Lecture #2 Applying Mendel’s Principles Unit: Mendelian Genetics.
-
Upload
eustace-abner-hancock -
Category
Documents
-
view
222 -
download
1
Transcript of Lecture #2 Applying Mendel’s Principles Unit: Mendelian Genetics.
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.