- Slides: 20
Mendelian Genetics Ø Gregor Mendel –monk, studied how traits are pass on. Ø Mendel successful - used statistics to guide and support his studies. (many trials = increases probability)
l Heredity = l traits/ genes passed on to successive generations l Genes = l “factors” or pieces of DNA that code for proteins which create the traits l two genes per trait l Chromosomes = l Whole strands of DNA that contain genes l 2 of each chromosome
Alleles • variation of a gene – Example: there is a gene that determines the texture of hair. – The alleles of the gene are curly and straight – Letters are used to symbolize the alleles (Example: C = curly)
Purebred or True-Breed • Purebred or true-breed = it produces the same trait generation after generation (i. e. both of the alleles that an individual has for that trait are the same example CC or cc) • Hybrid = the alleles that an individual has for that trait are different from each other (example Cc)
Mendel came up with three basic principles of genetics
Mendel came up with three basic principles of genetics 1. Principle of Dominance n n An allele is considered Dominant when only one of that allele is present and that trait is still expressed. An allele is considered Recessive when both alleles need to be present for that trait to be expressed.
• Mendel crossed purebred purple flower plants with purebred white flower plants. X • The purple allele is dominant and the white allele is recessive
• Since the purple allele is dominant it is symbolized by a “P” • The white is recessive and is symbolized by a “p” X PP pp Pp
X P generation PP F 1 generation pp Pp • The individuals in the P generation are said to be homozygous because their two alleles are the same PP and pp. (PP = homozygous dominant pp = homozygous recessive) • The F 1 generation is heterozygous Pp
Genotype & Phenotype Genotype – the gene type (allele combination) – example: Pp Phenotype - is the expression of the genotype (what the visible trait is) – example: purple flower
2. Principle of Segregation n n Only one of the parent’s two genes is passed on to the next generation. During gamete formation the two genes, for a trait, separate so that the gamete only carries one gene for every trait.
P generation X PP F 1 generation pp
If two of the F 1 generation are crossed what will the genotype be for the F 2 purple flowers? (in the F 2 there will be some white flowers too. ) F 1 generation X Pp Pp F 2 generation ? ? P_
Punnet Square • Punnet square is used to determine probability of heredity • If we are crossing a heterozygous Pp with another heterozygous Pp what are the probabilities for each of the genotypes and phenotypes? P p P PP p Pp pp
The resulting genotypes are: PP = 1 Pp = 2 pp = 1 (note the ratio is written 1 PP: 2 Pp: 1 pp the fraction of PP is ¼, the percentage of PP is 25%) The resulting phenotypes are: 3 purple and 1 white flower
• What would the phenotype and genotype be for the following crosses? • PPx. Pp (Purple is the dominant trait) • ttx. Tt (Tall is the dominant trait)
3. Principle of Independent Assortment n during gamete formation the segregation of the alleles of one pair of alleles is independent of the segregation of the alleles of another pair of alleles n example: just because a pea plant has purple flowers does not mean it is going to also be tall. n Traits that assort independently are called unlinked traits.
Dihybrid Cross • A dihybrid is an individual that is a hybrid for two traits (di = 2). Example: Tall plant with yellow seeds (Tt. Yy) • A dihybrid cross illustrates the principle of independent assortment • An example of dihybrid cross is Tt. Yy x Tt. Yy
Tt. Yy x Tt. Yy TY Ty t. Y ty TY TTYY TTy. Y t. TYY t. Ty. Y Ty TTYy TTyy t. TYy t. Tyy t. Y Tt. YY Tty. Y tt. YY tty. Y ty Tt. Yy Ttyy tt. Yy ttyy
Results Genotypes TTYY = 1 TTyy = 1 TTYy = 2 Tt. YY = 2 Tt. Yy = 4 Ttyy = 2 tt. YY = 1 ttyy = 1 Phenotypes 9 Tall and yellow seeds 3 Tall and green seeds 3 Short and yellow seeds 1 Short and green seeds