Meiosis and Genetics Jaime Crosby, CHS

Diploid and Haploid Cells Body cells of animals and most plants, chromosomes occur in pairs One chromosome in each pair comes from the male parent and one from the female parent A cell with two of each kind of chromosome is called diploid cell (2n) Organisms produce gametes that contain one of each kind of chromosome; called a haploid cell (n)

Homologous Chromosomes The two chromosomes of each pair in a diploid cell are known as homologous chromosomes Each pair of chromosomes has genes for the same traits Because there are different possible alleles for the same gene, the two chromosomes in a homologous pair are not always identical

Why Meiosis? Meiosis allows offspring to have the same number of chromosomes as their parents Produces gametes containing half the number of chromosomes as a parent’s body cell Occurs in specialized body cells of each parent that produce gametes Male gamete called sperm Female gamete called egg Fertilized egg called zygote

Phases of Meiosis During meiosis chromosomal activity is different than that during mitosis Mitosis produces cells that are genetically the same Meiosis produces cells that can be genetically different

Meiosis: Step by Step: IPMATPMAT Interphase I-replication of chromosomes Prophase I-homologous chromosomes form a four part structure called tetrad, can exchange genetic material Metaphase I-spindle fibers attach to centromere of each chromosome and pull chromosomes to the center equator of cell Anaphase I-separation of tetrads to opposite sides of the cell Telophase I-chromosomes uncoil, cytoplasm divides, two new cells formed with half the chromosome number

Meiosis II: Step by Step Prophase II-spindle fiber forms, attach to chromosomes Metaphase II-chromosomes are pulled to center of cell Anaphase II-centromere of each chromosome splits as chromosomes are pulled to poles of cell Telophase II-nucleus reforms, cytoplasm splits Four haploid cells are formed, become gametes

Mitosis versus Meiosis

Genetic Recombination; Crossing over Meiosis provides for the shuffling of the chromosomes, providing many possible genetic combinations Typically two or three crossovers per chromosome occur during meiosis

Cool Facts The number of different kinds of sperm or eggs a person can produce is more than 8 million When fertilization occurs 70 trillion different zygotes are possible Experimentation has allowed us to make XL RED tomatoes, broccoflower, seedless watermelon, etc.

Mistakes in Meiosis The failure of homologous chromosomes to separate properly during meiosis is called nondisjunction Trisomy is when a gamete with an extra chromosome is fertilized by a normal gamete, the zygote has an extra chromosome; will develop Down syndrome Monosomy is when a gamete with a missing chromosome fuses with a normal gamete during fertilization, the zygote lacks a chromosome; most do not survive

Meiosis Example II

Mendel and Meiosis

Gregor Mendel Gregor Mendel, who many consider the father of genetics, was an Austrian monk in the mid-nineteenth century Began to breed garden pea plants so that he could study the inheritance of their characteristics

Mendel’s Study Mendel conducted the first important studies of heredity Heredity is the passing on of characteristics from parents to offspring Genetics is the study of heredity Characteristics that are inherited are called traits

Mendel’s Choices Mendel chose the pea plants he used in experiments carefully They reproduce sexually, having male and female gametes or sex cells Male gamete is pollen grain Female gamete is the ovule, located in the pistil

Transfer of pollen grains to the pistil is called pollination When male and female gametes combine, called fertilization Peas normally reproduce by self-pollination If Mendel needed to cross two different plants he would open the petals, remove anthers, and dusted the pistil of the plant he wanted to cross it with.

Mendel’s Monohybrid Crosses Mendel crossed “pure bred” tall plants with “pure bred” short plants to produce what he called hybrids Hybrid is the offspring of parents that have different forms of a trait, such as tall or short height Monohybrid means a cross of only one trait

Mendel selected a tall plant and cross pollinated with a short pea plant Planted seeds All resulting offspring were tall As if short pea plant had not been parent First Generation

Second Generation Next, Mendel allowed the tall plants to self pollinate, form seeds, and planted Counted more than 1000 plants Found ¾ were as tall as the tall plants, ¼ were short as the short plants Shortness had reappeared Ratio of 3 tall to 1 short plant (3:1)

Generational Labels Original parents, “true tall” and “true short”, are known as the P1 generation (P stands for parent) The offspring of the parent plants are known as the F1 generation (F stands for filial) Their offspring become the F2 generation

The Rule of Dominance Refer to the Mendel’s experimentation Trait that was observed in the F1 generation is dominant Choose the letter of the dominant trait to represent the allele choices Trait that disappeared is recessive Tall allele is dominant to the allele for short plants, short is recessive For a single trait, a single letter is used to represent (capital represents dominant trait and lower case represents recessive trait)

The Law of Segregation/ Independent Assortment Mendel concluded that each tall plant in the F1 generation carried one dominant allele for tallness and one unexpressed recessive allele for shortness F1 generation plants receive one allele from the tall parent, one from short parent During fertilization, these gametes randomly pair to produce four combinations of alleles. These traits are all inherited independently from each other x

Phenotypes and Genotypes Tt x TT Two organisms can look alike but have different gene combinations Phenotype is the way an organism looks and behaves; phenotype of tall plant is tall, no matter what gene it contains Genotype is the actual gene combination an organism contains; genotype for a pure tall plant is TT, for non-pure tall plant it is Tt

Homozygous vs. Heterozygous An organism is heterozygous for a trait if its two alleles differ from each other A tall plant that has one allele for tall and one for short (Tt) is heterozygous An organism is homozygous for a trait if its two alleles for the trait are the same TT is homozygous for tall tt is homozygous for short

Mendel’s Dihybrid Crosses Mendel performed another set of crosses in which he used peas that differed from each other in two traits rather than only one This type of cross is known as a dihybrid cross x

Dihybrid Cross: F1 Generation Mendel took true-breeding pea plants that had round yellow seeds (RRYY) and crossed them with true-breeding pea plants that had wrinkled green seeds (rryy) All seeds produced in the F1 generation were yellow round seeds Round and yellow were dominant traits

Dihybrid Cross: F2 Generation Mendel allowed F1 plants to self pollinate All possible combinations were found in F2 generation Definite phenotypic ratio present 9 round green 3 round yellow 3 wrinkled green 1 wrinkled yellow CALLED THE PRODUCT RULE

Incomplete Dominance When one trait is not completely dominant over the other. Example: red + white snapdragons = PINK

Punnett Squares T T TT TT T t Tt Tt

Punnett Squares Method of finding the expected proportions of possible genotypes in the offspring of a cross Monohybrid crosses Cross Gg x gg, G = green, g = blue Cross Rr x Rr, R = round, r = wrinkled; record genotypic and phenotypic ratio Dihybrid crosses

Probability Punnett squares show the likelihood that a possible combination of gametes will occur Reality is that results will follow the rules of chance Flipping a coin gives you a possibility of heads or tails 1:2 or 1/2 chance of landing on heads 1:2 or 1/2 chance of landing on tails