Page 1 of 20 Genetics: Heredity: Trait: The scientific Study of Heredity. The passing of traits from one generation to the next. Any observable characteristic on organism may have. Ex: eye colour, hair colour, height, tongue rolling Q. Who was the first person to study genetics? A. Gregor Mendel Who was Gregor Mendel and what did he do? Name: Gregor Mendel Lived in the 1800 s A monk who studied pea plants. What did he do? Mendel studies pea plants and the traits they passed on from one generation to the next. He studied 7 different traits in peas and he was able to discover several important laws about genetics and how traits are passed on. Why did Mendel study peas? There are 4 main reasons why Mendel studied peas. o They reproduced quickly o Their traits were easily observable o They were readily available o They could self pollinate meaning he could control which plants pollinated. Mendel s Experiment Took the seeds of a purebred tall plant and the seeds of a purebred short plant and cross pollinated them and collected the seeds produced by this cross and planted them. Results: All the offspring of this cross were tall (No short plants) Did the trait for short disappear?? Next, Mendel self pollinated the plants from the F 1 generation. These seeds were then planted and grown.
Page 2 of 20 Results: Three quarters were tall and one quarter was short. To describe his findings Mendel said there was a dominant factor at play causing the plants to be all tall in the first cross and that a recessive factor was being expressed in the second cross. Principle of Dominance: When an organism is crossed for a pair of contrasting traits ONLY the dominant trait can be seen in the hybrid. The recessive factor was hidden. Terminology Associated With Mendel s Experiments and Genetics a. Unit Factors: These were the 2 factors that Mendel said affected the expression of traits in organisms. Each organism has two factors for each trait. We now call them GENES. b. Unit Theory of Inheritance: This was Mendel s theory where he believed Unit factors (now called genes) control the expression of traits in offspring. c. Dominant Factor: This was a factor that was ALWAYS expressed in an organism. d. Recessive Factor: This was a factor that SOMETIMES was expressed in a organism. e. Allele This is the form a gene can be. It can be Dominant or recessive, BUT NOT BOTH. f. F 1 Generation Also called First Filial Generation. This is the first set of offspring that are produced by parents. g. F 2 Generation Also called the Second Filial Generation. This is the second set of offspring created from the offspring of the parents. These offspring are the same thing as grandchildren. h. Pure Breed/Strain A term that refers to an organism having EITHER two dominant factors OR two recessive factors for a trait. Ex: Ex: A plant that is pure tall has two dominant factors for tallness - One from the mother and one from the father. A plant that is pure short has two recessive factors for tallness - One from the mother and one from the father.
Page 3 of 20 i. Hybrid A term meaning that an organism has one dominant and one recessive factor for a trait. j. Homozygous A term meaning that an organism has the SAME alleles for a gene. Ex: both are recessive or both are dominant. Same as Pure breed. k. Heterozygous A term meaning that an organism has DIFFERENT alleles for a gene. Ex: One is dominant and one is recessive. Same as hybrid. l. Genotype This is the genetic makeup of an organism. It is the combination of alleles for an organism. For example a person might be homozygous dominant (TT) for tallness, but at the same time be homozygous recessive (cc) for straight hair. m Phenotype This is the physical characteristics displayed by an organism. It is the appearance of a trait in an organism. For example: The organism above will be tall and have curly hair. n. Punnet Square A square that is used to help determine the possible/probable outcomes of a cross between two individuals. See the diagram for a sample punnet square. o. Cross This is another way of saying that a female has been mated with a male.
Page 4 of 20 Mendel s Laws l. Principle of Dominance A principle proposed by Mendel that stated that the Dominant form of a gene(trait) will ALWAYS be expressed. 2. Law of Segregation A law proposed by Mendel that stated that the pair of factors for a trait separated (segregated) during the formation of gametes (sperm and egg) and then recombined during fertilization. See diagram below. (Copy this diagram) Male Parent Tt Female Parent TT Gametes segregate Tsperm tsperm Tegg Tegg Either of these sperm can join with either of the eggs. The letters are used to show the different forms of the traits. T stands for the dominant form of the trait for TALLNESS. t stands for the recessive form of the trait for tallness. 3. Law of Independent Assortment: A law stating that during meiosis, genes for different traits are separated and distributed to gametes independently of one another.
