Mendel & Heredity Gregor Mendel “Father of genetics”

Mendel Successes 1st to predict how traits are transferred from one generation to next Successful b/c studied only one trait at a time to control variables & analyzed his data mathematically

Mendel’s experiments Self-pollination  true-breeding parent, only 1 plant Cross-pollination (making a cross)  offspring from two different parent plants

More on Mendel’s experiments Mendel studied 7 traits on pea plants - Seed shape & color, pod color & shape, flower color, flower position, & plant height Conducted monohybrid crosses looking at only 1 trait Example – Flower color

What’s going on? The white flower trait had reappeared. Mendel repeated for other traits and in every case he found that one trait seemed to disappear in the 1st generation only to reappear unchanged in ¼ of the 2nd generation.

Mendel’s hypothesis Organisms have 2 factors that described each trait  alleles Inherited one allele from father & other from mother

Explaining Mendel’s results A purebred parent plant would have 2 identical alleles for purple flowers (PP) or 2 alleles for white flowers (pp) F1 offspring have 1 allele for purple flowers & 1 allele for white flowers (Pp) F2 offspring - pp, Some PP, Pp

Mendel’s Law of dominance Some alleles are dominant (trait is expressed in the offspring) & other alleles are recessive (trait is only exhibited if dominant is absent) Ex: Purple & white flowers

Law of segregation Alleles for a trait will separate when the sex cells (gametes) are formed during meiosis. Evidence – F2 offspring show recessive trait.

Law of Independent Assortment Genes for different traits are inherited independently from each other Mendel discovered this through his experiments w/dihybrid crosses (2 parents w/2 different traits) Ex: tall, yellow pea plant with a short, green pea plant

Environment affects gene expression Internal - Age, gender (hormonal differences) Ex: male bird feathers colorful External - Temperature, nutrition, light, radiation, chemicals, viruses Ex: leaves at top of trees

Review Phenotype Genotype What the organism looks like (physical appearance) Ex: brown hair Genotype genetic makeup of an organism (we cannot see this) Ex: Bb, BB

Two forms of Genotype Homozygous both identical alleles are either dominant or recessive. Ex: BB or bb Heterozygous different alleles, one dominant & one recessive Ex: Bb

Punnett Squares

Punnett Squares (PSq) Developed in 1905 by Reginald Punnett. Used to predict & compare possible genetic variations resulting from a cross Probabilities, not exact results

Symbols used in PSq Original parents  P1 generation Offspring of the parent plants  F1 generation Offspring of the F1 generation  F2 generation

Example

Steps to solving PSq Identify the dominant & recessive alleles Write the genotypes of the parents Determine the possible gametes the parents can produce (how the alleles will separate)

Enter the possible gametes at top (#1 parent) & side (#2 parent) of the PSq Complete the PSq  write the alleles from the gametes in the appropriate boxes Determine the phenotypes of the offspring and percentages of each

PSq Practice Heterozygous yellow seeds x homozygous green seeds Key : Y yellow seeds y green seeds Y y

Answer Y y Yy yy

Now try problems on: Punnett Square Practice: Monohybrid Crosses worksheet STOP click here

Dihybrid cross – like a puzzle Can 2 parents with Heterozygous round, yellow seeds produce offspring with wrinkled, green seeds? 1st – determine possible gametes/ allele partners First Outside Inside Last Set up PSq boxes

Practice Dihybrid Cross Heterozygous Purple Flower & Homozygous Tall X Homozygous white flower & Heterozygous Tall Key: F purple flower T tall f white flower t short