Genetics: The Science of Heredity

Heredity: The passing of physical

characteristics from parents to offspring

•Every organism is a collection of traits

all inherited from its parents.

What have you inherited?

Eye color, hair color, nose shape, and

many other physical features are types

of traits that you inherit

from your parents.

Traits are controlled by genes

• Genes are made up of DNA.

• Genes are located on the chromosomes.

• Our genes determine our traits (hair color, eye color, etc.)

Our genes come from our parents

Alleles: Different forms of a gene

• Alleles are different forms of genes.

• We inherit two alleles for each trait (example hair color)

• One allele comes from our mother and the other from our father

Alleles control traits

• Some alleles are dominant, while others are recessive.

• A dominant allele is one whose trait always shows up in the organism when it is present.

• A recessive allele is hidden whenever the dominant allele is present.

Dominant traits control recessive traits

• In this diagram the trait for black fur dominates the trait for white fur.

• Even though one of the parents has white fur none of the offspring will have white fur.

Dominant and Recessive Alleles Continued.

• Dominant traits are represented by a CAPITAL letter.

• Example from diagram: the CAPITAL letter B represents the dominant trait for black fur.

• Recessive traits are represented by a lower case letter.

• Example from diagram: the lower case letter b represents the recessive trait for white fur.

TWO Alleles for each trait

• In the diagram each animals alleles are represented by TWO letters.

• One of the letters came from that animals mother and one from its father.

• Notice how the offspring are a COMBINATION of the parents alleles.

• The parents are BB for the black fur and bb for the white, but the offspring are Bb a combination of the parents alleles.

Genotype

• Genotype: The genetic makeup of an organism or allele combinations

• Example from diagram The genotype of the animal with black fur is BB and the one with white fur is bb.

• Genotype is represented by either a capital or lowercase letter.

Phenotype

• Phenotype is your physical appearance or visible traits.

• Example from diagram. Black fur and white fur are the phenotypes of the animals.

• The offspring in this example have a genotype of _____ and a phenotype of ______ for their fur color.

Homozygous and Heterozygous

• Homozygous: Two alleles for a trait that are exactly the same.

• - Genotype: BB or bb• Also called PUREBRED• The parents in this example

are homozygous purebreds.• Heterozygous: Two different

alleles for a trait.• - Genotype: Bb• - Also called HYBRIDS• The offspring in this example

are Heterozygous Hybrids.

Punnett Squares

• A punnett square is a tool to predict results in Mendelian genetics.

Step one: Punnett Squares

• Step one: Add the Genotypes of each parent on the top and left side of the punnett square.

Step Two: Punnett Squares

• Step two: Copy the genotype letters of the parent on the top down into the boxes below each letter

Step Three: Punnett Squares

• Step Three: Copy the genotype letters of the parent on the left side into the boxes next to the letters.

Punnett Square Practice

• Show a cross between a Heterozygous (Bb) animal with a homozygous purebred (bb).

• B = Black Fur• b = White Fur• What are the genotypes

of the offspring? • What are the phenotypes

of the offspring.

Punnett Squares Predict Probability

• In a genetic cross, the allele that each parent will pass on to its offspring is based on probability.

• You can determine what percentage chance you have of passing on certain alleles.

• Since there are four squares each square represents 25% (4 x 25 = 100)

Probability Example

• What percentage of the offspring are Heterozygous?

• What percentage of the offspring are homozygous?

• What would the ratio of the offspring be?

• 2:2 or 1:1

Probability Examples