The passing of traits from parents to offspring. The scientific study of the inheritance

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1 Inheritance The passing of traits from parents to offspring Genetics The scientific study of the inheritance Gregor Mendel -Father of modern genetics -Used peas to successfully identify the laws of heredity Why use Peas as an Experimental Organism? -Short life span -Bisexual -Cross- and self- pollinating so can have purebred (homozygous) and hybrid (heterozygous) -Many traits known

2 Mendel s Work -Studied crosses of seven characters, each with two expressions or traits Ex.: Character- Height Traits- Tall or short Monohybrid or Mendelian Crosses -Crosses that work with a single character at a time Ex.: Tall or short P Generation The Parental generation or the first two individuals (true breeding= homozygous parents) used in a cross Offspring -F1: first filial (brother) generation ( offspring of the P generation) -F2: second filial generation, bred by crossing two F1 plants together or allowing a F1 to selfpollinate

3 Another Sample Cross P1- Tall x short (TT x tt) F1- All tall (Tt) F2- Three tall to one short (1TT:2Tt:1tt) Result Summary: -In all crosses, the F1 generation showed only one of the traits (Dominant) regardless of which male or female -The other trait (Recessive) reappeared in the F2 at 25% (3:1 rate) Mendel s Hypothesis explaining the 3:1 inheritance pattern in the F2 offspring Homozygous 1.Genes can have alternate versions called alleles 2.Each offspring inherits two alleles, one from each parent 3.If the two alleles differ, the dominant allele is expressed. The recessive allele remains hidden unless the dominant allele is absent 4.The two alleles for each trait separate during gamete formation. This is called: Mandel s Law of Segregation When organisms have two of the same alleles for a particular trait (dominant traits are usually capitalized (TT) and recessive traits are usually lower case (tt). Heterozygous When the two alleles for that trait are different (Tt). In complete dominance, the dominant allele will be expressed.

4 Phenotype vs Genotype -The physical appearance of the organism: Phenotype -The genetic makeup of the organism, usually shown in a code: Genotype Ex.: -P allele= Purple -p allele= White Monohybrid Cross -A cross involving only one character Ex.: What happens when you cross two pea plants heterozygous for flower color? -Genotype of parents? Pp xpp -Results: Genotype ratio: 1:2:1 Phenotype ratio: 3:1 6 Mendelian Crosses are Possible Cross Genotype Phenotype TT x tt All Tt All Don Tt xtt 1TT:2Tt:1tt 3 Dom: 1 Res TT x TT All TT All Dom ttxtt All tt All Res TT x Tt 1TT:1Tt All Dom Tt x tt 1Tt:1 tt 1 Dom: 1 Res Test Cross -Purpose: Done to determine if an individual showing a dominant phenotype has a homozygous or heterozygous genotype -How to: Cross of an unknown dominant parent with a homozygous recessive parent Ex.: T_ x tt If TT- all dominant offspring If Tt- 1 dominant: I recessive offspring

5 Dihybrid Cross -Cross intended to study two genetic traits Ex.: Tall (T) or Short (t) and Red (R) or White (r) -Need four letters to code for the cross Ex.: TtRr -Each gamete must get one letter for each trait -Use the FOIL method to figure out the possible gamete combinations from a parent Ex.: A parent TtRr can produce TR, Tr, tr, tr offspring due to independent assortment Dihybrid Cross: TtRr x TtRr -Each parent can produce 4 types of gametes -Cross is a 4x4 with 16 possible offspring -Parent genotype: TtRr -Possible offspring: TR, Tr, tr, tr -Results: Genotype ratio: Tall red=9, Tall white=3, Short red=3, Short white=1 Phenotype ratio: 9:3:3:1 Law of Independent Assortment -The inheritance of 1 st genetic traits is NOT dependent on the inheritance of the 2 nd trait -Inheritance of height is independent of the inheritance of flower color Probability -Genetics is a specific application of the rules of probability -The chance that an event will occur out of the total number of possible events

