Chapter 10 Sexual Reproduction and Genetics 10.1 Meiosis

Chromosomes and Chromosome Number Human body cells have 46 chromosomes DNA on the chromosomes are arranged in sections that code for a trait; these sections are genes Humans have approximately 23,000 genes

Chromosomes and Chromosome Number Of the 46 human chromosomes each parent contributes 23 chromosomes These chromosomes are paired Homologous chromosomes—one of two paired chromosomes, one from each parent

Homologous Chromosomes Same centromere position Same length Homologous chromosomes contain the same genes; although they may contain different versions of the gene

Chromosomes and Chromosome Number Haploid cells contain only one of the homologous pair of chromosomes (half the number of chromosomes) Diploid cell contain the chromosomes in pairs (di=two)

Chromosomes and Chromosome Number Gametes (sex cells) contain the haploid number of chromosomes (n) Body cells contain the diploid number of chromosomes (2n) Human gametes (sperm and egg) have 23 chromosomes and body cells have 46 chromosomes

Meiosis Gametes are formed during the process of meiosis Meiosis reduces diploid cells to haploid cells Fertilization restores the diploid number

Meiosis Meiosis involves two consecutive sets of cell divisions Meiosis only occurs in the reproductive structures of organisms who reproduce sexually: Most animals Most plants Most fungi Most protists

Meiosis I and Meiosis II Meiosis I is the reduction division; cells start out diploid and end up haploid In Meiosis II sister chromatids are separated (much like mitosis)

Meiosis I Interphase Chromosomes replicate. Chromatin condenses.

Meiosis I Prophase I Pairing of homologous chromosomes occurs. Each chromosome consists of two chromatids. Prophase I The nuclear envelope breaks down. Spindles form.

Meiosis I Prophase I Crossing over produces exchange of genetic information. Crossing over—chromosomal segments are exchanged between a pair of homologous chromosomes. Tetrads are groups of four sister chromatids

Meiosis I Metaphase I Chromosome’s centromere attach to spindle fibers. Metaphase I Homologous chromosomes line up at the equator in tetrads

Meiosis I Anaphase I Homologous chromosomes separate and move to opposite poles of the cell. Anaphase I

Meiosis I Telophase I The spindles break down. Chromosomes uncoil and form two nuclei. The cell divides.

Meiosis II Prophase II A second set of phases begins as the spindle apparatus forms and the chromosomes condense. Prophase II

Meiosis II Metaphase II A haploid number of chromosomes line up at the equator. Metaphase II

Meiosis II Anaphase II The sister chromatids are pulled apart at the centromere by spindle fibers and move toward the opposite poles of the cell. Anaphase II

Meiosis II Telophase II The chromosomes reach the poles, and the nuclear membrane and nuclei reform. Telophase II

Meiosis II Cytokinesis results in four haploid cells, each with n number of chromosomes. Cytokinesis

Meiosis Meiosis consists of two sets of divisions Produces four haploid daughter cells that are not identical Results in genetic variation

Meiosis Depending on how the chromosomes line up at the equator, four gametes with four different combinations of chromosomes can result. Genetic variation also is produced during crossing over and during fertilization, when gametes randomly combine.

Meiosis and Variation Number of possible genetic variations in the gametes equals: 2n where n is the haploid number In humans number of possible genetic combinations in gametes is 223 Add the genetic combinations that exist when crossing over exists (at 3 per meiosis) and get (223)3

Meiosis and Variation, cont The possibility that (223)3 variations exists for each gamete When fertilization occurs this number must be doubled 2 x (223)3 You are unique; no one else exists or ever has existed that is just like you (unless you have an identical twin).

