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Genetics Overview Acton Biology 1.

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Presentation on theme: "Genetics Overview Acton Biology 1."— Presentation transcript:

1 Genetics Overview Acton Biology 1

2 Heredity Mendelian Genetics 9

3 Reproduction Asexual reproduction
Single parent passes copies of all its genes to offspring Offspring arise by mitosis (clone of parent) Sexual Reproduction Two individuals (parents) contribute genes to offspring Results in greater genetic variation of offspring than asexual reproduction 2

4 Asexual Reproduction Binary Fission - Bacteria Budding - Yeast
Vegetative Reproduction - Strawberry Spore Formation - Fungi Asexual reproduction is reproduction which does not involve meiosis, ploidy reduction, or fertilization. 3

5 Sexual Reproduction Combination of genetic material
Results in increased genetic diversity Processes: Meiosis Fertilization Sexual reproduction is characterized by processes that pass a combination of genetic material to offspring, resulting in increased genetic diversity. The main two processes are: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the restoration of the original number of chromosomes. During meiosis, the chromosomes of each pair usually cross over to achieve homologous recombination. 4

6 Meiosis 2 stages of cell division:
Meiosis I Meiosis II Results in daughter cells which has half as many chromosomes as parent cell 5

7 Meiosis I Meiosis I separates homologous chromosomes, resulting in reduction from diploid to haploid cells Crossover occurs during Prophase I  increasing genetic diversity - In prophase I, homologous chromosomes pair up into tetrads in a process called synapsis. Crossing over of DNA segments occurs at chiasmata, and genetic information is exchanged between the homologues. In metaphase I, the paired tetrads, formed by synapsis during prophase I, line up on the metaphase plate. Note the alignment of maternal and paternal homologues along the metaphase plate is random. In anaphase I, the homologous pairs move toward opposite poles of the cell. Notice sister chromatids remain attached, and some of the chromatids have exchanged genetic material. At the end of telephase I and cytokinesis I, two haploid cells are produced with two chromatids still attached to each chromosome. 6

8 Meiosis II Meiosis II is similar to mitosis, except the cell undergoing division is haploid rather than diploid Meiosis II has produced 4 haploid gametes, each of the chromosomes has one chromatid 7

9 Fertilization Sexual life cycle of animals
Haploid gametes fuse by fertilization to form a diploid zygote. Zygote undergoes many rounds of mitosis to produce the diploid multicellular organism. Diploid germ cells undergo meiosis to produce haploid gametes. 8

10 Genetic Variation Sexual reproduction results in increased genetic variation Specific aspects affect how genes are varied Alleles Crossover 10

11 Alleles For each observable trait (phenotype), organism inherits 2 alleles, one from each parent. Alleles make up its genotype. If the 2 alleles at a locus (the region on a chromosome where a gene is found) differ, the organism is heterozygous, otherwise it is homozygous. 11

12 Crossover Crossing over occurs in Prophase I
Nonsister chromatids of homologous chromosomes exchange portions. The recombinant chromosomes carry genes derived from different parents. 12

13 Terminology Homozygous: contain 2 of the same alleles for particular trait Heterozygous: 2 different alleles for a trait Phenotype: expressed traits Genotype: Genetic Make-up Testcross: determine if individual showing dominant trait is homozygous or heterozygous by crossing it with homozygous recessive individual Monohybrid Cross: Cross involving study of one trait Dihybrid Cross: Cross involving study of 2 traits 13

14 Testcross 19

15 Inheritance Patterns Mendel used scientific approach to identify 2 laws of inheritance 4 concepts that make up Mendel’s model Alternative versions of genes cause variations in inherited characteristics among offspring (e.g. Alleles) For each character, every organism inherits one allele from each parent If the 2 alleles are different, then the dominant allele will be fully expressed in the offspring, recessive allele will have no noticeable effect on the offspring The 2 alleles for each character separate during gamete production  Law of segregation 14

16 Mendel’s Law of Segregation
Each plant inherits 1 allele for flower color from each parent. The 2 alleles segregate (separate) and end up in different gametes during meiosis. Random fertilization between gametes yield predictable ratios in the offspring. 15

17 Mendel’s Law of Independent Assortment
States that each pair of alleles will segregate independently during gamete formation 2 traits assort independently of each other, assuming they are on different chromosomes 16

