Introduction to Mendelian Genetics

1. Introduction to Mendelian Genetics Ritika Goyal

2. Gregor J. Mendel • Father of Genetics • He was an Austrian monk and a teacher of natural science • With the help of various simple crosses between pea plants, he developed the concept of genetics • He worked for 7 years and did numerous crosses. • He gave the laws of inheritance. Image Source: http://creationrevolution.com/2012/05/gregor-mendel/

3. Why pea plant? • They grow easily. • They have short generation time. • They have many different visible trait. • There mating can be easily controlled. • They grow in large numbers. • Figure shows 7 characteristics which were studied by Mendel Image adapted from: http://gathabiotens.wordpress.com/2010/08/12/ipadio-brookes-phlog-3rd-phonecast/

4. Mendel’s experiment • Mendel crossed two plants having contrasting traits. These plants are known as parental generation. • The offsprings were observed and it was found that all of them were of the same type. This generation is the F1 generation. • Then the offsprings were self fertilized. There he found that plants of both the types were produced. This generation is the F2 generation. Image Source: Book: Biology in Context: The spectrum of life by Kennedy, Aubusson & Hickman; Oxford Press.

5. Terminology • Trait • It can be defined as a distinguishing characteristic of an individual. • In pea plants, there are two traits for each character. • Examples: • For plant height character, the two traits are tall and dwarf. • For flower color character, the two traits are purple color and white color.

6. Terminology • Factors or alleles • It can be defined as the heritable units which control specific traits. • Each trait is controlled by one factor. • Examples: • For plant height, the two traits are tall and dwarf. Tall trait is controlled by a factor called “T” and dwarf trait is controlled by a factor called “t”. • Each individual plant has two factors for a trait, one from each parent.

7. Terminology • Heterozygous and Homozygous • Homozygous plants are the plants in which both the alleles of a character are same. • Example: For plant height, a plant is homozygous if the plant has the alleles as TT or tt. • Heterozygous plants are the plants in which both the alleles of a character are different. • Example: For plant height, a plant is heterozygous if the plant has the alleles as Tt. • Therefore, • Homozygous plant can be TT or tt • Heterozygous plant can be Tt

8. Terminology • Dominant and recessive traits • Dominant trait can be defined as a trait which is expressed in heterozygous condition. • Recessive trait is defined as the trait which is not expressed in heterozygous condition. • Examples: • If a plant is heterozygous for plant height character, it has alleles Tt. Such plants are always tall. This indicates that tall trait (T) is dominant over recessive trait (t). • Therefore, • Tall is the dominant trait & dwarf is the recessive trait • T is the dominant allele & t is the recessive allele

9. Terminology • Phenotype and Genotype • Phenotype can be defined as the external appearance of a character. • Example: • For plant height character, the phenotypes are Tall and Dwarf. • For flower color character, the phenotypes are purple colored and white colored. • Genotype can be defined as the actual set of alleles present in an individual. • Example: • For plant height character, the genotypes are TT, Tt and tt. • For flower color character, the genotypes are PP, Pp and pp

10. Genotype to phenotype • We can identify the phenotype of the plant by knowing the genotype of the plant. • Example: For plant height, T is the dominant allele and codes for tall plant and t is the recessive allele and codes for dwarf plant.

11. Sample Question • For plant seed shape, round shape is dominant over wrinkled shape. Round shape is coded by “R” allele and wrinkled shape is coded by “r” allele. • What will be genotype of a heterozygous plant? • RR • Rr • rr • What will be the phenotype of plant with genotype RR? • Round seeds • Wrinkled seeds • What will be the phenotype of plant with genotype Rr? • Round seeds • Wrinkled seeds