Chapter 9 Patterns of Inheritance - Mendel and His Discoveries

Gregor Mendel Father of Genetics In the years 1856 – 1863 carried out his experiment –Genes are arranged in the chromosomes –Genes are responsible for passing the parental characters (traits) to the offsprings –2 genes are responsible for each characteristic. –These 2 genes separate at the time of gamete formation

–Heredity - the transmission of genetic traits from one generation to the next. –Locus - the location of a gene on chromosome. –Alleles -Two pairs of genes which will be responsible for particular trait. –Genetics - the study of heredity - explains how characteristics are passed on from parent to child.

Mendel conducted his experiments using garden pea plants (Pisum sativum) He applied mathematics to his observations, and could soon predict the outcome of his experiments

Reason for selecting pea plant Can be self or cross pollinated ( the deposition of pollen grain on the stigma is pollination) Can be grown quickly (easily cultivated) They are annuals - complete their life cycle within one year; all the stages of growth observed within a year Note: Self pollination – pollen grain of a flower is deposited on its own stigma. Cross pollination – pollen grain of one flower is deposited on the stigma of another flower.

Self Pollination Pollen from the anther (male) of a plant falls on the stigma (female) of that same plant.

Cross Pollination (Manual) Pollen from the anther (male) of one plant is transferred to the stigma (female) of another plant of the same species.

Phenotype External appearance or the outward appearance of a living organism. Example: –Tall plant –Dwarf plant –Wrinkled seed –Smooth seed –Colored seed

Genotype Genetic makeup of a living organisms. Examples: – TT – tt – SS – Ss – YY – yy

Dominant and Recessive Traits In his studies, Mendel recognized physical characters of pea plants. There were seven characteristics for seed and flower color, seed shape and plant height.

Parental, F 1 and F 2 Generations Parental Generation – P Offspring of parental generation – F 1 (first filial generation) Offspring of first filial generation – F 2 (second filial generation)

When an organism has two identical alleles (genes) for a given trait, the organism is Homozygous. –For example TT and tt If an organism has two different alleles (genes) for a given trait, the organism is Heterozygous. – For example Tt

Monohybrid Cross Mendel took single trait for crossing i e., height of the plant. He crossed two true-breeding (pure), homozygous plants (parents) having contrasting traits –crossed, homozygous (pure) tall plant (TT) with homozygous (pure) dwarf plant (tt), all the individuals of F 1 generation were tall and heterozygous (Tt) in condition.

In heterozygous condition ( T t ), one trait (T) masked the effect of the other (t) - the expressed character is Dominant and the one not expressed is Recessive. Dominant allele “T”, masked the presence of the recessive allele “t”. - All the plants in F 1 were tall.

F 1 individuals of heterozygous tall condition were intercrossed (Tt x Tt) - producing F 2 generation 1 (TT) : 2 (Tt) : 1 (tt) - F 2 genotypic ratio 3Tall : 1 dwarf - F 2 phenotypic ratio

Mendel’s P generation & F 1 Cross Homozygous male and female parents with pure yellow seed and pure green seeds when crossed resulted in heterozygous yellow showing only the dominant character in the F1 generation. Above mentioned cross can be graphically represented in the Punnett square.

Mendel’s P generation & F 1 Cross

From F 1 to F 2 Generation When F 1 heterozygous yellow seeded plants were crossed among themselves, the following monohybrid genotypic ratio is expressed for F 2 generation: - I homozygous yellow seed (YY) : 2 heterozygous yellow seed (Yy) : 1 homozygous green seed (yy). Monohybrid phenotypic ratio for F 2 generation is 3 yellow seed :1 green seed.

From F 1 to F 2 Generation

The Law of Dominance Any one factor may mask the effect of another factor in a given pair of alleles. The expressed allele is referred to as dominant, and the one that is not expressed is recessive. Example - in the cross between yellow seed and green seed variety, yellow seed variety is dominant over the green seed

The Law of Segregation Each character is governed by two alleles –Alleles separate at the time of gamete formation.

Dihybrid Cross Dihybrid cross is one in which two characteristics from each parent were considered for crossing. In this cross, a parent with pure yellow color smooth seed is crossed to a parent with pure green color wrinkled seed. All F 1 generation will have yellow and smooth seeds.

Dihybrid Cross (cont’d) When F1 generation plants are intercrossed, F2 generation produced 9 yellow smooth seed : 3 yellow wrinkled seed : 3 green smooth seed : 1 green wrinkled seed. Dihybrid phenotypic ratio : - 9:3:3:1 Dihybrid genotypic ratio : - 1SSYY : 2SSYy : 1SSyy : 2sSYY : 4SsYY : 2Ssyy : 1ssYY : 2ssYy : 1ssyy

Dihybrid Cross (cont’d)

Law of Independent Assortment At the time of gamete formation, alleles assort independently. –Dihybrid cross parents with yellow round and green wrinkled seeds produce yellow wrinkled and green round seeds in F 2 generation

Incomplete Dominance Cases in which characteristics blend - Snap Dragon Flowers Red (RR) X White (rr) = 1 Red (RR) : 2 pink (Rr) : 1 white (rr)

Incomplete Dominance

Co-Dominance Instances where neither allele is capable Of dominating over the other, but share (express) equal contributions to the trait

Multiple Alleles/Polygenic Inheritance Multiple Alleles - Three or more Alleles determine any individual characteristic –height, weight, eye color, skin color Variations within a characteristic often present in a normal bell curve.

Multiple Alleles/Polygenic Inheritance

Pleiotropy One gene capable of expressing many effects Results - mental retardation - various facial anomalies