Transcript Laws of Probability and Inheritance Patterns

How Much Do
You
Remember???
A heritable feature
Character
A variant for a character
Trait
Plants that produce offspring of
the same variety when they
self-pollinate
True-breeding
Crossing of two true-breeding
varieties
Hybridization
Parental generation; the truebreeding parents
P Generation
First filial generation; hybrid
offspring from the P generation
F1 Generation
Second filial generation;
offspring of F1 hybrids that selfpollinate
F2 Generation
Alternative versions of a gene
Allele
When 2 alleles at a locus are
different, it determines the
organism’s appearance
Dominant allele
Recessive
allele
When two alleles at a locus are
different, it has no noticeable
effect on the organism’s
appearance
*both alleles must be recessive
to see this trait
Law of
Segregation
The two alleles for a heritable
character separate during
gamete formation and end up
in different gametes
*an egg or sperm only gets
one of the two alleles that are
present in the somatic cells
A diagram for predicting the
allele composition of offspring
from a cross between
individuals of known genetic
makeup
Punnett Square
*Practice…
An organism having a pair of
identical alleles for a character
Homozygous
An organism that has two
different alleles for a gene
Heterozygous
An organism’s traits
Phenotype
And organism’s genetic
makeup
Genotype
Test Cross
Breeding of a recessive
homozygote with an organism
of dominant phenotype but
unknown genotype to
determine the unknown
genotype
Organism that is heterozygous
for one character
Monohybrid
Organism that is heterozygous
for two characters
Dihybrid
Law of
independent
assortment
Each pair of alleles segregates
independently of other pairs of
alleles during gamete
formation
Laws of Probability
and Inheritance
Patterns
Laws of Probability
 Probability: 1 = will occur, 0 = will NOT occur
 Probabilities of all possible outcomes must add up to 1.
 For a coin,
 If the sides are both “heads,” the probability of landing on that side is 1;
and the probability of landing on “tails” is 0.
 If the coin has two different sides, there is a ½ chance of landing on a
particular side.
 For a stack of 52 different cards, there is a 1/52 chance that you
will select any given card, and there is a 51/52 chance of selecting
a card other than the one you want.
 Outcome is not affected by previous trials.
Laws of Probability
 Just as each coin toss’s outcome is independent of the others,
so the alleles of a gene segregate into gametes
independently of another gene’s alleles. (law of independent
assortment)
 Two rules will help predict the outcome of the fusion of
gametes:
 Multiplication Rule
 Addition Rule
Multiplication Rule
 Take the individual probabilities of the given outcome and
multiply them together
 Example: For a monohybrid cross Rr x Rr (R is dominant and r is
recessive) the possibility of each allele for a particular gamete
being given to the offspring is ½.
 The probability of both gametes giving the same allele to the offspring is
½ x ½ = ¼.
Addition Rule
 Add the individual possibilities together when determining if
one of two or more mutually exclusive events is going to occur.
 For example: In a monohybrid cross (Rr x Rr), the probability of
the dominant allele being passed on by one of the gametes is
½ x ½ = ¼ , and the probability of the dominant allele being
passed on by the other gamete ½ x ½ = ¼ .
 Probability of a heterozygote (Rr): ¼ + ¼ = ½.
Solving Complex Genetics Problems
with the Rules of Probability
 Extending Mendelian Genetics for a Single Gene: the
Spectrum of Dominance
 Complete Dominance – heterozygote and dominant homozygote
are indistinguishable
 Mendel’s pea crosses (white OR purple, round OR wrinkled)
 Codominance – both phenotypes are exhibited at the same time
 Human blood surface molecules (MN has M AND N molecules)
 Incomplete Dominance – phenotype is between the phenotypes
of the parents
 Snapdragons with red and white parents have pink offspring
Multiple Alleles
 Human Blood Type: A, B, AB, O
 Determined by which of two carbohydrates (A or B) are found
of the surface of a person’s red blood cells
 An enzyme (I) attaches the carbohydrates
A
B
 I adds A, I adds B, i adds neither
 Each person has 2 alleles, so there are 6 possible genotypes
and 4 phenotypes
Genotype
Phenotype
The End 