Published on December 14, 2007
CHAPTER 9: CHAPTER 9 Patterns of Inheritance What is Inheritance?: What is Inheritance? Passing on genetic information from parents to offspring Humans have 23 pairs of chromosomes (46 total) 1 of each pair is from one parent and the other is from the other parent Chromosomes: Chromosomes Long strands of DNA that carry the genetic information on building and sustaining a living human being. Each chromosome contains many genes What is a Gene?: What is a Gene? A segment of a chromosome that containing the code for a single protein (enzyme) - The enzyme causes a chemical reaction that allows a trait to be expressed. ie. Production of a pigment Alleles: Alleles Alleles are alternate forms of a gene Homozygous individuals have two identical alleles for a trait Heterozygous individuals have two different alleles for a trait Gregor Mendel (1822 - 1884): Gregor Mendel (1822 - 1884) Mendel was a monk who was trained in both biology and mathematics. He used his knowledge of biology to do test crosses on garden peas and count the progeny. He used his knowledge of mathematics to manipulate his data and derive his “postulates” for inheritance. In an Abbey Garden: Mendel studied garden peas These plant are easily manipulated These plants can self-fertilize In an Abbey Garden Figure 9.5 Stamen Carpel Slide9: Mendel carried out some cross-fertilizations Figure 9.6 Removed stamens from purple flower 1 2 3 Transferred pollen from stamens of white flower to carpel of purple flower 4 Planted seeds from pod Pollinated carpel matured into pod White Stamens Carpel Parents (P) Purple Offspring (F1) Slide10: Mendel’s started with traits with homozygous alleles He called these true breeding plants Mendel’s plants showed simple dominance Presence of the dominant allele meant that the trait would be expressed whether homozygous or heterozygous For the recessive form of the trait, expression only occurs in the homozygous form Dominant alleles are shown as uppercase letters Recessive alleles are shown as lower case letters Mendel’s Principles of Segregation: Mendel performed many experiments He tracked several characteristics in pea plants from which he formulated several hypotheses Mendel’s Principles of Segregation Slide12: Figure 9.7 Dominant Recessive White Pod shape Inflated Constricted Flower position Pod Color Dominant Recessive Flower color Purple Axial Terminal Green Yellow Tall Dwarf Seed shape Round Wrinkled Seed color Yellow Green Stem length Monohybrid Crosses: A monohybrid cross is a cross between parent plants that differ in only one characteristic Monohybrid Crosses Figure 9.8a P Generation (true-breeding parents) All plants have purple flowers Fertilization among F1 plants (F1 F1) F2 Generation 3/4 of plants have purple flowers 1/4 of plants have white flowers Purple flowers White flowers F1 Generation (a) Mendel’s crosses tracking one characteristic (flower color) Slide14: Mendel developed four hypotheses from the monohybrid cross There are alternative forms of genes, now called alleles For each characteristic, each organism has two genes Gametes carry only one allele for each inherited characteristic Alleles can be dominant or recessive Slide15: An explanation of Mendel’s results, including a Punnett square Wild type traits are those most often seen in nature: Wild type traits are those most often seen in nature Selective breeding brings out selected alleles for a trait Slide18: Phenotype An organism’s physical traits How does it look? Genotype An organism’s genetic makeup What alleles are present? Mendel’s principle of segregation: Mendel’s principle of segregation Pairs of alleles segregate (separate) during gamete formation; the fusion of gametes at fertilization creates allele pairs again Mendel’s Principle of Independent Assortment: Two hypotheses for gene assortment in a dihybrid cross Dependent assortment Independent assortment Mendel’s Principle of Independent Assortment Using a Testcross to Determine an Unknown Genotype: Using a Testcross to Determine an Unknown Genotype A testcross is a mating between An individual of unknown genotype and A homozygous recessive individual Family Pedigrees: Mendel’s principles apply to the inheritance of many human traits Family Pedigrees Figure 9.14 Freckles No freckles Widow’s peak Straight hairline Free earlobe Attached earlobe Slide24: A family pedigree Shows the history of a trait in a family Allows researchers to analyze human traits Human Disorders Controlled by a Single Gene: Many human traits Show simple inheritance patterns Are controlled