Fundamentals of Genetics Genetic Crosses Fundamentals of Genetics

What is Genetics? The science of heredity and of the mechanisms by which traits are passed from parents to offspring.

The passing of genetic traits from parents to offspring. What is Heredity? The passing of genetic traits from parents to offspring.

Who was Gregor Mendel? Monastery in Brunn Austria His task was to tend to the gardens He began to observe the growth of many plants University of Vienna to study statistics Mendel returned to the monastery Taught high school and kept a garden plot Studied many plants BUT remembered most for his experiments with Pisum sativum a species of garden peas “Father” of modern genetics!

Why Study Peas? 7 easy traits to study Grow many generations in a short time Can control mating, self-fertilization or cross pollination Lots of offspring in a short time

Seven Characteristics of Pisum sativum Observed by Mendel

Pisum sativum or Garden Peas

Mendel’s Work- Background In peas, male and female parts in every flower Pollen = plant sperm Ovary contains eggs Self-pollination- flower fertilizes itself Cross-pollination- flower fertilized by another Mendel hand-pollinated flowers using a paintbrush! He could snip the stamens (contain pollen) to stop self-pollination

Mendel’s work with garden peas led to his discovery of the basic principles of genetics!

Mendel’s Conclusions Law of Segregation- “factors” (genes) that occur in homologous pairs are separated during gamete formation & recombined at fertilization Law of Dominance- a dominant trait will overpower a recessive trait Two alleles coding for the same trait separate during gamete formation (Yy) - - Y then y *member of each pair of alleles are separated when gametes are formed

Mendel’s Conclusions Law of Independent Assortment- “factors” genes separate to gametes in a way that is completely independent of other factor pairs (modern terms- genes for different traits are separated and distributed to gametes independently of one another) Pairs of alleles separate independently of one another during gamete formation

Which law? Which law is proven by the following examples: Alleles R and r separate Alleles Rr and Yy separate R & r – segregation Rr & Yy – independent assortment

Chromosomes, Genes, and Alleles Chromosomes- rod-like structures in cells that contain hereditary info Genes- (used in place of Mendel’s “factors”) distinct unit of hereditary information found on chromosomes Alleles- Alternate forms of a gene (one from mom and one from dad)

Genotype and Phenotype Genotype- genetic makeup of an organism which consists of the alleles that the organism inherits from its parents (actual gene pair). Dominant trait = capital letter…written FIRST Recessive trait = lower case letter Phenotype- the physical trait that an organism develops as the result of its genotype. Phenotype does NOT always indicate genotype!

Homozygous vs. Heterozygous Homozygous- If the alleles for a certain trait in an organism are the same the organism is homozygous for that trait. Can be either homozygous dominant or homozygous recessive When a pea plant is homozygous for tallness what would its genotype be? When a pea plant is homozygous for shortness what would its genotype be? Heterozygous- describes an individual that has two different alleles for a trait. If a pea plant in heterozygous for tallness what would be its genotype?

Genotypic ratio vs. Phenotypic ratio Genotypic ratio- the ratio of the genotypes that appear in an offspring. What is the genotypic ratio in this Punnett Square? Phenotypic ratio- the ratio of phenotypes produced by the cross. What is the phenotypic ratio in this Punnett Square?

Punnett Squares Show possible gene pairings (from possible gametes) Show probability of particular offspring in one reproduction event Dominant trait = capital letter…written FIRST Recessive trait = lower case letter

How do you predict the likelihood than an event will occur? Probability- the likelihood that a possible future event will occur at any given instance. Can be expressed as a decimal, percentage, or a fraction. Probability = Number of times an event is expected to happen Number of times an event could happen The results predicted by probability are MORE likely to occur when there are MANY trials!

Predicting Results of Monohybrid Crosses Monohybrid cross- a cross between individuals that involve one pair of contrasting traits. Example: Punnett Squares- a visual diagram used to predict the results of a genetic cross. Used to predict the outcomes of different types of crosses.

Baby Steps to Creating a Punnett Square Determine the genotype of the parent organisms Write down your “cross” (mating) Draw a “p” square known as a Punnett Square “split” the letters of the genotype for each parent and put them outside the “p” square Determine the possible genotypes of the offspring by filling in the “p”-square Summarize the results (Genotypes and phenotypes of the offspring) Bask in the glow of your accomplishment!! http://users.adelphia.net/~lubehawk/BioHELP!/psquare.htm

Monohybrid Cross Homozygous x Homozygous

Monohybrid Crosses Homozygous x Heterozygous Half of the offspring would be homozygous dominant Half of the offspring would be heterozygous dominant What would a Punnett Square for this cross look like?

Monohybrid Crosses Heterozygous x Heterozygous The genotypic ratio 1 BB: 2 Bb: 1 bb The phenotypic ratio 3 Purple: 1 White

Dihybrid Crosses Dihybrid cross- a cross between individuals that have different alleles for the same trait. Two characteristics are tracked Predicting the outcomes of a dihybrid cross is more difficult because more combinations of alleles are possible

Dihybrid Crosses Homozygous x Homozygous

Dihybrid Crosses Heterozygous x Heterozygous