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Sexual Reproduction and Genetics

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Presentation on theme: "Sexual Reproduction and Genetics"— Presentation transcript:

1 Sexual Reproduction and Genetics
Chapter 10

2 Meiosis 10.1

3 Chromosomes Human body cells have 46 chromosomes. Each parent contributes 23 chromosomes resulting in 23 pairs. The chromosomes that make up a pair (one chromosome from each parent) are called homologous chromosomes. Homologous chromosomes have the same length and centromere position and they carry genes that control the same traits.

4 Chromosomes

5 Chromosome Number To maintain the same chromosome number from generation to generation, organisms produce gametes. Gametes: sex cells that have half the number of chromosomes. The symbol n is used to represent the number of chromosomes in a gamete.

6 Chromosome Number An organism with n number of chromosomes is called a haploid cell. Fertilization: the process by which one haploid gamete combines with another haploid gamete. As a result of fertilization the cell will now contain a total of 2n chromosomes (n from the female parent and n from the male parent). An organism with 2n chromosomes is called a diploid cell.

7 Chromosome Number

8 Asexual vs Sexual Reproduction
Asexual Reproduction: only one parent, offspring identical to parent. Sexual Reproduction: two parents combine their genetic material to produce offspring that are genetically different from both parents.

9 Mitosis vs Meiosis Mitosis maintains the chromosome number.
Meiosis reduces the chromosome number by half.

10 Mitosis vs Meiosis

11 Meiosis Meiosis: cell division resulting in a reduced number of chromosomes, gamete formation. Occurs in the reproductive structures of organisms that reproduce sexually. Involves two cell divisions called Meiosis I and Meiosis II.

12 Sexual Life Cycle

13 Meiosis I Chromosome pairs replicate resulting in two haploid (n) daughter cells with duplicated chromosomes different from the sets in the original diploid (2n) cell. Occurs in 4 stages: prophase I, metaphase I, anaphase I, and telophase I. Begins with interphase, like in mitosis. Cells increase in size, produce RNA, synthesize proteins, and replicate DNA.

14 Meiosis I

15 Meiosis I: Prophase I Replicated chromosomes become visible.
Pairing of homologous chromosomes occurs, each chromosome consists of two chromatids. Crossing over: chromosomal segments are exchanged between a pair of homologous chromosomes producing an exchange of genetic information. Nuclear envelope breaks down and spindles form.

16 Meiosis I: Prophase I

17 Meiosis I: Metaphase I Chromosome centromeres attach to the spindle fibers. Homologous chromosomes line up at the equator.

18 Meiosis I: Metaphase I

19 Meiosis I: Anaphase I Homologous chromosomes separate and move to the opposite poles of the cell. Sister chromatids remain attached. Each daughter cell will receive only one chromosome from each homologous pair.

20 Meiosis I: Anaphase I

21 Meiosis I: Telophase I The spindles begin to break down.
Chromosomes uncoil and form two nuclei. The cell divides.

22 Meiosis I: Telophase I

23 Meiosis II Each of the two daughter cells from Meiosis I contains only one chromosome from each parental pair. Each daughter cell from Meiosis I will undergo Meiosis II.

24 Meiosis II

25 Meiosis II: Prophase II
The chromosomes condense and the spindles form in each new cell. The spindle fibers attach to the chromosomes.

26 Meiosis II: Prophase II

27 Meiosis II: Metaphase II
The centromeres of chromosomes line up randomly at the equator of each cell.

28 Meiosis II: Metaphase II

29 Meiosis II: Anaphase II
The centromeres split. The sister chromatids separate and move to opposite poles.

30 Meiosis II: Telophase II
Four nuclei form around chromosomes. The spindle fibers begin to break down. The cell divides.

31 Anaphase II & TelophaseII

32 Meiosis

33 Mendelian Genetics 10.2

34 How Genetics Began Inheritance: the passing of traits to the next generation. Gregor Mendel: published findings on the method and mathematics of inheritance in garden pea plants.

35 Why Pea Plants? Pea plants are easy to grow and many are true- breeding which means they consistently produce offspring with only on form of a trait. Pea plants usually reproduce by self-fertilization (male and female gametes in same flower). Pea plants can easily be cross-pollinated by hand (transferring a male or female gamete in one flower to another flower).

