Genetics & Heredity 11/16/2017

Transcription

1 Genetics & Heredity Biology I Turner College & Career High School 2017 Fertilization is the fusion of an egg and a sperm. Purebred (True breeding plants) are plants that were allowed to selfpollinate and the offspring will be exactly like the parent. 1

2 The Work of Gregor Mendel Austrian monk Born in Studied heredity. Heredity: the passing on of characteristics from parents to offspring. Characteristics that are inherited are called traits. First person to successfully predict how traits are transferred from generation to generation. Used garden peas in his experiments. The Father of Genetics Mendel s Work Mendel carried out his work with garden peas. 2

3 Mendel s Observations Mendel noticed differences in: Flower color Flower position Seed color Seed shape Pea pod shape Pea pod color Stem height This led him to further experiment on the plants. Why pea plants? Reproduce sexually, which means that they produce male and female sex cells, called gametes. In a process called fertilization, the male gamete unites with the female gamete. The resulting fertilized cell, called a zygote, then develops into a seed. 3

4 Mendel s Experiment He took pollen from a male plant and dusted it onto a female plant. Parental generation (p) = the original pair of plants Female part Pollen grains Transfer pollen Male parts Cross-Pollination Offspring (F1) = first filial generation (F2) = second filial generation His first experiments are called monohybrid crosses because they only deal with ONE single trait (height, color) mono means one 4

5 Mendel s Experiments Purebred: offspring where all previous generations have the same trait. Ex: a purebred short plant came from short parent plants. First, he crossed a purebred purple with a purebred white. Result of F1 Generation: all plants had purple flowers. Parent Plants F1 Generation Mendel s 2 nd Experiment Crossed two of the purple offspring from the F1 generation. The F2 generation resulted in some offspring having purple flowers and others having white flowers Parent Plants F1 Generation F2 Generation 5

6 Conclusions Each organism has two factors that control each of its traits. These factors are genes and that they are located on chromosomes. Genes exist in different forms called alleles. The principal of dominance states that some alleles are dominant and others are recessive. P 1 Short pea plant Tall pea plant F 1 All Tall pea plants F 2 Tall pea plant 3 tall: 1 short Mendel called the observed trait dominant and the trait that disappeared recessive. Mendel concluded that the allele for tall plants is dominant to the allele for short plants. P 1 Tall pea plant Short pea plant T T t t T t F 1 All tall plants T t 6

7 Law of Segregation Tt x Tt Cross F 1 Tall Tall T t T t F 2 Tall Tall Tall Short T T T t T t t t 3 1 The way an organism looks and behaves is called its phenotype. Ex. Tall, yellow The allele combination an organism contains is known as its genotype. Ex. TT, Tt An organism s genotype can t always be determined by its phenotype. An organism is homozygous for a trait if its two alleles for the trait are the same. (Truebreeding) Exp. TT or tt An organism is heterozygous for a trait if its two alleles for the trait differ from each other. (Hybrid) Exp. Tt 7

8 Segregation Summary Each trait has two genes, one from the mother and one from the father. Traits can be either dominant or recessive. A dominant trait only needs one gene in order to be expressed. A recessive trait needs two genes in order to be expressed. Egg and sperm are sex cells called gametes. Segregation is the separation of alleles during gamete formation. Genetics & Probability Probability = the likelihood that a particular event will occur Ex. Coin flipping: 1/2 probability that coin will flip head/tail If you flip the coin 3 times what's the probability of flipping 3 heads? 1/2 x 1/2 x 1/2 = 1/8 *Past outcomes do not affect future ones!!* The principles of probability can be used to predict the outcomes of genetic crosses. What is the probability of parents having two male offspring in a row? (1/2 x 1/2=1/4) 8

9 Genetics & Probability The likelihood that a particular event will occur is called probability. Each trait has two genes, one from the mother and one from the father. Alleles can be homozygous; having the same traits. Alleles can be heterozygous; having different traits. Question 1 The passing on of characteristics from parents to offspring is. A. genetics B. heredity C. pollination D. allelic frequency The answer is B. Genetics is the branch of biology that studies heredity. 9

10 Question 2 What are traits? Answer Traits are characteristics that are inherited. Height, hair color and eye color are examples of traits in humans. Question 3 Gametes are. A. male sex cells B. female sex cells C. both male and female sex cells D. fertilized cells that develop into adult organisms The answer is C. Organisms that reproduce sexually produce male and female sex cells, called gametes. 10

11 Question 4 Which of the following genotypes represents a animal that is homozygous dominant for a trait? a. KK b. Kk c. kk Question 5 Which of the following genotypes represents a plant that is homozygous recessive for height? a. TT b. Tt c. tt 11

12 Punnett Squares The gene combination that might result from a genetic cross can be determined by drawing a diagram known as a Punnett square. These are used to predict and compare the genetic variations that will result from a cross. The types of gametes go on the top and left sides of the square. The possible gene combinations appear in the four boxes. Punnett Squares If you know the genotypes of the parents, you can use a Punnett square to predict the possible genotypes of their offspring. 12

