1. Chapter 10 Mendel and Meiosis

2. 10.1 Gregor Mendel Gregor Mendel was an Austrian monk who carried out extensive studies of heredity Gregor Mendel was an Austrian monk who carried out extensive studies of heredity –He was the 1 st person to successfully predict how traits are transferred from one generation to the next –Mendel experimented with garden peas  Because they can self pollinate (breed with themselves) or  Could be cross pollinated (bred with a different plant) manually, but would not cross pollinate on their own.

3. 10.1 Controlled Experiments Mendel controlled his experiments very carefully in the following ways: Mendel controlled his experiments very carefully in the following ways: –He only studied one trait at a time –He analyzed his data mathematically –He used only true breeding plants  He only used tall plants that had been tall for many generations  He only used short plants that had been short for many generations

4. 10.1 Monohybrid crosses Mendel’s early experiments involved monohybrid crosses, where 2 parent plants (P 1 ) differed in only one trait, specifically height Mendel’s early experiments involved monohybrid crosses, where 2 parent plants (P 1 ) differed in only one trait, specifically height –He observed all offspring in the first generation (F 1 ) were tall –He observed that when he cross pollinated plants in the first generation, of the plants in the second generation (F 2 ) 75% were tall and 25% were short (a 3:1 ratio)

5. 10.1 Studied Traits Mendel studied 7 different traits in garden peas in this manner, one pair at a time Mendel studied 7 different traits in garden peas in this manner, one pair at a time –Seed shape (round or wrinkled) –Seed color (yellow or green) –Flower color (purple or white) –Flower position (axial (side) or terminal (tips)) –Pod color (green or yellow) –Pod shape (inflated or constricted) –Plant height (tall or short) He found every time that one trait disappeared in the F 1 generation and came back in the F 2 generation He found every time that one trait disappeared in the F 1 generation and came back in the F 2 generation

6. 10.1 Mendel’s Rules From his observations and data, Mendel created 2 rules of heredity: From his observations and data, Mendel created 2 rules of heredity: –The rule of unit factors: states that each organism has 2 genes that control each trait in alternative forms called alleles  These 2 genes come one from each parent –The rule of dominance says that one trait is dominant and will be expressed if it’s allele is present and the other trait is recessive and will only be expressed if the dominant allele is absent  There are other circumstances, such as incomplete dominance, where an intermediate trait is observed

7. 10.1 Law of Segregation Mendel also created the law of segregation Mendel also created the law of segregation –This states that each individual has 2 alleles of each gene and when gametes are produced, each gamete receives one of these 2 alleles  humans have 46 chromosomes  Each parent contributes 23 chromosomes to the gamete –During fertilization, the gametes randomly pair producing 4 combinations of alleles

8. 10.1 Phenotypes & Genotypes Phenotype is the way an organism looks and behaves Phenotype is the way an organism looks and behaves –Examples include blond hair, curly hair, attached earlobes, etc. Genotype is the allele combination an organism contains Genotype is the allele combination an organism contains –Examples include TT (homozygous tall), Tt or tT (heterozygous tall) and tt (homozygous short  Heterozygous means that the 2 alleles are different  Homozygous means that the 2 alleles are the same

9. 10.1 Dihybrid Crosses Mendel later carried out crosses involving 2 traits, called dihybrid crosses Mendel later carried out crosses involving 2 traits, called dihybrid crosses –He crossed homozygous dominant plants for both traits (RRYY) with homozygous recessive plants for both traits (rryy)  All offspring in the F 1 generation were heterozygous (RrYy), showing the dominant traits –He then crossed the F 1 generation with itself  He found a ratio of 9:3:3:1 in the F 2 generation where 56.25% showed both dominant traits, 37.50 % showed one dominant trait and one recessive trait (18.75% for each trait) and 6.25% showed both recessive traits

10. 10.1 Law of Independent Assortment As a result of his dihybrid crosses, Mendel created the Law of Independent Assortment As a result of his dihybrid crosses, Mendel created the Law of Independent Assortment –This law states that genes for different traits are inherited independently of each other  This means that 2 traits are not connected (having one does not mean you will or will not have the other)

11. 10.1 Punnett Squares Punnett Squares are a short way of determining the expected genotypes of a cross (monohybrid or dihybrid or higher) Punnett Squares are a short way of determining the expected genotypes of a cross (monohybrid or dihybrid or higher) –A monohybrid cross has a 2x2 grid –A dihybrid cross has a 4x4 grid –The gametes from one parent are placed above the columns on the top and the gametes from the other parent are placed next to the rows on the side –The gametes are then paired in the empty boxes to shows the possible genotypes

12. 10.1 2 x 2 Punnett Squares RR RR Homozygous round Rr Rr Heterozygous round Rr Rr Heterozygous round rr rr Homozygous wrinkled R r R r

13. 10.1 4 x 4 Punnett Squares RRYY Round, yellow RRYy RrYY RrYy RRYy RRyy Round, green RrYy Round, yellow Rryy Round, green RrYY Round, yellow RrYy rrYY Wrinkled, yellow rrYy RrYy Round, yellow Rryy Round, green rrYy Wrinkled, yellow rryy Wrinkled, green RY Ry rY ry RY Ry rY ry

14. 10.1 Probability Actual results of genetic crosses can vary from the expected results shown in a Punnett square Actual results of genetic crosses can vary from the expected results shown in a Punnett square –The Punnett square only shows the probability, or chance, that a specific offspring will occur  In actual data, the ratios should be close to the expected results  The more offspring tested, the closer the actual results will be

