Jim Twins

Mendelian Genetics Integrated Biology

What is genetics? What is heredity? Science of heredity and the mechanisms by which traits are passed from parent to offspring. The passing on of characteristics from parent to offspring. Inherited characteristics = “traits” What is heredity?

Who was Gregor Mendel? Monk Studied mathematics, taught science early on in life Tended the gardens (at monastery in Austria) Observed growth of plants  garden peas, Pisum sativum 1st to predict how traits were transferred from one generation to another Father of Modern Genetics

Why pea plants? 1. 7 easy traits to study – could examine one variable at a time

Why pea plants? 2. Grows many generations in a short period of time 3. Lots of offspring are produced in a short period of time

Why pea plants? 4. Reproduce sexually Similar to other organisms (i.e. humans) Male and female parts in every flower Plant sperm = pollen ; eggs = contained in ovary

Why pea plants? 5. Can control mating  self – pollination / fertilization or cross pollination Self = flower fertilizes itself Cross = flower fertilized by another flower Mendel hand-pollinated flowers w/a paintbrush He snipped the stamens (contain pollen) to stop self-pollination

Mendel’s Conclusions: Rule of Unit Factors Each organisms has 2 factors that control its trait Review  different forms of genes = alleles Ex: height (short vs. tall)

Mendel’s Conclusions: Law of Segregation “Factors “ (genes) that occur in homologous pairs are separated during gamete formation and recombined at fertilization Alleles segregate separately!

Mendel’s Conclusions: Law of Dominance One allele (letter) is dominant over another observed trait A dominant trait will overpower a recessive trait

Mendel’s Conclusions: Law of Independent Assortment Genes for different traits (i.e. hair color, eye color) are inherited independently of each other Don’t influence each other’s inheritance Example  P1: Round Yellow x Wrinkled Green F1: all Round Yellow F2: 9 Round Yellow, 3 Round Green, 3 Wrinkled Yellow, 1 Wrinkled Green

Which law? Which law is proven by the following examples: Alleles R and r separate Alleles Rr and Yy separate

Chromosomes, Genes, Alleles Chromosome  rodlike structures in cells that contain hereditary information Genes  distinct unit of hereditary information found on a chromosome Alleles  alternate form of a gene (one from mom, one from dad) i.e. gene = height  allele: T, for tallness ; t, for shortness

Alleles : Refresher For height  tall versus short Can have: 2 alleles for tallness 2 alleles for shortness 1 allele for tallness, 1 allele for shortness ** located on different chromosomes – 1 from mom, 1 from dad **

Dominance vs. Recessiveness DOMINANT  ALWAYS shown if allele is present One or two uppercase letters (TT, Tt) RECESSIVE  shown ONLY if both alleles are recessive 2 lowercase letters (tt) *Mendel’s tall plants (T) are dominate over short plants (t)

Recap: Use SAME letters for different alleles (of the same gene) Uppercase = dominant Lowercase = recessive Dominant (capital letter) always written first

Genotype vs. Phenotype GENOTYPE  Genetic makeup ; Allele comb. what the actual genes are ex: TT , Tt, or tt PHENOTYPE  Physical appearance ; what is actually shown/seen ex: tall, short * results from genotype *

Homozygous vs. Heterozygous HOMOZYGOUS  Both alleles are the SAME (TT, tt) aka: purebred *can either be homozygous dominant or homozygous recessive HETEROZYGOUS  alleles are DIFFER. (Tt) aka: hybrid

Let’s try it! 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? When a pea plant is heterozygous, what would its genotype be?

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?

Genotype/Phenotype Homozygous/Heterozygous WORKSHEET

Punnett Squares Short hand way to find expected proportions of possible genotypes (and phenotypes) in offspring of a cross Monohybrid  1 allele considered in cross Dihybrid  2 alleles considered in cross

Probability Method of calculating the expected outcome of an event Just like in math! Can be expressed as  decimal, percentage, fraction, or ratio 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

Probability Chosen outcome / all possible outcomes 1/6 Fraction  1 / 6 Ratio  1 : 5 ( 1 to 5 ) Percentage  16.7%

Mendel’s Monohybrid Crosses Crossed tall pea plants with short pea plants Mono = one ; hybrid = differ by a single trait Only one allele will be seen Mendel did monohybrid crosses with all 7 pairs of traits Yielded the same results in each cross! What was the result??

Mendel’s Cross: Parents F1 = filial  “daughter, son” of parents F2 = filial  “daughter, son” of first generation CLUE: Your parents = P1 You = F1 Your children = F2

Mendel’s Cross: Parents : TALL x SHORT F1: TALL Cross-pollination F1: TALL Self-pollination F2: (3) TALL : (1) SHORT Parents : PURPLE x WHITE Cross-pollination F1: PURPLE Self-pollination F2: (3) PURPLE: (1) WHITE

Mendel’s Cross: 1 trait always disappeared in the F1 generation RESULT  1 trait always disappeared in the F1 generation But, that trait always reappeared in the F2 generation In the ratio of 1 : 4 ( ¼ , 25% )

Mendel’s Monohybrid 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) Write on your paper how awesome your teacher is 

Monohybrid Cross Homozygous x Homozygous 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 would be: The phenotypic ratio would be:

Dihybrid Crosses Homozygous x Homozygous

Dihybrid Crosses Heterozygous x Heterozygous

Review Questions: What is genetics? Heredity?? Why did Mendel use pea plants? Differentiate between self-pollination and cross-pollination AKA purebred? AKA hybrid? The “F” in F1 and F2 refers to what? What observation did Mendel make for all monohybrid crosses? What does the rule of unit factors state? Rule of dominance? What is an allele? Differentiate between dominant and recessive. Differentiate between genotype and phenotype. Differentiate between homozygous and heterozygous. What does the law of segregation state? Law of independent assortment? Define probability and state the various ways it can be expressed.