Lecture #2 Applying Mendel’s Principles

Unit: Mendelian Genetics

Gene- a section of a chromosome that codes forthe production of aparticular protein.

Allele - an alternate formof a gene that can exist at asingle locus.

Locus – the region of achromosome where aparticular gene is located.

The locus is like the ‘address’ of the

gene.

Individuals can be homozygous or heterozygous for each gene

Homozygous- having twoidentical alleles at aparticular locus.

Heterozygous- having twodifferent alleles at aparticular locus

Phenotype VS. Genotype

Phenotype - The physical appearance of the trait.

Example: Tall Plant

Genotype - the set ofalleles an individual has. • In genetics, LETTERS are

used to represent them.

The genotype of a tallplant may be representedas TT or Tt.

Traits can be DOMINANT or RECESSIVE Dominant – the expressedform 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 isdominant to white coatcoloring, so ‘B’ is used torepresent BLACK

Recessive – the form of a traitthat has NO effect onphenotype unless present inthe HOMOZYGOUS condition.

Recessive alleles arerepresented by lower caseletters. It should be the sameletter that is used to express

thedominant 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) orHomozygous Recessive (Ex: bb) or Heterozygous (Ex: Bb)

Take 2 minutes to summarize how the traits anindividual has are determined as well as thethree 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 predictsthe genotype & phenotypecombinations of a geneticcross. Monohybrid cross - a cross that involves onepair of contrasting traits.

What would be theexpected results be from across between twoheterozygous black dogs?

Parents: Bb x Bb

Expected Results

Genotypic ratio# homozygous dominant : # heterozygous: # homozygous recessive

BB: Bb:bbExample Bb x Bb

Bb

Genotypic ratio: 1:2:1

BB Bb

Bb bb

B b

Phenotypic ratio # of Dominant: # of recessive 3: 1

B bBb

BB BbBb bb

Test Cross What is a test cross? A way to determine the genotype of anindividual whose phenotype is dominant but whose genotypeis 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 havelearned about the way we make predictions

about the outcome of genetic crosses and howindividuals with an ‘unknown’ genotype can

have their genotype determined.