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This article explores the major concepts of inheritance, including genes being the units of inherited information, the role of chromosomes in gene expression, and the importance of genetic variation for evolution. It also discusses the ethical considerations of genetic engineering.
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Major Concepts: • Genes are discrete sequences of DNA on chromosomes; chromosomes consist of DNA and associated proteins. • Genes are the units of inherited information. • Genes code for several RNA types; mRNA is the template for proteins. • Inheritance of genes occurs in regular patterns that can be predicted by the rules of probability. • Genetic variation, from mutation and recombination, is essential for evolution. • The products of genetic engineering give rise to ethical consideration of benefits and risks to human well-being and environmental integrity.
Genes • Nucleotides Genes (DNA) Chromosomes
Heredity and Environment • Genetics: branch of biology that deals with inherited variation.
Heredity and Environment Nature vs. Nurture: • Both heredity and environment influence an individual’s development. • e.g. Siamese cats inherit genes for enzymes that produce dark pigment for fur. • The enzymes function best at temps below normal body temp., thus dark markings are at extremities: ears, face, paws, tail. • One can change coloration by keeping certain portions of cat’s body cool.
Heredity and Environment • The environment has a strong impact on gene expression.
Heredity and Environment Studies of twins: • Fraternal twins: develop from separate eggs each fertilized by separate sperm cells. • Identical twins: develop from one zygote forming two complete embryos.
Heredity and Environment • If a trait shows up more often in identical twins than fraternal: characteristic is probably genetic. • If a trait differs in identical twins: probably environmentally influenced.
Heredity and Environment • Blended inheritance: notion that mixing of parents’ genes resulted in an “averaging” of parental characteristics • no longer seriously considered, as it would not allow for passing on of traits separately to future generations, which is observed.
Genes • Information is stored in genes in the sequence of nucleotide bases that make up DNA: a molecular code. • The code directs the cell processes involved in development and function of cells and, thus, the entire organism. • Genes provide instructions for the structure, function and development of a cell/organism.
Genes • Many genes code for the synthesis of specific proteins, e.g. an enzyme, muscle protein, pigment, regulatory proteins, etc.; other genes code for various forms of RNA • Through processes of meiosis and fertilization, chromosomes are passed on from generation to generation.
Genes Determine Biological Potential • Heredity - passing of traits from parent to offspring • Genes – basic units of genetic info • Genetics - study of heredity • Involves predictions • referred to as probability - predicts the chances that a certain event will occur • Geneticist – one who studies heredity and the actions of genes.
Genes and Chromosomes • Prokaryotic chromosomes: single circular DNA molecule with little protein; generally no introns. • ~ 90% of DNA is translated.
Prokaryotic Chromosomes • Often have small circles of additional DNA: plasmids. • Plasmids may move from one bacterial cell to another, thereby introducing genetic variation. • Geneticists use plasmids to introduce modified genetic material into bacterial cells: genetic engineering, e.g. insulin production. • Plasmids carry genes for antibiotic resistance.
Genes and Chromosomes • Eukaryotic chromosomes: consist of long molecules of DNA wrapped around proteins. • Only part of the DNA codes for proteins. • Some noncoding sections of DNA consist of sequences repeating thousands of times.
Genes and Chromosomes • Only ~ 1.5% of human DNA codes for proteins. (We don’t know importance of rest.) • Some introns are involved with gene expression. • Repetitive sequences may serve to stabilize DNA’s bond with associated proteins. • Mutations can convert inactive DNA sequences into active genes, or inactivate functional genes may be a source of new alleles in natural selection.
Genes and Chromosomes • Homologous chromosomes carry same genes, though not necessarily the same alleles for those genes.
Genes and Chromosomes • Chromosomes may be distinguished by their banding pattern (pattern of dye that occurs when chromosome is stained (Fig. 8.9, p. 191). Ea. chromosome has a distinctive banding pattern.
Genes and Chromosomes • Karyotype: (Fig. 13.11, p. 349) a display of human chromosomes arranged as homologous pairs.