Page 5 of 20 THE SINGLE FACTOR CROSS Single Factor Cross: A cross that is done for ONE TRAIT. Also called a Single Factor Cross Sample: Mary is Homozygous dominant for Straight Hair (SS). Steve is Heterozygous for Straight hair (Ss). What are the possible ratios of offspring for these two individuals? Answer: Parents: SS X Ss Gametes: S S S s Punnet Square: S s S SS Ss S SS Ss Ratios: Genotypic: 1:1 Phenotypic: 4:0 100% straight hair Homozygous Straight : Heterozygous Straight 50% homozygous straight: 50% heterozygous straight
Page 6 of 20 All will have straight hair
Page 7 of 20 The Monohybrid Cross Monohybrid Cross: Ex: A single factor cross where the parents are Heterozygous or Hybrid for the trait in question. Crossing two plants that are heterozygous for tallness Tt Parents Tt X Tt Gametes T t T t F 1 Generation: T t T TT Tt T Tt tt Phenotypic Ratio: 3: 1 Tall: Short Genotypic Ratio: 1: 2: 1 Homozygous Dominant: Heterozygous Dominant: Homozygous Recessive NOTE: In a MONOHYBRID Cross, the Ratios are ALWAYS as above. Genotypic and Phenotypic Ratios for a Monohybrid Cross Phenotypic: Genotypic: recessive 3: 1 (Dominant to recessive) 1: 2 : 1 (homozygous dominant : heterozygous dominant : homozygous
Page 8 of 20 The Product Rule A rule that uses the principles of probability to determine the possible outcomes of a genetic cross. Product Rule: The probability, or chance, that two or more independent events will occur together is the product of their individual probabilities of occurring alone. Example: Toss a single coin: Tossing coins Chances of getting heads = ½ Chances of getting tails = ½ Toss two coins together: Chance of getting two heads = ½ x ½ = ¼ Chance of getting two tails = ½ x ½ = ¼ Chance of getting heads and tails: ½ x ½ + ½ x ½ = ½ The same can be applied to Genetics crosses. Let s assume you have a monohybrid cross that you are performing. You can use the product rule to predict your outcomes. Parents: TT X Tt Gametes: T T T t Use the FOIL rule to get your possibilities: First (T and T) ½ T x ½ T = ½ TT Outside (T and t) ½ T x ½ t= ¼ Tt Inside (T and T) ½ T x ½ T = ¼ TT (You add this one to the one above for firsts because they are the same) ¼ TT + ¼ TT = ½ TT Last: (T and t) ½ T x ½ t = ¼ Tt (You add this to the one above for Tt because they are the same) ¼ Tt + ¼ Tt = ½ Tt
Page 9 of 20 Overall Results: ½ TT and ½ Tt (50% chance of TT and 50% Tt) Test Cross Test Cross: A cross in which an individual of unknown genotype is crossed with a homozygous recessive individual. This is done to determine the genotype of the unknown individual. Two crosses are performed. a. Cross a Homozygous dominant individual with the homozygous recessive individual. b. Cross a Heterozygous dominant individual with the homozygous recessive individual. Examine the results to tell the genotype of the unknown individual. TEST CROSS A -- Assume the unknown genotype is Homozygous Dominant. (We will use the letters TT for homozygous dominant and tt for homozygous recessive). Parents: TT X tt Gametes: T T t t F 1 Generation: T T t Tt Tt t Tt Tt Phenotypic Ratio: 4:0 All Tall Genotypic Ratio: 4:0 All Heterozygous Dominant
Page 10 of 20 TEST CROSS B: Assume Unknown Genotype is Heterozygous Dominant. Parents: Tt X tt Gametes: T t t t F 1 Generation: T t t Tt tt t Tt tt Phenotypic Ratio: 1:1 ½ tall: ½ short Genotypic Ratio: 1:1 ½ heterozygous dominant: ½ homozygous recessive
