genetic crosses – single gene. genotype and phenotypegenotype and phenotype –genotype is the...

Genetic Crosses – single gene

• Genotype and Genotype and PhenotypePhenotype

– Genotype is the Genotype is the genetic makeup genetic makeup of the organism.of the organism.

– Phenotype is the Phenotype is the physical physical appearance of appearance of the organism.the organism.

Heterozygous Vs Heterozygous Vs HomozygousHomozygous

• HomozygousHomozygous– Same gene or allele Same gene or allele

for the pair (AA or for the pair (AA or aa)aa)

• HeterozygousHeterozygous– Different genes or Different genes or

alleles in the pair alleles in the pair (Aa)(Aa)• Capital letter for Capital letter for

dominant, lower dominant, lower case for recessive case for recessive

ProbabilityProbability

• Likelihood that an Likelihood that an event will occur.event will occur.

• Can be expressed as Can be expressed as a fraction, decimal, a fraction, decimal, or a percentage.or a percentage.

• Calculated by:Calculated by:– Prob. = # times event Prob. = # times event

occurs/ # of times occurs/ # of times opportunity occurred.opportunity occurred.

Crosses Involving 1 Pair of Crosses Involving 1 Pair of GenesGenes

percent outcomespercent outcomes• P = TT x TTP = TT x TT

– 100% tall100% tall

• P = TT x TtP = TT x Tt– 100% tall100% tall

• P = TT x ttP = TT x tt– 100% tall100% tall

• P = Tt x ttP = Tt x tt– 50% tall, 50% tall,

50% short50% short

• P = Tt x TtP = Tt x Tt– 75% tall, 75% tall,

25% short25% short

• P = tt x ttP = tt x tt– 100% 100%

shortshort

Memorize these crosses – it will Memorize these crosses – it will save you time.save you time.

Crosses Involving 1 Pair of Crosses Involving 1 Pair of Genes Punnett squaresGenes Punnett squares

TT.5.5

tt.5.5

TT.5.5 tt.5.5

T T

T t

T

t

t

t

P = Tt x TtP = Tt x Tt

Gametes of 1 parent (1N)

Gametes of the other parent (1N)

What is inside the box are the prosible results of fertilization (2N)

Test Cross – a breeding experiment Test Cross – a breeding experiment

Pair share: Pair share: •How could you How could you determine if a black determine if a black guinea pig is guinea pig is heterozygous or heterozygous or homozygous? homozygous? •(black is dominant over white)(black is dominant over white)•Hint look at the outcomes of Hint look at the outcomes of different crosses…. different crosses….

B = Black b = whiteB = Black b = white(answer on next slide)(answer on next slide)

Test CrossTest Cross• P = P = B?B?

xxbbbb

If F1 is 100% If F1 is 100% black then black then genotype of genotype of unknown parent unknown parent isis

BBBB

If F1 is 50% black and 50% white then If F1 is 50% black and 50% white then

unknown parent is unknown parent is BbBb..

X

P = Bb x bbP = Bb x bbP = BB x bbP = BB x bb

BB BB

bb

bb

BBbb

bbBB

BBbb

BBbb

BB bb

bb

bb

BB

BB

bb

bb

bb

bb bb

bb

Incomplete Incomplete DominanceDominance

XX ==

RRRR WWWWXX == RWRW

Neither Neither ““RR”” allele or allele or ““WW ”” allele is dominant. allele is dominant. Phenotype is intermediate. Phenotype is intermediate.

Incomplete Dominance

X =

RW RWX

Neither Neither ““RR”” allele or allele or ““WW ”” allele is allele is dominant. Phenotype is dominant. Phenotype is intermediate.intermediate.

25% RR25% RR

25% WW25% WW

50% RW50% RW

Codominance • When neither genotype is dominant and

both traits are expressed.– Human blood type

Review

1. What is Phenotype?Phenotype?

2.2. What is the probability What is the probability of getting a white of getting a white guinea pig if both guinea pig if both parents are parents are heterozygous? heterozygous?

3.3. What is incomplete What is incomplete dominance?dominance?