Anatomy & Physiology

Anatomy & Physiology Study Guide affected by that gene. That allele will then indeed be expressed in your phenotype. For example, if you receive a gene for curly hair from your father and a gene for curly hair from your mother, you will be homozygous for curly hair—and you will have curly hair. About 80 percent of an individual’s genome consists of homozygous alleles. In simple inheritance, the phenotype is determined by interactions between a single pair of alleles. Because the chromosomes of a homologous pair have different origins, one paternal and the other maternal, they do not necessarily carry the same alleles. When you have two different alleles for the same gene, you are heterozygous for the trait determined by that gene. The phenotype that results from a heterozygous genotype depends on the nature of the interaction between the corresponding alleles. For example, if you received a gene for curly hair from your father, but a gene for straight hair from your mother, whether you will have curly hair, straight hair, or even wavy hair depends on the relationship between the alleles for those traits. In strict dominanc , an allele that is dominant will be expressed in the phenotype, regardless of any conflicting instructions carried by the other allele. For instance, an individual with only one allele for freckles will have freckles because that allele is dominant over the “non-freckle” allele. An allele that is recessive will be expressed in the phenotype only if that same allele is present on both chromosomes of a homologous pair. When an allele can be neatly characterized as dominant or recessive, you can predict the characteristics of individuals on the basis of their parents’ alleles. A simple box diagram, known as a Punnett square , enables us to predict the probabilities that children will have particular characteristics by showing the various combinations of parental alleles they can inherit. However, many phenotypic characters are determined by interactions among several genes. Such interactions constitute polygenic inheritance . The risks of developing several important adult disorders, including hypertension and coronary artery disease, are linked to polygenic inheritance. Genetic Recombination During meiosis, various changes can occur in chromosome structure, producing gametes with chromosomes that differ from those of each parent. This phenomenon, called genetic recombination, greatly increases the range of possible variation among gametes. Thus, among members of successive generations, there would be even greater range of variation since genotypes are formed by the combination of gametes in fertilization. Genetic recombination can also complicate the tracing of the inheritance of genetic disorders. Mutatio Variations at the level of the individual gene can result from mutations—changes in the nucleotide sequence of an allele. Random errors in DNA replication are called spontaneous mutations. Such errors are relatively common, but in most cases the error is detected and repaired by enzymes in the nucleus. Those errors that go undetected and unrepaired have the potential to change the phenotype in some way. The vast majority of mutations make the zygote incapable of completing normal development. Mutation, rather than chromosomal abnormalities, is probably the primary cause of the high mortality rate among pre-embryos and embryos. Roughly 50 percent of all zygotes fail to complete cleavage, and another 10 percent fail to reach the fifth month of gestation. Achieve Page 359 of 368 ©2018