by genes on autosomes Human Disorders Controlled by a Single Gene Slide26: Table 9.1 Recessive Disorders: Recessive Disorders Most human genetic disorders are recessive Individuals can be carriers of these diseases Dominant Disorders: Some human genetic disorders are dominant Achondroplasia is a form of dwarfism Dominant Disorders Figure 9.17 BEYOND MENDEL: Some patterns of genetic inheritance are not explained by Mendel’s principles Molecular genetics has provided some explanations BEYOND MENDEL Incomplete Dominanceor Codominance: Incomplete Dominance or Codominance In incomplete dominance F1 hybrids have an appearance in between the phenotypes of the two parents Slide31: Hypercholesterolemia Is a human trait that is incompletely dominant Multiple Alleles and Blood Type: The ABO blood groups in humans are examples of multiple alleles Possible alleles are A, B, or O Two of the human blood type alleles exhibit codominance Both alleles are expressed in the phenotype Multiple Alleles and Blood Type Figure 9.20 Pleiotropy: Pleiotropy is the impact of a single gene on more than one characteristic Sickle-cell disease is an example Pleiotropy Slide35: Figure 9.21 Individual homozygous for sickle-cell allele Sickle-cell (abnormal) hemoglobin Abnormal hemoglobin crystallizes, causing red blood cells to become sickle-shaped Sickled cells Breakdown of red blood cells Accumulation of sickled cells in spleen Physical weakness Anemia Heart failure Pain and fever Brain damage Damage to other organs Clumping of cells and clogging of small blood vessels Spleen damage Impaired mental function Paralysis Pneumonia and other infections Rheumatism Kidney failure Polygenic Inheritance: Polygenic Inheritance Polygenic inheritance is the additive effects of two or more genes on a single phenotype The Role of Environment: Many human characteristics result from a combination of heredity and environment The Role of Environment THE CHROMOSOMAL BASIS OF INHERITANCE: THE CHROMOSOMAL BASIS OF INHERITANCE The chromosome theory of inheritance states that Genes are located on chromosomes The behavior of chromosomes during meiosis and fertilization accounts for inheritance patterns Slide39: Figure 9.23 P Generation Yellow-round seeds (YYRR) Meiosis Fertilization Gametes Green-wrinkled seeds (yyrr) F1 Generation Principle of Segregation: Follow the long chromosomes (carrying R and r) taking either the left or right branch. All round yellow seeds (RrYy) Principle of Independent Assortment: Follow both the long and the short chromosomes. The R and r alleles segregate in anaphase I of meiosis. Meiosis Metaphase I (alternative arrangements) They are arranged in either of two equally likely ways at metaphase I. Only one long chromosome ends up in each gamete. Metaphase II They assort independently, giving four gamete types. Gametes Fertilization recombines the r and R alleles at random. Fertilization among the F1 plants Fertilization results in the 9:3:3:1 phenotypic ratio in the F2 generation. F2 Generation Gene Linkage: Gene Linkage In 1908, British biologists discovered an inheritance pattern inconsistent with Mendelian principles Slide41: This inheritance pattern was later explained by linked genes, which are Genes located on the same chromosome Genes that are typically inherited together SEX CHROMOSOMES AND SEX-LINKED GENES: SEX CHROMOSOMES AND SEX-LINKED GENES Sex chromosomes Influence the inheritance of certain traits Sex Determination in Human and Fruit Flies: Sex Determination in Human and Fruit Flies Sex chromosomes Are designated X and Y Determine an individual’s sex Sex-Linked Genes: Sex-linked genes Are any genes located on a sex chromosome Were discovered during studies on fruit flies Sex-Linked Genes Figure 9.28 (a) (b) Sex-Linked Disorders in Humans: A number of human conditions result from sex-linked (X-linked) genes Sex-Linked Disorders in Humans Slide47: Red-green color blindness Is a malfunction of light-sensitive cells in the eyes Figure 9.30 Slide48: Hemophilia Is a blood-clotting disease Figure 9.31 Queen Victoria Albert Alice Louis Alexandra Czar Nicholas II of Russia Alexis BIOLOGY AND SOCIETY: TESTING YOUR BABY: BIOLOGY AND SOCIETY: TESTING YOUR BABY Genetic testing Allows expectant parents to test for possibilities in their unborn child Includes amniocentesis and CVS Has risks associated with it Leads to Ethical Questions: Leads to Ethical Questions If you knew that your fetus had Down’s Syndrome or some other genetic defect, would you end the pregnancy? If you wanted a boy but testing showed the fetus to be a girl????