Download Study Guide Genetics

Study Guide: Genetics Exam 1 MEIOSIS: Meiosis I ● Prophase I: ○ Chromatin condense to form chromosomes and the nuclear membrane begins to disappear. ■ Sister chromatids are present and are connected by centromeres to form each chromosome. ○ Homologous chromosomes pair together and crossing over occurs at the chiasmata. ○ Two centrosomes, each with a pair of centrioles move toward the opposite poles of the cell and the mitotic spindle begins to form. ○ The chromosomes move toward the metaphase plate. ○ Diploid number of chromosomes. ● Metaphase I: ○ The homologous pairs are lined up at the metaphase plate. ○ The microtubules of the spindle are attached to the kinetochores on the centromeres. ○ Diploid number of chromosomes ● Anaphase I: ○ The homologous pairs split and move toward opposite ends of the cell. ○ Sister chromatids are still attached. ○ Diploid number of chromosomes. ● Telophase I and Cytokinesis: ○ The cleavage furrow forms (in plants the cell plate forms) and the splitting of the cytoplasm occurs ○ Two daughter cells are formed ○ There is a haploid number of chromosomes in each cell. 2 Meiosis II: ● Prophase II: ○ The spindle apparatus forms again ○ Sister chromatids are still present, but not homologs anymore. ○ The chromosomes move toward the metaphase II plate. ○ Haploid number of chromosomes. ● Metaphase II: ○ The chromosomes are lined up at the metaphase II plate. ■ Note: because of crossing over in Meiosis I, the sister chromatids are not genetically identical. ○ The kinetochores are attached to the microtubules. ○ Haploid number of chromosomes. ● Anaphase II: ○ The sister chromatids split and move toward opposite poles. ○ Technically there is a haploid number of chromosomes when the sister chromatids split because as soon as they are split they are no longer called chromosomes but chromatids. But on each side of the cell there is a haploid number. ● Telophase II and Cytokinesis: ○ Cleavage furrow (or cell plate) ○ Cytoplasm splits. ○ Haploid number of chromosomes. ○ Two daughter cells result from each of the two daughter cells produced in meiosis I. So the meiotic division of one parent cell produces four daughter cells total that are NOT GENETICALLY IDENTICAL. 3 Genetic Disorders related resulting from Meiosis: THEORETICAL GENETICS: Basics: ● Every organism is not identical because of variation in meiosis. ● Each species though is related in that they have the same number of chromosomes, number of nucleotide pairs, number of genes, etc. ● Traits like eye color and hair color arise from the expression of genes, which are a section of DNA on a chromosome. ○ Within a species, the location of these genes are on the same place of the chromosome or on the same locus. ○ Different variations of the gene are called alleles. ● When examining traits, we look at the genotype, which are the alleles of an organism and we look at the phenotype which is the appearance/characteristics of the organism. A little look into Alleles: ● All organisms have two alleles for every gene present. There can be many different alleles giving rise to different traits, but only two can show up in the genotype at once. ○ For example the genotype of a round seeded pea is Rr ■ The big R represents the dominant allele of round and little r represents the recessive allele of wrinkled. ○ Homozygous: the alleles are the same ■ in the example above, it is not homozygous because the genotype is not RR or rr. ○ Heterozygous: the alleles are different. ■ The genotype however, is heterozygous because there is both the dominant and recessive allele present. ○ Dominant and Recessive: ■ Dominant is usually uppercase, and recessive is lowercase. ● In most cases, the dominant masks the recessive allele. And so if both a dominant and recessive trait are present, then the organism’s phenotype is that of the dominant