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GENETICS GENETICS • Purpose: to understand how traits in our DNA are passed on (parent to child) • Used to predict possible outcomes of a genetic cross. – This means that what we predict and what we see could be different! HISTORY OF GENETICS (AS A SCIENCE) Gregor Mendel - “Father of genetics” Conducted experiments Used phenotype (studied what pea plants looked like( and probability (statistics) to determine the principles of genetics (how parents passed traits to their offspring) Studied many plants including Pisum sativum (peas) WHY PEAS (WHY CHOOSE THIS MODEL)? The Garden pea - Model system to study heritability small easily cultured short life span exhibits great variability true-breeding strains dominant/recessive alleles MENDEL’S EXPERIMENTS Looked at seven characteristics Characteristics are an inheritable factor, such as color, size, seed texture, etc. Each characteristic occurred in only two contrasting traits A trait is a genetically determined variant of a characteristic (allele) CHARACTERISTICS STUDIED (SEE 2ND PAGE OF NOTES) • Height • Flower position • Pod color • Pod appearance constricted • Seed texture • Seed color • Flower color tall or short axial or terminal green or yellow inflated or smooth or wrinkled yellow or green purple or white MENDEL’S METHOD Manual pollination (Selective breeding) Occurs when anthers are removed from the flowers of a plant (contain the pollen grains at the top). Then you choose which flowers to pollinate. HOW DID MENDEL CONTROL FERTILIZATION? THE NEXT FEW SLIDES SHOW YOU HOW MENDEL “hand fertilize” the peas You will need to know the definitions of Pollination Cross-pollination Self-pollination Other than that, the next six slides are for your interest ONLY (they are NOT in the notes) HOW DO PLANTS MAKE OFFSPRING? What is “natural pollination”? Pollination Pollen grains produced on anther are transferred to the stigma (top of the female reproductive system) Self pollination: Pollen from a plant pollinates a stigma on the same plant (same flower or different flower) Cross pollination Pollen from a plant pollinates a stigma on a totally different plant. RESULTS OF POLLINATION Flowers bloom- produce a pistil a stamen Female pistil: Stigma (sticky top) Style Ovule (seeds form) Male stamen Anther Filament FERTILIZATION EMBRYO FORMATION WHAT ARE PLANT EMBRYOS? Seeds! These are 2N! HIS SCIENTIFIC METHOD • Utilized monohybrid crosses – ONE characteristic, two alleles, selective breeding • Carefully recorded his data (PHENOTYPES). – Parental characteristics and offspring characteristics – 3 or more generations (P, F1, F2) • Formed testable hypotheses. • Tested hypotheses “statistically” • Utilized seven traits in the garden pea. Parents: (both true breeding) white x purple Expect ed : Light purple What he got: All purple So… he crossed two of them…. Expect??? All purple What he got: ¾ purple, ¼ white! A carpal is another name for _________? These crosses showed that there were “factors” being passed from parent to offspring even if it wasn’t being “used” Now we call these factors GENES Genes – control a heritable feature; characteristic Example of characteristic: Hair color, seed shape, height; Allele – controls the variation of a feature (characteristic) – AKA trait. Example of trait: brown, blonde, black hair Characteristic/Gene? Trait/Allele? CHARACTERS (characteristics) AND VARIANTS (traits) TRAITS ? TRAITS ? RARE DOMINANT PHENOTYPE - Polydactyly A chromosome = folded up string of many genes What are alleles? Variations of a gene that occupy the same locus on homologous chromosomes Locus = position on a chromosome. GENE = STEM LENGTH SHORT t T LONG GENE = FLOWER COLOR P p VOCAB WORDS – BE APPLY THESE TO GENETICS AND PUNNETT SQUARES Diploid (2n) Haploid(n) Egg Sperm Parent Meiosis Testes Gamete Zygote Progeny Offspring Fertilization Ovary LINKING VOCABULARY – PRACTICE PARAGRAPH Review Mendel’s process and substitute in all words on the previous slide, (used in mitosis and meiosis) to