1 UNIT III (Notes) : Genetics : endelian. (HR Biology p. 526-543) Heredity is the transmission of traits from one generation to another. Traits that are passed on are said to be inherited. Genetics is the branch of biology that deals with the principles of variation and inheritance in animals and plants. Traits are distinguishing characteristics that make a unique individual. Gregor endel, an Austrian monk, was the first to correctly interpret patterns of inheritance. Because traits are inherited as independent units, endel s factors of inheritance are sometimes referred to as unit characters and his theory as the unit theory of inheritance. endel s work : - took over 7 years to complete - involved collecting data from over 30,000 plants - took place in the 1800's when little was known about genetics Traits are sometimes called characteristics and are inherited. The gametes are the reproductive cells that carry genetic information such as the sperm (male) and egg (female). Remember : sperm + egg = fertilization (carries information from both gametes). endel studied peas. Pea flowers produce male and female gametes and are capable of self fertilization. It was also possible for him to cross fertilize plants. Therefore, he could mate or breed plants in a controlled manner. endel bred plants that were distinctive for a particular trait. He studied pea plants which was an advantage because : (A) contained sexual organs of the plant which were entirely enclosed within the flower ; (B) were commercially available ; (C) they grew and matured quickly ; (D) they self-pollinate, which allowed endel to control which plants reproduced ; (E) he could cross-pollinate plants ; () different varieties of pea plants had different traits that could be oerved easily from one generation to the next. endel examined seven different traits. Each trait had only two possible forms or variations. or Example: He used tall pea plants. They always produced tall plants. We call them pure lines. He used short pea plants that always produced short plants. We call them pure lines. When parents of two different pure lines are crossed we get hybrids. He used two distinct traits: (A) tall vs. short and (B) smooth vs. wrinkled. endel s experiment: The first thing he did was obtain purebred plants for the trait he wanted to study. Purebred means descended from ancestors of a distinct type, or breed. Example : He used tall pea plants that when fertilized always produced tall plants.* At the beginning the tall plants produced some short plants but endel selected tall plants and bred them until only tall plants were produced. He did the same with short offspring. The same is true for short plants. If he bred short plants that produced other short plants he knew that he had pure lines to deal with. Once endel had his purebred plants he designated them the parent or P generation. When he crossed a pure tall with a pure short the offspring were called the first filial generation or the 1 generation. endel called these
2 hybrid plants to indicate that they were the result of a cross between two different purebred plants. onohybrid cross (or one factor cross) is a cross which involves only one trait. Example : P 1 generation : Purebred tall x Purebred short What do you think were the results of crossing a pure tall with a pure short plant? endel thought plants would have been medium in height. 1 or first filial generation : All offspring were tall ( no short or medium) His results were 100 % tall. Did 1 plants lose their ability to produce short plants? endel s next step: The next experiment involved breeding the 1 generation. Hybrid plants from the 1 were allowed to self- pollinate producing the second filial or 2 generation. Tall 1 x Tall 1 2 generation : Three of four plants in the 2 generation were tall while one was short. The trait for shortness had reappeared! The second filial generation resembled one parent from the P generation 75 % of the time and resembled the other parent 25 % of the time. This ratio of 3:1 is known as the edelian ratio. endel s conclusion was that the trait for tall must be dominant and the trait for short must be recessive. A recessive trait may be expressed if it is the only trait present as in endel s short -growing plants. endel repeated the experiments many times and with different traits and concluded that heredity was more than a blending of traits. He concluded that when plants with two contrasting traits are crossed one trait is always dominant over the other. Principle of dominance : When individuals of contrasting traits are crossed, the offspring will express only the dominant trait. Another Explanation endel : Distinct units of heredity or factors( now known as genes) were responsible for inherited traits. Two factors controlled any single trait (ie.) one factor produced tall and one factor produced short. The tall (T) factor was dominant. The short (t) factor was recessive. Dominant trait is always expressed in an individual. Recessive trait is a trait that is latent (present but inactive) and is not usually expressed in an individual Generally, the dominant factor only appeared in 1 generation and the recessive factor appeared in 2. Two letters are necessary to describe each combination of alleles (dominant or recessive form which a gene may take). The combination of alleles is called the genotype (genetic make-up of an organism).