36 Pea Plants

37 The Inheritance of Traits
Mendel noticed certain varieties of garden pea plants produced specific forms of a trait, generation after generation. Mendel performed cross pollination by transferring male gametes from the flower of a true-breeding green-seed plant to the female organs of a flower from a true-breeding yellow- seed plant. Mendel called the green-seed plants and the yellow-seed plants the parent generation or P generation.

38 The Inheritance of Traits
When Mendel grew the seeds from the cross between the green-seed and yellow-seed plants, all of the offspring had yellow seeds. The offspring of the P cross are called the first filial (F1) generation.

39 The Inheritance of Traits
Mendel then planted the F1 generation of yellow seeds to determine whether the green- seed trait was no longer present or if it was being masked. The results of the second filial (F2) generation are as follows: 6022 were yellow and were green (a nearly perfect 3:1 ratio of yellow to green seeds).

40 Inheritance of Traits Mendel studied seven different traits, including: seed or pea color, flower color, seed pod color, seed shape or texture, seed pod shape, stem length, and flower positions. He found that the F1 generation plants from these crosses also showed a 3:1 ratio.

41 F1 and F2 Generations

42 Genes in Pairs From his experiments, Mendel concluded that there must be two forms of the seed trait in the pea plants and that each was controlled by a factor. Allele: an alternative form of a single gene passed from generation to generation. The green-seed and yellow-seed are different forms of a single gene or alleles.

43 Genes in Pairs Mendel called the form of the trait that appeared in the F1 generation dominant and the form that was masked recessive. The yellow-seed was dominant and the green-seed was recessive.

44 Law of Dominance If the allele for a trait is dominant it is represented by a capital letter. In Mendel’s case the yellow-seeds were dominant and therefore represented by a Y. If the allele for a trait is recessive it is represented by a lowercase letter. Therefore the green-seeds in Mendel’s experiment would be represented by a y.

45 Law of Dominance An organism with two of the same alleles for a particular trait is homozygous. Homozygous yellow-seed plants would be YY and homozygous green-seed plants would be yy.

46 Law of Dominance Heterozygous organisms have two different alleles for a trait. In Mendel’s experiment a plant with an allele for yellow-seeds and green-seeds would be represented by Yy. When an organism is heterozygous the dominant allele will be expressed.

47 Genotype and Phenotype
Genotype: an organism’s allele pairs. The genotype of a plant with yellow seeds would be YY or Yy. Phenotype: the observable characteristics or outward expression of allele pair. The phenotype of pea plants with the genotype yy would be green seeds.

48 Genotype and Phenotype

49 Law of Segregation Two alleles for each trait separate during meiosis and during fertilization two alleles for that trait unite.

50 Law of Independent Assortment
A random distribution of alleles occurs during gamete formation. Genes on separate chromosomes separate independently during meiosis.


52 Punnett Squares Used to predict the possible offspring of a cross between two known genotypes.

53 Monohybrid Cross Examines the inheritance of one trait.
Parent generation could both be homozygous or heterozygous or one could be homozygous and the other heterozygous.

54 Monohybrid Cross

55 Dihybrid Cross Examines the inheritance of two different traits.

56 Dihybrid Cross ry RY RrYy
R = dominant round, r = recessive wrinkled; Y = dominant for yellow, y = recessive green (rryy x RRYY)

57 Gene Linkage and Polyploidy 10.3

58 Genetic Recombination
The new combination of genes produced by crossing over and independent assortment. The possible combination of genes due to independent assortment can be calculated by the formula 2n where n is the number of chromosomes.

59 Gene Linkage Linked genes are genes that are located close to each other on the same chromosomes. These genes tend to travel together during gamete formation. Linked genes do not segregate independently and are therefore an exception to Mendel’s Law of Independent Assortment.

60 Chromosome Maps Chromosome Maps: drawings that show the sequence of genes on a chromosome and represent the relative position of genes. The higher the crossover frequency, the farther apart the two genes are because crossing over occurs more frequently between genes that are far apart. The frequency correlates with the relative distance between the two genes.

61 Chromosome Maps

62 Polyploidy Polyploidy: the occurrence of one or more extra sets of chromosomes in an organism. Ex) 3n: 3 complete sets of chromosomes. Rarely occur in animals but sometimes occur in earthworms and goldfish. Approximately 1 in 3 species of flowering plants are polyploid. Polyploidy is always lethal in humans.

63 Polyploidy 8n

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