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CHAPTER 11 INTRODUCTION TO GENETICS
CHAPTER MYSTERY – video on phsuccessnet.com
SECTION 11-1 The Work of Gregor Mendel
Key Concept Questions What is the principle of dominance? What happens during segregation?
Modern genetics began in the mid-1800s in an abbey garden, where a monk named Gregor Mendel documented inheritance in peas Traits 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 next Gregor Mendel showed that traits are inherited as discrete units. Many in Mendel’s day thought traits were blended.
Mendel made three key decisions in his experiments. –use of purebred plants –control over breeding –observation of seven “either-or” traits
The peas were TRUE-BREEDING or purebred –If they were allowed to self- pollinate, they would produce offspring identical to themselves –all purple flowers –all tall –all short –all green seeds –all yellow seeds
Quick plant reproduction
Fertilization - process in sexual reproduction in which male and female reproductive cells join to form a new cell Self-pollination – pollen fertilizes egg cells in the same flower Cross-pollination – pollen from one flower fertilizes egg cells of a different flower = seeds with two parents
Original pair of pea plants = P generation = parental generation Offspring = F1 = first filial generation Mini Latin Lesson: Filius = son
Mendel controlled the fertilization of his pea plants by removing the male parts, or stamens. He then fertilized the female part, or pistil, with pollen from a different pea plant. Mendel used pollen to fertilize selected pea plants. –He crossed purebred purple flowers to purebred white flowers –P generation crossed to produce F 1 generation –The F1 generation are hybrids – offspring of parents with different characteristics. –interrupted the self-pollination process by removing male flower parts
Mendel allowed the resulting plants to self- pollinate. –Among the F 1 generation, all plants had purple flowers –Among the F 2 generation, some plants had purple flowers and some had white
McDougall Littell video – Mendel’s experiment
Mendel 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
Mendel observed the same patterns in the first and second generations of his 7 crosses.
purplewhite Mendel 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.
The 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.
CHAPTER 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
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. Principle of Dominance
Law of segregation –when gametes are produced during meiosis, homologous chromosomes separate from each other –each allele for a trait is packaged into a separate gamete PP P P pp p p PpPp P p
McDougall Littell video – Mendel’s law of Segregation
Fun 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. WpsJ1cA&feature=related 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.
Key Concept Questions What is the principle of dominance? –The dominant allele gets expressed and the recessive allele is masked What happens during segregation? –each allele for a trait is packaged into a separate gamete
SECTION 11-2 Genetics and Probability
Key Concept Questions How do geneticists use the principles of probability? How do geneticists use Punnett squares?
Heredity 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.
Probability = number of ways a specific event can occur number of total possible outcomes
What is the probability of flipping a coin and landing on heads three times in a row? –½ x ½ x ½ = 1/8
What 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
Punnett 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.
Genes 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 phenotype –PP and Pp are the genotypes for purple flower color
–Each parent donates one allele for every gene. –Homozygous describes two alleles that are the same at a specific locus. PP or pp –Heterozygous describes two alleles that are different at a specific locus. Pp
Both homozygous dominant and heterozygous genotypes express dominant phenotypes. (PP, Pp)
homozygous dominant X homozygous recessive –all heterozygous, all dominant phenotype
Heterozygous X heterozygous –1:2:1 homozygous dominant genotype –3:1 purple:white phenotype 2 organisms can have the same phenotype but have different genotypes FF and Ff show the same color but have different alleles
Heterozygous X homozygous recessive –1:1 heterozygous:homozygous recessive –1:1 purple:white
Key Concept Questions How do geneticists use the principles of probability? –They use probability to try to determine the genetic outcome of a cross between two organisms How do geneticists use Punnett squares? –They use Punnett squares to determine all the possible outcomes of a particular cross
SECTION 11-3 Exploring Mendelian Genetics
Key Concept Questions What is the principle of independent assortment? What inheritance patterns exist aside from simple dominance?
A dihybrid cross involves two traits. Mendel’s dihybrid crosses with heterozygous plants yielded a 9:3:3:1 phenotypic ratio.
