Genetic CrossesGenetic Crosses

Shows possible outcomes of genes the new generation will have

We use: Punnett Squares

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TerminologyTerminology

Genotype Phenotype Parent Gamete Offspring

dominant recessive allele

Gene control of characteristicsGene control of characteristicsAll characteristics are controlled by a

minimum of 2 genes.

These genes give the characteristic a different way of presenting itself.

Different genes that control the same feature are called ALLELES

Punnett SquarePunnett Square

Another method of showing crosses

Gamete genotypes are inserted

Parent genotypes are inserted B=black b=white

Bb male black

bb white female

B b

b

b

What are the crosses

Bb bb

Bb bb

2 white and 2 black offspring

50:50 chance with these parents

QuestionQuestion

2 black rats are mated, they have 13 offspring. All of which are black, what are the genotypes of the parents

The male is re-mated with a white female, the offspring are

6 black and 7 white

What were the genotypes of the original parents

Rats have 2 coat colors – black B, white b.

Both BB BB

BUT

one could be Bb Female BB Male Bb

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Mendel’s LawsMendel’s Laws

1. Law of Dominance1. Law of Dominance2. Law of Segregation2. Law of Segregation3. Law of Independent 3. Law of Independent

assortmentassortment

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Law of DominanceStates that on crossing homozygous organisms for a single pair of contrasting characters, only one character make its appearance in F1

generation and is the Dominant character.

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Law of DominanceLaw of Dominance

In a cross of parents that are In a cross of parents that are pure for contrasting traitspure for contrasting traits, , only one form of the trait will only one form of the trait will appear in the next generation.appear in the next generation.

All the offspring will be All the offspring will be heterozygous and express heterozygous and express only the only the dominant trait.dominant trait.

RR x rr RR x rr yieldsyields all Rr (round all Rr (round seeds)seeds)

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..

Law of Segregation

The two alleles for each trait separate during

gamete formation.

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Law of SegregationLaw of Segregation

During the During the formation of gametesformation of gametes (eggs or (eggs or sperm), the sperm), the two allelestwo alleles responsible for a responsible for a trait trait separateseparate from each other. from each other.

Alleles for a trait are then Alleles for a trait are then "recombined" "recombined" at fertilizationat fertilization, producing the genotype , producing the genotype for the traits of the offspringfor the traits of the offspring.

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Law of Independent AssortmentLaw of Independent Assortment

Alleles for Alleles for differentdifferent traits are traits are distributed to sex cells (& offspring) distributed to sex cells (& offspring) independently of one another.independently of one another.

This law can be illustrated using This law can be illustrated using dihybrid crossesdihybrid crosses..

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Dihybrid CrossDihybrid CrossTraits: Seed shape & Seed colorTraits: Seed shape & Seed colorAlleles:Alleles: R round

r wrinkled Y yellow y green

RrYy x RrYy

RY Ry rY ryRY Ry rY ry RY Ry rY ryRY Ry rY ry

All possible gamete combinationsAll possible gamete combinations

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Dihybrid CrossDihybrid Cross

RYRY RyRy rYrY ryry

RYRY

RyRy

rYrY

ryry

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Dihybrid CrossDihybrid Cross

RRYY

RRYy

RrYY

RrYy

RRYy

RRyy

RrYy

Rryy

RrYY

RrYy

rrYY

rrYy

RrYy

Rryy

rrYy

rryy

Round/Yellow: 9

Round/green: 3

wrinkled/Yellow: 3

wrinkled/green: 1

9:3:3:1 phenotypic ratio

RYRY RyRy rYrY ryry

RYRY

RyRy

rYrY

ryry

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Dihybrid CrossDihybrid Cross

Round/Yellow: 9Round/green: 3wrinkled/Yellow: 3wrinkled/green: 1

9:3:3:1

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Question:Question:How many gametes will be produced How many gametes will be produced for the following allele arrangements?for the following allele arrangements?

Remember:Remember: 22nn (n = # of heterozygotes) (n = # of heterozygotes)

1.1. RrYyRrYy

2.2. AaBbCCDdAaBbCCDd

3.3. MmNnOoPPQQRrssTtQqMmNnOoPPQQRrssTtQq

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Answer:Answer:1. RrYy: 21. RrYy: 2nn = 2 = 222 = 4 gametes = 4 gametes

RY Ry rY ryRY Ry rY ry

2. AaBbCCDd: 22. AaBbCCDd: 2nn = 2 = 233 = 8 gametes = 8 gametes

ABCD ABCd AbCD AbCdABCD ABCd AbCD AbCd

aBCD aBCd abCD abCDaBCD aBCd abCD abCD

3. MmNnOoPPQQRrssTtQq: 23. MmNnOoPPQQRrssTtQq: 2nn = 2 = 266 = = 64 gametes64 gametes

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Summary of Mendel’s lawsSummary of Mendel’s laws

LAWLAW PARENT PARENT CROSSCROSS OFFSPRINGOFFSPRING

DOMINANCEDOMINANCE TT x ttTT x tt tall x shorttall x short

100% Tt 100% Tt talltall

SEGREGATIONSEGREGATION Tt x TtTt x Tt tall x talltall x tall

75% tall 75% tall 25% short25% short

INDEPENDENT INDEPENDENT ASSORTMENTASSORTMENT

RrGg x RrGgRrGg x RrGg round & round & green x green x round & round & greengreen

9/16 round seeds & green 9/16 round seeds & green pods pods 3/16 round seeds & yellow 3/16 round seeds & yellow pods pods 3/16 wrinkled seeds & 3/16 wrinkled seeds & green pods green pods 1/16 wrinkled seeds & 1/16 wrinkled seeds & yellow podsyellow pods