dihybrid crosses
Post on 23-Feb-2016
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Dihybrid Crosses
A monohybrid cross involves only one trait. This is what we have been looking at so far. Ex. Only dimples, OR only tongue rolling ability. A dihybrid cross involves two traits. Also known as two-factor cross. Ex. dimples AND tongue rolling ability.
• Dihybrid crosses illustrate Mendel’s Principle of Independent Assortment.
• The Principle of Independent Assortment states that genes for different traits segregate independently during the formation of gametes.
• New combinations of gametes are formed, creating genetic variety.
Meiosis creates genetic variety in 3 ways******
1. Crossing over
2. Independent assortment
3. creating gametes for fertilization – ultimately combining the genes of 2 organisms
ExampleEx. Cross a male heterozygous for dimples
AND tongue rolling ability with a female of the same genotype.
Presence of dimples: D= dimples d= no dimples
Tongue Rolling ability: A =able to roll a= not able
STEP 1: WRITE EQUATION
father DdAa x mother DdAa
STEP 2: GENERATE GAMETES
STEP 2: GENERATE GAMETES using the FOIL method for each parent. Make sure you get one allele for each trait in your gamete (D and A). You should always have 4 gametes, even if they repeat.
FOIL = First, Outside, Inside, Last = remember from math?
STEP 2: GENERATE GAMETES FOIL = First, Outside, Inside, Last
moms gametes for DdAa First: DA Outside: Da Inside: dA Last: da dad’s gametes for DdAa (the same as mom this
case)DA Da dA da
***STEP 2 Illustrates independent assortment, as D allele has the same chance of being in a gamete with A or a. (DA or Da equally likely).
STEP 3: 16 Box PunnettSTEP 3: DRAW a 16 box Punnett Square. COMPLETE the Punnett Square as normal. Gametes on the outside, offspring on the inside. Hint: Make sure you list the dominant allele first, and the same trait first for every box.
STEP 4: DRAW A TABLE
• LIST the possible phenotypes – there will always be 4• Assign them each a COLOR. Color the the box on the
table. Possible Phenotypes genotypes that produce the phenotype
phenotypic frequency
Dimples, Able to Roll
DDAA DDAa DdAA DdAa
9
Dimples, unable to roll
DDaa Ddaa
3
no dimples, able to roll
ddAA ddAa
3
no dimples, unable to roll
ddaa 1
• LIST all the genotypes that would produce that phenotype.
Possible Phenotypes genotypes that produce the phenotype
phenotypic frequency
Dimples, Able to Roll
DDAA DDAa DdAA DdAa
9
Dimples, unable to roll
DDaa Ddaa
3
no dimples, able to roll
ddAA ddAa
3
no dimples, unable to roll
ddaa 1
COLOR the genotypes the correct color on the Punnett square.
DA Da dA daDA DDAA DDAa DdAA DdAaDa DDAa DDaa DdAa DdaadA DdAA DdAa ddAA ddAada DdAa Ddaa ddAa ddaa
• COUNT the number of boxes you have colored in the Punnett.
• RECORD this in the phenotypic frequency column.
Possible Phenotypes genotypes that produce the phenotype
phenotypic frequency
Dimples, Able to Roll
DDAADDAaDdAA DdAa
9
Dimples, unable to roll
DDaaDdaa
3no dimples, able to roll
ddAAddAa
3no dimples, unable to roll
ddaa 1
STEP 5
• STEP 5: RECORD the phenotypic frequency as a ratio. Make sure your numbers add up to 16!!!!
phenotypic ratio 9 dimples, able to roll :3 dimples, unable to roll : 3 no dimples, able to roll :1 no dimples, unable to roll
Independent Assortment occurs in meiosis I.
• The genes for different traits are on different chromosomes.
• Homologous pair for tongue rolling lines up independently of the homologous pair for dimples.
• Two different alignments can occur, which produce different combinations of gametes.
ORD d
A aD da A
DA
aaD A AD
a
d d
d Da
aaD D
dA
d dA A
Gametes DA and da Gametes Da and dA
• Exception: Genes on the same chromosome do not sort independently. They are linked genes. (Of course they do not sort independently. They are physically attached!)
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