trait. ○ Incomplete Dominance: Neither alleles are dominant. And so what results is a blend of the phenotypes. ■ If a red and white snap dragon are crossed, the resulting offspring are going to be pink snapdragons. ○ Codominance: ■ For example, in sickle cell, if a person possesses the trait of sickle cell, both normal shaped blood cells and sickle shaped blood cells are present. Punnett squares: ○ The “parents” of the offspring have the genotypes Rr and Rr. They can each can produce gametes of R and r. ○ A punnett square is basically a table used to determine the potential genotypes and phenotypes of offspring. ○ On the top of all the columns, place the possible gametes for one parent, and on the left of all the rows, place the possible gametes for the other parent. 4 Sex linked genes: ● Some genes only appear on either the x or the y chromosome. ○ We denote the allele of a sex linked gene with a superscript on the X or Y, depending on what sex chromosome it is located on. ● For example the allele of color blindness occurs on the X chromosome. ○ XC is the allele for normal vision and Xc is the allele for color blindness. ● Why is color blindness so prevalent in men and why does it hardly occur in women? ○ For men, it is easy to possess the phenotype of colorblindedness because since it is only on the x chromosome, if the mother gives the allele for colorblindedness, there is no other chance for a dominant allele to mask the effects. ○ In women, there is a much lower chance of being colorblind because the father would have to be color blind, and the mother would either have to be colorblind or be heterozygous for that gene, to even have a chance of being colorblind. There is a fairly high chance that if the woman is a carrier for the colorblindedness trait, that it would be masked by the dominant allele for normal vision. Solving Linked Gene problems: ● Some genes are located on the same chromosome. If two alleles are linked on a chromosome, most likely those alleles will stay linked. But there are some times when there are different combinations that arise. This is due to recombination during crossing over. ● Most linked gene problems include one parent crossed with a homozygous recessive parent. ○ this is known as a test cross. ○ a test cross can determine if the genes are linked because it gives all the same gametes. Also the recessive is always true breeding (the dominant, there is possibility that it is heterozygous) ● The recombination frequency is the amount of recombinants over the total number of offspring (there has to be actual data of offspring) steps: 1. define alleles 2. determine recombinants 3. diagram linkages for the parents and any other generation before the test cross. 4. draw the linkages of all possible gametes. 5. combine them with the gametes from the homozygous recessive 6. write the number of offspring for each genotype. 7. calculate the recombination frequency by dividing the number of recombinants over the total number of offspring. 8. determine how far apart the two genes are. The recombination frequency is equivalent to the distance in map units or centimorgans 5 1.
In tomatoes, vine height and fruit shape are linked. Tall vines are dominant to dwarf vines, and round fruit (tomatoes) is dominant to pear-­shaped tomatoes. When a dwarf tomato plant with pear-­shaped tomatoes is crossed to a true-­breeding tall tomato plant with round tomatoes, all the first generation offspring are tall with round tomatoes. When these first-­generation plants are subjected to a test cross with the parent dwarf pear-­shaped tomato plant, these progeny are obtained: 229 tall, round 23 tall, pear-­shaped 27 dwarf, round 221 dwarf, pear-­shaped a)
Diagram the linkage groups of all the tomato plants in this problem (parents, F-­1, and test cross offspring) b)
Calculate % crossover between the height and shape genes.
