Mendel's Laws Of Heredity (Genetics1 two laws 1 PDF)

General Genetics Sheet 1. Mendel's laws Third class (Zoology & Microbiology) Prepared by: Dr. Ibrahim A. Ishag  Introduction Genetics is the study of inheritance or heredity. It defines as science deals with the transmission of characters, specific for tha...

General Genetics Sheet 1. Mendel's laws Third class (Zoology & Microbiology) Prepared by: Dr. Ibrahim A. Ishag  Introduction Genetics is the study of inheritance or heredity. It defines as science deals with the transmission of characters, specific for that particular type of plant or animal, from the parent to the offspring of the next generation. The history of Genetics is closely linked with the ancient cultural history of man. It began ten thousand years ago, when human beings changed from a nomadic life to the life of settlers. Early ideas about inheritance: Corn A need for food forced them to explore nature and adopt agriculture. The agricultural revolution in early human Wheat civilization opened the doors for very rapid improvements in human culture.  Early ideas about inheritance: They learned to identify plants that could be grown and cultivated (Rice, Rice Wheat, Barely). Barely They started selecting suitable varieties for agriculture.  Early ideas about inheritance: Simultaneously they started domestication of animals (Horse, Cattle, Camel, Dog and ect.) Many animals were brought under human control. Gradually they understood their reproductive methods.  Early ideas about inheritance: Hybridization and generation of new varieties of animals like horses, dogs, cats and cattle happened. The idea of Genetics was adopted in everyday practice. The Greek influence on ideas of inheritance: Hippocrates – “Humors”, which could be altered during an individuals lifetime and therefore diseased or normal, were drawn from various parts of the body to the semen and passed on to the offspring. This “pangenesis” theory even formed the basis of Darwin’s early ideas of inheritance. Aristotle – semen produced a “vital heat” that cooked and shaped the menstrual blood giving it the capacity to produce offspring with the same “form” as the parent. Later ideas of inheritance (1600- 1850): Pre-formationism: sex cells contain a complete miniature adult (the homunculus) Epigenesis: presumably put forth by Harvey, held that body structures were not present in the sex cells, but were formed anew. Blending Inheritance: the belief that characteristics of parents blended like paint, e.g., mix blue and yellow and get green paint.  Mendelian Genetics Austrian monk considered the “Father of Genetics” He had studied science and mathematics at the University of Vienna. Did his experiments in 1866 He did plant experiments using the garden pea plant He applied statistical methods to biological research. He discovered the pattern in which organisms inherit traits.  Mendel Breaks With Past Most likely his background in mathematics Mendel to add a statistical basis to his breeding experiments. He then chose to work with the garden pea, Pisum sativum. The garden pea was a good choice for genetic study because: - Easy to cultivate - Had a short generation time. - Has male (stamens) and female (carpal) sexual organs (normally self-pollinate or cross-pollinate) - Many varieties of peas were available with distinct heritable features (characters) with different variants (traits). What traits did Mendel study? Production of true-breeding plants (pure breeds): By self-pollinated (fertilizing ova with their own sperm for many generations), he produced true- breeding plants True-breeding plant: the offspring were like the parents plants and like each other. True-breeding organisms are genetically identical and have identical alleles for specific trait. Monohybrid Cross: After ensuring that his pea plant were true-breeding, Mendel was then ready to perform a cross-pollination experiment between two strains. He performed reciprocal crosses: first he dusted the pollen of purple followers plants on the stigmas of white plants, and then he dusted the pollen of white plants on the stigmas of purple plants. monohybrid crosses, because the offspring are hybrid – they are product of two different strains that differ in regard to only one trait.  In a typical breeding experiment, Mendel would cross-pollinate (hybridize) two contrasting, true- breeding pea varieties. – The true-breeding parents are the P generation and their hybrid offspring are the F1 generation. Mendel would then allow the F1 monohybrids to self-pollinate to produce an F2 generation. It was mainly Mendel’s quantitative analysis of F2 plants that revealed the two fundamental principles of heredity  Mendel did cross-pollination (hybridization) between two contrasting pea plants (True-breeding), Purple flowers X White flowers  Mendel found that the all plants (F1) from above crossing had purple flowers  When Mendel allowed the F1 plants to self-fertilize, the F2 generation included both purple- flowered and white-flowered plants. – The white trait, absent in the F1, reappeared in the F2.  