Mendel’s Laws of Heredity Why we look the way we look...

The “Father of Genetics” Gregor Mendel 1822-1884, Austrian monk The “Father of Genetics”

Mendel used peas... They are easy to grow They reproduce sexually They have two distinct, male and female, sex cells called gametes Differences in traits are easy to recognize. Their traits are easy to isolate

What is heredity? The passing on of characteristics (traits) from parents to offspring Genetics is the study of heredity

Mendel crossed them Fertilization - the uniting of male and female gametes Cross - combining gametes from parents with different traits

Questions What did Mendel cross? What are traits? What are gametes? What is fertilization? What is heredity? What is genetics?

What Did Mendel Find? He discovered different laws and rules that explain factors affecting heredity.

Rule of Unit Factors Each organism has two alleles for each trait Alleles - different forms of the same gene Genes - located on chromosomes, they control how an organism develops

Alleles: alternative versions of a gene. The gene for a particular inherited character resides at a specific locus (position) on homologous chromosome. For each character, an organism inherits two alleles, one from each parent

Dominant - a term applied to the trait (allele) that is expressed irregardless of the second allele. Recessive - a term applied to a trait that is only expressed when the second allele is the same (e.g. short plants are homozygous for the recessive allele). Dominant allele Recessive allele

“The Rule of Dominance” He also concluded that some of these particles are “Dominant”over the other. He called the non-dominant ones “Recessive” So, the only way a plant will show the Recessive characteristic is if it has none of the Dominant factors at all.

Rule of Dominance The trait that is observed in the offspring is the dominant trait (uppercase) The trait that disappears in the offspring is the recessive trait (lowercase)

“The Principle of Segregation” That each plant has two “factors” (alleles) for any genetic trait. One of these alleles comes from female parent and the other from the male parent (even if same plant). So the female sex cell (called egg or ovule) and the male sex cell (pollen or sperm)… each contain only one of these alleles.

Law of Segregation The two alleles for a trait must separate when gametes are formed A parent randomly passes only one allele for each trait to each offspring

Law of Independent Assortment The genes for different traits are inherited independently of each other.

Questions... How many alleles are there for each trait? What is an allele? How many alleles does a parent pass on to each offspring for each trait

Questions... What do we call the trait that is observed? What case (upper or lower) is it written in? What about the one that disappears? What case is it written in?

Phenotype & Genotype Phenotype - the way an organism looks, how a genetic trait outwardly expresses itself Tall or short plant Green or yellow pea pod

Phenotype & Genotype AA or Aa or aa genotype - the gene combination of an organism; the chemical make up of that gene AA or Aa or aa

Heterozygous & Homozygous Heterozygous - if the two alleles for a trait are different (Aa) Homozygous - if the two alleles for a trait are the same (AA or aa)

Dihybrid vs Monohybrid Monohybrid Cross - crossing parents who differ in only one trait (YY with yy)

Questions... What is the phenotype? What is the genotype? What is homozygous? What is heterozygous? What is monohybrid crossing?

First, he made sure that each variety bred “true” (was stable for two generations). Then he started making crosses between plants that were different for the same trait…..for instance, tall and short.

Tall x Tall Short x Short Observation 1 = Tall x Tall Short x Short Tall Short Parents = P parental generation First filial generation F

Mendel made a cross between a Tall plant and a Short plant….. Observation 2 Mendel made a cross between a Tall plant and a Short plant….. X Progeny were all Tall > …not medium height!

Offspring were: 75% Tall 25% Dwarf F2 generation Observation 3 = If these Tall-Short hybrids were self-pollinated: F2 generation Offspring were: 75% Tall 25% Dwarf He observed this same phenomenon for all 7 traits How could such a thing happen?

Mendel concluded that each plant carries two “particles” ..... One of these particles comes from female parent and theother from the male parent (even if same plant). He also concluded that one of these particles are “Dominant” over the other. He called the non-dominant ones “Recessive” It turns out there is a lot of truth to this conclusion....we now call these “particles” genes....to be specific they are alleles.