Heredity and Molecular Genetics: Chapters 14 through 19 Remington Grenier, Mercedes Cote, Tyana Nowlan, and Rebecca Isaacs
Gregor Mendel found his theories of inheritance through experiments with pea plants. • It was Mendel’s idea that parents pass discrete genes onto their children that preserve the parents’ identities through the generations. • He found that characters, or genetically inherited characteristics that differs from person to person, were what was passed on. • For each character, an organism inherits two alleles (different versions of the same gene), one from each parent. • Alternative versions of genes account for variations in inherited characters. • A trait is any variant of a character, such as blue or yellow color for a flower
Law of Segregation • One of Mendel’s major hereditary laws. • States that: • Every organism carries a pair of alleles for each trait • The members of this pair separate during gamete formation. • Example: If an individual is Bb for eye color, during gamete formation, one gamete would receive a B, and the other would receive a b. • Mendel determined his law of segregation while performing monohybrid crosses. • Monohybrid Cross: a cross that involves a single character in which both parents are heterozygous (BbxBb).
Dominant vs. Recessive • If two alleles at a locus differ, then one, the dominant allele (B), determines the organism's appearance. • The other, the recessive allele (b), has no noticeable effect on the organism's appearance. • Also known as complete dominance. • Example: A purple flower that has been crossed with a white flower could either be BB or Bb because the dominant allele (B) would overpower the recessive allele (b).
Homozygous vs. Heterozygous • Homozygous (pure): an individual is homozygous for a gene if both of the given alleles are the same. • Example: BB (homozygous dominant) or bb (homozygous recessive). • Heterozygous (hybrid): an individual is heterozygous for a gene if the two alleles are different. • Example: Bb
Phenotype vs. Genotype • Genotype: an organism’s genetic makeup for a given trait. • Example: Considering fur color where B represents the allele for brown and b represents the allele for black, the possible genotypes include homozygous brown (BB), heterozygous brown (Bb), and homozygous recessive (bb). • Phenotype: the physical expression of the trait associated with a particular genotype. • Example: Phenotypes for Mendel’s peas included round or wrinkled, green or yellow, and purple or white flower.
Intermediate Inheritance • Intermediate inheritance occurred when an individual heterozygous for a trait showered characteristics not exactly like either parent. • Two major types of inheritance include incomplete dominance and codominance. • Incomplete Dominance: the heterozygous genotype produces an intermediate phenotype rather than the dominant phenotype; neither allele dominates the other. • Also known as “blending inheritance” • Example: Crossing a snapdragon plant with red flowers with one that has white flowers yields offspring with pink flowers. • Codominance: both alleles express themselves fully in a heterozygous organism. • Example: Human Blood Groups
Other Forms of Inheritance • Polygenice Inheritance: a single phenotypic character is affected by two or more genes • Example: Skin Color • Multiple Alleles: in the whole population, some genes have more than two alleles • Example: ABO Blood Group Alleles • Epistasis: one gene affects the expression of another • Example: Coat Color of Mice • Pleiotropy: One gene is able to affect multiple phenotypic characters • Example: Sickle-Cell Disease
Trait-Any detectable variant in a genetic character. • Hybridization- In genetics, the mating, or crossing, of two true-breeding varieties. • P Generation- The parent individuals from which offspring are serived in studies of inheritance; P stands for parental. • F1 Generation- The first filial, or hybrid, offspring in a series of genetic crosses. • F2 Generation- Offspring resulting from interbreeding of the hybrid F1 generation.
Alleles- Any of the alternative versions of a gene that produce distinguishable phenotypic effects. • Dominant Allele- An allele that is fully expressed in the phenotype of a heterozygote. • Recessive Allele- An allele whose phenotypic effect is not observed on a heterozygote. • Homozygous- Having two identical alleles for a given gene. • Heterozygous- Having two different alleles for a given gene.
Phenotype- The physical and physiological traits of an organism, which are determined by its genetic makeup. • Genotype- The genetic makeup, or set of alleles, of an organism. • Mono hybrids -An organism that is heterozygous with respect to a single gene of interest. All the offspring from a cross between parents homozygous for different alleles are monohybrids. • Dihybrids- An organism that is heterozygous with respect to two genes of interest. All the offspring from a cross between parents doubly jomozygous for different alleles are dihybrids.