Based on a large sample size, Mendel recorded:  705 purple-flowered F2 plants and  224 white-flowered F2 plants from the original cross. With Ratio 3 purple:1 white Notice: See the below Table  Table. All crosses done by Mendel and results Trait Characteristic F2 results Dominant Recessive Stem length Tall Short 787 277 Pod shape Inflated Constricted 882 299 Seed shape Round Wrinkled 5474 1850 Seed color Yellow Green 7022 2001 Flower position Axial Terminal 651 207 Flower color Purple White 705 224  Mendel knew that the same ratio was obtained among the F2 generation time and time again for each trait (Table), and he sought and explanation for these results A 3:1 ratio among F2 offspring was possible if: 1- the F1 generation contain two separate copies of each heredity factor, one of these being dominant and one being recessive; 2- the factors separated when the gametes were performed, and each gamete carried only one copy of each factor; and 3- random fusion of all possible gametes occurred upon fertilization. Mendel also explained that:  If the blending model or theory is correct, the F1 hybrids from a cross between purple-flowered and white-flowered pea plants would have pale purple flowers.  Instead, the F1 hybrids all have purple flowers, just a purple as the purple-flowered parents.  According to above Mended coined his first law or the law of segregation, the two alleles for a characters are packaged into separate gametes The first law of Mendel or Law of Segregation: Each organism contain two factors for each trait, and the factors segregate during the formation of gametes so that each gamete contains only one factor for each trait. By the other words: the two alleles for a characters are packaged into separate gametes  Genetic notation In genetic notation, the alleles are identified by letters, - the dominant allele (so named because of its ability to mask the expression of its allele) with an uppercase (capital) letter - the recessive allele with the same but lowercase (small). With the reference to the cross being discussed, there is an allele for purple followers (P) and an allele for white followers (p). Alleles occur on a homologous pair of chromosomes at particular location that is called the gene locus.  Monohybrid Crosses in dogs BB = brown eyes Bb = brown eyes bb = blue eyes P : BB ♀ (female with brown eyes) X bb♂ (male with blue eyes) F1: Bb (all offspring with brown eyes) Cross two Alaskan huskies that are heterozygous for brown eyes (Bb). female gametes B b Genotypes % 25% BB, 50% Bb, 25% bb B BB Bb Cross Ratios Bb x Bb 1BB:2Bb:1bb b Bb bb Phenotypes % 75% Brown eyed, 25% Blue eyed male gametes Ratio 3 Brown eyed: 1 Blue eyed  Genetic Vocabulary Character: heritable feature Trait: each variant for a character True-breeding: plants that self-pollinate all offspring are the same variety Monohybrid cross: a cross that tracks the inheritance of a single character P generation: (parental) true-breeding F1: (first filial) offspring of P generation F2: (second filial) offspring from F1 cross  Vocabulary (continued) Gene: the basic unit of inheritance; the factors that carry all traits. Now: segment of DNA or short section of chromosome. Allele: alternate version of a gene or different forms of the same gene. Locus: the site of particular gene. Chromosome: very long strand of DNA and associated proteins present in nucleus of every cell of an organism. Deoxyribonucleic Acid (DNA): very complex molecule that forms genetic code for all living things.  Vocabulary (continued) Phenotype: appearance of an organism Genotype: genetic makeup (all genes carried by an organism) Homozygote: pair of identical alleles for a character Homozygous dominant- BB Homozygous recessive - bb Heterozygote: two different alleles for a character (Bb) Dominate allele: expressed in the heterozygote Recessive allele: not expressed in the heterozygote  Genes on Chromosomes Gene Locus – Alleles – two forms of location of the same gene specific gene  Test cross Mendel developed a way to determine whether an organism that expressed a dominant trait was a heterozygote or a homozygote is called the test cross. This technique is still used by plant and animal breeders. By performing a test cross, one can determine whether the individual is homozygous or heterozygous dominant  Test cross In a test cross, the dominant-expressing organism is crossed with an organism that is homozygous recessive for the same characteristic. 