3The Work of Gregor Mendel SECTION 11-1The Work of Gregor Mendel
4Key Concept Questions What is the principle of dominance? What happens during segregation?
5Modern genetics began in the mid-1800s in an abbey garden, where a monk named Gregor Mendel documented inheritance in peasTraits are distinguishing characteristics that are inherited.Genetics is the study of biological inheritance patterns and variation.Heredity is the study of inheritance of characteristics from one generation to the nextGregor Mendel showed that traits are inherited as discrete units.Many in Mendel’s day thought traits were blended.
6Mendel made three key decisions in his experiments. use of purebred plantscontrol over breedingobservation of seven “either-or” traits
7The peas were TRUE-BREEDING or purebred If they were allowed to self-pollinate, they would produce offspring identical to themselvesall purple flowersall tallall shortall green seedsall yellow seeds
9Fertilization - process in sexual reproduction in which male and female reproductive cells join to form a new cellSelf-pollination – pollen fertilizes egg cells in the same flowerCross-pollination – pollen from one flower fertilizes egg cells of a different flower = seeds with two parents
10Original pair of pea plants = P generation = parental generationOffspring = F1 = first filial generationMini Latin Lesson:Filius = son
11Mendel used pollen to fertilize selected pea plants. He crossed purebred purple flowers to purebred white flowersP generation crossed to produce F1 generationThe F1 generation are hybrids – offspring of parents with different characteristics.interrupted the self-pollination process by removing male flower partsMendel controlled thefertilization of his pea plantsby removing the male parts,or stamens.He then fertilized the femalepart, or pistil, with pollen froma different pea plant.
12Mendel allowed the resulting plants to self-pollinate. Among the F1 generation, all plants had purple flowersAmong the F2 generation, some plants had purple flowers and some had white
14Mendel wondered why the recessive alleles seem to disappear in the F1 generation and then reappear in the F2 generation?He thought that at some point the alleles for purple and white in the F1 plants were SEGREGATED – separated from each other during the formation of GAMETES – sex cells
15Mendel observed the same patterns in the first and second generations of his 7 crosses.
16Mendel drew three important conclusions. Traits are inherited as discrete units.Organisms inherit two copies of each gene, one from each parent.The two copies segregate during gamete formation.The last two conclusions are called the law of segregation.purplewhite
17The same gene can have many versions. A gene is a piece of DNA that directs a cell to make a certain protein.Each gene has a locus, a specific position on a pair of homologous chromosomes.An allele is any alternative form of a gene occurring at a specific locus on a chromosome.
18CHAPTER MYSTERY CLUE #1 – Parakeets come in four colors: white, green, blue, and yellow.How many alleles might there be for feather color?Two alleles because there are four possible phenotypes
19Principle of Dominance Alleles can be represented using letters.A dominant allele is expressed when at least one allele is dominant.A recessive allele is expressed only when two copies are present.Dominant alleles are represented by uppercase letters; recessive alleles by lowercase letters.
20Law of segregation P PP p pp Pp when gametes are produced during meiosis, homologous chromosomes separate from each othereach allele for a trait is packaged into a separate gametePPPpppPp
21McDougall Littell video – Mendel’s law of Segregation
22Fun Fact!Basenjis are small dogs with pointed ears, short silky hair, and rows of wrinkles on their foreheads.Basenjis cannot bark, but they can make yodeling type sounds.Basenjis can mate with barking breeds to produce puppies that bark.Hypothesize a genetic explanation for why basenjis cannot bark.The ability to bark is a dominant trait in dogs. Basenjis have two recessive genes for this character.
23Key Concept Questions What is the principle of dominance? The dominant allele gets expressed and the recessive allele is maskedWhat happens during segregation?each allele for a trait is packaged into a separate gamete
24Genetics and Probability SECTION 11-2Genetics and Probability
25Key Concept QuestionsHow do geneticists use the principles of probability?How do geneticists use Punnett squares?
26Heredity patterns can be calculated with probability. Probability is the likelihood that something will happen.Probability predicts an average number of occurrences, not an exact number of occurrences.Probability applies to random events such as meiosis and fertilization.
27number of ways a specific event can occur Probability =number of ways a specific event can occurnumber of total possible outcomes
28What is the probability of flipping a coin and landing on heads three times in a row? ½ x ½ x ½ = 1/8
29What is the probability of rolling a die and landing on 3 three times in a row? 1/6 x 1/6 x 1/6 = 1/216
30Punnett squares illustrate genetic crosses. The Punnett Square is a grid system for predicting all possible genotypes resulting from a cross.The axes represent the possible gametes of each parent.The boxes show the possible genotypes of the offspring.The Punnett square yields the ratio of possible genotypes and phenotypes.