15. 10.2 Genes & Chromosomes Genes are lined up on chromosomes within an organism Genes are lined up on chromosomes within an organism –Each chromosome can have over 1000 genes on it Chromosomes occur in pairs Chromosomes occur in pairs –One from the male parent and one from the female parent –A cell with 2 (2n) of each type of chromosome is called diploid –A cell with one (n) of each type of chromosome is called haploid  Gametes contain haploid numbers of chromosomes  The numbers for n and 2n vary by organism and some are shown on p. 265 in table 10.1

16. 10.2 Homologous Chromosomes The 2 chromosomes in each pair are called homologous chromosomes The 2 chromosomes in each pair are called homologous chromosomes –Each pair has genes for the same trait  These genes are in the same order on the homologous chromosomes, but the alleles can be different –If the alleles are different on homologous chromosomes, then the organism is heterozygous for that trait –If the alleles are the same on homologous chromosomes, then the organism is homozygous for that trait

17. 10.2 Meiosis Meiosis is a type of cell division that produces gametes with half the number of chromosomes as the parent’s body cells Meiosis is a type of cell division that produces gametes with half the number of chromosomes as the parent’s body cells –Meiosis occurs in specialized cells –Meiosis occurs in 2 stages: meiosis I and meiosis II –It starts with one diploid cell having 2n chromosomes –It ends with 4 haploid cells having n chromosomes  These haploid cells are sex cells called gametes –Male gametes are sperm –Female gametes are called eggs –Zygotes result from the combination of egg and sperm and have diploid (2n) chromosomes

18. 10.2 Phases in Meiosis I Meiosis I has 5 phases: Meiosis I has 5 phases: –Interphase- the cell replicates its chromosomes –Prophase I- the chromosomes coil up and a spindle forms; homologous chromosomes line up, forming tetrads  Crossing over occurs in prophase I, where non- sister chromatids in genetic material break and exchange genetic material

19. 10.2 Phases in Meiosis I (con’t) –Metaphase I- centromeres attach to spindle fibers and are pulled to the middle –Anaphase I- homologous chromosomes separate and move to opposite poles of the cell –Telophase I- the spindle breaks down, the chromosomes uncoil and the cytoplasm divides to form 2 new cells  Each new cell contains one chromosome from each homologous pair

20. 10.2 Phases in Meiosis II There are 4 phases in Meiosis II There are 4 phases in Meiosis II –Prophase II- a spindle forms and spindle fibers attach to the chromosomes –Metaphase II- chromosomes pulled to the center of the cell and line up randomly at the equator –Anaphase II- centromeres split allowing the sister chromatids to separate and move to opposite ends of the cell –Telophase II- nuclei re-form, spindles break down, cytoplasm divides  4 haploid cells result after meiosis II containing 1 chromosomes from each homologous pair  These become gametes

21. 10.2 Genetic Variation Since Meiosis creates cells that are not identical to the parent cells, there is the possibility of forming new combinations of alleles Since Meiosis creates cells that are not identical to the parent cells, there is the possibility of forming new combinations of alleles –This is done by crossing over, which increase the variety of offspring in the species

22. 10.2 Possible Offspring The number of possible gametes is dependent on the number of chromosomes in each cell The number of possible gametes is dependent on the number of chromosomes in each cell –2 to the power of the number of chromosome pairs tells you the number of gametes possible for that organism –You can then square that number to determine the number of possible different offspring –For example: humans have 23 pairs of chromosomes  2 23 = 8388608 possible different male or female gametes  8388608 2 = 70368744177700 possible different offspring!!  These numbers increase if crossing over occurs

23. 10.2 Genetic Recombination Genetic recombination is the re- assortment of chromosomes by crossing over or by independent segregation of homologous chromosomes Genetic recombination is the re- assortment of chromosomes by crossing over or by independent segregation of homologous chromosomes –It is the major source for variation among organisms

24. 10.2 Nondisjunction The failure of homologous chromosomes to separate properly during meiosis is called nondisjunction The failure of homologous chromosomes to separate properly during meiosis is called nondisjunction –In nondisjunction, both chromosomes from a homologous pair move to the same side of the cell in meiosis I rather than separating and moving to opposite poles of the cell –One type of nondisjunction occurs when one gamete is missing a chromosome and the other has an extra chromosome  The zygote that forms with the extra chromosome will cause a trisomy defect, which includes Downs syndrome  The zygote that forms lacking a chromosome will cause a monosomy defect, which usually results in the zygote’s death –Turner’s syndrome can result however, and the organism survives

25. 10.2 Polyploidy In the form of nondisjunction where homologous chromosomes do not separate at all, polyploidy results In the form of nondisjunction where homologous chromosomes do not separate at all, polyploidy results –A triploid zygote results from one of the 2 gametes has double chromosomes –A tetraploid zygote results from both of the gametes having double chromosomes  Polyploidy in animals almost always results in the death of the organism  Polyploidy in plants, however, often results in healthier, more productive plants –These are commonly available for purchase without the consumer’s knowledge

26. 10.2 Gene Linkage Some genes, particularly those that are located near each other on the same chromosome, can be inherited together rather than independently Some genes, particularly those that are located near each other on the same chromosome, can be inherited together rather than independently –This means that the genes do not follow Mendel’s law of independent assortment, although the chromosomes do –Linked genes can be separated by crossing over and are used to map the location of the genes on the chromosomes