18 Random Fertilization When a heterozygote (Rr) forms gametes, segregation of alleles is like the toss of a coin. We can determine the probability for any genotype among the offspring of 2 heterozygotes by multiplying the individual probabilities of a gamete having a particular allele (R or r). 18

19 Laws of Probability Rule of multiplication: When calculating probability that 2 of more independent events will occur together, you multiply the probabilities e.g. AABbCc x AaBbCc Rule of Addition: Calculating the probability that any of two or more mutually exclusive events will occur you add together the individual probabilities. 20

20 Inheritance Patterns Complete Dominance Codominance
Incomplete Dominance 22

21 Complete Dominance A monohybrid cross yields a 3:1 phenotypic ratio in the F2, assuming purple flower color is dominant and white is recessive. The genotypic ratio is 1:2:1, since there are 2 types of purple- flowered plants: PP (homozygous) and Pp (heterozygous). The true-breeding P generation must have identical alleles for that gene and are homozygous. In the heterozygous F1 and F2 indivuduals, the dominant purple allele determines the phenotype. 23

22 Codominance The A, B, AB, or O phenotypes are affected by 3 different alleles. IA and IB alleles produce different antigens on the surface of red blood cells, thus are dominant to the i allele which produces no antigen. IA and IB are codominant to each other because the RBCs bear both antigens. 24

23 Incomplete Dominance When red snapdragons are crossed with white ones, the F1 hybrids have pink flowers. Superscripts indicate alleles for flower color: CR for red and CW for white. The F2 generation produces a 1:2:1 ratio for both genotype and phenotype. 25

24 Epistasis A gene at one locus may affect phenotypic expression of a gene at another locus by epistasis. The B/b gene determines the pigment color (B for black and b for brown) The epistatic C/c gene controls whether or not any pigment will be deposited in the hair. A homozygous recessive cc mouse has no hair pigment and is albino regardless of its B/b genotype. 17

25 Multifactorial Traits: Melanin
Human skin pigmentation is influenced by multiple genes which produce different melanin pigment molecules and shows quantitative variation. This polygenic inheritance also exhibits incomplete dominance. 26

26 Sample Questions A couple has six children, all daughters. If the woman has a seventh child, what is the probability that the seven child will be a daughter? A) 6/7 B) 1/7 C) 1/36 D) 1/49 E) 1/2 Answer: E 27

27 Questions Which of the following is NOT true of meiosis?
During metaphase, spindle microtubules first come into contact with chromosomes. The chromosome number in the newly formed cells is half that of the parent cell The homologous chromosomes line up along the metaphase plate, or equator of the cell The cytoplasm of the cell and all its organelles are divided approximately in half. In anaphase II, the sister chromatids travel to opposite ends of the cell. Answer: A 28

28 http://www.mhhe.com/biosci/genbio/virtual_lab s/BL_15/BL_15.html
Virtual Fly Lab s/BL_15/BL_15.html 29

29 Pedigree Charts Pedigree analysis is a way to solve genetic puzzles
Useful when traits of many generation of offspring have been recorded can be used to trace the passing of an allele from parents to offspring 30

30 Pedigree Symbols 31

31 Pedigree A recessive trait such as attached earlobe may skip a generation. A dot may be placed in a symbol to represent known heterozygotes (carriers who do not exhibit the recessive phenotype). 32

32 Linked Genes 33

33 Linked Genes If the 2 genes were on different chromosomes, independent assortment should yield equal numbers of the 4 types of F2. If the 2 genes were on the same chromosome, each allele combination (B+ vg+ and b vg) should stay together and only yield parental phenotypes in the F2. The high percentage of parental phenotypes in the F2, with some nonparental (recombinant) phenotypes imply the 2 genes are physically close to each other on the same chromosome. wild-type crossed with black, vestigial-winged flies produce heterozygous F1 dihybrids, all of which appear wild-type. A testcross of F1 females with black, vestigial-winged males produced 2,300 F2 offspring. If the 2 genes were on different chromosomes, independent assortment should yield equal numbers of the 4 types of F2. If the 2 genes were on the same chromosome, each allele combination (B+ vg+ and b vg) should stay together and only yield parental phenotypes in the F2. The high percentage of parental phenotypes in the F2, with some nonparental (recombinant) phenotypes imply the 2 genes are physically close to each other on the same chromosome. 34

