Genetics

Prehistoric Pooches

• Dogs and humans have been companions for thousands of years. The domestication of dogs may have been one of the earliest human experiences with genetics.

• Genetics - scientific study of how traits are passed from parent to offspring

• Millions of years ago there were no dogs. Today’s domestic dogs are descended from a wild, wolfish ancestor.

• Certain wolves were selected to mate which affected the traits that were passed from the parents to the pups.

• Trait – a characteristic that can be passed on

• With each generation, the traits of the wolves living with humans became more distinct from the traits of the wild wolves. Eventually, this led to a new breed – the domestic dog.

• We’ve known for centuries that traits are passed from parent to offspring, but we haven’t always known how those traits are determined.

• Blending Hypothesis – traits of parents were blended in the offspring

Chromosomes and cell division

• The chromosomes make copies of themselves and then pass those on to the daughter cells.

• We know that traits are passed on in these chromosomes but the relationship between chromosomes and traits was not always understood.

Gregor Mendel

• Austrian monk in 1860’s• Experimented with pea plants

- Structure of pea flowers- Traits showed obvious differences that were easy to see- Reproduced quickly

Mendel’s Experiments

Structure of pea plants

• the relatively closed structure of the pea flower petals makes it very easy for pollen from the anther to fertilize the pistil of the same flower

Self-fertilization

What’s the difference between self fertilization and asexual reproduction? Are they the same?

Structure of pea plants

• Purebred – receives same genetic traits from both parents

• Hybrid – receives different forms of a trait from each parent

Cross-pollination

Traits are easy to see

• Mendel studied 7 traits that only come in 2 distinct forms

• Example: pea pods were either yellow or green (there were no in-between colors)

Short reproductive cycle - 90 days

• Quick results• Could repeat the experiments many times

Mendel’s Observations

• Mendel began his experiments using two different purebred plants

• P – the parental generation or first group to fertilize

• F1 – the filial generation or offspring• F2 – the offspring of the F1 generation

Findings:

• When crossing 2 different purebred parents, F1 generation were only one form of the trait

• F2 generation showed 75% of F1 trait, and 25% of other trait

P

F1

F2What should have happened according to the blending hypothesis?

Repeated his experiments with the other 6 traits

Mendel’s Conclusions

• Blending hypothesis was wrong – forms of a trait must remain separate in offspring

P

F1

F2

P

F1

F2

• Genes – sections of a chromosome that code for a trait

• Allele – distinct form of a gene

• Dominant allele – form of a gene that is fully expressed when two different alleles are present

• Recessive allele – form of a gene that is not expressed when paired with a dominant allele

Unnoticed

• Mendel performed these experiments for 8 years and published his work. Unfortunately, at the time scientists were focused on the work of Charles Darwin and so Mendel’s work went unnoticed for the next 37 years.

Walter S. Sutton

• Observed stained cells under a microscope and noticed that the chromosomes acted like Mendel’s factors.

Chromosome Theory of Heredity

• material of inheritance is carried by the genes in chromosomes

To help explain the new theory, scientists came up with some new terms and symbols

• Gene – section of chromosome that code for traits

• Allele – different forms of that trait

• Pea color is controlled by a single gene with two alleles – one for green and one for yellow

Letters are used…

• Dominant allele – capital letter- Example: Yellow is the dominant pea color and would be represented by Y

• Recessive allele – lowercase letter- Example: green is the recessive pea

color and would be represented by y

• What letters would be used for other traits?

Genotype

• the genetic makeup of an organism (YY, Yy, yy)

• It includes both genes in a homologous pair of chromosomes

Phenotype

• the physical appearance (yellow, green)

Homozygous

• the two alleles in a gene are identical (YY or yy)

Heterozygous

• the two alleles in a gene are different (Yy)

Mendel’s Laws

Law of Segregation

• each pair of genes separates during meiosis

F1 generation Meiosis 1 Meiosis 2

Law of Independent Assortment

• gene pairs separate into gametes randomly and independently of each other

PPpp

PPpp

PP

Pp

Pp

pp

Law of Dominance

• the dominant allele is expressed and the recessive allele is hidden. The recessive allele is only expressed when no dominant allele is present.