III. Exploring Mendelian Genetics Review : Dominant trait – stronger trait that can hide a weaker trait. Represented as a capital letter, ex : B. Recessive trait – weaker trait that can be hidden by a stronger trait. Represented by a lower case letter, ex : b. Must have 2 in order to be shown. Does segregation of alleles occur independently?

A. Independent Assortment *Mendel performed Dihybrid (2-factor) crosses to determine if alleles segregate independently. Dihybrid cross – cross in which 2 traits are considered at once, ex : B = round seed/b = wrinkled A = yellow color/a = green AaBb x AaBb It involves 16 possible offspring.

Mendel’s Dihybrids Mendel crossed true-breeding (purebred) plants that produced only round yellow peas (RRYY) with plants that produced wrinkled green peas (rryy). F1 result = All hybrids with genotype of RrYy. *This cross only showed which traits were dominant & which were recessive. In order to examine allele segregation, a dihybrid cross must be done with the F1 hybrid generation.

Results of Mono vs. Dihybrid crosses -In a heterozygous cross of single traits, the genotypic ratio would be 1 : 2 : 1 & the phenotypic ratio would be 3 : 1. -Mendel’s dihybrid crosses of F1 heterozygotes resulted in values that were very close to the phenotypic ratio of 9 : 3 : 3 : 1 predicted in the Punnett square. -These findings proved that the alleles for seed color & shape segregate independently, a principle known as independent assortment.

Dihybrid cross showing 9 : 3 : 3 : 1 ratio

B. Summary of Mendel’s Principles *Mendel’s principles are the basis of modern genetics as we know it today. Mendel’s principles can be summarized into 3 basic laws : 1. Law of Segregation – states that each pair of genes segregates (separates), during meiosis/gamete formation. 2. Law of Independent Assortment – states that alleles for different genes segregate into gametes randomly & independently of each other. 3. Law of Dominance – states that the dominant allele in a pair is expressed & the recessive allele can be hidden.

C. Beyond Dominant & Recessive Alleles *Exceptions to Mendel’s principles exist, ex : many important traits are controlled by more than one gene. -Some alleles are neither dominant nor recessive, & many traits are controlled by multiple alleles or multiple genes. Incomplete Dominance – when one allele is not completely dominant over another, ex : snap dragons can produce red flowers (RR) or white flowers (WW). The heterozygous form (RW) produces pink flowers. *Color seems intermediate to parents.

Incomplete Dominance & Crosses -Both alleles are partially expressed. -Phenotypes of incompletely dominant organisms appear to be a blend of the parents, ex : Snapdragon flowers. -When pure red snapdragon flowers are crossed with pure white ones, heterozygous offspring look pink. Problem : Cross a red snapdragon with a white snapdragon & determine the phenotypic & genotypic ratios of the offspring. What if we crossed 2 pink?

Codominance Codominance – when both alleles of a gene contribute to the phenotype of the organism, ex : chickens & erminette coloring, horses with roan coloring & human proteins for cholesterol level.

Codominance & Crosses -Both are dominant, so both are expressed at the same time. -Phenotypes of codominant organisms will show both dominant alleles, ex : roan coloring in horses. -When pure white horses are bred with pure red horses, heterozygous offspring look strawberry blonde. -When examined closely one can see both red and white hairs in a roan horse’s coat. Problem : Cross a red horse with a white horse, then cross two of their offspring & find genotypes & phenotypes.

Multiple Alleles & Polygenic Traits -Some human traits are controlled by a single gene that has more than two alleles, or multiple alleles. Multiple alleles – when genes have more than two alleles that code for a single trait, ex : coloring in rabbits & human blood types. Polygenic Traits – traits controlled by two or more genes. Means, “Having many genes”. Examples include brownish-red eyes in fruit flies & skin color in humans.

Multiple Alleles & Blood types *Human blood type is a great example of multiple alleles. -There are 4 major blood types : A, B, AB, and O. -The allele for blood type A and the allele for blood type B are codominant & the allele for blood type O is recessive. -A person who receives the alleles for A & O will be type A. -A person who receives alleles for both A & B will be type AB. -A person who receives alleles for B & O will be type B. -The only way that a person can have type O blood is if they receive two recessive alleles for type O.

Universal Donors & Recipients *Since type AB people can receive all other types of blood, they are called the Universal recipients. *Since type O people can donate blood to all other blood types, they are called the Universal donors.

Rh Factor in Blood Types *Besides the blood group ABO in humans there is also a blood group known as Rh factor. *Knowing a person’s blood group is critical because using the wrong type of blood for a transfusion can be fatal. -The Rh blood group is controlled by 1 gene with 2 alleles – positive & negative. -Rh stands for “rhesus monkey”, the animal in which the factor was 1st found. -The positive (Rh+)allele is dominant, so people who are Rh+/Rh+ or Rh+/Rh- are said to be Rh-positive. -Only individuals with 2 recessive Rh- alleles are Rh-negative. -When medical workers refer to blood groups, they usually mention both at the same time, ex : If a person has AB - negative blood, they have the AB genotype for blood type & two Rh- alleles.

Rh Factor Crosses Problem : Knowing that people who are : Rh+ = have the Rh antigen (dominant) Rh- = do NOT have the Rh antigen (recessive) Cross a parent who is heterozygous for the Rh factor with another heterozygous parent. Use the 6 steps.

D. Genetics & the Environment -Characteristics in organisms are determined by interaction between genes and the environment, ex : Plant may inherit tall/good fruit genes, but if in poor soil conditions with little water, it may be stunted or become diseased.