Transcript Probability and Heredity

Probability, Heredity and
Punnett Squares
Coin Toss
• I call “Heads”
• What are my chances that I win?
Probability
• A number that describes how likely it is that an
event will occur
• Example: My coin toss
▫ Probability that a tossed coin
lands with heads up is 1 in 2
 ½ or 50%
Independence of Events
• When you toss a coin more than once, the results
of one toss do not affect the results of the next
toss
▫ Each event occurs independently
▫ The results of previous tosses do NOT affect the
result of the next toss
Probability and Genetics
• Mendel was the first scientist to recognize that
the principles of probability can be used to
predict the results of genetic crosses
▫ Cross TALL (Tt) and SHORT (tt) pea plant
 2 out of 4 were TALL
 2 out of 4 were SHORT
▫ Cross TALL (Tt) and TALL (Tt)
 3 out of 4 were TALL
 1 out of 4 were SHORT
Punnett Squares
• A chart that shows all the possible combination
of alleles that can result from a genetic cross
▫ To determine the probability of a particular
outcome
How to make a Punnett Square
1. Draw a box and divide it into four squares
2. Write the male parent’s alleles along the top
and the female parent’s alleles along the left
side
3. Copy the Female alleles into the boxes and
copy the Male alleles into the boxes
Let’s try one together….
• Create a Punnett square to show the cross
between a Black guinea pig (BB) and a white
guinea pig (bb)
b
b
Probability
B
Bb
Bb
• 4 out of 4 are Bb
• 100% of the Guinea
pigs will be Black
B
Bb
Bb
Practice on your own….
1. Create a Punnett square to show the cross
between a Purple flower (Pp) and Purple
flower (Pp)
1. Create a Punnett square to show the cross
between yellow seed (Yy) and green seed (yy)
Phenotype vs. Genotype
• Phenotype: physical appearance
• Genotype: genes or allele combination
• Example: G (green pea pod) g (yellow pea pod)
Genotype
Phenotype
GG
Green pea pod
Gg
Green pea pod
gg
Yellow pea pod
Homozygous and Heterozygous
• Homozygous: has two identical alleles for that
trait
• Heterozygous: has two different alleles for
that trait (sometimes called hybrid)
Genotype
Phenotype
GG
Green pea pod
Homozygous dominant
Gg
Green pea pod
Heterozygous
gg
Yellow pea pod
Homozygous recessive
• Codominance: alleles are neither dominant
nor recessive. As a result, both alleles are
expressed in the offspring
• Incomplete Dominance – one allele is not
completely dominant over the other. Results in a
third phenotype
Sex-linked traits
• 23rd pair of chromosomes are the sex chromosomes
▫ XX – Female
▫ XY – Male
• Sex-linked genes
▫ Ex. Red- green colorblindness, hemophilia, muscular
dystrophy, Fragile X
• X and Y chromosomes are different and most of the
genes on the X chromosome are not on the Y
chromosome.
▫ In males, there is usually not matching allele on their
Y chromosome.
▫ Any allele on their X chromosome (even a recessive)
will be seen in that male.
Multiple Alleles
• When more than one allele determines a
trait
▫ ex. Hair and skin color
▫ The more active alleles present, the more
color pigment
Multifactorial
• “Many factors”
• Both genetic and environmental
• Example: Height, neural tube defects (spina
bifida)