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CHAPTER 23: PATTERNS OF INHERITANCE LECTURE OUTLINE 23.1 Mendel’s Laws Gregor Mendel developed certain laws of heredity after doing crosses between garden pea plants. Gregor Mendel Mendel kept careful and complete records of his plant crosses and applied mathematics to the outcome. We now recognize that chromosomes come in pairs, called homologous pairs of chromosomes, which carry the genes for the same traits in the same order. Alternate forms of a gene for the same trait are called alleles. On the basis of his studies Mendel formulated the law of segregation. It states that each individual has two factors for each trait. These factors segregate during the formation of the gametes and that each gamete contains only one factor from each pair of factors. Fertilization gives each new individual two factors for each trait. The Inheritance of a Single Trait The phenotype of an individual refers to what the person actually looks like. The genotype refers to the alleles the chromosomes carry that are responsible for that trait. A capital letter indicates a dominant allele and a lowercase letter indicates a recessive allele. Dominant means that this allele will mask the expression of the recessive allele when they are together in the same organism. If the two members of the allelic pair are the same, the organism is said to be homozygous. If they are different, the organism is said to be heterozygous. Gamete Formation The gametes have only one allele for each trait in accordance with Mendel’s law of segregation. One-Trait Crosses A Punnett square can be used to keep track of all the alleles in a one-trait, or monohybrid, cross. One-Trait Crosses and Probability Another method of calculating the expected ratios uses the rules of probability or chance. The product rule of probability and the sum rule of probability are useful. The One-Trait Testcross A testcross is used to determine if an animal showing the dominant phenotype is homozygous or heterozygous. The Inheritance of Two Traits When the homologues separate during meiosis, each gamete receives one member of each pair of homologues. Independent Assortment Mendel’s second law is the law of independent assortment. It states that each pair of factors separates independently and that all possible combinations of factors can occur in the gametes. Two-Trait Crosses The genotype WwSs is called a dihybrid because the individual is heterozygous in two regards. A Punnett square represents all the possible sperm/egg combinations. Two-Trait Crosses and Probability It is also possible to use the product rule and the sum rule of probability to predict the results of a dihybrid cross. Pedigrees A pedigree is a chart of a family’s history with regard to a particular genetic trait. 23.2 Beyond Simple Inheritance Patterns There are many other types of inheritance patterns other than simple dominant or recessive inheritance. Incomplete Dominance and Codominance Incomplete dominance occurs when the heterozygote is intermediate between the two homozygotes. Codominance occurs when alleles are equally expressed in a heterozygote. Multiple Allele Inheritance Even though a gene may exist in several allelic forms, each person has only two of the possible alleles. ABO Blood Types Three alleles for the same gene control the inheritance of ABO blood types. Sex-Linked Inheritance Traits controlled by genes on the sex chromosomes X and Y are said to be sex-linked. The Y chromosome from the father often does not carry an allele for a trait found on the X chromosome. Sex-Linked Alleles When considering X-linked traits, the allele on the X chromosome is shown as a letter attached to the X chromosome. Polygenic Inheritance Polygenic inheritance occurs when a trait is governed by two or more sets of alleles. These often result in a continuous variation of phenotypes. Skin Color Skin color is an example of a polygenic trait that is likely controlled by many pairs of alleles. 23.3 Environmental Influences Environmental factors, such as nutrition or temperature, can also influence the expression of genetic traits. Investigators try to determine what percentage of various human traits is due to nature and what percentage is due to nurture. 23.4 Inheritance of Linked Genes All the alleles on one chromosome form a linkage group because they tend to be inherited together.