Theoretical Genetics

Topic 4.3

4.3.1: Key Definitions

Key Definitions

4.3.2: Monohybrid crosses: an example

Determine the genotypes and phenotypes of the offspring of a monohybrid cross between a homozygous, dominant yellow pea plant with a homozygous, recessive green pea plant. Shown below are the steps, numbered 1 to 4, to complete this problem. Test yourself: determine the genotypes and phenotypes of a monohybrid cross between the F1 offspring (i.e., Yy x Yy). Placing your mouse pointer on the figure below will show the genotypes and phenotypes of the offspring of the F1 cross.

Monohybrid Crosses

4.3.3 & 4.3.4: ABO Blood groups

ABO blood groups are an example of codominance and how some genes have more than two alleles (i.e., multiple alleles). Test yourself: if a man has blood group O and a woman has blood group AB, what is the probability that their child will be blood group O? Hint use a Punnett grid to determine the genotypes and phenotypes of a monohybrid cross between the man and woman. Placing your mouse pointer on the figure below will show the answer.

abo blood groups link abo blood groups animation
ABO blood groups

4.3.5: How do the sex chromosomes control gender?

Sex chromosomes and gender determination

4.3.6: Difference between the X chromosome and Y chromosome in humans?

Place mouse pointer on the figure to see chromosome conclusion.

X and Y chromosome difference

4.3.7: Colour blindness and haemophilia

Gender is determined by the sex chromosomes where males have an X and a Y chromosome and females have two X chromosomes. It is important to note that some genes are present on the X chromosome and absent on the shorter Y chromosome in humans. This can lead to diseases that are sex-linked where the disease is caused by a gene located on a sex chromosome (e.g., colorblindness and hemophilia). Test yourself: a woman's father has hemophilia, but her husband does not. What is the probability of the woman having a child with hemophilia? Hint use a Punnett grid to determine the genotypes and phenotypes of a monohybrid cross between the woman and her husband. Placing your mouse pointer on the figure below will show the answer.

4.3.8 - 4.3.10: The inheritance of colour blindness and haemophilia

Sex Linkage explained

4.3.12: Pedigree Key to Symbols

Use the pedigree symbol below to help deduce the genotypes and phenotypes of individuals in pedigree charts. For dominant and recessive alleles, upper-case and lower-case letters, respectively, should be used. Letters representing the alleles should be chosen to avoid confusion with upper and lowercase. For codominance, the main letter should refer to the gene and the suffix to the allele, both uppercase. For example, red and white codominant flower colors should be represented as CR and CW, respectively. For sickle cell anemia, HbA is normal and HbS is sickle cell.

pedigree link pedigree animation
Pedigree Key to Symbols
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