describe how Mendel arrived at the F1 generation. Draw a Punnett square and link terms on the previous page to the Punnett square MENDEL’S LAWS OF GENETICS 1. Law of segregation: only one allele for each gene is passed from a parent to the offspring. Why? Has to do with separation of homologous chromosomes during meiosis. Segregation of Alleles Tongue Rolling 2. Law of independent assortment: Alleles for one gene are passed to offspring independently of alleles from other genes. The result is that new combinations of genes present in neither parent is possible. This only applies to SOME genes, not all. 3. Law of complete dominance – some alleles overpower others. So even if both alleles are present, we only “see” the dominant one. - the “hidden” allele is called recessive This only applies to SOME genes, not all. Remember Mendel’s pea plants? - Purple was crossed with white and we got ALL purple. So which allele is dominant? Genotype: the alleles that an organism has. - alleles are abbreviated using the first letter of the dominant trait. (with some exceptions that we will get to) - a capital letter represents the dominant ex: P for purple flower allele - a lower case represents the recessive. ex: p for white flower allele All diploid organisms have two alleles for each trait: - you can have two of the same alleles Ex: PP or pp - such an individual is described as Pure or Homozygous. OR All diploid organisms have two alleles for each trait: - you can have two different alleles Ex: Pp - such an individual is described as hybrid or heterozygous Phenotype: physical appearance Examples: brown hair, widows peak - the trait that “shows” in the case of complete dominance; - depends on the combination of alleles Terminology for Genetic Crosses P generation: “parents;” First generation in the cross F generations: results of the cross; - F1 – 1st generation; offspring of P generation - F2 – 2nd generation; offspring of F1 generation Monohybrid cross: cross that focuses on the alleles of a single characteristic; How do we show the possibilities? - punnett square PUNNETT SQUARE Allele in sperm 1 Allele in sperm 2 Allele in Egg 1 Allele in Egg 2 Zygote formed if sperm 1 fertilizes egg 1 Zygote formed if sperm 1 fertilizes egg 2 Zygote formed if sperm 2 fertilizes egg 1 Zygote formed if sperm 2 fertilizes egg 2 In pea plants, tallness is dominant to short or dwarf. Cross a pure tall male to a pure dwarf female pea plant. Show both ratios phenotype & genotype for the offspring. Now cross two of the F1. Take it step by step until you “get it” Step 1: what are the parent’s genotypes? Mom? Dad? tt TT Step 2: Set up Punnett Square t t T Tt Tt T Tt Tt Step T T 3: ANSWER THE QUESTION Offspring t t genotypes: Tt Tt Tt Tt Offspring phenotypes: Step T t 4: Part II T t TT Tt Tt t t Offspring genotypes: Offspring phenotypes: Inheritance Patterns: Every gene demonstrates a distinct phenotype when both alleles are combined (the heterozygote) Complete dominance is one - when both alleles are present, only the dominant trait is seen. This is the dominance pattern seen in the characteristics Mendel used. The problems you were given exhibit this pattern. Go do the problems OTHER DOMINANCE PATTERNS Incomplete Dominance Still use Capital and Small letters Heterozygous offspring ARE blended Other Inheritance Patterns: Incomplete dominance - when both alleles are present, the two traits blend together and create an intermediate trait (Red + White = Pink) Codominance - When both alleles are present you see both traits of the characteristic are visible. Red + White = Red and White INCOMPLETE DOMINANCE Inheritance Patterns: Co-dominance - when both alleles are present, both traits are visible Different notation: Use first letter of the feature with a superscript for the trait. Example: CW or CB for white coat or black coat; Inheritance Patterns: Co-dominance - when both alleles are present, both traits are visible Inheritance Patterns: Each gene has a specific inheritance pattern. - you will either be