The Basics of Genetics…the passing of traits Our genes determine our traits or characteristics. Your genotype (Dd) or genetic makeup for a particular trait determines your phenotype (dimples) or physical appearance for that trait. Alleles are different forms that a gene may have for a single trait. For example, having dimples or not having dimples.

Alleles can be represented by letters Alleles can be represented by letters. You get one allele from each parent; that’s why there are 2 letters. Alleles can be dominant, represented by a capital letter or recessive, represented by a lowercase letter. Homozygous (pure) = 2 of the same alleles for a trait; for example: dd or DD Heterozygous (hybrid) = 2 different alleles for a trait; for example: Dd

Punnett squares and Pedigree Charts

Punnett squares A chart that shows the chances or probability of a particular trait being expressed in offspring. Each box of the Punnett square represents a 25% chance for that trait being expressed. Example: Cross Tt and tt T = dominant allele = tall T = recessive allele = short The Tt and tt along the top and sides of the Punnett square are the genotypes for the parents. The genotypes for the offspring are inside of the Punnett square.

Interpreting Punnett squares Remember that T is dominant over t, which is recessive. Parent 1’s genotype = tt; phenotype = short Parent 2’s genotype = Tt; phenotype = tall There is a 50% chance of producing short offspring = tt because there are 2 boxes with that trait. There is a 50% chance of producing tall offspring = Tt.

Pedigree Charts (also see Pedigree notes) There are four generations shown on this pedigree chart. Squares = males and circles = females; if the shape is totally shaded then he/she has the condition; if the shape is shaded ½ way then he/she is a carrier for that condition. Generation I parents had 2 boys and 3 girls of which only 1 has the condition.