34 Genetic Recombination
Recombinant phenotypes explained by crossing over of homologous chromosomes No new allele combinations in males  sex linked trait Genetic recombination. Recombinant phenotypes can be explained by crossing over of homologous chromosomes In the female, crossing over can occur between b and vg in Meiosis I, which then segregate in Meiosis II and yield recombinant gametes. In the male, crossing over yields no new allele combinations. 35

35 Genetic Recombination
The recombinant chromosomes from the female explains the small percentage of recombinant phenotypes in the F2 offspring. 36

36 Recombinant Frequencies
Recombination frequencies can be used to construct a linkage map of the chromosome The farther apart genes are, the more likely they are to be separated during crossing over, and the higher their frequency of recombination. cn- cinnabar – gene for red eye colour b – body colour vg – wing-szie The recombination frequencies between 3 gene pairs (b-cn 9%, cn-vg 9.5%, and b-vg 17%) best fit a linear order in which cn is positioned about halfway between the other 2 genes. Recombination frequencies can be used to construct a linkage map of the chromosome, since the farther apart genes are, the more likely they are to be separated during crossing over, and the higher their frequency of recombination. 37

37 Karyotype 38

38 What is a karyotype? Organized profile of a person's chromosomes
Chromosomes are arranged and numbered by size, from largest to smallest Arrangement helps scientists quickly identify chromosomal alterations that may result in a genetic disorder 39

39 http://learn.genetics.utah.edu/content/begin/tr aits/karyotype/
Karyotype Activity Try it yourself! aits/karyotype/ 40

40 X-Linked Inheritance X-linked inheritance.
A father with the disorder will transmit the mutant allele to all daughters but to no sons. If the mother is a dominant homozygote, the daughters will have the normal phenotype but will be carriers of the mutation. 41

41 Errors and Exceptions Errors in Meiosis
Aneuploidy - Missing copy of a chromosome Polyploidy - Extra copy of a chromosome Disorders due to Chromosomal Alterations Certain cancers are due to alterations in chromosome configuration 42

42 Nondisjunction Nondisjunction can produce gametes with an extra or missing chromosome, or aneuploidy. Homologous chromosomes may fail to separate during meiosis I. Sister chromatids may fail to separate during meiosis II. 43

43 Alterations of Chromosome structure
Deletion – removes chromosomal segment Duplication – repeats segment Inversion – reverses segment within chromosome Translocation – moves segment from one chromosome to another, nonhomologous one Deletions and duplications are likely to occur during meiosis – homologous chromatids sometimes break and rejoin at ‘incorrect’ places so that one partner gives up more genes than it receives – results with one chromatid with deletion and other with duplication Large deletions are typically lethal in embryo Duplications are translocations tend to have harmful effects – altered phenotype because gene’s expression can be influenced by location among neighboring genes 44

44 Human Disorders Trisomy 21 – Down Syndrome
Extra X chromosome in a male – Kleinfelter syndrome Turner’s Syndrome – Monosomy X Triple X Syndrome – Trisomy X Cri du chat – specific deletion in Chromosome 5 Chronic myelogenous leukemia (CML) – Portion of chromosome 22 switched places with small fragment from tip of chromosome 9 Cri du chat: mentally retarded, small head, unusual facial features, cry like a distressed cat – don’t survive past infancy/early childhood CML: cancer affected cells that give rise to white blood cells 45

45 Sample Questions During the first meiotic division (Meiosis I)
Homologous chromosomes separate The chromosome number becomes haploid Crossing over between nonsister chromatids occurs Paternal and maternal chromosomes assort randomly All of the above occur Answer:E 46

46 If an individual with blood type O, whose mother has blood type A, The father must have which of the following blood types? A, B or O AB or A AB or B AB only O only Answer: A 47

47 The mother carries an allele for hemophilia
In humans, hemophilia is a sex-linked recessive trait. If a man and a woman have a son who is affected with hemophilia, which of the following is definitely true? The mother carries an allele for hemophilia The father carries an allele for hemophilia The father is afflicted with hemophilia Both parents carry an allele for hemophilia The boy’s paternal grandfather has hemophilia Answer: A 48

48 Review of Free-Response Question
Genes are located on chromosomes and are the basic unit of heredity that is passed on from parent to child, through generations Explain how a chromosome mutation could occur and why mutations are detrimental to the organism in which they take place. Explain why it is that – Although there are very few genes located on the Y chromosome – human males may suffer from having just one copy of the X chromosome, whereas females have two X’s and do not suffer as much as males. 49


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