Advanced Biology
Punnett Square Problems
For each of the following genetics problems, construct a Punnett square and analyze the results of eachby answering the questions. Please remember that heterozygous/hybrid refers to one dominant and one recessive allele (Bb), homozygous/pure bred can either be two dominant alleles (BB) or two recessive alleles (bb).
1. A pure dominant father and a pure recessive mother are expecting a baby. What are their chances of having a blonde-haired child if B = brown and b = blonde?
2. A hybrid dwarf and a normal person have a baby. What are the chances that the child will be normal if D = dwarfism and d = normal? This type of dwarfism called achondroplasia is inherited by a dominant gene.
3. A mother has blue eyes. The father has brown eyes. The child has blue eyes. What is the father’s genotype?
4. A mother with sickle cell anemia (SS) marries a normal father (AA). How many of their children will have the disease? How many children are carriers? How many are normal?(AA = normal, AS = carrier of sickle cell, and SS = has sickle cell disease)
5. A couple has a child born with Tay-Sachs disease. Tay-Sachs is a recessive condition (t) and kills all victims by age 3 – 5 because an enzyme necessary for fat reabsorption in the brain is not present. What probable genotypes do the parents have? What are their odds of having another child with Tay-Sachs?
6. A heterozygous dominant rose (R = red) crosses with a homozygous recessive (r = white) rose. How many of the offspring will be pink (Rr) and how many will be white?
7. Hemophilia is a disease in which the blood does not clot properly and is carried on the X sex chromosome and not the Y chromosome. A carrier mother of hemophilia marries a normal father. (XH = normal and Xh = hemophilia gene) What are the chances of having a son with hemophilia? What are the chances of having a daughter with hemophilia? What are the chances of having a child that is a carrier?
8. An albino rabbit mates with another albino rabbit. Both parents are homozygous recessive or pure bred for albinism. How many of their offspring will be albino? (A = dark fur and a = albino)
9. Colorblindness is another sex-linked trait carried on the X chromosome. A mother carries the trait for colorblindness but has normal vision. Her husband carries no genes for color blindness. Will her children be affected by colorblindness? How many sons will be affected? How many daughters will be affected? (XC = normal, Xc = colorblind)
10. Two tall pea plants produce short offspring. How can you explain this? (T = tall and t = short)