Page 11 of 20 Two Factor Crosses Two Factor Cross: A cross performed where two traits are involved at the same time. Sample Two Factor Cross A cow that is homozygous dominant for Brown Hair coat (BB) and Homozygous dominant for split Hoof (SS) is crossed with a cow that is homozygous recessive for brown hair (bb) and homozygous recessive for split hoof (ss). What are the possible genotypic and phenotypic ratios for the offspring? Parents: BBSS X bbss Gametes: BS BS BS BS bs bs bs bs F 1 Generation: bs bs bs bs BS BS BS BS BbSs BbSs BbSs BbSs Black /Split hoof Black /Split hoof Black /Split hoof Black /Split hoof BbSs BbSs BbSs BbSs Black /Split hoof Black /Split hoof Black /Split hoof Black /Split hoof BbSs BbSs BbSs BbSs Black /Split hoof Black /Split hoof Black /Split hoof Black /Split hoof BbSs BbSs BbSs BbSs Black /Split hoof Black /Split hoof Black /Split hoof Black /Split hoof Phenotypic Ratio: Genotypic Ratio: 16:0 (All black coat and with split hoof) 16: 0 (All Heterozygous Black and heterozygous Split hoof)
Page 12 of 20 DIHYBRID CROSS Dihybrid Cross: A two factor cross where both parents are Heterozygous for BOTH traits. Sample Dihybrid Cross In cows, the gene for black coat colour (B) is dominant to the gene for white coat colour (b). The gene for split hoof(s) is dominant to the gene for solid hoof (s). What are the expected genotypic and phenotypic ratios for a Dihybrid cross between two cows? Answer: Parents: Recall that Dihybrid means that both parents are heterozygous for the two traits. BbSs X BbSs Gametes: BS Bs bs bs BS Bs bs bs F 1 Generation: BS Bs bs bs BS Bs bs bs BBSS BBSs BbSS BbSs Black /Split hoof Black /Split hoof Black /Split hoof Black /Split hoof BBSs BBss BbSs Bbss Black /Split hoof Black /Solid hoof Black /Split hoof Black /Solid hoof BbSS BbSs bbss bbss Black /Split hoof Black /Split hoof White /Split hoof White /Split hoof BbSs Bbss bbss bbss Black /Split hoof Black /Solid hoof White /Split hoof White /Solid hoof Phenotypic Ratio: 9:3:3:1 Black/Split Hoof: Black/ Solid Hoof: White/Split Hoof: White/Solid Hoof Genotypic Ratio: 1:2:2:4:1:2:1:2:1 Homozygous Black/Homozygous Split hoof 1 Homozygous Black/Heterozygous Split hoof 2 Heterozygous Black/Homozygous Split hoof 2
Page 13 of 20 Heterozygous Black/Heterozygous Split hoof 4 Dihybrid cross continued Homozygous Black/Homozygous solid hoof 1 Heterozygous Black/Homozygous solid hoof 2 Homozygous white/homozygous split hoof 1 Homozygous White/Heterozygous split hoof 2 Homozygous white/homozygous solid hoof 1 NOTE: THESE RATIOS REMAIN CONSTANT FOR ANY DIHYBRID CROSS!!!!
Page 14 of 20 INCOMPLETE DOMINANCE This is a situation where neither of the two alleles for a trait is dominant. Examples of incomplete Dominance: a. Snapdragon Flowers (Heterozygous = pink) b. Four O Clock Flowers (heterozygous = pink) Let s Look at a cross involving Incomplete Dominance Snapdragon Flowers The gene for White petal colour is incompletely dominant while the gene for Red petal colour is also incompletely dominant. When you cross a white flower with a red flower you get all PINK flowers. Let s see how this occurs. Let s make the allele for Red petal colour (R). Let s make the allele for White petal colour (R ). Because both of these genes are incompletely dominant, they both have a capital letter. We use R because both are dominant, but not one over the other. Now, let s say we cross a white flower (Homozygous - the only thing it can be) with a Red flower (again, homozygous). Here are the results. Parents: RR X R R Gametes: F 1 Generation: R R R R R R R R R R R R R R R R Phenotypic Ratio = 4:0 for all PINK flowers. Genotypic Ratio = 4:0 for Heterozygous Pink flowers (R R ).