Mendel’s dihybrid crosses led to his second law, the law of independent assortment. round wrinkled Law of independent assortment – allele pairs separate into gametes independently non-homologous chromosomes align independently classes of gametes produced in equal amounts –YR = Yr = yR = yr only true for genes on separate chromosomes yellow green :11:1:1 Yr yR YR yr YyRr
Most traits occur in a range and do not follow simple dominant-recessive patterns. INCOMPLETE DOMINANCE –One allele is not completely dominant over another
Ex) Four o’clock plants Red flowers = RR; white flowers = WW; pink flowers RW The heterozygous phenotype is somewhere between the two homozygous phenotypes What phenotypic ratio would you expect to see if two heterozygous plants with pink flowers were crossed? –1 red:2 pink:1 white
CODOMINANCE –Both alleles contribute to the phenotype of the organism Ex) Cattle that are roan colored – red hairs and white hairs are both displayed in the coat color Ex) chickens will have an allele for black feathers and one for white feathers that are codominant and you see a speckled looking chicken
MULTIPLE ALLELES –Many genes are said to have more than two alleles Ex) coat color in rabbits – has four different alleles – the rabbit can only have two of those alleles but in various possible combinations
CHAPTER MYSTERY CLUE #2 –Green 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
POLYGENIC TRAITS –Traits controlled by two or more genes Ex) range of skin color in humans – four different genes control this trait What Happens in Albino people? –Gene for skin pigment has dominant allele ‘A’ for skin coloration –Homozygous recessive have aa = albino – person who lacks the pigment melanin that gives human skin its color
If 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
A A aa
Aa AAAAa a aa 1 in four probability the next child will be albino
Genes and the environment Environmental conditions can change gene expression and influence genetically controlled traits.
Key Concept Questions What is the principle of independent assortment? –different genes separate into gametes independently What inheritance patterns exist aside from simple dominance? –Incomplete dominance, codominance, multiple alleles, polygenic traits, environment
SECTION 11-4 Meiosis
Key Concept Questions What happens during the process of meiosis? Why is crossing over important? How is meiosis different from mitosis?
You 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 cells sex cells (sperm) sex cells (egg)
Your 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.
Body 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.
Haploid (n) cells have one copy of every chromosome. –Gametes are haploid. –Gametes have 22 autosomes and 1 sex chromosome.
Chromosome 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 makes more diploid cells.
Meiosis makes haploid cells from diploid cells. –Meiosis occurs in sex cells. –Meiosis produces gametes.
Cells go through two rounds of division in meiosis. Meiosis reduces chromosome number and creates genetic diversity.
Meiosis 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 chromosomes sister chromatids sister chromatids
Meiosis I occurs after DNA has been replicated. Meiosis I divides homologous chromosomes in four phases.
Crossing over 3 steps –cross over –breakage of DNA –re-fusing of DNA New combinations of traits Tetrad
Meiosis II divides sister chromatids in four phases. DNA is not replicated between meiosis I and meiosis II.
McDougall Littell video - Meiosis
Haploid 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.
The value of sexual reproduction Sexual reproduction introduces genetic variation –genetic recombination during meiosis independent assortment of chromosomes –random alignment of homologous chromosomes in Meiosis 1 –crossing over mixing of alleles across homologous chromosomes –random fertilization which sperm fertilizes which egg? Driving evolution –variation for natural selection
McDougall Littell video – Law of Independent Assortment
Meiosis 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.
Mitosis vs. Meiosis
Mitosis –1 division –daughter cells genetically identical to parent cell –produces 2 cells –2n 2n –produces cells for growth & repair –no crossing over Meiosis –2 divisions –daughter cells genetically different from parent –produces 4 cells –2n 1n –produces gametes –crossing over
mitosis zygote Putting it all together… egg sperm 46 meiosis fertilization mitosis & development meiosis fertilization mitosis + development 46 gametes
Variation from random fertilization Sperm + Egg = ? –any 2 parents will produce a zygote with over 70 trillion (2 23 x 2 23 ) possible diploid combinations
Sexual reproduction creates variability Sexual reproduction allows us to maintain both genetic similarity & differences. Martin & Charlie Sheen, Emilio Estevez Michael & Kirk Douglas Baldwin brothers
Key Concept Questions What happens during the process of meiosis? –The chromosome number is reduced by half Why is crossing over important? –It creates new genetic combinations and more variety. How is meiosis different from mitosis?
Key Concept Question What structures actually assort independently?
Gene linkage was explained through fruit flies. Morgan found that linked traits are on the same chromosome. Chromosomes, not genes, assort independently during meiosis. Wild type Mutant
CHAPTER MYSTERY CLUE #3 –White 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
The 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
Linked genes are not inherited together every time. Chromosomes exchange homologous genes during meiosis.
Linkage 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.
Cross-over frequencies can be converted into map units. gene A and gene B cross over 6.0 percent of the time – gene A and gene C cross over 18.5 percent of the time – gene B and gene C cross over 12.5 percent of the time
Key Concept Question What structures actually assort independently? –Chromosomes