Chapter 11: The Basic Principles of Heredity

Mendel’s Principles of Inheritance

  • Genes are in chromosomes, and the site a gene occupies in the chromosome is its locus.
    • Different forms of a particular gene are alleles; they occupy corresponding loci on homologous chromosomes.
  • An individual that carries two identical alleles is homozygous for that locus.
    • If the two alleles are different, that individual is heterozygous for that locus.
  • One allele, the dominant allele, may mask the expression of the other allele, the recessive allele, in a heterozygous individual.
    • For this reason two individuals with the same appearance, or phenotype, may differ from each other in their genetic makeup (i.e., their combination of alleles), or genotype.
  • According to Mendel’s principle of segregation, during meiosis the alleles for each locus separate, or segregate, from each other.
    • When haploid gametes are formed, each contains only one allele for each locus.
  • According to Mendel’s principle of independent assortment, alleles of different loci are distributed randomly into gametes.
    • The result can be genetic recombination, the production of new allele combinations that were not present in the parental (P) generation.
  • A cross between homozygous parents (P generation) that differ from each other with respect to their alleles at one locus is called a monohybrid cross; if they differ at two loci, it is called a dihybrid cross.
    • The first generation of offspring, called the F1 generation, is heterozygous; the generation produced by a cross of two F1 individuals is the F2 generation.
  • A test cross between an individual of unknown genotype and a recessive individual helps determine the unknown genotype.
  • Segregation of alleles is a direct result of homologous chromosomes separating during meiosis.
  • Independent assortment occurs because there are two ways in which two pairs of homologous chromosomes can be arranged at metaphase I of meiosis.
    • The orientation of homologous chromosomes on the metaphase plate determines the way chromosomes are distributed into haploid cells.

Using Probability to Predict Mendelian Inheritance

  • According to the product rule, the probability of two independent events occurring together can be calculated by multiplying the probabilities of each event occurring separately.
  • According to the sum rule, the probability of an outcome that can be obtained in more than one way can be calculated by adding the separate probabilities.
  • Naturally, if there is more than one way to get a result, the chances of its being obtained improve; we therefore combine the probabilities of mutually exclusive events by summing (adding) their individual probabilities.

Inheritance and Chromosomes

  • Linkage is the tendency for a group of genes on the same chromosome to be inherited together.
    • Independent assortment does not apply if two loci are linked close together on the same pair of homologous chromosomes.
  • Recombination of linked genes can result from crossing-over (breaking and rejoining of homologous chromatids) in meiotic prophase I.
    • (Recall from Section 11.1 that recombination can also result from an independent assortment of unlinked genes.)
  • To distinguish between independent assortment of unlinked genes and linked genes, perform a two-point test cross between an individual that is heterozygous at both loci and an individual that is homozygous recessive for both.
  • Linkage is recognized when an excess of parental-type offspring and a deficiency of recombinant-type offspring are produced in a two-point test cross.
  • The sex of humans and other mammals is determined by the X and Y chromosomes.
    • Normal female mammals have two X chromosomes; normal males have one X and one Y.
    • The fertilization of an X-bearing egg by an X-bearing sperm results in a female (XX) zygote.
    • The fertilization of an X-bearing egg by a Y-bearing sperm results in a male (XY) zygote.
  • The Y chromosome determines male sex in mammals.
    • The X chromosome contains many important genes unrelated to sex determination that are required by both males and females.
    • A male receives all his X-linked genes from his mother.
    • A female receives X-linked genes from both parents.

Extensions of Mendelian Genetics

  • Pleiotropy is the ability of one gene to have several effects on different characters.
    • Most cases of pleiotropy can be traced to a single cause, such as a defective enzyme.
    • Alternatively, alleles of many loci may interact to affect the phenotypic expression of a single character.
  • Dominance does not always apply; some alleles demonstrate incomplete dominance, in which the heterozygote is intermediate in phenotype, or codominance, in which the heterozygote simultaneously expresses the phenotypes of both homozygotes.
  • Multiple alleles, three or more alleles that can potentially occupy a particular locus, may exist in a population.
    • A diploid individual has any two of the alleles; a haploid individual or gamete has only one.
  • In epistasis an allele of one locus can mask the expression of alleles of a different locus.
  • In polygenic inheritance multiple independent pairs of genes may have similar and additive effects on the phenotype.
  • The range of phenotypic possibilities that can develop from a single genotype under different environmental conditions is known as the norm of reaction.
  • Many genes are involved in the inheritance of height in humans.
    • Also, height is modified by a variety of environmental conditions, such as diet and general health.
    • The genes that affect height set the norm of reaction—that is, the limits—for the phenotype, and the environment molds the phenotype within its norm of reaction.