Chapter 9.2

Genetic Crosses

Genotype and Phenotype

• RECALL: All traits are found in genes made of DNA• An organisms genetic makeup (DNA, genes,

chromosomes) is called its genotype• This consists of the alleles that an organism

inherits from its parents• Example: white-flowering pea plant = pp• Purple-flowering pea plant = either Pp or PP• An organisms physical appearance based on the

genotype is called is phenotype (physical)

Homozygous or Heterozygous

• When an organism has two of the SAME alleles for a gene, it is called homozygous (PP or pp)

• When the two alleles of a gene are DIFFERENT, it is called heterozygous(Pp)

Probability

• Probability: The likelihood that a specific event will occur

• These are usually expressed as a decimal, percentage or fraction:– Probability = number of times an event is expected to

happen/number of times an event could happen• In Mendel’s experiments, the recessive trait is

expected to be seen in the F2 generation 0.25 or 25% of the time (1/4); the dominant trait is expected 0.75 or 75% of the time (3/4)

Monohybrid Crosses

• Based on the probabilities that Mendel put into place, we can make predictions about the outcomes of genetic crosses

• Monohybrid cross: A cross where only ONE characteristic is tracked

• We use Punnett squares in order to diagram crosses and predict the outcomes of the cross

• So, now we will explore some examples of monohybrid crosses (pgs 182-184 in textbook)

Example 1: FlowersHomozygous Purple (PP) X

Homozygous White (pp)

p

P

P

p

Example 2: Guinea PigsHomozygous Black (BB) X Heterozygous Black (Bb)

B

B

B

b

Example 3: RabbitsHeterozygous Black (Bb) X

Heterozygous Black (Bb)

B

B

b

b

Example 4:Testcross 1

In guinea pigs, BOTH BB and Bb give a black color. So, if a guinea pig is black, how do you know if it’s homozygous dominant or heterozygous? We do a TESTCROSS!

Bb Bb

Bb Bb

Example 4:Testcross 2

In guinea pigs, BOTH BB and Bb give a black color. So, if a guinea pig is black, how do you know if it’s homozygous dominant or heterozygous? We do a TESTCROSS!

Bb Bb

bb bb

Example 5: FlowersIncomplete Dominance

Incomplete dominance occurs when a heterozygote is a mixture of the homozygotes.

R

R

r

r

Example 6: Blood TypingCodominance

Codominance occurs when there is no dominant or recessive allele. Each allele is an equal, BUT there is NO MIXTURE! Blood types are a great example.

LM

LM

LM

LN

Dihybrid Crosses

• We learned how to do many types of monohybrid crosses

• There are really only a couple of types of dihybrid crosses

• A Dihybrid Cross is a cross where TWO characteristics are tracked instead of just one

• These are a little bit more complicated, but we still use Punnett Squares in order to tract them

Round, Yellow (RRYY) X Wrinkled, Green(rryy)

ry ry ry ry

RY

RY

RY

RY

Round, Yellow (RrYy) X Round, Yellow (RrYy)

RY Ry rY ry

RY

Ry

rY

ry

PRACTICE!

• To practice your Punnett squares, please complete the following questions in your book. If you would like more practice, it can be arranged

• Page 188 - #s 12, 14, 16, 17