Karyotypes • Used in genetic studies of disease to search for hereditary causes. • Members of each pair have a specific banding pattern when dyed, as the stains bind to specific regions of the chromosomes.
Karyotypes • White blood cells frequently used for such studies: • They can be made to divide easily. • Grow well in culture. • Chemicals interrupt cell cycle at metaphase. Why? • Cells are placed on microscope slide and treated with water to spread chromosomes apart. • Stains cause the banding pattern to appear. • Unique banding of each chromosome pair enables researchers to detect missing or extra chromosome parts and extra chromosomes themselves. • Have also helped with mapping of genes on chromosomes.
Genes and Chromosomes Karyotype: • Allows one to count and identify chromosomes, and spot any unusual, missing or extra chromosomes fairly quickly.
Genes and Chromosomes • Human karyotype made up of 22 pairs of autosomes, chromosomes that are the same in ♂ & ♀, and one pair of sex chromosomes, chromosomes that are different in ♂ & ♀. • ♀: XX; ♂: XY
Probability • Probability: an area of mathematics that predicts the chances that a certain event will occur. • Using the rules of probability, one can predict the most probable outcome of randomly ordered events; the actual outcome, however, may not match the prediction, i.e. the prediction is simply that: a prediction, not a guarantee. • Investigation 13A: Probability, pp. 748 – 49 (Lab write-up due ___)
Gregor Mendel • 1822 – 1884 • Austrian monk • Studied science & math at the University of Vienna • Formulated the laws of heredity in the early 1860's • Did a statistical study of traits in garden peas over an eight year period • Mendel’s work led to the concept of the gene - Mendelian Genetics
Why garden peas (Pisum sativum)? • Can be grown in a small area • Produce lots of offspring • Produce pure plants when allowed to self-pollinate several generations (true-breeding) • Can be artificially cross-pollinated
Garden pea flowers contain both male & female reproductive parts • Self-pollination (pollinates itself) • Cross-pollination (collect pollen from flowers of one pea plant & transfer to another)
Mendel and the Idea of Alleles • 4 ways experiments were unique: • Looked at only one trait at a time • Used large numbers • Combined results of many identical experiments • Analyzed results using rules of probability • Thus, Mendel was able to see patterns of inheritance
Mendel and the Idea of Alleles • Mendel studied 22 simple traits of pea plants (e.g. seed color & shape, pod color & shape, etc.). • Mendel traced the inheritance of individual traits & kept careful records of numbers of offspring. • He used his math principles of probability to interpret results. • Mendel studied pea traits, each of which had a dominant & a recessive form.
Inheritance of Alleles • Trait: any characteristic that can be passed from parent to offspring • Allele: one of two or more possible forms of a gene (e.g. dominant & recessive) • Dominant: an allele that masks the presence of another allele of the same gene in a heterozygous organism, represented by capital letter, e.g. B • Recessive: a trait (allele) whose expression is masked (hidden) in a heterozygous organism, represented by lower-case letter, e.g. b • Genotype: genetic makeup of an organism; gene combination for a trait (e.g. RR, Rr, rr) • Phenotype: observable appearance or trait determined by the genotype; the physical feature resulting from a genotype (e.g. tall, short)
Inheritance of Alleles • Homozygous: The condition (genotype) in which both alleles are the same form, e.g. RR, rr; can produce only one type of gamete; also called “pure.” • Heterozygous: The condition in which two alternate forms (alleles) of a gene are contained within the organism, e.g. Rr; also called “hybrid.” • Monohybrid cross: a cross (mating) involving a single trait. • Dihybrid cross: a cross (mating) involving two traits. • Punnett Square: graphic tool (grid) used to solve genetics problems.
