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Meiosis & Mendel
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Homologous Chromosomes
23 from “mom” 23 from “dad” 46 total The 23 chromosomes have different sizes, shapes and number of genes
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Your cells have autosomes and sex chromosomes
Autosomes: chromosome that contains genes for characteristics not directly related to the sex of an organism In humans, pairs 1-22 Sex Chromosomes: chromosomes that directly controls the development of sexual characteristics In humans, pair 23
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You have body cells & gametes
Somatic cells (body cells): cells that make up the body tissues and organs (except gametes) 2 copies of 23 different chromosomes 46 total chromosomes Gametes (sex cells): egg or sperm cells 1 copy of 23 different chromosomes Combination of mom and dad’s genes
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Body cells are diploid; gametes are haploid
Haploid: A cell or organisms that only has one set of unpaired chromosomes Diploid: A cell that has a set of paired chromosomes; one from each parent
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Formation of Haploid Cells
In order to pass on genes, organisms pass on haploid cells to offspring Meiosis: From the Greek meiōn, meaning “less” The process of cell division that halves the number of chromosomes Occurs when forming specialized reproductive cells, such as gametes or spores
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Cells go through two rounds of division in meiosis
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Homologous chromosomes and sister chromatids
Homologous chromosomes: one from each parent; separated in meiosis I Sister chromatids: duplicated chromosomes; separated in meiosis II
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Meiosis i
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Right before meiosis begins…
The DNA in the original cell is replicated Therefore, meiosis starts with homologous chromosomes
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Prophase I The chromosomes condense The nuclear envelope breaks down
Homologous chromosomes pair along their length (tetrad) Crossing-over occurs Portions of a chromatid on one homologous chromosome are broken and exchanged with the corresponding chromatid portions of the other homologous chromosome
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Crossing Over DNA exchange
Adds even more recombination to independent assortment of chromosomes The number of genetic combinations amongst gametes is practically unlimited
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Metaphase I The spindle moves the pairs of homologous chromosomes to the equator of the cell The homologous chromosomes remain together
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Anaphase I The homologous chromosomes separate
Spindle fibers pull the chromosomes to opposite poles Sister chromatids do not separate at their centromeres Each chromosome is composed of two chromatids, the genetic material, however, has recombined
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Telophase I Individual chromosomes gather at each of the poles
The cytoplasm divides (cytokinesis) Two new cells are formed Both cells or poles contain one chromosome from each pair of homologous chromosomes
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Chromosomes DO NOT replicate between meiosis I and meiosis II
Round 2 Chromosomes DO NOT replicate between meiosis I and meiosis II
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Meiosis ii
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Prophase II Nuclear membrane breaks down
A new spindle forms around the chromosomes
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Metaphase II The chromosomes lines up along the equator
The chromosomes are attached at their centromeres to spindle fibers
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Anaphase II The centromeres divide
The chromosomes (now called chromatids) move to opposite poles of the cell
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Telophase II A nuclear envelope forms around each set of chromosomes
The spindle breaks down The cell undergoes cytokinesis Four haploid cells result
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Comparing mitosis & meiosis
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Haploid cells develop into mature gametes
Gametogenesis: the production of gametes
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Meiosis in Males: Spermatogenesis
The process by which sperm are produced in male animals Occurs in the testes (male reproductive organs)
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Meiosis in Females: Oogenesis
The process by which eggs are produced in female animals Occurs in the ovaries (female reproductive organs)
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The Importance of Genetic Variation
Meiosis and the joining of gametes are essential to evolution No genetic process generates variation more quickly Breeding (i.e. domestic animals) slows down genetic variation
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Mendel and heredity Gregor Mendel
“Father of Genetics” “Mendelian Genetics” Austrian monk Bred different varieties of the garden pea The first to develop rules that accurately predict patterns of heredity
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Heredity The passing of genetic traits from parents to offspring
Breeding animals and crops was done before DNA and chromosomes were discovered
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Genetics The science of heredity and of the mechanisms by which traits are passed from parent to offspring
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Mendel laid the groundwork for genetics
Before Mendel people thought offspring were a “blend” of their parents Example: a tall plant crossed with a short plant would get a medium plant…wrong!
