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Chapter 10: Sexual Reproduction and Genetics

Chapter Worksheet

Ch. 10.1 Meiosis

Meiosis produces haploid gametes.

Chromosomes and Chromosome Number

Each characteristic such as eye color is called a trait. The instructions for traits are in the nucleus on chromosomes. Genes- segments of DNA on chromosome that control the production of proteins. Each gene determines the characteristics and functions of cells.

Homologous Chromosomes

Human body cells have 23 pairs of chromosomes (1 from each parent) for a total of 46. Homologous chromosomes- chromosome that make up a pair, one from each parent. Have the same: length, genes for traits, and same centromere position. Although they have genes for the same characteristics, each chromosome may have a different version.

Haploid and Diploid Cells

Gametes- sex cells that have half the number of chromosomes. Number of chromosomes varies among species, in humans each gamete contains 23. (n) is symbol to represent the number of chromosomes in a gamete. Haploid- a cell with n chromosomes.

Fertilization- process which one haploid gamete combines with another. The cell produced is 2n (diploid) and contains a set of chromosomes from each parents.

Meiosis I

Process that forms gametes. Meiosis- a type of cell division that reduces the number of chromosomes. Occurs in reproductive structures of organisms that reproduce sexually. Meiosis reduces the number of chromosomes by half through separation of homologous chromosomes. Includes two cell divisions; meiosis I and meiosis II.

Interphase- Carry out metabolic processes (make proteins, replicate DNA).

Prophase I- Replicated chromosomes consist of two sister chromatids. Synapse; homologous chromosomes pair up as they condense. Crossing over- chromosomal segments are exchanged between a pair of homologous chromosomes. Centrioles migrate, spindle fibers form and bind at centromere.

Metaphase I- Homologous chromosomes line up as pairs at the cell's equator. Spindle fibers attach to centromere.

Anaphase I- Homologous chromosomes separate, each consisting of both chromatids. Chromosome number is reduced from 2n to n.

Telophase I- Homologous chromosomes with both chromatids reach separate poles of cell. May be different after synapse. Cytokinesis occurs next and cells enter interphase before Meiosis II.

Meiosis II

Prophase II- Spindle apparatus forms, chromosomes condense.

Metaphase II- Chromosomes line up at equator of cell.

Anaphase II- Sister chromatids are pulled apart at centromere and move towards poles of the cell.

Telophase II- Chromosomes reach poles of cell, nuclei reform. At end, cytokinesis occurs to form four haploid cells.

The Importance of Meiosis

Consists of two sets of divisions and produces four haploid daughter cells that are not identical. Results in genetic variation.

Meiosis Provides Variation

How the chromosomes line up at the equator is a random process that results in gametes with different combinations of chromosomes. Four gametes with four different combinations of chromosomes can result.

Genetic variation also produced during crossing over and during fertilization.

Sexual Reproduction v. Asexual Reproduction

Asexual reproduction involves a single parent. Offspring is genetically identical to parent. Bacteria reproduce asexually. Most protists reproduce asexually and sexually. Most plants and of the more simple animals reproduce asexually and sexually. More advanced animals reproduce only sexually.

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Ch. 10.2 Mendelian Genetics

Mendel explained how a dominant allele can mask the presence of a recessive allele.

How Genetics Began

In 1866, Gregor Mendel, an Austrian monk published his findings on the method of inheritance in garden pea plants. Inheritance (heredity)- the passing of traits to the next generation. Pea plants are true-breeding; pass on only one form of a trait.

Pea plants self-fertilize, but can be cross-pollinated by hand. Mendel experimented with many cross-pollinations. Genetics- the science of heredity. Mendel is regarded as the father of genetics.

The Inheritance of Traits

Mendel noticed that some pea plants produced specific forms of a trait, generation after generation.

Mendel performed cross-pollination from a true-breeding green-seed plant to a true-breeding yellow-seed plant. To prevent self-fertilization Mendel removed the male organs from the flower of the yellow-seed plant. Mendel called them the parent generation or P generation.

F1 and F2 Generation

When the yellow and green plants were crossed, all of the offspring had yellow seeds. These offspring are called first filial (F1) generation. Mendel planted the seeds and allowed them to self-fertilize. They produced a 3:1 ratio of yellow to green seeds.

