Chapter 10 Mendel and Meiosis

10.1 Mendel’s Laws of Heredity

Mendel was first person to succeed in predicting how traits would be transferred from one generation to the next Used pea plants to solve the problem of heredity Mendel controlled his experiments, studying one trait at a time

Cross pollination – breed (cross) one plant with another

Mendel cross-pollinated using tall and short pea plants to create new plants - hybrids –Tall pea plants were from population that produced tall pea plants for generations –Short pea plants were also from population that was short for generations

Mendel’s Monohybrid Crosses 1 st experiment with pea plants dubbed monohybrid Mono = One Two parents differed by a single trait - height

The First Generation (F 1 ) Cross pollinated pea plants –One six foot tall pea plant (P 1 ) –One short pea plant -  2 feet tall (P 1 ) All offspring (F 1 ) grew as tall as taller parent Almost as if shorter parent didn’t exist

The Second Generation (F 2 ) Tall plants from first generation, (F 1 ), self pollinated and seeds were planted ¾ plants were as tall as the tall plants in the parent, (P 1 ), and 1 st generation (F 1 ) ¼ plants were as short as short plants in parent generation, (P 1 )

P 1 = Grandma & Grandpa F 1 = Mom F 2 = You

Grandma (TT) & Grandpa (tt) Mom (Tt) & Dad (??) You = either (T?) or (t?)

Mendel did several experiments testing seven different traits The trait seemed to disappear in each F 1 generation, only to reappear in the F 2 generation

The rule of unit factors Each organism has two factors that control each of its traits  genes Genes exist in alternative forms Gene forms are called Alleles There are two alleles for every gene –One inherited from each parent

The Rule of Dominance If an organism is heterozygous for a trait (one dominant allele and one recessive allele) the dominant trait will be the one observed

Mendel’s Pea Plant Experiment –Tall P 1 & Short P 1 produced Tall F 1 Only one trait was observed = dominant Unobserved trait = recessive

The Law of Segregation Organisms have two different alleles for a trait; it can produce two different types of gametes During fertilization male and female gametes randomly pair to produce four combinations of alleles

Phenotypes & Genotypes Genotype = what you are (Tt) Phenotype = what you look like (Tall)

Heterozygous & Homozygous Heterozygous = two different alleles for a trait (Tt) Homozygous = two of the same alleles for a trait (tt)

Mendel’s Dihybrid Crosses Crosses in which the organism has two traits that differ from each other Question is: –Will the two traits stay together or will they be inherited independently from one another?

The First Generation Parent Pea Plants (P1) –RRYY (round yellow seeds) –rryy (wrinkled green seeds) F1 (first generation) all round and all yellow

The Second Generation Ratio of phenotypes were –9 round yellow –3 round green –3 wrinkled yellow –1 wrinkled green

Law of Independent Assortment Genes for different traits are inherited independently –If alleles for seed shape and color were inherited together than only two kinds of pea seeds would be produced; however, four types of pea seeds were produced

Punnett Squares Proves fertilization occurs at random If you know genotypes of parents; you can predict the possible genotypes of offspring

Monohybrid crosses – one trait –Two kinds of gametes from one parent are on one side of the square and the two from the other parent are on the other

Dihybrid crosses –More than one trait

Probability Genetics is like flipping a coin – it follows the rules of chance The chance that an event will occur can be determined by dividing the number of desired outcomes by the total number of possible outcomes The probability of getting a female child would be 1 in 2 chances, written as 1:2

10.2 Meiosis

Genes, Chromosomes & Numbers Genes are lined up on chromosomes by the thousands Chromosomes occur in pairs – one from mom & one from dad

Diploid cell = cell with two of each kind of chromosome Diploid cells are called 2n

Haploid cell = cell that contains one of each kind of chromosome (sex cells) Haploid cells are called 1n or n

Homologous Chromosomes Paired chromosomes with genes for the same traits arranged in the same order Not always identical, due to the fact that there are different alleles for a trait

Meiosis –Cell division that allows offspring to have same amount of chromosomes as parents –Produces gametes containing ½ the number of chromosomes as parent’s body –Two divisions Meiosis I Meiosis II

Meiosis I & II Meiosis I begins with one diploid (2n) cell and completes Meiosis II with four haploid (n) cells called gametes Gametes –Sperm  male sex cell –Egg  female sex cell Sperm fertilizes egg and resulting cell is Zygote

Phases of Meiosis

Interphase I – cell replicates chromosomes Prophase I –Chromosomes coil up & condense –Spindle Forms –Homologous chromosomes come together to form four-part structure called tetrad

Tetrads = two homologous chromosomes made up of sister chromatids –Are paired tightly and crossing over can occur –Crossing over  chromosomes exchange genetic material

Metaphase I –Spindle fiber attaches to centromere –Tetrads line up on middle of cell Anaphase I –Tetrads, homologous chromosomes, separate and are pulled to opposite ends of the cell Telophase I –spindle broken down, chromosomes uncoil –Cytoplasm divides to make two cells

The Phases of Meiosis II Prophase II – spindle forms Metaphase II – chromosomes line up on equator Anaphase II – chromosome splits and sister chromatids pulled to opposite ends of cell Telophase II – nuclei re-form, spindle breaks down, cytoplasm divides

Genetic Recombination Crossing over occurs during prophase I creating variation in chromosomes Major source of variation among organisms

Mistakes in Meiosis Non-disjunction – failure of chromosomes to separate correctly

Trisomy Extra chromosome In humans, chromosome #21 individual will develop Down’s Syndrome (AKA Trisomy-21)

Monosomy lacking a chromosome zygote usually does not survive, if it does the organism will not survive –Turner’s Syndrome

PolyPloidy Rare in humans; however, frequently occurs in plants –Triploid - lack of separation of chromosomes and offspring inherits 3 sets of chromosomes (apples) –Tetraploid – Offspring with 4 sets of chromosomes (wheat)