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Mendel’s data revealed patterns of inheritance
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The Seven Characters Mendel Studied and Their Contrasting Traits
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Useful Features in Peas
Male & female reproductive parts of garden peas are on the same flower Self-fertilization: mating can be controlled Cross-pollination: pollen can be transferred to another plant The garden pea is small, grows easily, matures quickly and produces many offspring
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Monohybrid Cross A cross that involves one pair of contrasting traits
Example: cross a plant with purple flowers and a plant with white flowers Mendel did his experiments in 3 steps
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Step 1 True-breeding: Allow each variety of garden pea to self-pollinate for several generations Ensure offspring will only show one form of the character Example: a true breeding purple-flowering plant should produce only purple flowers P generation: The parental generation
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Step 2 Cross-pollinate two P generation plants with contrasting traits
Example: cross purple flowers with white flowers F1 generation: offspring of the P generation
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Step 3 Allow the F1 generation to self pollinate
F2 generation: offspring of the F1
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results
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Mendel’s Results Each of the F1 showed only one form of the trait
One of the traits had disappeared!
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Mendel’s Results When the F1 self-pollinated, the missing trait reappeared in some of the flowers! Mendel saw that there was a ratio of 3 purple flowers to 1 white flower (3:1) Mendel looked at the 7 traits and found the same 3:1 ratio in the F2 generation
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conclusions 1. Traits are inherited as discrete units, which explains why individual traits persist without being blended or diluted over successive generations Law of Segregation 2. Organisms inherit 2 copies of each gene, one from each parent 3. Organisms donate only 1 copy of each gene in their gametes
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Traits, genes, & alleles
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The same gene can have many versions
Allele: two versions of a gene Individuals receive one allele from each parent Each allele can be passed on when the individual reproduces
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Homozygous Two alleles of a particular gene are present in an individual Homozygous recessive (pp for pea flowers) Homozygous dominant (PP for pea flowers)
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Heterozygous Two different alleles of a particular gene are present in an individual Pp for pea flowers (still shows purple)
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genome All of an organism’s genetic material
Unless you have an identical twin, you have a unique genome that determines all of your traits!
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Genotype and Phenotype
The set of alleles that an individual has for a character (PP, Pp, pp) Phenotype The physical appearance of a character Determined by the genotype PP = purple, Pp = purple, pp = white
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Dominant alleles Dominant
The expressed form of the character (Mendel’s flowers: purple) Represented by a capital letter
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Recessive alleles Recessive
The trait that was not expressed when the dominant form of the character was present (Mendel’s flowers: white) Represented by a lower case letter
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Traits & probability
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Punnett squares illustrate genetic crosses
Diagram that predicts the outcome of a genetic cross by considering all possible combinations of gametes in the cross
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A monohybrid cross involves one trait
Homozygous-Homozygous 4:0 dominant to recessive Heterozygous-Heterozygous 3:1 dominant to recessive Heterozygous-Homozygous 1:1 dominant to recessive
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Homozygous-homozygous
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heterozygous-heterozygous
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Heterozygous-homozygous
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Test cross determining unknown genotypes
Used to determine if a dominant phenotype is heterozygous or homozygous Cross the individual with a homozygous recessive
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A dihybrid cross involves two traits
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Heredity patterns can be calculated with probability
Probability: the likelihood that a specific event will occur Probability = number of one kind of possible outcome total number of all possible outcomes Coin toss: either heads or tails, probability of heads is ½ and probability of tails is 1/2
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Probability of the Outcome of a Cross
Two parents are involved in a genetic cross so both must be considered when calculating the probability To find the probability that a combination of two independent events will occur, multiply the separate probabilities of the two events
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Coin tossing…2 Coins Tossed
Probability of 2 heads: ½ x ½ = ¼ Probability of 2 tails: ½ x ½ = ¼ Probability of 1 heads and 1 tails: ½ x ½ = ¼ Probability of 1 tails and 1 heads: ½ x ½ = ¼ Probability of all possible outcomes must add up to 1
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Meiosis and genetic variation
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Law of Independent Assortment
The inheritance of one character does not influence the inheritance of any other character The alleles of different genes separate independently of one another during gamete formation
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Independent Assortment
The random distribution of homologous chromosomes Each of the 23 pairs of chromosomes segregates (separates) independently About 223 (about 8 million) gametes with different gene combinations can be produced from one original cell
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Random Fertilization The zygote that forms a new individual is created by the random joining of two gametes Because fertilization of an egg by a sperm is random, the number of possible outcomes is squared (223 x 223 = 64 million)
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Sexual reproduction creates unique gene combinations
Crossing over Genetic linkage
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