Mendel and

HeredityChapter 8

Ms. Hogg, Biology

The Origins of Genetics

• Heredity – the passing of characteristics from parent to offspring– Before DNA and chromosomes were discovered,

heredity was one of the greatest mysteries of science!

Gregor Mendel• The scientific study of heredity began with

Gregor Mendel– Austrian monk– Carried out experiments with garden peas– He was the 1st to develop rules that accurately

predict patterns of heredity.– “Father of Genetics”

Pea Plant

7 Characteristics Mendel Studied:

Pollen Transfer in Mendel’s Experiments

Mendel’s Hypotheses#1

• For each inherited characteristic, an individual has two copies of the gene – one from each parent.

Mendel’s Hypotheses#2

• There are alternative versions of genes.– These different “versions” are called alleles.– An individual receives one allele from each parent.

Mendel’s Hypotheses#3

• When two different alleles occur together, one of them may be completely expressed, while the other may have no observable affect on the organisms appearance.

– Dominant = expressed trait– Recessive = trait that is NOT expressed

Mendel’s Hypotheses#4

• When gametes are formed, the alleles for each gene in an individual separate independently of one another.

Terms of Genetics

• Homozygous = two alleles of a gene are the SAME– Example: BB or bb

• Heterozygous = two alleles of a gene are DIFFERENT– Example: Bb

• Genotype = the set of alleles that an individual has for a trait– Example: BB

• Phenotype = the physical appearance of a characteristic – Example = Brown hair

Terms of Genetics

Punnett Square

• A punnett square is a diagram that predicts the outcome of a genetic cross

• Monohybrid cross:

The Hardy-Weinberg Principle

p2 + 2pq + q2 = 1

You can use this equation to predict genotype frequencies in a

population.

The Hardy-Weinberg Principle

p2 + 2pq + q2 = 1

The sum of allele frequencies must always equal 1.

p + q = 1

Exceptions…

• The Hardy-Weinberg principle holds true for any population as long as evolutionary forces are not acting…– Mutations– Gene flow– Nonrandom mating– Genetic drift– Natural selection

A Review…Monohybrid Cross

A Review…Dihybrid Cross

NOTE: Must use the “FOIL” method to correctly set up the parent genotype combinations on the Punnett Square

Incomplete Dominance

• An organism can display a phenotype that is intermediate between the two parents

• Example: Snapdragon flowersRR = redRr = pinkrr = white

Incomplete Dominance

Codominance

• Two dominant alleles can be expressed at the same time

• Example: Human Blood groups (ABO blood groups)

Type A = IAIA or IAi Type B = IBIB or IBiType AB = IAIB (Universal Recipient)Type O = ii (Universal Donor)

Codominance – ABO Blood Groups

Rh FactorSource: NobelPrize.org

Human Blood Type FrequencySource: RedCrossBlood.org

Sex-linked Traits• In sex-linked inheritance, the gene responsible

for the disease is located on the X chromosome.

• Usually, the abnormal gene is recessive. – For these reasons, the resultant disorder is called

an X-linked recessive disease

Sex-linked Disorders

• Color blindness– A person with the most common form of color

blindness cannot distinguish red from green hues.

• Hemophilia– In hemophilia, the blood does not clot properly.

Color blindness Tests

Hemophilia

Pedigrees

Polygenic Traits

• Polygenic traits are controlled by two or more genes, usually on different chromosomes.

• Examples of Polygenic Traits:– Height, skin color, weight, – Many common diseases, such as heart disease,

are caused by polygenic genes and environmental factors (Lifestyle choices such as diet and exercise)

Human skin color is controlled by at least eight different loci on different chromosomes...