11.1 The Work of Gregor Mendel

An individual's characteristics are determined by factors that are passed from one parental generation to the next.

During gamete formation, the alleles for each gene segregate from each other so that each gamete carries only one allele for each gene.

• genetics (308)

• allele (310)

• fertilization (309)

• principle of dominance (309)

• trait (310)

• hybrid (309)

• segregation (312)

• gene (310)

• gamete (312)

11.2 Applying Mendel's Principles

Punnett squares use mathematical probability to help predict the genotype and phenotype combinations in genetic crosses.

The principle of independent assortment states that genes for different traits can segregate independently during the formation of gametes.

Mendel's principles of heredity, observed through patterns of inheritance, form the basis of modern genetics.

• probability (313)

• homozygous (314)

• heterozygous (314)

• phenotype (315)

• genotype (315)

• Punnett square (315)

• independent assortment (317)

11.3 Other Patterns of Inheritance

Some alleles are neither dominant nor recessive. Many genes exist in several different forms and are therefore said to have multiple alleles. Many traits are produced by the interaction of several genes.

Environmental conditions can affect gene expression and influence genetically determined traits.

• incomplete dominance (319)

• multiple allele (320)

• polygenic trait (320)

• codominance (319)

11.4 Meiosis

The diploid cells of most adult organisms contain two complete sets of inherited chromosomes and two complete sets of genes.

In prophase I, replicated chromosomes pair with corresponding homologous chromosomes. At metaphase I, paired chromosomes line up across the center of the cell. In anaphase I, chromosome pairs move toward opposite ends of the cell. In telophase I, a nuclear membrane forms around each cluster of chromosomes. Cytokinesis then forms two new cells. As the cells enter prophase II, their chromosomes become visible. The final four phases of meiosis II result in four haploid daughter cells.

In mitosis, when the two sets of genetic material separate, each daughter cell receives one complete set of chromosomes. In meiosis, homologous chromosomes line up and then move to separate daughter cells. Mitosis does not normally change the chromosome number of the original cell. Meiosis reduces the chromosome number by half. Mitosis results in the production of two genetically identical diploid cells, whereas meiosis produces four genetically different haploid cells.

Alleles of different genes tend to be inherited together from one generation from the next when those genes are located on the same chromosome.

• homologous (323)

• tetrad (324)

• diploid (323)

• crossing-over (324)

• haploid (323)

• zygote (325)

• meiosis (324)

Think Visually Use the following terms to create a concept map: alleles, genes, chromosomes, dominant, traits, recessive.