A. An Introduction to Heredity
1. Offspring acquire genes from parents by inheriting chromosomes.
a. Parents endow their offspring with coded information in the form of genes.
b. Genetic information is transmitted as specific sequences of the four nucleotides in DNA. It is the sequence of the nucleotides that is the code.
c. Most genes are codes for enzymes and other proteins that produce an organism�s inherited traits.
d. Each chromosome has hundreds or thousands of genes and each gene is at a specific location on a specific chromosome. The gene's location is called its locus.
e. The transmission of hereditary traits from parents to offspring requires the precise replication of DNA.
1. DNA replication produces copies of genes that can be passed from parents to offspring.
f. In plants and animals, sperm and ova (unfertilized eggs) transmit genes from one generation to the next.
g. After fertilization (fusion) of an ovum with a sperm cell, genes from both parents are present in the nucleus of the fertilized egg.
2. Comparison of asexual and sexual reproduction
a. In asexual reproduction, a single individual passes along copies of ALL its genes to its offspring.
1. Single-celled eukaryotes reproduce asexually by mitotic cell division to produce two identical daughter cells.
2. Even some multicellular eukaryotes, like hydra, can reproduce asexually by budding cells produced by mitosis. (Fig. 13.1)
b. Sexual reproduction results in greater variation among offspring than does asexual reproduction. (Fig. 13.2)
1. Two parents produce offspring that have unique combinations of genes inherited from the parents.
2. Offspring of sexual reproduction vary genetically from their siblings and from both parents.
B. The Role of Meiosis in Sexual Life Cycles
1. Fertilization and meiosis alternate in sexual life cycles.
a. In humans, each somatic cell (all cells other than sperm or ovum) has 46 chromosomes.
1. Each chromosome can be distinguished by its size, position of the centromere and by pattern of staining with certain dyes.
b. A karyotype display of the 46 chromosomes in humans shows 23 pairs of chromosomes each pair with the same length, centromere position, and staining pattern.
c. We have 2 copies of each of our genes, one copy on each chromosome of a pair. These pairs are called homologous chromosome pairs and carry genes that control the same inherited characters. (Fig. 13.3)
d. An exception to the rule of homologous chromosomes is found in the sex chromosomes, the X and the Y.
1. The pattern of inheritance of these chromosomes determines an individual�s sex.
2. Human females have a pair of X chromosomes (XX).
3. Human males have an X and a Y chromosome (XY).
4. Only small parts of these chromosomes have the same genes.
e. The other 22 pairs of chromosomes are called autosomes.
f. We inherit one chromosome of each homologous pair from each parent.
1. The 46 chromosomes in a somatic cell can be viewed as two sets of 23, a maternal set and a paternal set.
2. Sperm cells or ova (gametes) have only one set of chromosomes � 22 autosomes and an X or a Y.
g. A cell (sperm or egg) with a single chromosome set is haploid.
1. For humans, the haploid number of chromosomes is 23 (n = 23).
h. A life cycle is the sequence of stages in the reproductive history of an organism. (Fig. 13.4)
1. It starts at the conception of an organism and continues until it produces its own offspring.
i. A haploid sperm fertilizes a haploid ovum.
1. These cells fuse (syngamy) resulting in fertilization.
2. The fertilized egg (zygote) now has two haploid sets of chromosomes bearing genes from the maternal and paternal family lines.
3. The zygote divides by mitosis to become the embryo and the embryonic cells continue to divide to eventually produce the multicellular adult.
j. The zygote and all cells with two sets of chromosomes are diploid cells.
1. For humans, the diploid number of chromosomes is 46 (2n = 46).
k. As an organism develops from a zygote to a sexually mature adult, the zygote�s genes are passed on to all somatic cells by mitosis.
l. Gametes, which develop in the ovaries and testes, are not produced by mitosis.
1. If gametes were produced by mitosis, the fusion of gametes would produce offspring with four sets of chromosomes after one generation, eight after a second and so on.
2. Instead, gametes undergo the process of meiosis in which the chromosome number is halved.
3. Human sperm or ova have a haploid set of 23 different chromosomes, one from each homologous pair.
m. Fertilization restores the diploid condition by combining two haploid sets of chromosomes. (Fig. 13.4)
2. Meiosis reduces chromosome number from diploid to haploid.
a. Similar to mitosis, meiosis is preceded by the replication of chromosomes, producing sister chromatids.
b. However, in meiosis, there are two consecutive cell divisions, meiosis I and meiosis II, that result in four daughter cells.
1. Each final daughter cell has only half as many chromosomes as the parent cell.
c. Meiosis reduces chromosome number by copying the chromosomes once, but dividing twice. (Fig. 13.6)
1. During interphase of meiosis the chromosomes are replicated to form sister chromatids. These are genetically identical and joined at the centromere.