Advantages of Asexual Reproduction The organism inherits all of its chromosomes from a single parent. The new individual is genetically identical to its parent. Usually occurs more rapidly than sexual reproduction

Advantages of Sexual Reproduction Beneficial genes multiply faster over time. The organisms inherits genes from two parents and is not genetically identical to either parent. Ensures genetic variation

Chapter 10 Sexual Reproduction and Genetics 10.2 Mendelian Genetics

Gregor Mendel Lived in Europe in what is now Czech Republic near the Austrian border. Father of Genetics Monk, entered monastery 1843

Gregor Mendel Failed teacher’s exam When to U of Vienna Studied with physicist Doppler- science through experiment, applied math to science Studied with botanist Unger- interest in causes of variation in plants Passed teacher’s exam and taught at monastery’s school; also responsible for school’s garden Published 1866, mathematics and plant breeding

Mendel Studied Peas Available in many varieties Self pollinating (can manipulate pollination) “either-or” inherited traits Had “true breeder” for parental generation (P) due to flower structure

Mendel Studied Peas The petals enclose the stamen (with pollen) so that cross pollination does not occur Cross pollination is easily accomplished by peeling back the petals and moving pollen with a paint brush

Inheritance of Traits

Inheritance of Traits The offspring of this P cross are called the first filial (F1) generation. The second filial (F2) generation is the offspring from the F1 cross.

Pea Traits Studied by Mendel

Inheritance of Traits Mendel studied thousand of pea plants for the seven traits. He concluded that: Genes are in pairs Different versions of genes (alleles) account for variation in inherited characteristics Alleles can be dominant or recessive

Dominant and Recessive Alleles can be dominant or recessive. An allele is dominant if it appears in the F1 generation when true breeder parents are crossed. An allele is recessive if it is masked in the F1 generation.

Symbols To help make genetics easier symbols are used Capital letters are used for dominant alleles Lower case letters are used for recessive alleles The letter to use is based on the dominant trait Example: purple is dominant to white, P would be the dominant allele and p the recessive allele

Homozygous and Heterozygous Dominant traits can be homozygous or heterozygous Homo= same; the alleles would be the same, PP Hetero=different; the alleles would be different, Pp For the recessive trait to be expressed both alleles would be recessive, pp

Genotype and Phenotype Genotype is the organism's gene pairs: PP, Pp or pp Phenotype is the outward physical appearance or expression of the genotype: purple or white

Genotype and Phenotype If the phenotype displays the recessive trait (white) then you know the genotype; pp If the phenotype displays the dominant trait (purple) then the genotype could be homozygous dominant (PP) or heterozygous (Pp) Genotype is PP or Pp Genotype is pp

Punnett Squares Mathematical device for predicting the results of genetic cross Male gametes are written across the top Female gametes are written along the side Genetic possibilities of the offspring are in the boxes Expect 3:1 phenotypic ratio

Monohybrid Cross Mono= one One trait is studied at a time This one is seed color Monohybrid crosses provided evidence for the Law of Segregation

Mendel’s Law of Segregation Two alleles for each trait separate during meiosis

Dihybrid Cross Di= two Two traits are studied at a time This one is seed color (yellow or green) and seed shape (wrinkled or round)

Dihybrid Cross Four types of alleles from the male gametes and four types of alleles from the female gametes can be produced. The resulting phenotypic ratio is 9:3:3:1 which gave evidence for the Law of Independent Assortment

Mendel’s Law of Independent Assortment Random distribution of alleles occurs during gamete formation Genes on separate chromosomes sort independently during meiosis. Each allele combination is equally likely to occur.

Mendel’s Law of Independent Assortment

Probability Genetic crosses predict what to expect in the phenotypes and genotypes of the offspring. Observed results are what you actually see with the organisms. The larger the number of offspring the closer the expected and observed results usually are.

Chapter 10 Sexual Reproduction and Genetics 10.3 Gene Linkage and Polyploidy

Gene Linkage The linkage of genes on a chromosome results in an exception to Mendel’s law of independent assortment because linked genes usually do not segregate independently.

Polyploidy Polyploidy is the occurrence of one or more extra sets of all chromosomes in an organism. Approximately 30% of flowers are polyploidy Strawberries are octoploidy (8n)

Polyploidy Horticultural important plants are forced to polyploidy to increase the size and flavor of flowers and fruits and overall vigor of the plants. Polyploidy is uncommon in animals.