3 Genes are specific portion of DNA molecule that determine the characteristics of an organism. They may be dominant (ie.) determine the expression of genetic trait in an offspring. It prevents the expression of the recessive trait. It is represented by an upper case letter. They may be recessive (ie.) overruled by the dominant gene. It is masked by the dominant gene. It is represented by a lower case letter; (eg.) brown eyes (BB or Bb) ; blue eyes (bb). Homozygous : Heterozygous : alleles are the same BB, bb alleles are different Bb The physical expression of a trait is called the phenotype ; (ie.) brown eyes, blue eyes. An organism can have the same phenotype (brown eyes) but have a different genotype (BB or Bb). endel s conclusions : (1) Each parent in the 1 generation starts with two hereditary factors. One is dominant and one is recessive. (2) The factors separate in the parent. Only one factor from each parent is contributed to the offspring. (3) Each offspring inherits one factor from each parent. If the dominant factor is present it will be expressed even if the recessive factor is also present. (4) The recessive factor will be expressed only if a pair of recessive factors are present. endel s results from the 2 generation gave rise to his first law of heredity. endel s irst Law: The law of segregation states that inherited traits are determined by pairs of factors. These factors segregate (separate) in the gametes, with one in each gamete. Segregation refers to the separation of paired genes during meiosis. odern form : alleles segregate in meiosis. We know today that endel s factors were genes. Genes are the part of a chromosome that governs the expression of a particular trait. An allele is an alternate form of a gene. Two letters are necessary to describe each combination of alleles. Example : T - tall t- short TT- Tall Tt- Tall tt - short When two alleles are the same (both dominant or both recessive), we say the plant (or animal) is homozygous. When two alleles are different (one dominant and one recessive), we say the plant (or animal) is heterozygous. Principle of Segregation Key Points : 1. Hereditary characteristics are determined by distinct units or factors. 2. or each characteristic, an individual carries two factors, one inherited from each parent. 3. The two factors of each pair segregate from each other and end up in separate gametes. 4. Organisms inherit two alleles for each trait; one from each parent.
Probability and genetics : Prediction is important in genetics. It involves probability or the chance that a given event will occur or is the study of the likelihood of the occurrence of a particular event or offspring. The chance or probability that an event will take place can be expressed as a fraction (1/4), ratio (1:4) or % (25%). 4 Probability = # of chances for anevent # of possible combinations The 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. Calculating Probabilities : Remember that the concept of probability is involved in genetics. This concept refers to the chance that a given event will occur. or example in a coin toss, the probability of a heads (H s) or tails (T s) is ½ or 50%. Example 1 : What is the probability of flipping two coins and getting two heads at the same time? Answer : ½ x ½ = ¼ Example 2 : Rr x Rr (heterozygous monohybrid cross) Probability of RR is ½ from the mother and ½ from the father, thus ½ x ½ = ¼. Probability of rr is ½ from the mother and ½ from the father, thus ½ x ½ = ¼. Probability of Rr is R : ½ from the mother and ½ from the father (½ x ½ = ¼ ) ; r : ½ from the 2 mother and ½ from the father (½ x ½ = ¼ ) thus ¼ + ¼ = 4 or ½. Our phenotypic ratio of 3:1 is met; (ie.) 3 dominant to 1 recessive. As is our genotypic ratio of 1:2:1, 1 RR to 2 Rr to 1 rr, as seen in a punnet square (below). Punnett Square : You may use a punnet square to show results of a monohybrid (or one-factor) cross. A Punnett square is a chart that shows all the possible genotypes of a cross. It was devised by R.C. Punnett, a British mathematician. It is used to calculate the probability of inheriting a particular trait. All possible gametes of one parent are listed across the top and all the possible gametes for the other parent are listed down the side. When the square is filled in by copying the row and column-head letters across or down the empty squares the outcome for a cross for a given set of alleles is given. Example 1 : Given two parents with the following genotypes. Genotypes : TT and tt r According to principle of segregation, each parent contributes only one allele. The tall parent gives T or T ( tall dominates). The short parent gives t or t.
