IB Biology Topic 9: Monohybrid Crosses

1. Define the following terms: gene, locus, allele, genotype, phenotype

Gene -  short sections of DNA on chromosomes that code for specific traits

Locus - gene’s location on a chromosome

Allele - different forms of a gene (e.g. tall vs short alleles for height)

Genotype - combination of 2 alleles for a particular trait

Phenotype - the physical trait shown

2. State the number of alleles for a particular gene that are found within a gamete (sex cell).  The answer is ONE allele, and you should be able to explain why!

Gamete cells do not have homologous chromosome pairs, which means that there is only one allele for each trait that is coded. This allows for the gametes to come together, forming a zygote that has the homologous chromosomes (that have 2 alleles for a gene).

3. Describe how dominant and recessive alleles interact with one another.

Dominant alleles show in the phenotype whereas recessive alleles do not, however when a genotype is homozygous recessive the phenotype will be recessive.

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4. Distinguish between the following genotypes–homozygous dominant, homozygous recessive, and heterozygous.

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5. Complete Punnett square problems for traits that show complete dominance (aka simple dominance).

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6. Describe the purpose of a test cross and interpret the results of a test cross.

Purpose: To determine if an organism with a dominant phenotype has a homozygous dominant genotype or heterozygous genotype

Process: Cross organism with homozygous recessive genotype

Interpreting results:

If the results of the test cross yield ONLY dominant phenotypes then the parent organism has a homozygous dominant genotype

If the results of the test cross yield BOTH dominant and recessive phenotypes then the parent organism has a heterozygous genotype

7. State that cystic fibrosis is caused by a recessive allele, whereas Huntington’s disease is caused by a dominant allele.  Complete Punnett square problems regarding inheritance of these two diseases.
8. Describe how codominant alleles interact with one another.

Codominant alleles yield both phenotypes when the genotype of heterozygous. When the genotype is homozygous the phenotype yields the only allele that is in the genotype.

9. Complete Punnett square problems involving codominant alleles.
   10. Describe how the blood type alleles (IA, IB, and i) interact with each other.  Predict phenotypes (blood types) based on genotypes (ex: IBi).

IA - Blood type A, codominant

IB - Blood type B, codominant

i - Blood type O, recessive

11. Complete blood type Punnett square problems.
12. Explain how hemophilia and red-green colorblindness (both sex-linked recessive traits) are inherited. Explain why (with reference to the different sex chromosomes in males and females) these types of traits are more common in males.  Also explain how females can be carriers of the disease allele and males cannot.

(Also, you need to know that males inherit their X chromosome from their mother and their Y chromosome from their father)

Hemophilia and red-green colorblindness is controlled by the recessive allele on the X chromosome. Hemophilia only presents in the phenotype when the male has one recessive allele or when the genotype of a phenotype is homozygous recessive.

13. Complete sex-linked Punnett square problems.

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