Complete Dominance- The situation in which the phenotypes od the heterozygote and dominant homozygote are indistinguishable. • Incomplete Dominance- The situation in which the phenotype of heterozygotes is intermediate between the phenotypes of individuals homozygous for either allele. • Co-dominance- The situation in which the phenotypes of both alleles are exhibited in the heterozygote because both alleles affect the phenotype in separate, distinguishable ways.
Pleiotropy- The ability of a single gene to have multiple effects. • Epistasis - A type of gene interaction in which one gene alters the phenotypic effects of another gene that is independently inherited. • Polygenic Inheritance- An additive effects of two or more genes on a single phenotypic character. • Carriers- In genetics, an individual who is heterozygous at a given genetic locus, with one normal allele and one recessive allele. The heterozygote is phenotypically dominant for the character determined by the gene but can pass on the recessive allele to offspring.
Chorionic Villus Sampling (CVS)- A technique of prenatal diagnosis in which a small sample of the fetal portion of the placenta is removed and analyzed to detect certain genetic and congenital defects in the fetus.
Chapter 14 Graphic • In his garden Mendel bred two different colored flowers. While in the first generation the hybrids were all the same color because they were heterozygous in the F1 generation. His breeding of the plants also shows that in the F2 generation they follow the same guidelines that punnett squares do resulting in a 3:1 with heterozygote purple flowers and homozygous recessive white flowers.
Mendelian Inheritance and Chromosome Behavior • Meiosis produces haploid gametes with Meiosis 1 including the separation of homologous pairs and crossing over and Meiosis 2 including the separation of sister chromatids. • The behavior of chromosomes during meiosis accounts for the law of segregation and independent assortment. • This is also known as the chromosome theory of inheritance, which states that Mendelian genes have specific loci along chromosomes and these chromosomes undergo segregation and independent assortment.
The Chromosomal Basis of Sex • The sex of a organism is an inherited phenotypic character usually determined by which sex chromosomes are present. • The sex chromosomes carry genes for some traits that are unrelated to sex characteristics. • Example: Recessive alleles causing color blindness are carried on the X chromosome. Fathers transmit this and other sex-linked alleles to all daughters but no sons. Any male who inherits such an allele from his mother will express the trait.
X-Inactivation • X-inactivation: during the development of the female embryo, one of two X chromosomes in each cell remains coiled as a Barr body whose genes are not expressed. A cell expresses the alleles of the active X chromosome only. • Not all cells inactivate the same X. As a result, different cells will have different active X chromosomes.
Linkage • Linked genes are genes along the same chromosome that tend to be inherited together because the chromosome is passed a long as a unit. • Linked genes lie on the same chromosome and do not follow Mendel’s law of independent assortment. • Recombinant offspring exhibit new combinations of traits inherited from two parents. • Due to the law of independent assortment of chromosomes, unlinked genes show a 50% frequency of recombination in the gametes. • Linked genes experience crossing over between nonsisterchromatids which accounts for the observed recombinants, which is always less that 50% of the total.
Common Disorders • Simple recessive disorders in which a person must be homozygous recessive for the gene in question to have the disease include: • Tay-Sachs disease: a fatal genetic storage disease that renders the body unable to break down a particular type of lipid. • Cystic Fibrosis: a recessive disorder that is the most common lethal genetic disease in the United States. A defective version of a gene on chromosome 7 results in the excessive segregation of a thick mucus, which accumulates in the lungs and digestive tract.
Common Disorders (cntd.) • Sickle Cell Anemia: a recessive disease caused by the substitution of a single amino acid in the hemoglobin protein of red blood cells, leaving hemoglobin less able to carry oxygen and also causing the hemoglobin to deform to a sickle shape when the oxygen content of the blood is low. • Phenylketonuria: an autosomal recessive disease caused by a single gene defect that leaves a person unable to break down phenylalanine, which results in a by-product that can accumulate to toxic levels in the blood and cause mental retardation. • Huntington disease: an autosomal dominant degenerative disease of the nervous system that shows itself when a person is in their 30s or 40s and is both irreversible and fatal.
