traits. Mendelian genetics is derived from the mathematical ratios that describe the segregation and assortment of hereditary material.
Mendel’s model states that each parent transmits a set of information about its traits in its gametes. Therefore, each individual possesses two factors (genes) for each trait. Each factor exhibits many possible forms (alleles) that do not influence one another; each remains discrete within the cell. An individual may be homozygous and possess two identical alleles, or heterozygous and have two different alleles. The presence of a factor does not ensure its expression; dominant traits are expressed while recessive traits are generally not expressed. The existence of the recessive allele in a heterozygote causes that factor to be masked for a generation. Additionally, there is a difference between an individual’s phenotype, or overall appearance, and its genotype, its precise genetic blueprint. Mendel’s First Law of Heredity explains how alleles randomly segregate in the gametes, each gamete has an equal chance of receiving either allele. His second law explains that different alleles assort into gametes independently of one another, the presence of an allele of one trait does not preclude the presence or absence of any other allele of any other trait.
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