2. The first division, meiosis I, separates homologous chromosomes.
3. The second, meiosis II, separates sister chromatids.
d. Division in meiosis I occurs in four phases: prophase I, metaphase I, anaphase I, and telophase I.
1. Prophase I (Fig. 13.7)
a. The chromosomes condense and homologous chromosomes pair to form tetrads.
b. In a process called synapsis, special proteins attach homologous chromosomes tightly together.
c. At several sites the chromatids of homologous chromosomes are crossed (chiasmata) and segments of the chromosomes are traded between the maternal and paternal homologous chromosomes of a pair.
d. A spindle forms from each centrosome and spindle fibers attached to kinetochores on the chromosomes begin to move the tetrads around.
2. Metaphase I
a. The tetrads are all arranged at the metaphase plate.
b. Microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad, while those from the other pole are attached to the other.
3. In anaphase I, the homologous chromosomes separate and are pulled toward opposite poles.
4. Telophase I
a. Movement of homologous chromosomes continues until there is a haploid set at each pole.
b. Each chromosome consists of linked sister chromatids.
e. Cytokinesis by the same mechanisms as mitosis usually occurs simultaneously.
f. In some species, nuclei may reform, but there is no further replication of chromosomes.
g. Meiosis II is very similar to mitosis.
1. Prophase II
a. A spindle apparatus forms, attaches to kinetochores of each sister chromatid, and moves them around.
b. Spindle fibers from one pole attach to the kinetochore of one sister chromatid and those of the other pole to the other sister chromatid.
2. Metaphase II
a. The sister chromatids are arranged at the metaphase plate.
b. The kinetochores of sister chromatids face opposite poles.
3. At anaphase II, the centomeres of sister chromatids separate and the now separate sisters travel toward opposite poles.
4. Telophase II
a. Separated sister chromatids arrive at opposite poles.
b. Nuclei form around the chromatids.
h. Cytokinesis separates the cytoplasm.
i. At the end of meiosis, there are four haploid daughter cells. (Activity 13B pages 1-10 = stages of meiosis)
j. Mitosis and meiosis have several key differences. (Fig. 13.8)
1. The chromosome number is reduced by half in meiosis, but not in mitosis.
2. Mitosis produces daughter cells that are genetically identical to the parent and to each other. Meiosis produces cells that differ from the parent and each other.
3. During prophase I, homologous chromosomes pair in a process called synapsis.
a. A protein zipper, the synaptonemal complex, holds homologous chromosomes together tightly.
b. At X-shaped regions called chiasmata, sections of nonsister chromatids are exchanged.
4. At metaphase I of meiosis homologous pairs of chromosomes, not individual chromosomes are aligned along the metaphase plate.
a. In humans, you would see 23 tetrads.
5. At anaphase I of meiosis, it is homologous chromosomes, not sister chromatids, that separate and are carried to opposite poles of the cell.
a. Sister chromatids remain attached at the centromere until anaphase II.
6. Mitosis produces two identical daughter cells, but meiosis produces 4 very different cells.
C. Origins of Genetic Variation
1. Sexual life cycles produce genetic variation among offspring.
a. The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises each generation during sexual reproduction.
b. Three mechanisms contribute to genetic variation.
1. Independent assortment of chromosomes contributes to genetic variability due to the random orientation of tetrads at the metaphase plate. (Fig. 13.9)
a. There is a fifty-fifty chance that a particular daughter cell of meiosis I will get the maternal chromosome of a certain homologous pair and a fifty-fifty chance that it will receive the paternal chromosome.
b. Each homologous pair of chromosomes is positioned independently of the other pairs at metaphase I.
c. Therefore, the first meiotic division results in independent assortment of maternal and paternal chromosomes into daughter cells.
d. The number of combinations possible when chromosomes assort independently into gametes is 2n, where n is the haploid number of the organism.
1. For a human with n = 23, there are 223 or about 8 million possible combinations of chromosomes.
2. Crossing over produces recombinant chromosomes, which combine genes inherited from different parents into the same chromosome. (Fig. 13.10)
a. For humans, this occurs two to three times per chromosome pair.
b. One sister chromatid may undergo different patterns of crossing over than its match.
c. Independent assortment of these nonidentical sister chromatids during meiosis II increases still more the number of genetic types of gametes that can result from meiosis.
3. The random nature of fertilization adds to the genetic variation arising from meiosis.
a. Any sperm can fuse with any egg.
b. An ovum is one of approximately 8 million possible chromosome combinations (actually 223).
c. The successful sperm represents one of 8 million different possibilities (actually 223).
d. The resulting zygote is composed of 1 in 70 trillion (223 x 223) possible combinations of chromosomes.
e. Crossing over adds even more variation to this.