Chromosomal Complications • A change in the number of chromosomes in the individual structure of chromosomes, such as nondisjunction and aneuploidy, can affect the phenotype. • Examples: down syndrome (aneuploidy), trisomy 21 (nondisjunction), and turner syndrome (nondisjunction). • The breaking of chromosomes can result in deletions, inversions, duplications, and translocations. • Examples: cri-du-chat (deletion) and chronic myelogenous leukemia (chromosomal translocation).
Law of Segregation -Mendel’s first law, stating that the two alleles in a pair segregate into different gametes during gamete formation. • Law of Independent Assortment - Mendel’s second law, stating the each pair of alleles segregates, or assorts, independently of each other pair during gamete formation; applies when genes for two characters are located on different pairs of homologous chromosomes.
Chromosome theory of inheritance- states that mendillian genes have specific loci along chromosomes and it’s the chromosomes that undergo segregation and independent assortment • Wild type- the phenotype for the characteristic most commonly observed in natural populations • Sex-linked gene- a gene located on either sex chromosome • Duchenne muscular dystrophy- a disease characterized by a progressive weakening of the muscles and loss of coordination • Hemophilia- a sex linked disorder defined by the absence of one or more of the protein required for the blood clotting factor
Barr body- a dense object lying along the inside of the nuclear envelope in cells of female mammals, representing a highly condensed, inactivated X chromosome • Linked genes- genes located close enough together on a chromosome that they tend to be inherited together • Genetic recombination- general term for the production of offspring with combinations of traits that differ from those found in either parent • Parental types- an offspring with a phenotype that matches on of the parental phenotypes • Recombinant types- an offspring whose phenotype differs from that of the parents
Crossing over-the reciprocal exchange of genetic material between nonsisterchromatids during prophase I of meiosis • Genetic map- an ordered list of genetic loci along a chromosome • Linked map- a genetic map based on the frequencies of recombination between markers during crossing over of homologous chromosomes • Map units-a unit of measurement of the distance between genes
Nondisjunction- an error in meiosis or mitosis in which members of a pair of homologous chromosomes or a pair of sister chromatins fail to separate properly from each other • Aneuploidy- a chromosomal aberration in which in which one or more chromosomes are present in extra copies or are deficient in number • Monosomic- referring to a cell that has only one copy of a particular chromosome instead of the normal two • Trisomic-referring to a diploid cell that has three copies of a particular chromosome instead of the normal two
Polyploidy-a chromosome alteration in which the organism possesses more than two complete chromosomes sets • Deletion- a mutational loss of one or more nucleotide pairs of genes • Duplication-An aberration in chromosome structure due to fusion with a fragment from a homologous chromosome, such that a portion of a chromosome is duplicated.
Inversion-An aberration in chromosome structure resulting from reattachment of a chromosomal fragment in a reverse orientation to the chromosome from which it originated. • Translocation- An aberration in chromosome structure resulting from attachment of a chromosomal fragment to a nonhomologous chromosome. • Genomic imprinting-A phenomenon in which expression of an allele in offspring depends on whether the allele is inherited from the male or female parent.
Chapter 15 Graphic • The graphic shows Meiosis and how it demonstrates Mendel’s laws exhibited during dihybrid crossing. • On one side you see the Law of Segregation which shows that the two alleles for each gene separate during gamete formation. As an example, follow the fate of the long chromosomes (carrying R and r). • On the other you see the Law of Independent Assortment which shows that alleles of genes on nonhomologous chromosomes assort independently during gamete formation.
DNA is the Genetic Material • Experiments with bacteria and with phages provided the first strong evidence that the genetic material is DNA. • Watson and Crick found that DNA is a double helix with two sugar-phosphate backbones. • The “rungs” on such a ladder would represent pairs of nitrogenous bases • Note that Adenine (A) always pairs with Thymine (T) and Cytosine (C) with Guanine (G).