31Genes influence the development of traits. All of an organism’s genetic material is called the genome.A genotype refers to the makeup of a specific set of genes.A phenotype is the physical expression of a trait.Purple is the phenotypePP and Pp are the genotypes for purple flower color
32Each parent donates one allele for every gene. Homozygous describes two alleles that are the same at a specific locus.PP or ppHeterozygous describes two alleles that are different at a specific locus.Pp
34homozygous dominant X homozygous recessive all heterozygous, all dominant phenotype
35Heterozygous X heterozygous 1:2:1 homozygous dominant genotype3:1 purple:white phenotype2 organisms can have the same phenotype but have different genotypesFF and Ff show the same color but have different alleles
36Heterozygous X homozygous recessive 1:1 heterozygous:homozygous recessive1:1 purple:white
37Key Concept QuestionsHow do geneticists use the principles of probability?They use probability to try to determine the genetic outcome of a cross between two organismsHow do geneticists use Punnett squares?They use Punnett squares to determine all the possible outcomes of a particular cross
39Key Concept Questions What is the principle of independent assortment? What inheritance patterns exist aside from simple dominance?
40A dihybrid cross involves two traits. Mendel’s dihybrid crosses with heterozygous plants yielded a 9:3:3:1 phenotypic ratio.
41Mendel’s dihybrid crosses led to his second law, the law of independent assortment. allele pairs separate into gametes independentlynon-homologous chromosomes align independentlyclasses of gametes produced in equal amountsYR = Yr = yR = yronly true for genes on separate chromosomesyellowgreenroundwrinkledYyRrYrYryRyRYRYRyryr1:1:1:1
42Most traits occur in a range and do not follow simple dominant-recessive patterns. INCOMPLETE DOMINANCEOne allele is not completely dominant over another
43Ex) Four o’clock plants Red flowers = RR; white flowers = WW; pink flowers RW The heterozygous phenotype is somewhere between the two homozygous phenotypesWhat phenotypic ratio would you expect to see if two heterozygous plants with pink flowers were crossed?1 red:2 pink:1 white
44CODOMINANCEBoth alleles contribute to the phenotype of the organismEx) Cattle that are roan colored – red hairs and white hairs are both displayed in the coat colorEx) chickens will have an allele for black feathers and one for white feathers that are codominant and you see a speckled looking chicken
45MULTIPLE ALLELESMany genes are said to have more than two allelesEx) coat color in rabbits – has four different alleles – the rabbit can only have two of those alleles but in various possible combinations
46CHAPTER MYSTERYCLUE #2Green feathers don’t actually contain green pigments. Rather, they contain a mixture of blue and yellow pigments.Could feather color be controlled by more than one gene?There is probably two genes – one for each color
47POLYGENIC TRAITSTraits controlled by two or more genesEx) range of skin color in humans – four different genes control this traitWhat Happens in Albino people?Gene for skin pigment has dominant allele ‘A’ for skin colorationHomozygous recessive have aa = albino – person who lacks the pigment melanin that gives human skin its color
48If two people with normal skin color have a child with albinism, what are the odds that a second child will also have albinism?Use a Punnett square to show your answer
501 in four probability the next child will be albino AAAaaa1 in four probability the next child will be albino
51Genes and the environment Environmental conditions can change gene expression and influence genetically controlled traits.
52Key Concept Questions What is the principle of independent assortment? different genes separate into gametes independentlyWhat inheritance patterns exist aside from simple dominance?Incomplete dominance, codominance, multiple alleles, polygenic traits, environment
54Key Concept Questions What happens during the process of meiosis? Why is crossing over important?How is meiosis different from mitosis?
55You have body cells and gametes. Body cells are also called somatic cells.Germ cells develop into gametes.Germ cells are located in the ovaries and testes.Gametes are sex cells: egg and sperm.Gametes have DNA that can be passed to offspring.body cellssex cells (sperm)sex cells (egg)
56Your cells have autosomes and sex chromosomes. Your body cells have 23 pairs of chromosomes.Homologous pairs of chromosomes have the same structure.For each homologous pair, one chromosome comes from each parent.Chromosome pairs 1-22 are autosomes.Sex chromosomes, X and Y, determine gender in mammals.