told or be given a hint; look at the heterozygote! STILL MORE INHERITANCE PATTERNS Sometimes depend on the gender (male/female) Reason: males have “non-homologous” sex chromosomes Women have two X’s but men only have one. How do we deal with the genes on the X chromosome? Sex-linked trait Alleles for the trait are located on the X chromosome in humans. - works the same in women as all the other traits. BUT – - men only inherit one such allele. Sex-linked trait For females: have to figure out phenotype based on inheritance pattern. For Males: phenotype is that of whatever allele they inherit. Example: color blindness Seeing color (XC) is dominant to c being color blind (X ) Identify the sex and trait of the following: XCY XCXc XcXc XcY XCXC Example: Color Blindness Set up a punnett square crossing a heterozygous normal female with a normal male: - what is mom’s genotype? - what is dad’s genotype? - what gametes can each give? - what are the offspring’s geno’s? Cross Number 1: XC Xc XC C C X X C c X X Y XC Y c X Y What % chance of having color blind daughter? Son? SEX-LINKED TRAITS COLOR BLINDNESS AFFLICTS 8% MALES AND 0.04% FEMALES. Based on phenotype, can you determine whether you are homozygous or heterozygous? WHAT ARE THE POSSIBILITIES? Can we use the inheritance pattern? Complete dominance Incomplete dominance Co-dominance If the gene has a complete dominance inheritance pattern, what phenotype should we breed to in order to determine whether the genotype is homozygous dominant or heterozygous???? Test cross: a cross that determines genotype of dominant parent - Cross unknown dominant parent (possibilities BB or Bb) with a recessive parent then analyze the offspring. If some of the offspring have the recessive trait, then the unknown parent has to be b heterozygous B ? Bb ?b b Bb ?b If all offspring are dominant, unknown parent HAS to be homozygous B ? b Bb ?b b Bb ?b MULTIPLE ALLELES GENES While you only inherit two alleles, some genes have 3, 4, 5 and even 6 possible alleles. BUT you can only inherit two (and they must come from your parents! Multiple alleles: Some genes have more than two variations that exist, although we still only inherit 2 Example: Human blood types Three alleles: IA IB i Genotype (6) IA IA IA i IB IB IB i IA IB ii Phenotype (4) A A B B AB 0 INHERITANCE PATTERN DEPENDS ON ALLELE IA and IB are codominant i (the O allele) is recessive to IA and IB POLYGENIC Several genes control a single character trait Phenotype shows a continuum (quantitative) rather than discrete categories of color Polygenic – Multiple genes each with 2 alleles Creates additive/ quantitative effect SKIN PIGMENTATION EXAMPLE Skin Color: Humans Height - humans Dihybrid cross: A cross that focuses on possibilities of inheriting two traits - two genes, 4 alleles Black fur is dominant to brown fur Short fur is dominant to long fur What is the genotype of a guinea pig that is heterozygous for both black and short fur? Dihybrid cross: Parent phenotypes: BbSs x BbSs Figure out the possible gametes: Then set up punnett square Dihybrid cross: BS BS Bs bS bs Bs bS bs Linked Genes: genes that are on the same chromosome. Does the law of independent assortment apply? Can they be separated? Will they always separate? WHAT DOES THIS MEAN? It means that you can pass on an allele that you got from your mom and an allele you got from your dad ON THE SAME CHROMOSOME However, it is more likely that two alleles that start on the same chromosome will get passed on together. MULTIPLE GENE INTERACTIONS One gene affects another On-off Darker lighter The combination of alleles phenotype EPISTASIS Expression of one gene is modified or influenced by one or several other genes. expression of another (on-off switch) Labradors: recessive expression in one gene turns off a second gene Labrador retrievers Black, chocolate and yellow labs LABRADOR RETRIEVERS Color of fur: Black (BB or Bb) with “black gums” Chocolate (bb) with “red gums” How do we get a Yellow lab? Second gene (E gene) If the E gene is recessive