Lecture #2 Applying Mendel’s Principles Unit: Mendelian Genetics

Gene- a section of a chromosome that codes for the production of a particular protein. Allele - an alternate form of a gene that can exist at a single locus. Locus – the region of a chromosome where a particular gene is located. The locus is like the ‘address’ of the gene.

Individuals can be homozygous or heterozygous for each gene Homozygous- having two identical alleles at a particular locus. Heterozygous- having two different alleles at a particular locus

Phenotype VS. Genotype Phenotype - The physical appearance of the trait. Example: Tall Plant Genotype - the set of alleles an individual has. In genetics, LETTERS are used to represent them.  The genotype of a tall plant may be represented as TT or Tt.

Traits can be DOMINANT or RECESSIVE Dominant – the expressed form of a trait. Dominant alleles are represented using capital letters. The first letter of the dominant trait is usually used. Ex: (from handout) Black coat coloring is dominant to white coat coloring, so ‘B’ is used to represent BLACK

Recessive – the form of a trait that has NO effect on phenotype unless present in the HOMOZYGOUS condition. Recessive alleles are represented by lower case letters. It should be the same letter that is used to express the dominant trait. B = black b = white Example White coat coloring in dogs (bb) Phenotype: White Genotype: bb

How to refer to an individual by the alleles that they carry For most traits, individuals can be Homozygous dominant (Ex: BB) or Homozygous Recessive (Ex: bb) or Heterozygous (Ex: Bb)

Take 2 minutes to summarize how the traits an individual has are determined as well as the three pairs of terms covered in this last section.

Probability Probability - the odds of something happening. – Example: Flip a penny. There is a ½ probability (50%) of it landing heads side up. Question: If you flip a coin three times in a row, what is the probability that it will land heads up every time?

Answer –Each coin flip is an independent event, with a one chance in two probability of landing heads up. –Therefore, the probability of flipping three heads in a row is: (flip 1) 1/2 × (flip 2) 1/2 × (flip 3) 1/2 = 1/8 –Past outcomes do not affect future ones. Just because you’ve flipped 3 heads in a row does not mean that you’re more likely to have a coin land tails up on the next flip.

How can we use probability to predict traits? Punnett Square - A diagram that predicts the genotype & phenotype combinations of a genetic cross. Monohybrid cross - a cross that involves one pair of contrasting traits.

What would be the expected results be from a cross between two heterozygous black dogs? Parents: Bb x Bb

Expected Results Genotypic ratio # homozygous dominant : # heterozygous: # homozygous recessive BB: Bb:bb Example Bb x Bb B b Genotypic ratio: 1:2:1 BBBb bb B b

Phenotypic ratio # of Dominant: # of recessive 3: 1 B b B b BBBb bb

Test Cross What is a test cross? A way to determine the genotype of an individual whose phenotype is dominant but whose genotype is unknown. Example (From handout): A black dog; it may be BB or Bb How does it work? You cross the individual in question with a homozygous recessive (bb) individual. If any offspring show the recessive phenotype (white coloring), the unknown individual had the heterozygous genotype.

Take 2 minutes to summarize what you have learned about the way we make predictions about the outcome of genetic crosses and how individuals with an ‘unknown’ genotype can have their genotype determined.