57Body cells are diploid; gametes are haploid. Fertilization between egg and sperm occurs in sexual reproduction.Diploid (2n) cells have two copies of every chromosome.Body cells are diploid.Half the chromosomes come from each parent.
58Haploid (n) cells have one copy of every chromosome. Gametes are haploid.Gametes have 22 autosomes and 1 sex chromosome.
59Chromosome number must be maintained in animals. Many plants have more than two copies of each chromosome.Mitosis and meiosis are types of nuclear division that make different types of cells.Mitosis makesmore diploid cells.
60Meiosis makes haploid cells from diploid cells. Meiosis occurs in sex cells.Meiosis produces gametes.
61Cells go through two rounds of division in meiosis. Meiosis reduces chromosome number and creates genetic diversity.
62Meiosis I and meiosis II each have four phases, similar to those in mitosis. Pairs of homologous chromosomes separate in meiosis I.Homologous chromosomes are similar but not identical.Sister chromatids divide in meiosis II.Sister chromatids are copies of the same chromosome.homologous chromosomessisterchromatidssisterchromatids
63Meiosis I occurs after DNA has been replicated. Meiosis I divides homologous chromosomes in four phases.
64Crossing over 3 steps New combinations of traits cross over breakage of DNAre-fusing of DNANew combinations of traitsTetrad
65Meiosis II divides sister chromatids in four phases. DNA is not replicated between meiosis I and meiosis II.
67Haploid cells develop into mature gametes. Gametogenesis is the production of gametes.Gametogenesis differs between females and males.Sperm become streamlined and motile.Sperm primarily contribute DNA to an embryo.Eggs contribute DNA, cytoplasm, and organelles to an embryo.During meiosis, the egg gets most of the contents; the other cells form polar bodies.
68The value of sexual reproduction Sexual reproduction introduces genetic variationgenetic recombination during meiosisindependent assortment of chromosomesrandom alignment of homologous chromosomes in Meiosis 1crossing overmixing of alleles across homologous chromosomesrandom fertilizationwhich sperm fertilizes which egg?Driving evolutionvariation for natural selection
69McDougall Littell video – Law of Independent Assortment
70Meiosis differs from mitosis in significant ways. Meiosis has two cell divisions while mitosis has one.In mitosis, homologous chromosomes never pair up.Meiosis results in haploid cells; mitosis results in diploid cells.
72Mitosis vs. Meiosis Mitosis 1 division daughter cells genetically identical to parent cellproduces 2 cells2n 2nproduces cells for growth & repairno crossing overMeiosis2 divisionsdaughter cells genetically different from parentproduces 4 cells2n 1nproduces gametescrossing over
73Putting it all together… meiosis fertilization mitosis + developmentgametes46234623464646464623meiosis4646egg464623zygotefertilizationmitosis & developmentmitosissperm
74Variation from random fertilization Sperm + Egg = ?any 2 parents will produce a zygote with over 70 trillion (223 x 223) possible diploid combinations
75Sexual reproduction creates variability Sexual reproduction allows us to maintain both genetic similarity & differences.Michael & Kirk DouglasBaldwin brothersMartin & Charlie Sheen, Emilio Estevez
76Key Concept Questions What happens during the process of meiosis? The chromosome number is reduced by halfWhy is crossing over important?It creates new genetic combinations and more variety.How is meiosis different from mitosis?
80Gene linkage was explained through fruit flies. Morgan found that linked traits are on the same chromosome.Chromosomes, not genes, assort independently during meiosis.Wild typeMutant
81CHAPTER MYSTERYCLUE #3White is the least common color found in parakeets.What does this fact suggest about the genotypes of both green parents?Parents were probably heterozygous to produce a homozygous recessive offspring
82The genes for this fruit fly’s reddish-orange eyes and miniature wings are almost always inherited together. The reason for this is the genes are close together on a single chromosome
83Linked genes are not inherited together every time. Chromosomes exchange homologous genes during meiosis.
84Linkage maps estimate distances between genes. The closer together two genes are, the more likely they will be inherited together.Cross-over frequencies are related to distances between genes.Gene linkage maps show the relative locations of genes.
85Cross-over frequencies can be converted into map units. gene A and gene B cross over 6.0 percent of the timegene B and gene C cross over 12.5 percent of the timegene A and gene C cross over 18.5 percent of the time
86Key Concept Question What structures actually assort independently? Chromosomes