for both alleles (ee) – pigment is not “expressed” in coat (but can be seen in “gums” Black nose Red nose What are the possible genotypes for each of the dogs? EXAMPLE 2 Could two black horses produce a white horse? Could a white horse be homozygous dominant for the color gene? Heterozygous? Could you produce a black horse from the mating of a white horse and a tan horse PEDIGREES Used to study past matings and transference of specific disease alleles PEDIGREE SYMBOLS PEDIGREES AND “DISEASE” Pedigrees are used to look for “patterns in certain diseases. There are three inheritance patterns X-linked Autosomal recessive Autosomal dominant PRACTICING WITH PEDIGREES Use the handout called “Hemophilia in the Descendants of Queen Victoria” This contains the pedigree of several royal families in Europe. What can you tell me about the pattern? HEMOPHILIA IS “X-LINKED” Found ONLY on X chromosomes Most diseases are recessive One “disease allele” causes disease in males (hemizygous) In females, two recessive required How does this affect the pedigree? Look at the carriers and the “affected” individuals What are their genders? Can the “allele” be hidden? In what gender? X-LINKED RECESSIVE DISEASE Pedigrees often show of those that have the disease, virtually all are males, and females as carriers If a female has hemophilia, her father must have the disease and her mother must be a carrier A male ALWAYS inherits the disease allele from his mother. QUESTIONS What inheritance pattern is demonstrated ? How many generations are shown? According to the pedigree, what is the earliest known origin of the hemophilia gene? Why do boys show the disease, and girls are carriers? How is it that some boys have the disease and others do not? REMEMBER HOW TO NOTATE X CHARACTERISTICS Xh XH Y has no alleles You must specify The chromosome (X or Y) The gender (shown by XX or XY) Whether the transferred allele is dominant or recessive DISEASES – AUTOSOMAL RECESSIVE Cystic Fibrosis Sickle Cell Disease (recessive or co-dominant) Tay Sachs Who must the alleles be inherited from? Can you have an allele, but not the disease? RECESSIVE Affected individuals may have 2 carrier parents (unaffected) Affected x affected 100% affected Unaffected parents of affected child may be related. CYSTIC FIBROSIS 1 in 2500 Caucasians affected Chloride transport is affected Thick mucous forms Why does the gene “survive”? Heterozygotic Advantage? (http://bric.postech.ac.kr/science/97now/98_5now/9805 06b.html) Protection from typhoid fever? Malaria? Cholera? Still uncertain. SICKLE CELL 1 in 10 African Americans almost 1:2 in some African countries REASON: protects against malaria (heterozygous) This is a good case of “natural selection” maintaining the allele TAY SACHS In Ashkenazic Jews, 1 out of every 30 people carry the allele Infantile form Symptoms appear at 3-6 months Child dies by age 4 or 5 Lacks hexosaminidase A, a protein that helps break down a chemical in nerves (gangliosides) Without this protein, gangliosides, particularly ganglioside GM2, build up in cells, especially nerve cells in the brain deterioration of the brain and nervous system death. PEDIGREES – AUTOSOMAL DOMINANT DISEASES Autosomal Dominant Requires ONE gene to show illness Frequency the same in Male/Female 50% chance offspring will inherit from either parent EXAMPLE – HUNTINGTON’S DISEASE Late onset – 30s or 40s Progressive How many generations are shown in pedigree 1? Does HD show up in each generation? nerve system damage Could it skip a generation (have a recessive allele) and then show up again? Explain What gender could be a carrier? EXAMPLE: DWARFISM Dominant autosomal Two dominant alleles lethal Heterozygous is a “dwarf” Only homozygous recessive is “normal” IDENTIFYING PEDIGREES WITH DOMINANT TRAIT DISORDERS Every Affected Individual has at least one Affected Parent Generations not skipped (in other words, no “hidden” genes If one parent is affected, 50% of children will be affected If two parents are affected, 75% of children are affected