c)
How far apart are these two genes? 6 Pedigrees: ● Pedigrees are basically the “family trees” of genetics. ● They help to figure out genotypes of previous generations by using the history of known genotypes. Polygenic Inheritance: GENETIC DISORDERS/MUTATIONS MUTATIONS: ● point mutations=base substitutions. Usually not harmful. ○ silent mutations: does not affect ● frameshift mutations: when one or more nucleotide is deleted, causing a shift in the reading of codons. HARMFUL!! Results in completely different proteins being synthesized because all of the following codons are shifted and they code for completely different amino acids. GENETIC DISORDERS: ● Nondisjunction: Occurs in mitosis or meiosis. ○ Nondisjunction is when the sister chromatids or the homologous chromosomes fail to separate, in which one receives a surplus of chromosomes and the other now has a lack of chromosomes. ○ A cell in which nondisjunction has occurred is called an aneuploid. Cells with 2n-­1 chromosomes is considered monosomy, and cells with one extra chromosome is trisomy. ■ Trisomy 21 is also known as Down Syndrome. ○ Nondisjunction in Meiosis I: Homologous chromosome fails to split. ○ Nondisjunction in Meiosis II: Sister chromatids fail to split. 7 VOCABULARY ABO blood groups: A blood types can result from IAi or IAIA genotypes. B blood types can either result from IBIB or IBi genotypes. AB blood types result from a IAIB genotype. O blood types result from a ii genotype. Blood type is an example of codominance. Allele: A form of a gene Amniocentesis: also known as the amniotic fluid test to determine prenatal chromosomal abnormalities and fetal infections. Anaphase I: the phase in meiosis I in which homologous chromosomes split up. Anaphase II: the phase in meiosis II in which sister chromatids split. Aneuploidy: an irregularity in chromosome number. This occurs in nondisjunction. There can either be extra chromosomes or a lack of chromosomes. Autosome: A chromosome that is not a sex chromosome (all other chromosomes except the X and Y chromosome). Barr body: the condensed inactivated x-­chromosome present in most females. Carrier: To be a carrier for a disease is to be heterozygous and possess the recessive allele. A carrier for a disease does not directly have the disease but has the potential to give a gamete that carries the disease. Centromere: the region in which the sister chromatids are connected. Chi Square: A statistical value to determine how far apart a set of data is from it’s expected value. Chiasmata: the region in which the chromosomes cross during crossing over. Chorionic villus sampling: another method of determining prenatal chromosomal abnormalities. Uses a sample of placental tissue. Chromosome: Condensed structures of chromatin. Contains DNA. Codominance: There are more than one dominant alleles. They each influence the phenotype in an individual way. Both get expressed. This is different from incomplete dominance because incomplete dominance is that neither masks the other and a blend is resulted. Color blindness: recessive X-­linked trait. More prevalent in males because of this. Females rarely have color blindness because it is easier to receive a dominant allele that will mask the recessive allele. Crossing Over: a section of a chromosome gets spliced and exchanged with another section of a homologous chromosome in prophase I of Meiosis. This is how variation and recombinants arise. Cytokinesis I and II: the phases in meiosis that are the splitting of the cytoplasm. Daughter Cell: a cell that results from cellular division. In mitosis the daughter cells are genetically identical. In meiosis they have one half the number of chromosomes and they are not genetically identical to the parents. Mitosis produces 2 daughter cells and mitosis produces 4 daughter cells. Dihybrid Cross: The cross of F1 offspring of two individuals differing in two traits. Diploid: an autosomal number of chromosomes, also known as 2n. DNA: the genetic code for everything in the body. Made up of four nitrogenous bases Adenine, Thymine, Guanine, and Cytosine. Dominant: An allele that masks the recessive allele. The dominant phenotype is expressed when a dominant and recessive allele are paired when the allele is completely dominant. Down Syndrome: Also known as Trisomy 21, this genetic disorder results when there is an extra chromosome 21 from meiosis. Results from nondisjunction in meiosis. Egg: The female sex cell or gamete. Can only be produced through meiosis. Only one out of four daughter cells in meiosis in a female makes an egg. In males, all of the daughter cells in meiosis end up as sperm. Eggs are made before birth. And females are born with a set number of eggs that they release monthly after puberty. F1: The first generation after the “parents” Stands for filial 1. Filial means son. The offspring of the parents. F2: The second generation after the “parents”. The offspring of the F1 generation. Gamete: Sex cell. Gametes only arise from meiosis. Gender: XX is usually female and XY is usually male. However there are some cases when XY can be female because a certain gene is not expressed in the Y chromosome. 