1. If the dominant-expressing organism is a homozygote, then all F1 offspring will be heterozygotes expressing the dominant trait. 2. Alternatively, if the dominant-expressing organism is a heterozygote, the F1 offspring will exhibit a 1:1 ratio of heterozygotes and recessive homozygotes.  Test cross A test cross can be performed to determine whether an organism expressing a dominant trait is a homozygote or a heterozygote.  Law of Independent Assortment Mendel’s law of independent assortment states that genes do not influence each other with regard to the sorting of alleles into gametes, and every possible combination of alleles for every gene is equally likely to occur. Independent assortment of genes can be illustrated by the dihybrid cross, a cross between two true-breeding parents that express different traits for two characteristics. DIHYBRID CROSS In pea plants round seeds are dominant to wrinkled seeds and green pods are dominant to yellow. If 2 plants that were heterozygous for both traits were crossed, what would be the phenotypic and genotypic ratios? Gene Trait Appearance R Round r Wrinkled G Green g Yellow Cross: RrGg X RrGg Round seeds, green pods X wrinkled seeds, yellow pods (RRGG) (rrgg) Genotype of parents: RRGG X rrgg Gametes of parent: RG rg F1 Genotype: RrGg (Dihybrid) F1 phenotype: all plants with round seed and green pod  Self-pollination of F1 plants: F1 X F1 Genotype of F1: RrGg X RrGg RG rg Rg rG Gametes of F1: In dihybrid crosses you will see more combinations of the two genes. And To predict genotypes from dihybrid crosses, we will use Punnett square Punnett square Female gametes RG Rg rG rg RG RRGG RRGg RrGG RrGg Male gametes Rg RRGg RRgg RrGg Rrgg rG RrGG RrGg rrGG rrGg rg RrGg Rrgg rrGg rrgg Summary of genotypes and phenotypes and their ratios: Genotypic ratio: 1 RRGG: 2 RRGg: 2 RrGG: 4 RrGg: 1 RRgg: 2 Rrgg: 2 rrGg: 1 rrGG: 1 rrgg Phenotypic ratio: 9 round, green: 3 round, yellow: 3 wrinkled, green: 1 wrinkled, yellow  (9:3:3:1) When you compared contrast traits of each character independently you can find this: Seed shape: 12 round: 4 wrinkled (3:1) Pod color: 12 green: 4 yellow (3:1) From these results Mendel coined his second inheritance law Second Mendel's Law or Law of Independent Assortment: Each set of alleles segregates independently By other words: The alleles for different genes usually separate and inherited independently of one another. Example 1: Angus cattle are polled and black (pure breed: homozygous in two loci PPBB), while Hereford cattle are horned and red (pure breed: homozygous in two loci ppbb). P allele for polled completely dominant on p allele horned B allele black color completely dominant on b allele red Angus bulls bred to horned Hereford cows or vise versa P: Angus ♂ X Herford ♀ PPBB X ppbb Gametes: PB X pb F1: PpBb (all are Polled and Black) F1: PbBb ♂ X PbBb ♀(Dihybrid cross) Gametes: PB, Pb, pB, pb Genotypes' ratio F2: results in Punnett square ¼ PP ½ Pp ¼ pp ¼ BB 1/16 PPBB 2/16 PpBb 1/16 ppBB ♂ ♀ PB Pb pB pb ½ Bb 2/16 PPBb 4/16 PbBb 2/16 ppBb PB PPBB PPBb PpBB PpBb ¼ bb 1/16 PPbb 2/16 Ppbb 1/16 ppbb Pb PPBb PPbb PpBb Ppbb pB PpBB PpBb ppBB ppBb Phenotypes' ratio ¾ P- ¼ pp pb PpBb Ppbb ppBb ppbb Nine distinct genotypes are produced, but ¾ B- 9/16 P-B- 3/16ppB- because of complete dominance; only four phenotypes are recognizable: polled/ ¼ bb 3/16P-bb 1/16ppbb black, polled/red, horned/black, and horned/red. These will occur with a ratio of approximately 9:3:3:1  Probabilities in genetics Probabilities are mathematical measures of likelihood. They are a way of quantifying how likely something is to happen. Empirical probability (P) of an event is calculated by counting the number of times that event occurs (m) and dividing it by the total number of times that event could have occurred (n), thus P=m/n Theoretical probability of an event is calculated based on information about the rules and circumstances that produce the event. – It reflects the number of times an event is expected to occur relative to the number of times it could possibly occur. Important rules of probabilities: The product rule: Which states that the probability of two (or more) independent events occurring together can be calculated by multiplying the individual probabilities of the events. For example, if you roll a six-sided dice once, you have a 1/6 chance of getting a six. If you roll two dice at once, your chance of getting two sixes is: (probability of a six on dice 1) x (probability of a six on dice 2) = (1/6)⋅(1/6)=1/36.  We can use the product rule to predict frequencies of fertilization events.  For instance, consider a cross between two heterozygous (Aa×Aa) individuals. What are the odds of getting an aa individual in the next generation?  The only way to get an aa individual is if the mother contributes an a gamete and the father contributes an a gamete.  