6 Genetic Ratios -The monohybrid ratios are actually the probabilities of the results of random fertilization Ex.: 3:1 75% chance of the dominant ¾ 25% chance of the recessive ¼ Rule of Multiplication Other than complete dominance -The probability that two alleles will come together at fertilization, is equal to the product of their separate probabilities Ex.: TtRr x TtRr -the probability of getting a tall offspring is ¾ -the probability of getting a red offspring is ¾ -the probability of getting a tall red offspring is ¾ x ¾ =9/16 Comment -Use the Product Rule to calculate the results of complex crosses rather than the Punnett square Inheritance patterns are often more complex than predicted by simple Mendelian Genetics which demonstrates complete dominance Incomplete Dominance -When the F1 hybrids show a phenotype somewhere between the phenotypes of the two parents -Unlike complete dominance problems, where only the allel letter is used, for these problems, the character is the letter we use and the allele is written as a superscript Ex.: Red (C R C R ) x White (C W x C W ) F1= all pink F2= 1 red: 2pink: 1 white -No hidden recessive

7 Codominance Multiple Alleles -Both alleles are expressed equally in the phenotype -You get both alleles expressed in the heterozygous form, here a pink flower ( C P x C P ) and a white flower (C W x C W ) form a pink and white flower (C P x C W ) Result: -No hidden recessive -3 phenotypes and 3 genotypes (but not a does effect) -A form of codominace where there are more than two alleles for a trait Ex.: ABO blood group --I A = A type antigen --I B =B type antigen --i= no antigen Result: -Multiple genotypes and phenotypes -Very common event in many traits Alleles and Blood Types Type Genotypes A B AB O I A I A or I A i (AA or Ai) I B I B or I B i (BB or Bi) I A I B ii Blood Type -Rh blood factor is a separate factor from the ABO blood group -Rh+ = Dominant -Rh- = Recessive -A+ blood = Dihybrid Trait

8 Polygenic Inheritance Environment affect -Two or more genes have an addictive effect on a single character in the phenotype -When several genes are involved, the phenotype described by a bell-shaped curve Ex.: Skin color, height Ex.: Skin color is likely controlled the trait by atleast 4 genes, each dominant gives a darker skin color Results: -Traits tend to run in families -Offspring often between parental types -Can affect the expression of genes Ex.: In terms of skin color, although your genes may code for a little melanin production, prolonged exposure to UV radiation may alter the expression rate of the gene and you will produce more as a response to you environment Morgan -Choose to use fruit flies as a test organism in genetics -All owed the first tracing of traits to specific chromosomes Morgan discovered: -Sex linked traits Sex- Linked Genes -Genetic traits whose expression are dependent on the sex of the individual -Are located on the sex chromosomes X and Y -If a gene is located on the X chromosome, fathers pass sex-linked genes on to their daughters but not sons( they get Y) -Females will express a sex-linked trait exactly like any other trait but males will express the allele on the X chromosome from mother -The vast majority of genes on the X chromosome have nothing to do with sex

9 Males -Hemizygouse: one copy of X chromosome -Show all X traits (Dominant or recessive) -More likely to show X linked recessive gene disorders than fermales Several Sex-Linked Disorders have Medical Significance: -Duchenne Muscular Dystrophy: is a sex-linked disorder resulting in progressive loss of muscular coordination until the late 20s when usually lethal -Hemophilia: Also a sex-linked disorder resulting in the inability to clot blood normally b/c of the absence of proteins required to do so -Colorblindness X-Linked Patterns Extranuclear Inheritance -Trait is usually passed from a carrier mother to ½ of sons -Affected father has no affected children, but passes the trait on to all daughters who will be carriers for the trait Comment -Watch how questions with sex linkage are phrased: Chance of children? All Possibilities Chance of males? Male Possibilities -Extranuclear genes are those found in the mitochrondria and chloroplasts. They were discovered in plants in Since then, they have been linked to several rare and severe inherited diseases in humans. Defects in the mitochrondrial DNA reduce the amount of ATP a cell can make, Therefore the organs most affect by these mutations are the ones that require the most energy: the nervous system and muscle