Page 15 of 20 What would be the result when two pink Flowers were crossed? Let s see. Remember: Pink Snapdragons are Heterozygous (R R ) Parents: RR x R R - Gametes: F 1 Generation: R R R R R R R RR R R R R R R R Phenotypic Ratio = 1:2:1 for Red: Pink: White Genotypic Ratio = 1:2:1 for Homozygous Red: Heterozygous Pink: Homozygous White. Note: Other ways of showing the alleles for incomplete dominance i) R red R = white ii) F R - red F w - white iii) R red W - white
Page 16 of 20 CO-DOMINANCE Co-Dominance This is a situation where both alleles for a trait may be dominant. The alleles are said to be co-dominant. BOTH alleles are expressed in the heterozygous individual. Examples of Co-dominance a. Feather colour in chickens (black and white feathers expressed at same time Barred plumage) b. Roan horses (Red and white hair expressed at same time) c. Blood type (AB Type A and B blood expressed at same time) Sample of Co-Dominance Cross In chickens, the gene for black feather colour is co-dominant to the gene for white feather colour. What are the expected ratios for a cross between a black feathered rooster and a white feathered hen? Answer: Recall that both of these traits are dominant so both will be expressed. Both traits receive a different capital letter. Parents: BB X WW Gametes: B B W W F 1 generation: B B W BW BW W BW BW Phenotypic Ratio: Genotypic Ratio: 4:0 All Barred feathers (Note: Barred = Black and white mix) 4:0 All heterozygous black/white (Barred)
Page 17 of 20 NOTE: If you cross two Barred coloured chickens, it would be possible to get black and white chickens produced. MULTIPLE ALLELES Some genetic traits are expressed by multiple (many) alleles. Such a trait is blood type in Humans. The blood type of humans is controlled by many alleles and not just one or two as is the norm. The human blood types/groups are as follows: 1. Type A 2. Type B 3. Type AB 4. Type O To have one of these blood types you must of course have received the genes/alleles from your parents. The alleles that you could have received are as follows: Genotype Phenotype (Blood type) I A I A or I A i A I B I B or I B i B I A I B AB (A & B are co-dominant) Most common blood type. ii O (recessive) Least common blood type
Page 18 of 20 MIXING BLOOD We all know that only certain types of blood cannot be mixed with other types of blood. This is because if you mix the wrong types of blood agglutination or clumping will occur. This will causes the blood cells in the person being affected to clump together and the person would die from a blood clot in the brain or heart etc. Q. What types of blood can be mixed together? A. To answer this question we should look at the alleles for each blood type. Blood Type/Allele Blood it can mix with- safely A - I A I A or I A i A or AB (Must have the A allele) B - I B I B or I B i B or AB (Must have the B allele) AB - I A I B AB only!!!! (Must have A and B alleles) O - ii Any of the blood types (Recessive, so it can mix with any blood type) (Universal donor)!!! NOTE: AB is the universal ACCEPTOR (it can accept all blood types) O is the universal DONOR (can donate to all blood types because it is recessive) Sample Cross involving Multiple Alleles (Blood types) A woman with Type A blood marries a man with type AB blood. What are the possible blood types of their offspring? Answer: Recall, Type A blood results from both a homozygous condition (I A I A ) or a heterozygous condition (I A i). A type AB person will have only one genotype (I A I B ). To figure out the results, we will have to complete two crosses. Cross # 1: Parents: We will assume the mother is homozygous for A blood. I A I A I A I A x I A I B Gametes: I A I A x I A I B I A I A I A I A I A I A I A I B I A I B I A I B Phenotypic Ratio: 1:1 (Type A and Type AB blood) Genotypic Ratio: 1:1
Page 19 of 20 F1 generation:
Page 20 of 20 Cross # 2 Parents: We will assume the mother is heterozygous for A blood. I A i Gametes: F1 generation: I A I A I A I A I A i I B I A I B I B i I A i x I A I B I A i x I A I B i Phenotypic Ratio: 2:1: 1 (Type A: Type AB: Type B) Genotypic Ratio: 1:1:1:1 Homozygous A: Heterozygous: HeterozygousAB: Heterozygous B