- Slides: 40
Mendelian Genetics n n Genetics – The branch of Biology that studies heredity. Heredity – The passing on of characteristics from parent to offspring. Since early days of farming humans have been involved with genetics and heredity. The first to actively study inheritance was Gregor Mendel
Mendel n n n Mendel was an Austrian monk who studied pea plants. Why would you choose to study peas rather than an elephant? Traits Mendel studied: n n n Seed shape Seed color Flower position Pod color Pod shape Plant height
Pure breeding and inbreeding n Pure breeding – Organisms that always produce the same type of offspring. Tall plants always produce tall plants n Red flowers always produced red flowers n n n Mendel did all of his genetic work before the discovery of DNA, chromosomes, genes, evolution and natural selection. Mendel used mathematics to analyze his data.
Monohybrid crosses n Monohybrid cross – Is a cross between two parent organisms that differ in only one trait. Mendel first crossed a pure breeding tall plant to a pure breeding short plant. Parental (P 1) tall x short n First generation (F 1) n Second generation (F 2) n n tall (crossed with another F ) 1 3 Tall : 1 short
Mendel concluded n n n There are two “factors” for each trait These factors or genes have different alleles Allele – A gene form for each variation of an organism. n n So a plant could have 2 alleles for tall or 2 alleles for short, or one from each. Mendel further concluded that “factors” were either dominant or recessive.
Dominant and recessive n n n Dominant alleles – Are traits that are visible. Observable traits of organisms that masks a recessive form (T) Recessive alleles – Hidden trait of an organism that is masked by a dominant trait. (t) Possible combinations of traits in Mendelian Genetics: n n n TT, tt, or Tt The dominant letter is always written first Once you select a letter to represent a trait you must not change it.
Mendel’s Law of Segregation n n A parent passes on at random only one allele for each trait to each offspring TT or Tt or tt T n T T t t t Alleles give an organism its individual appearance.
Phenotype and Genotype n n n Phenotype – the way an organism looks and feels. Its PHYSICAL appearance. Genotype – The allele (gene) combination an organism contains. Phenotype tall, short, round Genotype TT, tt, Rr Homozygous – Organism has two alleles for the trait that are the same (TT or tt) Heterozygous – Two alleles for a trait are different (Tt) n Always show the dominant trait
Mendelian genetics and Punnet squares n n n P = TT x tt Tt F 1 cross = Tt x Tt F 2 generation = 3 tall: 1 short T T t TT tall t Tt tall Tt tt tall short Genotypic ratio 1 TT: 2 Tt: 1 tt Phenotypic ratio 3 tall: 1 short
Practice problem n n Cross a homozygous blue flower with a heterozygous purple flower. What color is dominant? n n What color is recessive? n n Pp What is the genotype of the blue flower? n n The heterozygote always shows the dominant trait What is the genotype of the Purple flower? n n Blue How do you know? n n Purple pp What are the genotypic and phenotypic ratios of the cross?
Seat work n Cross the following: TT x Tt, tt x tt, Tt X tt n n T = tall, t = short, what are the genotypic and phenotypic ratios? Cross a homozygous red flower with a heterozygous blue flower Cross a homozygous recessive to a homozygous dominant pink flower Cross a heterozygous white flower to another heterozygous white flower
Dihybrid Cross n n n A dihybrid cross involves two different traits. Mendel crossed true breeding round and yellow seeds with green and wrinkled seeds. The Genotypes are: RR = round n rr = wrinkled n YY = yellow n yy = green n The genotypes for this dihybrid cross are: RRYY x rryy When writing dihybrid crosses, we always keep the alleles together Rr. Yy, rr. YY, Rryy
Dihybrid cross n n n True breeding P 1 = RRYY x rryy Resulting F 1 = Rr. Yy All are round and yellow Mendel then crossed the F 1 together and observed the following: 9 round and yellow: 3 round and green: 3 wrinkled and yellow: 1 wrinkled and green. Law of independent assortment – Genes for different traits are inherited independently of each other. n That means in the above example, the seed color does not affect the seed shape, and the seed shape does not affect the seed color.
Dihybrid cross n n See side board for how to use a punnet square for a dihybrid cross. RRYY x rryy, Rr. Yy x Rr. Yy, Rryy x Rr. Yy R=round, r=wrinkled Y=yellow, y=green
Non-Mendelian Genetics The exceptions to the rule
Non-Mendelian Genetics n n Even though a trait is not simply dominant or recessive, geneticists can still predict and explain their appearance. We will discuss 3 examples of non-mendelian genetics: Incomplete dominance n Co-dominance n Sex-linked traits n
Incomplete Dominance n n When the phenotype of the heterozygote is an intermediate of the two homozygotes. Sometimes called blending Red flower x white flower pink flower RR – red x WW – white W W R R RW pink Notice that I used a capital letter for both and distinguished them by using different letters The only difference is in how we express the genotype and the appearance of the organism
Incomplete dominance n n Cross two pink flowers, what do you think the phenotypes will be? RW x RW R RR red W RW pink WW white
Co-dominance n n n Both alleles are expressed equally. black rabbit x white rabbit spotted rabbit With co-dominance since both alleles are seen we use capital letters and the prime symbol (‘)
Co-dominance n n BB (black) x B’B’ (white) BB’ (black and white) BB’ x BB’ B B’ B BB’ BB Black B’ Black & white spots B’B’ White
Sex linked traits n n n Also called x linked. Sex linked traits are controlled by genes located on sex chromosomes. Sex chromosomes determine the gender of an individual. In humans, the sex chromosomes are the 23 rd pair. n n These are the only chromosomes that are not homologous! Autosomes – pairs of matching homologous chromosomes. (all the chromosomes except the sex chromosomes
Sex linked traits n n In humans as well as fruit flies, males are XY, females are XX. Males produce gametes that will have either the X or Y sex chromosome. Females produce gametes that will just carry the X sex chromosome. This means that the gender of a child is determined by the father.