Inheritance of Alleles • A Punnett Square:
Inheritance of Alleles • A Punnett Square:
The dominant (shows up most often) gene or allele is represented with a capital letter, & the recessive gene with a lower case of that same letter (e.g. B, b) • Mendel’s traits included: a. Seed shape --- Round (R) or Wrinkled (r) b. Seed Color---- Yellow (Y) or Green (y) c. Pod Shape --- Smooth (S) or wrinkled (s) d. Pod Color --- Green (G) or Yellow (g) e. Seed Coat Color --- Gray (G) or White (g) f. Flower position --- Axial (A) or Terminal (a) g. Plant Height --- Tall (T) or Short (t) h. Flower color --- Purple (P) or white (p)
Mendel’s Experiments (cont.) • 1st: Mendel tested each strain of plant he used to ensure that it was true-breeding (homozygous), i.e. genetically true, producing offspring identical to themselves generation after generation. • 2nd: He worked with strains that were true-breeding in all but one characteristic, e.g. tall vs. short plant form; green vs. yellow seeds, round vs. wrinkled seeds, etc. • This allowed him to follow the pattern of inheritance of one trait at a time from generation to generation, e.g. round vs. wrinkled seeds:
Mendel’s Experiments (cont.) • Parental generation (P1): Crossed round seed-producing plants with wrinkled seed-producing plants. • First filial generation (F1): All offspring of the above cross produced round seeds. • Second filial generation (F2): ¾ produced round seeds; ¼ produced wrinkled seeds. • Thus, wrinkled seeds seemed to disappear in one generation (F1), then reappear in the next (F2). • Mendel called the round seed condition, “dominant,” and the wrinkled seed condition, “recessive.” • He surmised that the recessive form of a trait could only be manifest when the individual inherited that trait from both parents, i.e. received two “doses” of the trait. • This is known as Mendel’s principle of dominance.
Mendel’s Experiments (cont.) • Alleles are not dominant or recessive; only their effects on a trait are. • At the molecular level, the genotype, both alleles are present. • At the level of the organism, the phenotype, the effects of one allele (the dominant form) may mask those of the other (recessive form) allele. • Mendel repeated this type of experiment for other traits (outlined in Fig. 6.10, p. 179). • He calculated the ratio of dominant to recessive forms for each trait and it was always essentially the same: the dominant form appeared in approx. ¾ of the F2 plants, while the recessive form appeared in ¼ of the F2 plants. Thus, the ratio was always3:1. • (See p. 179)
Inheritance of Alleles • Mendel did not know about genes. He referred to dominant and recessive “factors” to describe the results of his experiments. • He did not know where these “factors” were located in cells. • Hypothesized that only one copy of a factor went into each sperm or ovum, i.e. if a parent were true-breeding for round seeds, for example, all its gametes would have the “round-seed factor.” Similarly for “wrinkled-seed factor.” • Offspring of a round-seed by wrinkled seed cross would have one factor of each from each parent: the principle of segregation.
Inheritance of Alleles • Genotype: genetic makeup of an organism; made up of the alleles for any gene, e.g. RR, Rr, rr. • Phenotype: the observable appearance or trait that is determined by the genotype, e.g. Three distinct genotypes: • BB phenotype: purple flowers (dominant) • Bb phenotype: purple flowers (dominant) • bb phenotype: white flowers (recessive)
Inheritance of Alleles • Mendel also worked with plants that varied in two traits at a time, e.g. round vs. wrinkled seeds and yellow vs. green seed color. • Dihybrid cross: a cross that results in offspring that are heterozygous for two (di) traits. • See Fig. 13.14, p. 353; a Punnett square • F2 Phenotypes occur in 9:3:3:1 ratio (9/16, 3/16, 3/16, 1/16). • Each trait individually displays the 3:1 ratio of a monohybrid cross. • The genes for the various traits separate independently from one another . . . • Principle of independent assortment: alleles for one trait segregate independently of alleles for the other trait during gamete formation.
Patterns of Inheritance • Testcross: a cross between an organism with an unknown genotype and an organism with the recessive phenotype • The recessive phenotype individual must be homozygous recessive, thus one knows the genotype. • The resulting Punnett Square results will allow you to determine the genotype of the “unknown” parent. • Quick Lab: Using a Testcross (p. 185)