Each parent has a 1/2 chance of making an a gamete.  Thus, the chance of an aa offspring is: (probability of mother contributing a) x (probability of father contributing a) = (1/2)⋅(1/2)=1/4 The above result is the same result you’d get with a Punnett square. The sum rule of probability: The probability that any of several mutually exclusive events will occur is equal to the sum of the events’ individual probabilities. For example, if you roll a six-sided dice, you have a 1/6 chance of getting any given number, but you can only get one number per roll. You could never get both a one and a six at the same time; these outcomes are mutually or exclusive. Thus, the chances of getting either a one or a six are: (probability of getting a 1) + (probability of getting a 6) = (1/6)+(1/6)=1/3  Dominance  Dominance is the phenomenon of one allele of a gene masking or overriding the effect of other allele of the same gene (Interaction between an allelic genes).  Dominance relationships include: - Complete dominance, - Incomplete dominance, and - Co-dominance.  Complete dominance Complete dominance: The expression of heterozygous genotype is not different from the expression of the homozygous genotype, that having two dominant genes (flower colour of pea plant). Note: At the molecular level, these relationships between pairs of alleles depend upon patterns of gene expression.  Incomplete Dominance Incomplete dominance is an allelic relationship where dominance is only partial. - In a heterozygote, the recessive allele is not expressed. - The one dominant allele is unable to produce the full phenotype seen in a homozygous dominant individual. - The result is a new, intermediate phenotype.  Incomplete Dominance An example is plumage color in chickens: a. Crossing a true-breeding black chicken (CBCB) with a true-breeding white one (CWCW) produces an Andalusian blue F1 (CBCW). b. When the F1 interbreed, the F2 include black (CBCB), Andalusian blue (CBCW) and white (CWCW) birds, in a ratio of 1:2:1. c.At the molecular level, two copies of CB produce black, while 1 copy is sufficient to produce only the gray “Andalusian blue” phenotype. Fig. Incomplete dominance in chickens  Incomplete dominance in horse color Palomino horses (golden-yellow body with nearly white mane and tail) are another example. When palominos are interbred, the progeny are: a. 1/4 cremello (cream colored) with genotype Ccr/Ccr. b. 1/2 palomino with genotype C/ Ccr. c. 1/4 light chestnut with genotype C/C.  Incomplete dominance in plant Incomplete dominance often occurs in plants. - An example is flower color in snapdragons involving two alleles, CR and CW. - Red-flowered plants (CR/CR) crossed with white-flowered ones (Cw/Cw) produce all pink progeny (CR/Cw ).  Codominance o Two alleles affect the phenotype in separate and distinguishable ways. o Neither allele can mask the other and both are expressed in the offspring and not in an “intermediate” form.  Example in plant: Red flowers that are crossed with white flowers that yield red and white flowers.  Codominance Example in cattle: Coat color (mixed red and white hairs) occurs in the heterozygous (Rr) offspring of red (RR) and white (rr) homozygotes. When two roan cattle are crossed, the phenotypes of the progeny are found to be in the ratio of 1 red:2 roan:1 white.  Codominance In codominance, the heterozygote’s phenotype includes the phenotypes of both homozygotes. Examples include: a. The ABO blood series, in which a heterozygous IA/IB individual will express both antigens, resulting in blood type AB. b.The human M-N blood group involves red blood cell antigens that are less important in transfusions. There are three types: i. Type M, with genotype LM/LM. ii. Type MN, with genotype LM/LN. iii. Type N, with genotype LN/LN.  Molecular Explanations of Incomplete Dominance and Codominance Current explanations involve levels of gene expression for each allele in the pair. a. In codominance, both alleles make a product, producing a combined phenotype. b. In incomplete dominance, the recessive allele is not expressed, and the dominant allele produces only enough product for an intermediate phenotype.  Molecular Explanations of Incomplete Dominance and Codominance c. By contrast, a completely dominant allele creates the full phenotype by one of two methods: i. It produces half the amount of protein found in a homozygous dominant individual, but that is sufficient to produce the full phenotype. These genes are haplosufficient. ii. Expression of the one active allele may be upregulated, generating protein levels adequate to produce the full phenotype. Notice: At the molecular level, these relationships between pairs of alleles depend upon patterns of gene expression.