Mendel studied 7 traits; F2 generations all showed a 3:1 ratio.

Genes in Pairs

Mendel concluded there must be two forms for each trait in the pea plants (yellow seed & green seed). Each trait is controlled by a factor called allele. Allele- an alternative form of a gene. F1 generation was dominant generation. Dominant- the allele expressed over another. Recessive- the trait that is masked.

Dominance

In F1 self-fertilization the recessive trait didn't disappear, it was just masked by the dominant trait. In F2 the recessive trait returned.

Dominant allele is represented by a capital letter, and the recessive allele is represented by a lowercase letter. Homozygous- two of the same alleles for a trait; YY or yy. Heterozygous- two different alleles for a trait; Yy. In heterozygous organisms the dominant trait is expressed.

Genotype and Phenotype

The appearance of an organism does not always indicate which pair of alleles is present. Genotype- the organism's allele pairs; YY, Yy, yy. Phenotype- the observable appearance of an allele pair; yellow or green.

Mendel's Law of Segregation

States: The two alleles for each trait from a parent separate during meiosis. During fertilization both parent alleles unite for a trait. With YY x yy, F1 were Yy; these are hybrids.

Monohybrid Cross

A cross between hybrids that involves only one trait (Yy x Yy). The dominant allele is written first. When crossed, three genotypes are possible YY, Yy, yy with a genotypic ratio of 1:2:1 and a phenotypic ratio of 3:1.

Dihybrid Cross

After establishing inheritance patterns of single trait crosses Mendel began crosses for two or more traits. P generation of YYRR x yyrr produces F1 generation with genotype YyRr; phenotype Yellow Round. These offspring are dihybrids since they are heterozygous for two traits.

Law of Independent Assortment

Mendel self-fertalized the F1 generation and calculated the genotypic and phenotypic ratios of the offspring. Law of independent assortment- states that a random distribution of alleles occurs during gamete formation. In other words, genes on separate chromosomes sort independently during meiosis. Four possible gene combinations in gametes that can form four phenotypes at a 9:3:3:1 ratio.

Punnett Squares

Dr. Reginald Punnett developed a tool to predict the possible offspring of a cross between known genotypes; the punnett square.

Punnett Square- Monohybrid Cross

Monohybrid has four squares. Gametes of a parent goes on top, while other parent gametes go on the side. In the squares are the possible combinations when combining the gametes.

Punnett Square- Dihybrid Cross

With dihybrid parent four types of allele gametes are possible. The punnett square has 16 squares and the four allele combinations of one parent is on top, the other on the side. Producing a 9:3:3:1 phenotypic ratio.

Probability

Probability of heads when flipping a coin is 1 out of 2 (1/2). Actual results may not match probability but the higher the sample, the closer the results will be to the probability. The same occurred with Mendel's results when compared to the Punnett square.

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Ch. 10.3 Gene Linkage and Polyploidy

Genetic Recombination

Genetic Recombination- the new combination of genes produced by crossing over and independent assortment. Possible gene combinations can be calculated by the 2 to the n power. n is the number of chromosome pairs. Pea plants are 2 to the 7th power (128 combinations) for each parent. After fertilization is is 16,384. Humans are 2 to the 23rd, over 70 trillion possible combinations. Not including crossing over.

Gene Linkage

Genes that are located close to each other on the same chromosome are linked and usually travel together during gamete formation. Linked genes are exception to Mendel's law of independent assortment because they do not segregate independently.

The fruit fly Drosophila melanogaster was used in thousands of crosses confirming linked genes usually travel together during meiosis. Crossing over can separate linked genes.

Chromosome Maps

Crossing over occurs more frequently between genes that are far apart than those that are close together. A chromosome map shows the sequence of genes on a chromosome and can be created by using crossover data. The higher the crossover frequency, the farther apart the two genes are.

Polyploidy

Most species have diploid cells, but some have polyploid cells. Polyploidy- the occurrence of one or more extra sets of all chromosomes in an organism (example 3n). Rare in animals, lethal in humans.

One in three species of known flowering plants are polyploid. Polyploid plants often have increased vigor and size.

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