Genetic Diagrams

Genetic Diagrams

• Genetic diagrams are used to calculate the probability of offspring inheriting particular alleles.
• An allele is a different form of a gene - for example, the gene for eye colour has alleles for blue eyes and brown eyes.
• Dominant and recessive alleles are represented using letters in these diagrams: a capital letter for the dominant allele and a lowercase letter for the recessive allele.

Punnett Squares

• A Punnett square is a useful tool for visualising genetic crosses and determining probabilities of inheriting traits.
• In a Punnett Square, the alleles of one parent are written across the top, and the alleles of the other parent down the side.
• Each square within the Punnett Square then represents a possible genotype of the offspring.

Mono-hybrid Crosses

• A mono-hybrid cross involves a pair of contrasting traits.
• It’s used to show the inheritance of one gene in which each parent is heterozygous or carries two different forms of a specific gene.
• The alleles are separated during meiosis and then reunited at fertilisation.

Co-dominance and Multiple Alleles

• In co-dominance, both alleles contribute to the phenotype when they are together in the heterozygote. Neither is recessive or dominant.
• Cases where a gene has more than two possible alleles in a population are involved in multiple allele inheritance. An example is the ABO blood group system in human beings.

Sex-Linked Inheritance

• Some traits are determined by genes on the sex chromosomes and thus are inherited differently in males and females. This is termed sex-linked inheritance.
• In humans, the X chromosome carries many more genes than the Y chromosome, so males (XY) are more likely to express a sex-linked trait than females (XX).