5 or example Parents TT and tt Gametes T t T t Results onohybrid Cross. t t T Tt tt T Tt tt Example 2 : Do a cross showing the expected outcome of a cross between two tall pea plants. Both are heterozygous. Show the ratio of the genotype and the phenotype. Parents : Tt x Tt Gametes : T t T t Results : T t T TT Tt t Tt tt
6 Genotypic ratio (ratio of genotypes) : Phenotypic ratio (ratio of phenotypes) : 1 TT : 2 Tt : 1 tt 3 tall : 1 short endel s Law of Independent Assortment endel s Second Law: Genes are located on chromosomes. During gamete formation, chromosomes sort out such that one chromosome from each pair goes into a gamete. Chromosomes separate and sort out independently. Dihybrid Cross (or two factor cross) involves two sets of contrasting traits. If you cross two heterozygotes for two contrasting traits or two contrasting factors, a edelian ratio of 9:3:3:1 would be realized in the 2 generation. Example : In rabbits, the allele for black coat colour (B) is dominant over the allele for brown coat colour (b). The allele for straight hair (S) is dominant over that for curly hair (s). A male rabbit with black (heterozygous), straight (heterozygous) hair is bred (mated) with a brown curly hair female. Using a Punnett square, find the genotypic and phenotypic ratios for all the possible offspring of this cross. P 1 : BbSs x bs Gametes : BS Bs bs 1 : BS Bs bs () BbSs BbSs BbSs BbSs Bs Bs Bs Bs bbss bbss bbss bbss bs bs bs bs Genotype : BbSs = 4 Phenotype : black and straight Bs = 4 black and curly bbss = 4 brown and straight bs = 4 brown and curly Genotypic Ratio : ¼ : ¼ : ¼ : ¼ Phenotypic Ratio : 4 : 4 : 4 : 4 or 1:1 :1:1
7 Applications beyond edelian Genetics : (I) Incomplete Dominance : Incomplete dominance is a type of inheritance in which two contrasting alleles contribute to the individual a trait not exactly like either parent. It is inheritance in which an active allele does not entirely compensate for an inactive allele. It is the blending of two traits An example of this would be colour in flowers as in the Japanese 4 o clock plant. The active allele is a red pigment and the inactive allele is no pigment (or white). The hybrid condition (active + inactive) would be pink flowers. See igure 16.15, p.541. Another example would be snapdragon flowers (heterozygous is pink). (II) Co-dominance : Co-dominance is a condition in which both alleles of a gene are expressed; (ie.) both alleles for a trait may be dominant. Such alleles are said to be co-dominant because both alleles are expressed in a heterozygous individual. See igure 16.16, p. 541. 541. Other examples would be roan horses (red and white hair) and chickens (black and white feathers). (III) ultiple Alleles : This occurs when a gene has more than 2 alleles. It results form different mutations of the same gene. There are 3 or more different alleles for a given trait. One example would be the alleles for human blood type (ABO blood groups). The three alleles are A, B and O. A and B are said to be co-dominant (ie.) neither one prevents or masks the expression of the other. A is dominant over O. B is dominant over O. Genotype AA, AO (I A I A, I A i) BB, BO (I B I B, I B i) AB (I A I B ) Blood Type (Phenotype) A B AB OO (ii) O Example : A woman with blood type AO marries a man with blood type AB. What would be the genotypic ratio and phenotypic ratio for the resulting children or possible offspring? P 1 : AB X AO Gametes : A A B O
8 1 : A AA AO B AB BO Genotypic Ratio : ¼ : ¼ : ¼ : ¼ Phenotypic Ratio : 2 : 1 : 1 What is a test cross? You cannot determine a genotype of an individual expressing the dominant trait by appearance. It is a cross between the dominant phenotype and the recessive phenotype. It is an experiment which reveals genotypes (of the 1 generation). It is important for plant and animal breeders to determine whether new varieties of organisms have established pure lines. How do we determine the unknown genotype? (I) We need to determine the genotype of the dominant individual. (2) We cross the organism exhibiting the dominant trait with a homozygous recessive organism.(3) If all of the offspring produced appear normal then the individual of unknown genotype is homozygous (pure) dominant. (4) If half of the offspring are normal and half are exhibiting the recessive traits, then the individual s genotype is heterozygous. How do we perform a test cross to determine a genotype of an individual for two traits (a two trait test cross)? Perform the test cross as with 1 trait but use an individual that shows the dominant phenotype for two traits and cross it with an individual that is homozygous recessive for the same two traits. r endel crossed a plant of unknown genotype with a plant having a known genotype. P 1 : T _ x tt t t T Tt Tt (T)? Tt Tt
9 t t T Tt Tt (t)? tt tt