Sex linked traits n n Color blindness in humans and eye color in fruit flies are two examples of sex linked traits. See the side board for how to work on a sex linked trait
Karyotypes n n A karyotype is a picture of an individual’s chromosomes. Karyotypes can be used to determine if there have been mutations to a chromosome. n n Mutations like duplication or deletions Human males and females have different Karyotypes because of differences in the sex chromosomes (X and Y chromosomes).
Pedigree n n Pedigrees are graphical representations used to predict the likelihood of the appearance of a trait. Symbols of a pedigree: Affected male Unaffected male Affected female Unaffected female Known heterozygote, female I A mating offspring II 1 2 3 4
Pedigrees n n Used to determine the possibility of a genetic defect or probability of a specific trait. See side board for a pedigree
Odds and Ends n n n Test cross is cross of an individual of unkown genotype with an individual of known genotype in order to determine the unknown genotype A carrier is another term for a heterozygous individual. A hybrid is an offspring produced when two varieties of plants or animals are mated. Hybrids often exhibit greater vigor and size than their parents. A phenomena termed hybrid vigor.
Chromosome structure and allele locus Allele for purple flowers Locus for flower-color gene Allele for white flowers Homologous pair of chromosomes
Linked genes n Linked genes are genes for two different traits that are located close together on the same chromosome. n Linked genes tend to travel together during crossing over Closely linked Not Linked
Pleiotropy Most genes have multiple phenotypic effects, a property called pleiotropy n For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease n
Epistasis n n 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
Polygenic Inheritance • • • Quantitative characters are those that vary in the population along a continuum Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype Skin color in humans is an example of polygenic inheritance
Multiple Allele Traits n n n Most genes exist in populations in more than two allelic forms For example, the four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: IA, IB, and i. The enzyme encoded by the IA allele adds the A carbohydrate, whereas the enzyme encoded by the IB allele adds the B carbohydrate; the enzyme encoded by the i allele adds neither
Multiple Alleles n Any of a set of three or more alleles, or alternative states of a gene, only two of which can be present at any given time in a diploid organism. n Example: Physical characteristics such as eye color, skin color, height and coat color in animals …these are also POLYGENIC traits controlled by many genes interacting together! copyright cmassengale 35
Multiple Alleles and Codiminance BLOOD TYPING n Blood is a complex, living tissue that contains many cell types and proteins. Distinct molecules called agglutinogens (a type of antigen) are attached to the surface of red blood cells. n n There are two different types of antigen markers, type “A” and type “B”. Each type has different properties. The ABO blood type classification system uses the presence or absence of these molecules to categorize blood into four types. n type A type B type AB type O = = AA or AO BB or BO AB OO (sometimes ii) • A, B, AB, and O are the POSSIBLE PHENOTYPES • A and B are DOMINANT alleles • i or type O is the RECESSIVE allele
Chart of Blood Types
WHEN BLOOD TYPES MIX Blood plasma is packed with proteins called antibodies. The body produces a wide variety of antibodies that will recognize and attack foreign molecules that may enter from the outside world. A person’s plasma does not contain any antibodies that will bind to molecules that are part of his or her own body. When conducting a blood transfusion, it is important to carefully match the donor and recipient blood types. If the donor blood cells have surface molecules that are different from those of the recipient, antibodies in the recipient’s blood recognize the donor blood as foreign. This triggers an immune response resulting in blood clotting. If the donor blood cells have surface molecules that are the same as those of the recipient, the recipient’s body will not see them as foreign and will not mount an immune response. There are two special blood types when it comes to blood transfusions. People with type O blood are universal donors because there are no molecules on the surface of the red blood cells that can trigger an immune response. People with type AB blood are universal recipients because they do not have any antibodies that will recognize type A or B surface molecules. Note: Blood cells are covered with a variety of surface molecules. For simplicity, only type A and B surface molecules are shown here.
Nature and Nurture: The Environmental Impact on Phenotype n n n Another departure from Mendelian genetics arises when the phenotype for a character depends on environment as well as genotype The norm of reaction is the phenotypic range of a genotype influenced by the environment For example, hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity
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