Mendelian Genetics and Punnett Squares (tutorial)

[q labels=”top”]

Cystic fibrosis is caused by a defect in a protein found in cell membranes. The _______ for this protein is found on chromosome 7. That makes the gene an ____________ gene (because it’s not on the X or Y chromosome).

If you inherit a working ________ (version) of this gene, you’ll be fine, even if you inherit one defective allele. That’s because the disease-causing allele is ___________. If you inherit the working, ____________ allele, then this allele will be translated into the working membrane protein. Your cells will function as they should, and there’ll be no build-up of the sticky mucus that underlies the disease.

You can only have the disease if you inherit two defective alleles. Another way to say this is that to have cystic fibrosis, you have to be ______________ for the cystic fibrosis allele. Being homozygous means that you’ll have two copies of the allele. People who are _______________, with one dominant allele and one recessive allele, don’t get the disease. They have a normal ____________ (no sticky mucus).

Typically, we represent alleles with symbols. If we use C to represent the normal allele and c to represent the cystic fibrosis allele, then we have three ____________:

• _______________ dominant: CC
• ________________: Cc
• homozygous ___________: cc

[q] In peas, the tall allele (T) is dominant to the short allele (t). Cross a plant that is homozygous tall with a plant that is heterozygous tall.

Step 1: Determine the genotypes of the parents

Parent 1) Homozygous tall: ___ ___

Parent 2) Heterozygous tall: ___ ___

Steps 2 through 4: Complete your Punnett square. Note that for this quiz, you have to place the alleles of the gametes of the 1st parent to the left of the Punnett square, and put the alleles of the second parent’s gametes on top. On paper, you could do it in either order.

Step 5: Genotype ratio: _____ TT: ____Tt: ____tt

Step 6: Phenotype ratio: _____ Tall: ______Short

[q]In carnations, the gene for red flowers (R) is dominant to the gene for white flowers (r). What will happen if a plant homozygous for red flowers is crossed with a plant homozygous for white flowers?

Step 1: Determine the genotypes of the parents

Parent 1) homozygous red flowers: ___ ___
Parent 2) Homozygous white flowers: ___ ___

Step 2: Draw the Punnett square (already done below)

Step 3: Place the alleles of the gametes that the 1st parent can make to the left of the Punnett square. Put the alleles that the 2nd parent can make on top. (note: these required positions (left and top) are solely so that this quiz program will work. On paper, you can put them in either position.

Step 4: Move the gametes to the right or downward into the square, simulating fertilization and zygote formation.

Step 5: Genotype ratio: _____ RR: ____Rr: ____rr

Step 6: Phenotype ratio: _____ Red-flowered, ____white flowered

[q]In peas, the gene for tall (T) is dominant to the gene for short (t). What genotypes and phenotypes will result in the offspring if two heterozygous plants are crossed with each other?

Step 1: Determine the genotypes of the parents

Parent 1) Heterozygous tall: ___ ___
Parent 2) Heterozygous tall: ___ ___

Step 2: Draw the Punnett square (I did that one for you)

Step 3: Place the alleles of the gametes that the 1st parent can make to the left of the Punnett square. Put the alleles that the 2nd parent can make on top. (note: these required positions (left and top) are solely so that this quiz program will work. On paper, you can put them in either position).

Step 4: Move the gametes to the right or downward into the square, simulating fertilization and zygote formation.

Step 5: Genotype ratio: _____ TT: ____Tt: ____tt

Step 6: Phenotype ratio:

_____ Tall, _____medium, _____ short