10 Extranuclear Inheritance Cont. -Because the mitochrondria passed to the zygote all come from the cytoplasm of the egg, these dieases are always inherited from the mother and because of this, do not follow Mendelian rules of inheritance Genetic Studies in Humans Pedigree Chart Tips -Often done be pedigree charts -Pedigree: a chart that shows the relationship b/t parent and offspring across two or more generations -Can help determine the genome of individuals that comprise them and also genome of future offspring -Male = square - Female = circle -Person with trait = colored in -Dominant Trait: never skip generations -Recessive Trait: skips generations -Sex linked: Predominant gender -Autosomal: Both genders equally get disorder Human Recessive Disorders -Albinism -Sickle Cell Anemia -Tay-Sachs Disease -Cystic Fibrosis -PKU -Galactosemia

11 Sickle-cell Disease -Caused by an allele that codes for a mutant hemoglobin molecule that forms long rods when O 2 in the blood is low -Reduced O 2 carrying capacity -Codominant inheritance Tay-Sachs -Caused by an allele that codes for a dysfunctional enzyme that breaks down certain lipids in the brain -Brain cells unable to metabolize type if lipid accumulation and causes brain damage -Death in infancy or early childhood Cystic Fibrosis -Most common lethal genetic disease in U.S -Most frequent in Caucasian populations (1/20 a carrier) -Produces defective chloride channels in membranes causing high levels of chloride outside cells that make muclus thick and heavy leading to organ damage Recessive Pattern -Usually rare -Skips generations -Often an enzyme defect

12 Human Lethal Dominant Disorders -Less common than recessive. Require only one copy of the dominant allele in order for the disorder to be expressed Ex.: -Huntingtons Disease(progressive degeneration of brain cells) -Achondroplasia (Dwarfism) -Famllial Hyperchlosteroemia (High cholesterol) Inheritance Pattern -Each affected individuals had one affected parent -Doesn t skip generations -Homozygous causes show worse phenotype symptoms -May have post-maturity onset of symptons Genetic Screening -Risk assessment for an individual inheriting a trait -Uses probability to calculate the risk Carrier Recognition -Fetal Testing --Amiocentesis --Chorionic Villi Sampling -Newborn Screening Fetal Testing -Biochemical testing -Chromosome analysis

13 Amniocentsis -Administrated between weeks -Extract amnionic fluid = cells and fluid -Biochemical tests and karyotype -Requries culture time for cells Chorionic Villi Sampling -Administrated between 8-10 weeks -Extract tissue from chorian (placenta) -Greater risk but no culture time required Newborn Screening -Blood tests for recessive conditions that can have the phenotypes treated at avoid damagegenotypes are not changed Ex.: -PKU Multifactorial Diseases -Where genetic and environmental factors interact to cause the disease -Becoming more widely recognized in medicine Ex. -Genetic -Diet -Exercise -Bacterial infection

14 Chi-Square -Is a statistical test used to determine how well observed ratios of data that you collect fits expected ratios of data Expected Ratios -Come from a Punnett square -Can count offspring to see how well our observed results match the expected outcomes -In genetic crosses, b/c Mendel s Laws we assume that there is a random assortment of alleles from parents to offspring, so the probability of receiving a particular gene combination in an offspring is due to chance and a punnett square should predict the outcome Null Hypothesis The formula for Chi Square -N 0 : Our observed data should match our expected data -If our data matches the expected ratios of offspring within a reasonable amount, we accept our null hypothesis and our results are statistically significant -If our data does not match the expected ratios within a reasonable amount, we reject the null hypothesis and our data is not statistically significant -Where O is the observed frequency -E is the expected frequency -df is the degree of freedom (n-1) -x 2 is chi square

15 Critcial Value -P =.o5 always -If chi-square results are below the CV we accept the null hypothesis -If chi-square results are above the CV we reject the null hypothesis Use the row based on the df, which is one subtracted from the number of phenotypes possible