Gregor Mendel: The Father of Genetics!

Gregor Mendel 1822-1884 Austrian monk Experiments with garden peas lay the foundation for the future of genetics

Peas Mendel chose to work with peas because they had many traits Trait: a particular version of an inherited characteristic No intermediates

Peas could be crossed-fertilized & self-fertilized

Crossing Pea Plants Used only true breeding plants: when self fertilized, produce offspring identical in appearance to itself, generation after generation

Crossing Pea Plants

Crossing Pea Plants Sperm (pollen) of one plant fertilizes the eggs in the flower of a different plant Parental plants are the P generation The first generation of offspring is the F1 generation If the F1 offspring are crossed with each other, their offspring are called the F2 generation

Observations P Generation: F1 Generation: 100% of the offspring plants were tall! (Instead of a blending – where all plants would be medium sized...) X

Other Observations

Traits Lost? Mendel then let the F1 generation of offspring self-pollinate... this time most were tall, but some were short and ALWAYS 3:1 ratio

Traits NOT Lost In the F2 generation, the “lost” form of the trait always reappeared in about 1/4th of the plants!

3:1 Ratio

Observations Produced hybrids: offspring that differ from their parents in one or more traits Hybrids of Mendel's crosses were NOT a mix of both parents. Hybrids expressed one of the parental traits. Disproved previous theories. Found that certain traits dominated others

Terminology Genes can exist in different forms Two versions of the gene for seed colour in peas: yellow and green An allele is an alternative version of a gene So, peas have a yellow seed allele and a green seed allele

For each gene, an individual inherits two alleles, one from each parent If the two alleles are the same, the individual is said to be homozygous for that gene (TT or tt) If the two alleles are different, the individual is heterozygous for that gene (Tt)

Alleles: Alternate forms of a gene

An allele may be expressed (appear) or else hidden in a heterozygous individual The dominant allele determines the trait that a heterozygous individual expresses The recessive allele is not expressed in a heterozygous individual There must be two copies of the recessive allele for it to be expressed as a trait

Mendel’s Law of Segregation When organisms are crossed and offspring are produced, the offspring receive one gene from each parent for a particular trait

Law of Independent Assortment All traits are inherited independently of one another

Genotype The genetic makeup or combination of alleles for an individual Uppercase letters represent dominant allele Lowercase letters represent recessive allele Tall pea plant, T Dwarf pea plant, t Homozygous dominant for height: TT Heterozygous for height: Tt Homozygous recessive for height: tt

Phenotype Refers to an individual’s expressed traits Includes physical appearance In pea plant stem length, the actual height (tall) is the phenotype

Genotype vs. Phenotype

Which Allele is expressed?

Genetic Crosses Monohybrid Cross: cross involving a single trait Flower colour, plant height Dihybrid Cross: cross involving two traits Flower colour and plant height Punnett squares help determine the possible combinations of genotypes that can occur in the offspring It also shows the probability of each genotype occurring

Example 1 A cross between a pea plant that is heterozygous for purple flowers (Pp) is crossed with a pea plant with white flowers (pp). Determine the genotypes and phenotypes of the possible offspring.

Example 2 In guinea pigs a black coat (B) is dominant over a white coat (b). Determine the phenotype and genotype ratios if a homozygous dominant parent is crossed with a heterozygous parent.

Example 3 In humans, free ear lobes (F) are dominant over attached ear lobes (f). What are the phenotype and genotype ratios of the offspring: When a homozygous dominant female is crossed with a homozygous recessive male Based on these results, what is the chance (percentage)of the child having attached ear lobes What is the phenotype of the F2 generation

Example 4 In unicorns, a large horn (H) is dominant over a small horn (h). If two heterozygous parents are crossed, what are the phenotypic and genotypic ratios of the F1 generation. How many offspring will be born with a small horn if the parents have 100 offspring.