A monohybrid crossinvolves 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 in3 ways****** • Crossing over 2. Independent assortment 3. creating gametes for fertilization – ultimately combining the genes of 2 organisms
Example Ex. 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 DdAax 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 DadAda ***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 Punnett STEP 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.
COUNT the number of boxes you have colored in the Punnett. • RECORD this in the phenotypic frequency column.
STEP 5 • STEP 5: RECORD the phenotypic frequency as a ratio. Make sure your numbers add up to 16!!!! phenotypic ratio 9dimples, able to roll : 3dimples, unable to roll : 3no dimples, able to roll : 1no 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 alignmentscan occur, which produce different combinations of gametes.
D d D d OR a A A a D d D d A a A a D A A A D a a D d a D A d d a d 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!)