DNA Replication • DNA occurs in the S-phase in a semi-conservative fashion and in a 5’ to 3’ direction. • Semi-Conservative: the replicated double helix consists of one old strand and one new strand.
Steps of DNA Replication • 1. Helicase unwinds our double helix into two strands. • 2. Polymerase adds nucleotides to an existing strand. • Since DNA polymerase can only add DNA in the 5’ to 3’ direction, a leading strand and lagging strand are created. • 3. Ligase brings together the Okazaki fragments. • 4. Topoisomerase cuts and rejoins the helix. • 5. RNA primase catalyzes the synthesis of RNA primers.
Proofreading and Repairing DNA • DNA polymerases proofread new DNA, replacing incorrect nucleotides. • Two repair mechanisms are mismatch repair and nucleotide excision repair. • Mismatch Repair: DNA polymerases replace an incorrectly placed nucleotide with the proper nucleotide. • Nucleotide Excision Repair: enzymes cut out and replace damaged stretches of DNA.
Replicating the Ends of DNA Molecules • The ends of eukaryotic chromosomal DNA get shorter with each round of replication. • Telomeres postpone the erosion of genes. • Telomerase catalyzes the lengthening of telomeres in germ cells.
A Chromosome Consists of DNA Packed Together with Proteins • Eukaryotic chromatin is composed mostly of DNA, histones, and other proteins. • Histones bind to each other and to the DNA to create nucleosomes. • Additional folding leads to highly condensed chromatin. • In interphase cells, most chromatin is less compacted (euchromatin), but some remain highly condensed (heterochromatin).
Transformation- A change in genotype and phenotype due to the assimilation of external DNA by a cell. • Semiconservative Model- Type of DNA replication in which the replicated double helix consists of one old strand, derived from the old molecule, and one newly made strand. • Origins of Replication-Site where the replication of a DNA molecule begins, consisting of a specific sequence of nucleotides.
Replication Fork-A Y-shaped region on a replicating DNA molecule where the parental strands are being unwound and new strands are growing. • Helicases-An enzyme that untwist the double helix of DNA at the replication forks, separating the two strands and making them available as template strands. • Single-strand Binding Proteins- A protein that binds to the unpaired DNA strands during DNA replication, stabilizing them and holding them apart while they serve as templates for the synthesis of complementary strands of DNA.
Topoisomerase- A protein that breaks, swivels, and rejoins DNA strands. During DNA replication, topoisomerase helps to relieve strain in the double helix ahead of the replication fork. • Primer/Primase- An enzyme that joins RNA nucleotides to make the primer using the parental DNA strand as a template. Primer is a short stretch of RNA with a free 3’ end, bound by complementary base pairing to the template strand, that is elongated with DNA nucleotides during DNA replication.
DNA Polymerases- An enzyme that catalyzes the elongation of new DNA by the addition of nucleotides to the 3’ end of an existing chain. There are several different DNA polymerases; DNA polymerase III and DNA polymerase I play major roles in DNA replication in prokaryotes. • Lagging Strand- A discontinuously synthesized DNA strand that elongates by means of Okazaki fragments, each synthesized in a 5’-3’ direction away from the replication fork. • Leading Strand-The new complementary DNA strand synthesized continuously along the template strand toward the replication fork in the mandatory 5’-3’ direction.
Okazaki Fragments- A short segment of DNA synthesized away from the replication fork on a template strand during DNA replication, many of which are joined together to make up the lagging strand of newly synthesized DNA. • Mismatch Repair-The cellular process that uses specific enzymes to remove and replace in correctly paired nucleotides. • Nuclease- An enzyme that cuts DNA or RNA either removing one or few bases or hydrolyzing the DNA or RNA completely into its component nucleotides.
Nucleotide Excision Repair- A repair system that removes and then correctly replaces a damaged segment of DNA using the undamaged strand as a guide. • Telomeres- The tandemly repetitive DNA at the end of a eukaryotic chromosome’s DNA molecule that protects the organism’s genes from being eroded during successive rounds of replication. • Telomerase- An enzyme that catalyzes the lengthening of telomeres in eukaryotic germ cells.