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Extending Mendelian Genetics, moving into Human Genetics Chapter 7 • 1879 Walther Flemming – *German biologist *stained cells with dye and saw tiny, threadlike structures in the nucleus CHROMOSOMES!!! *observed and described MITOSIS -- noted that a full set of chromosomes was being passed on to each daughter cell. • 1900 -- Sixteen years after Mendel’s death in 1884, his paper is rediscovered -- scientists realize that the chromosomes are the carriers of heredity – Mendel’s FACTORS are ensuring the passing of traits from parents to offspring (remember, Mendel’s pea plant research spanned 1856-1863) • 1902 Walter Sutton – *American biologist; supports idea that “factors” are located on chromosomes *thought that Mendel’s concepts could be applied to all chromosomes at a cellular level • 1905 E.B. Wilson and Nettie Stevens -- Americans studying insect chromosomes -- they worked independently -- both saw that male insects always showed a chromosome that did not seem to have a match (females always had a perfect matching set of chromosomes.) Thus, they developed the chromosomal theory of sex *referred to the non-matching chromosomes as Sex Chromosomes. • In females the sex chromosomes do match XX • In males, one of the chromosomes looked as if it were missing a part, so called it a Y XY See board for Punnett square for sex determination: Autosomes vs. sex chromosomes The first 22 pairs of human chromosomes are autosomes ONLY the last pair are sex chromosomes. • 1909 Wilhelm Johannsen – Danish biologist who coined the term “gene” to define the physical units of heredity • GENE: segment of DNA molecules that carries the instructions for producing a specific trait • 1912 Thomas Hunt Morgan – Showed evidence that the presence of white eye color in fruit flies was associated with a particular gene on a particular chromosome. • Drosophila melanogaster -- scientific name for fruit fly Chose to study fruit flies for following reasons: • produces about 100 offspring per egg lay – good statistics! • Matures in only 15-20 days • Only have 8 chromosomes (4 pair) so less to look at • Easy/inexpensive to raise • Chromosomes are VERY large and easy to see and locate • Sexes are easily distinguished --female is larger -- color of abdomens identify sexes at a glance • Normally, fruit flies always have RED eyes, but Morgan saw a white eyed one show up, and it was MALE!! Thought that this was strange, so he conducted an experiment: P white eyed X red eyed F1 all red eyed offspring (thus concluded that red is dominant over white for color) F1 red eyed X red eyed F2 ¾ red eyed ¼ white eyed AND ALL OF THE WHITE EYED ONES WERE MALE!!! • Determined that this was a sex-linked trait – the trait for eye color in fruit flies is carried on the sex chromosome. • Examples of other sex-linked traits: hemophilia • Color blindness • C = normal vision, c = colorblindness • XcY crossed with XC Xc When are showing sex linked genes, MUST use the X and Y as base letters… Sex-linked genes in female mammals… In each cell of female mammals, one of the two X chromosomes is randomly “turned off” by the process of X chromosome inactivation Because of this, females are a patchwork of two types of cells: *one type with an active X chromo that came from the mother, and a second type with an active X chromo that came from the father Ex. Tortoiseshell and calico cat fur colors • X inactivation Epistasis • In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus • For example, in mice and many other mammals, coat color depends on two genes • One gene determines the pigment color (with alleles B for black and b for brown) • The other gene (with alleles C for color and c for no color) determines whether the pigment will be deposited in the hair Ex. double heterozygous black mouse (BbCc) X double heterozygous black mouse (BbCc) gametes: BC, Bc, bC, bc for each Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 14-12 BbCc BbCc Sperm 1/ 4 BC 1/ 4 bC 1/ 4 Bc 1/ 4 bc Eggs 1/ 1/ 1/ 1/ 4 BC BBCC BbCC BBCc BbCc BbCC bbCC BbCc bbCc BBCc BbCc BBcc Bbcc BbCc bbCc Bbcc bbcc 4 bC 4 Bc 4 bc 9 : 3 : 4 • Incomplete Dominance: when BOTH alleles in an individual affect the appearance of a trait and you get a brand new color that was not found in the original parents. Both possibilities are written in capitals and have different letters because BOTH control the appearance. The trait being studied is shown as a base letter • Example: flower color in snapdragons (C) Red = R, so CR is used White = W so CW is used • Cross a homozygous white snapdragon with a homozygous red snapdragon. Incomplete dominance is in effect. C WCW x C R CR Offspring are all CRCW, and are pink! Codominance • situation in which both alleles are equally strong and both alleles are visible in the hybrid genotype. • An example of codominance is found in chickens. • When white chickens are crossed with black chickens, the result is not a gray chicken, but a chicken with both black and white feathers. Codominance: when 2 alleles work together and BOTH are expressed without one masking the other (NO intermediate phenotype) Multiple Alleles: when more than two possibilities for a trait are present. • Example: Blood type – see pages 205 There are 3 alleles for blood type -- A, B, O Possible combinations: AA, AO -- Type A blood BB, BO -- Type B blood AB -- Type AB blood OO -- Type O blood • Here, A and B are dominant over O, but if A and B are present together, neither dominates!!! This is codominance – they share the power of expression. More on Blood types • The letters A, B, and O refer to 2 carbs found on the surface of RBC’s Link to website • Will often see the A,B designation as superscripts with a base of I; O is shown as i (since it is recessive to A and B) • Matching compatible blood groups is critical – proteins called antibodies are produced against foreign blood factors • Antibodies bind to foreign molecules and cause donated blood cells to clump together (agglutination) Figure 14.10 Multiple alleles for the ABO blood groups Rh factor • Another red blood cell chemical that can cause MAJOR trouble during pregnancy – all are called antigens • Antigens – substances that stimulate an immune system response in the body • A, B, and Rh are 3 types of antigens on RBC’s, but are 2 different categories of antigens If Mom is Rh negative and the baby is Rh positive…. • First baby will be fine, but the mother’s body is capable of “remembering” the mis-match, and if she gets pregnant again with ANOTHER baby that is Rh positive, then her body would see that fetus as a foreign invader and destroy it….basically destroys the baby’s RBC’s • Can take RoGam during 2nd and subsequent pregnancies to prevent problems with baby Polygenic Inheritance • Additive effect of two or more genes on a single phenotypic character • Ex. Skin color in humans Figure 14.12 A simplified model for polygenic inheritance of skin color Humans are difficult to study… Why? 1. # of human genes is extremely large (each cell has ~100,000 different genes) 2. Humans cannot be easily controlled by an investigator 3. Time span between generations is long 4. Only a small # of offspring are produced by each set of parents 5. Environment has a HUGE effect on a person’s development… Have developed ways to approach the difficulties… • Pedigree analysis – family history for a particular trait • Twin studies – Nature vs. nurture • Population Sampling • Study of Genetic diseases • Genetic Technology – start with this in spring 2014… • pedigree overview online Pedigree Analysis • A pedigree is a family tree that describes the interrelationships of parents and children across generations • Inheritance patterns of particular traits can be traced and described using pedigrees • Squares indicate males; circles indicate females • Roman numerals indicate generations; each individual in a generation has a separate number – should refer to individuals by the generation number and then the number that they occur in the pedigree line (ex. III-2) • Fully shaded individuals have a characteristic being studied (may be dominant or recessive) • May see half shaded individuals that indicate carriers (but won’t always see these) Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 14-15b 1st generation (grandparents) 2nd generation (parents, aunts, and uncles) Ww ww ww Ww ww ww Ww Ww Ww ww 3rd generation (two sisters) WW or Ww Widow’s peak ww No widow’s peak (a) Is a widow’s peak a dominant or recessive trait? Fig. 14-15c 1st generation (grandparents) Ff 2nd generation (parents, aunts, and uncles) FF or Ff ff Ff ff ff Ff Ff Ff ff ff FF or Ff 3rd generation (two sisters) Attached earlobe Free earlobe (b) Is an attached earlobe a dominant or recessive trait? Pedigree drag and drop practice Please note: This practice site does not use Roman Numerals and does not number each individual – you WILL have to do that for me on the exam and on the practice worksheet tonight… more Pedigree practice part 1 EVEN MORE practice -- part 2 Nature vs. Nurture • Phenotype depends on environment (nurture) AND genes (nature)… • See NORM OF REACTION: phenotypic range of possibilities due to environmental influences on genotype Ex. Blood count of RBC’s and WBC’s depends on altitude, physical activity, presence of infection Ex. Color of hydrangea blooms depends on soil acidity Figure 14.13 The effect of environment of phenotype pH of soil effects color of hydrangea blooms…. Twin studies • Fraternal twins – 2 separate eggs, 2 separate sperm fertilized these (no more similar than siblings born years apart….) • Identical twins – HAVE SAME DNA! One egg, one sperm….during mitosis, the one ball of cells separates into two distinct sets and continues to develop as separate organisms… Population sampling • Studies humans as a GROUP • Compares statistics • Ex. – Tasters vs. nontasters Chemical – Phenylthiocarbimide (PTC) Tasters – TT or Tt – bitter taste when this chemical is present Nontasters – tt – no taste Link on discovery of this Study of Genetic Diseases • Will do this overview in computer lab activity – each of you will go through the diseases given and be familiar with what they are in terms of inheritance and symptoms…