13 Making a Punnett Square You try this one A A A A A A AA AA a a Aa Aa Possible Children: ½ (50%) AA = trait A purebred ½ (50%) Aa = trait A hybrid 100% will show the trait 13

14 More Practice For a gene determining hair color (B); where both parents are heterozygous for blue hair. B = dominant allele; blue hair b = recessive allele; yellow hair Parent 1 = Bb B b Parent 2 = Bb B BB Bb Possible Children b Bb bb Possible Children: ½ (50%) Bb = blue hair ¼ (25%) BB = blue hair ¼ (25%) bb = yellow hair More Practice Seed Shape Round (R) Wrinkled (r) Seed Color Yellow (g) Green (G) Flower Position Flower Color Axial (A) Terminal (a) Purple (A) White (a) Pod Shape Smooth (N) Constricted (n) Stem Height Pod Color Yellow (g) Green (G) Tall (T) Short (t) 14

15 Exploring Mendelian Genetics Independent Assortment 1. Genes segregate independently. 2. The principle of independent assortment states that genes for different traits can segregate independently during the formation of gametes. 3. Independent assortment helps account for the many genetic variations observed in plants, animals and other organisms. 15

16 Summary of Mendel s Principals Genes are passed from parent to offspring. Some forms of a gene may be dominant and others recessive. In most sexually producing organisms, each adult has two copies of each gene: one from each parent. These genes are segregated from each other when gametes are formed. The alleles for different genes usually segregate independently of one another. Beyond Mendel: Dominant & Recessive Alleles Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or multiple genes. Cases in which one allele is not completely dominant over another are called incomplete dominance. Example: White (W) and Red (R) are both dominant in a flower. If WW x RR the F 1 generation would be WR = pink. 16

17 Incomplete Dominance RR x WW RW x RW RR : RW : WW Incomplete: Think PINK! Co-dominance Codominance is when both alleles contribute equally to the phenotype. Examples: Feathers, flowers, cattle. 17

18 Polygenic Inheritance Polygenic inheritance the kind of inheritance in which the trait is produced from the cumulative effects of many genes. In humans, height, weight, and skin and eye color are examples of polygenic inheritance MEIOSIS A type of cell division that results in four daughter cells each with half the number of chromosomes of the parent cell, as in the production of gametes and plant spores. 18

19 Review: Characteristics of Living Things Living things are made up of cells. Unicellular: one celled organisms. Multicellular: many celled organisms. Review: Characteristics of Living Things Living things reproduce to make offspring of the same species. Asexual reproduction Sexual reproduction 19

20 Review: Characteristics of Living Things Living things grow and develop 1. What type of cell division is the diagram above? 2. What phase of cell division the arrow pointing to? 20

21 Meiosis is Reduction Division Chromosome number is cut in half by separation of homologous chromosomes in diploid cells Meiosis Every individual has two sets of chromosomes. One from the mother; one from the father. When the chromosomes pair up for the same trait they are called homologous chromosomes. A cell that contains pairs of homologous chromosomes is said to be diploid or 2n. Gametes (egg /sperm) have only one chromosome and are said to be haploid or n. 21

22 Chromosome Number in Body Cells vs. Gametes Gametes Haploid (n) - have 1 set Body Cells Diploid (2n) - have 2 sets Phases of Meiosis Meiosis is a process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell. Meiosis I: The homologous chromosomes line up but, then they crossover, exchanging genetic information. Meiosis II: The two cells produced now enter a second division. Start with 2 the two new cells and get 4 different cells each with 23 chromosomes. 22

23 Variation During normal cell growth, mitosis produces daughter cells identical to parent cell (2n to 2n) Meiosis results in genetic variation by shuffling of maternal and paternal chromosomes and crossing over. No daughter cells formed during meiosis are genetically identical to either mother or father During sexual reproduction, fusion of the unique haploid gametes produces truly unique offspring. Crossing Over: Meiosis I Chiasmata: site of crossing over, occur in synapsis. Exchange of genetic material between non-sister chromatids. Crossing over produces recombinant chromosomes. Results in variations in daughter cells. 23

24 Meiosis I Meiosis II Crossing Over Results of Meiosis Spermatogenesis: the process in which spermatozoa are produced from male germ cells by way of meiosis. Oogenesis: the production of an egg cell (ovum). Results: 4 viable haploid (n) sperm and 1 viable haploid (n) egg. All gametes are genetically different. 24

25 Summary of Meiosis II Results in 4 haploid (n) cells. Differs in male and female. Genetically different from each other & the original cell. Human Cells 2n=46. Sperm Cell 23 chromosomes Gamete is haploid (n) Egg Cell 23 chromosomes Gamete is haploid (n) White Blood Cell 46 chromosomes Body cell is diploid (2n) 25