8 Gene: A section of a chromosome that expresses a trait. One gene does not have to code for only one trait. Generation: Same birth cohort. Cousins and siblings are of the same generation. Aunts, uncles and their nephews and nieces are of different generations. Parents and children are in different generations. etc. Genetic Testing: Tests of the DNA to determine genotypes. Genome: the genetic material of an organism. When the entire human genome is sequenced means that the number of chromosomes, the genes on the chromosomes, and the number of nucleotides is known. Genotype: The alleles present. The genetic makeup. Haploid: The number of chromosomes in a sex cell or a gamete. Also known as n. Half the number in the somatic cells. Hemizygous: Only one member of a chromosome pair is present. Refers particularly to x-­linked genes in males. In which under usual circumstances only have one X chromosome. So when referring to an x-­linked allele present in a male, it is hemizygous for that allele. Hemophilia: A recessive X-­linked trait in which the blood cannot clot normally. Heredity: The passing of physical or mental characteristics to offspring genetically. Heterozygous: The genotype has both the dominant and recessive allele. Homologous: Chromosomes of the same type. Homozygous: The same alleles are present in the genotype. Huntington’s Disease (HD): Autosomal dominant mutation. Affects muscle coordination. Incomplete Dominance: There is no dominant allele. Neither masks the other. Inheritance: the process of passing characteristics to offspring. Karyotype: An image showing the chromosomes. They are arranged in pairs of homologs. Linkage group: grouping of linked alleles. Linked genes: genes located on the same chromosome. Locus: the location of a gene on a chromosome. Meiosis: the process of cellular division resulting in gametes or sex cells. Mendel’s law of independent assortment: Allele pairs separate during the formation of gametes. Mendel’s law of segregation: allele pairs separate during gamete formation and reunite during fertilization. Metaphase I: The phase in meiosis I in which homologous chromosomes line up at the metaphase plate. Metaphase II: The phase in meiosis II in which sister chromatids line up at the metaphase plate. Metaphase plate: The place in which the chromosomes line up before splitting in anaphase. Mitosis: Cellular division resulting in somatic cells. Mitotic spindle: The group of microtubules that guide the chromosomes when they split. Monohybrid Cross: Cross between two individuals differing in one trait. Mutation: the changing of the structure of a gene. Either substitution, deletion, or rearranging of base pairs. Non-­disjunction: An abnormality in chromosome number. Pedigree: The “family tree” of genetics. Used to figure out unknown genotypes based on the history of the family genetics. Phenotype: the physical expression of a trait. Polygenic inheritance: More than one gene affects one trait. Prophase I: the phase in Meiosis I in which chromatin condenses into chromosomes, crossing over occurs, and the mitotic spindle forms. Prophase II: The phase in Meiosis II in which the mitotic spindle forms. Punnett Square: A “table” to determine the potential genotypes of offspring. Purebred: Also known as true breeding. Homozygous. Recessive: Allele that is masked by the dominant allele. When a dominant allele is present, the recessive allele is not expressed in the phenotype in a usual case. Recombination Frequency: also known as the percent crossover. This is determined by dividing the number of recombinants over the total number of offspring. Reduction division: Cellular division in which the chromosome number is reduced. Meiosis is a reduction division because the daughter cells have half the number of chromosomes as the parent cell. 9 Sex chromosome: The chromosomes that determine the gender of an individual. X and Y are the sex chromosomes in humans. Sex linkage: A gene present only on one of the sex chromosomes. Sickle-­cell anemia: A recessive x-­linked disease in which the hemoglobin are sickle shaped and cannot carry oxygen well, resulting in a lack of oxygen delivered to somatic cells. Sister Chromatids: each of the two strands in a chromosome that divides during mitosis and meiosis II. Somatic Cell: also known as body cells, these cells have a complete set of chromosomes or 2n or diploid. These are all cells except sex cells. Sperm: The male sex cell. In male meiosis, all of the daughter cells form sperm, whereas in female meiosis, only one out of four daughter cells become the egg. T-­test: A statistical test of two population means. Telophase I and II: The phases in meiosis I and II that the cell actually divides, the cleavage furrow or the cell plate forms. Trisomy 21: Also known as down syndrome. This is a genetic disorder resulting from an extra chromosome 21. True-­breeding: also known as pure-­breeding, it can only pass down one version of gamete. Homozygous. Unlinked Genes: Genes that are not on the same chromosome. X-­Chromosome: Sex chromosome. Females have two X-­Chromosomes in usual cases. Males only have 1. Females can only pass on X chromosomes. Y-­Chromosome: The sex chromosome that usually only males possess. 10 11