Genetics - Gene Segregation and Integration PDF

} Gregor Mendel – responsible for the Laws governing Inheritance of traits. An Austrian Monk and studied the inheritance of traits of pea plants. } Trait – any characteristics that can be passed from parent to offspring. } Heredity – passing of traits from parent to offsp...

} Gregor Mendel – responsible for the Laws governing Inheritance of traits. An Austrian Monk and studied the inheritance of traits of pea plants. } Trait – any characteristics that can be passed from parent to offspring. } Heredity – passing of traits from parent to offspring. } Genetics – study of heredity. } Gene – a unit of heredity which is transferred from a parent to offspring and kis held to determine some characteristic of the offspring. } Alleles – two form of a gene (dominant & recessive). Different form of the same gene. } Gene specifies trait, allele specifies form gene takes. } Dominant – stronger of two genes expressed in the hybrid: represented by a capital letter “R” } Recessive – gene that shows up less often in a cross: represented by a lower case letter “r” } Genotype – gene combination for a trait ( e.g. RR, Rr, rr ) } Phenotype – the physical feature resulting from a genotype (e.g. red, white) } Genotype of alleles: R = red flower r = yellow flower All genes occurs in pairs, so 2 alleles affect a characteristics } Possible combination are: Genotypes RR Rr rr Phenotypes RED RED YELLOW } Homozygous genotype – gene combination involving 2 dominant or 2 recessive genes ( e.g. RR or rr) also called pure. } Heterozygous genotype – gene combination of one dominant & one recessive allele ( e.g Rr) also called hyrid } Monohybrid cross – cross involving a single trait e.g. flower color } Dihybrid cross – cross involving two traits e.g flower color and plant height } The passing of traits to the next generation is called inheritance or heredity. } Mendel performed cross-pollination in pea plants. } Mendel followed various traits in the pea plants. }Mendel studied seven different traits: v Seed or pea color v Flower color v Seed pod color v Seed shape or texture v Seed pod shape v Stem length v Flower position } Parent Generation is also known as the P generation. Each of the original pair of plants. } The offspring of this P cross are called the F1, or first filial generation. } The second filial generation or F2 is the offspring from the F1 cross. } In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation. Offspring that are hybrid for a trait will have only the dominant trait in the phenotype. } When Mendel was crossing his pea plants , he noticed something interesting. } When he crossed pure tall plants with pure short plants, all the new pea plants were tall. } So what was observed is that when there is a dominant trait present in a genotype, the phenotype is affected by the dominant allele and not the recessive one. Mendel state that one factor in pair may mask the expression of the other. } During the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other. } Alleles for a trait are then “recombined” at fertilization, producing the genotype for the traits of the offspring. } The law of segregation states that the two alleles of one trait segregate from each other during gamete formation. } Alleles for different traits are distributed to sex cells (& offspring) independently of one another. } Mendel noticed during all his work that the height of the plant and the shape of the seeds and the color of the pods had no impact to one another. } The different traits seem to be inherited INDEPENDENTLY. } The genotypes of the parent pea plants will be: } RrGg x RrGg Where: “R” =dominant allele for round seeds “r” = recessive allele for wrinkled seeds “G”= dominant allele for green pods “g”= recessive allele for yellow pods } Round/Green: 9 } Round/yellow: 3 } wrinkled/Green: 3 } Wrinkled/yellow: 1 Phenotypic ratio: 9:3:3:1 } So you can see from the results, a green pod can have round or wrinkled seeds, and the same is true of a yellow pod. } The different traits do not influence the inheritance of each other. They are inherited INDEPENDENTLY. } Meiosis is the process of creating sperm and egg gametes. Each gamete contains half (haploid) the number of chromosomes an individual needs, so fertilization results in the offspring receiving one allele for a trait from each parent. } The law of segregation states that the parental genes must separate randomly and equally into gametes during meiosis so there is an equal chance of the offspring inheriting either allele. No allele is favored or has an advantage over another. } Like segregation, independent assortment occurs during meiosis, specifically in prophase I when the chromosomes line up in random orientation along the metaphase plate. } Crossing over, the exchange and recombination of genetic information between chromosomes also occurs in prophase I and adds to the genetic diversity of the offspring. 1. Complete Dominance 2. Incomplete Dominance 3. Co-Dominance 4. Over Dominance 5. No Dominance } Dominance is a relationship between alleles of one gene, in which one allele is expressed over a second allele at the same locus. } The first allele is dominant and the second allele is recessive. } Dominance is a key concept in Mendelian inheritance. } Incomplete dominance (also called partial dominance) occurs when the phenotype of the heterozygous genotype is distinct from and often intermediate to the phenotypes of the homozygous genotypes. } Example: ◦ Snap Dragon or Morning Glory or Primrose (Flowers) ◦ A child with wavy hair due to one parent’s curly hair and the other’s straight hair. ◦ Tay-Sachs disease } A snapdragon flower is a pink flower as a result of cross-pollination between a red flower and a white flower when neither the white nor the red alleles are dominant. } Genotypic ratio = 1RR:2RW:1WW } Phenotypic ratio = 1Red:2Pink:1White } When both alleles of a pair are fully expressed in a heterozygote, they are called co-dominant alleles. } Such alleles exhibit a unique pattern of expression with heterozygous being phenotypically distinguishable from both of the homozygous and expressing both alleles equally. } Example: ◦ Roan coat color in horses and cattle ◦ Chestnut and white color in horses are both dominant traits; the heterozygous individual is a palomino (golden) } C=Color Gene ◦ CRCR = Red ◦ CWCW = White ◦ CRCW = Red Roan } Genotypic Ratio = All CRCW } Phenotypic Ratio = All Red Roan } Chestnut and White color in horses are both dominant traits; the heterozygous individual is a palomino (golden) } Expression of heterozygotes is more extreme than both of homozygotes. } Heterozygote expresses the phenotype outside of the range of homozygote phenotypes. } BUT most closely resembles the expression of the homozygous dominant phenotype } Often called “Superior heterozygote” but “Extreme” might be more correct. } Example: Survivability in wild rats