Introduction to Genetics

Genetics: The scientific study of heredity

Heredity – the passing of traits from parents to offspring

Inheritance

• You get your genes from your parents• In meiosis, half of the chromosomes in a

pair come from the Dad, half come from the Mom

• What we know today is based on the work of Gregor Mendel – Austrian monk

Key terms to know

• Allele – each form of a gene for a certain trait (R or r)

• Gene – sequence of DNA that codes for a protein a thus determines a trait

• Genotype – combination of alleles for a given trait ( RR or Rr or rr)

• Phenotype – Appearance of trait ( round seeds or wrinkled seeds

Terms continued• Homozygous - when you have 2 or the same alleles for

a given trait (RR or rr)• Heterozygous – when you have 2 different alleles for a

trait (Rr)• Incomplete dominance – blending of alleles to produce a

different phenotype (red and white produce pink)• Codominance – Both alleles are expressed equally

(produces combinations of each- red and white produce red spotted white)

• Multiple alleles – a set of 3 or more different alleles controlling a trait ( eye color, skin color)

Painting of Mendel

Gregor Mendel

• Born in 1822 in Czech Republic• Became a priest and studied math and

science at the University of Vienna • Worked for next 14 years in the monastery

as head of monastery garden and taught at the high school

Mendel studied seven/eight different pea plant traits…

• Trait – a specific characteristic that varies from one individual to another

(ex. Seed color, height, hair color)• Mendel’s studied traits had two contrasting

characters or “alleles” -- different forms of a gene

Seed Shape

Flower Position

Seed CoatColor

Seed Color

Pod Color

Plant Height

PodShape

Round

Wrinkled

Round

Yellow

Green

Gray

White

Smooth

Constricted

Green

Yellow

Axial

Terminal

Tall

Short

Yellow Gray Smooth Green Axial Tall

Section 11-1Figure 11-3 Mendel’s even F1 Crosses on Pea Plants

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Seed coat color and flower color are often put in for one another – thus, the EIGHT traits!!!

*Flower color – purple (P) vs. white (p)

A genetic cross

Alleles, alternative versions of a gene

Mendel’s test crosses….• Testcross: experimental cross between an

individual with the dominant phenotype for a given trait (genotype unknown, though) and another individual with the recessive phenotype (homozygous recessive)

• P1 generation – parents that Mendel cross-pollinated

• F1 generation – offspring of P1 that were allowed to self-pollinate

• F2 generation – offspring of F1 generation

Mendel tracked heritable characters for three generations

Mendel’s Conclusions….• Biological inheritance is determined by “factors”

that are passed from one generation to a next – today, called genes

• Law of Dominance: where there are two or more forms of a gene for a single trait, some alleles are dominant and other alleles are recessive

• Law of Segregation: alleles segregate (separate) from each other during the process of meiosis (gamete formation)

• Principle of independent assortment: genes for different traits can segregate independently during the formation of gametes

concluded that

which is called the

which is called the

GregorMendel

Law ofDominance

Law ofSegregation

Peaplants

“Factors”determine

traits

Some alleles are dominant,

and some alleles are recessive

Alleles are separated during gamete formation

Section 11-3

Concept Map

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experimented with

Principle of Independent Assortment

This one follows from the law of segregation – all alleles are not permanently associated with one another….

Punnet Squares

• Used to predict the possible gene combinations for a a cross

• Traits are represented by letters– Lower case letters = recessive traits– Upper case letters = dominate traits

How to do punnett squares

1. determine the genotypes of the parent organisms 2. write down your "cross" (mating) 3. draw a p-square 4. "split" the letters of the genotype for each parent & put them "outside" the p-square 5. determine the possible genotypes of the offspring by filling in the p-square 6. summarize results (genotypes & phenotypes of offspring) 7. bask in the glow of your accomplishment !

Beyond Mendel

• Walther Flemming – German biologist who stained cells with dye and saw tiny, threadlike structures in the nucleus CHROMOSOMES!!!

• 1902 Walter Sutton – American biologist who supports idea that “factors” are located on chromosomes

• 1909 Wilhelm Johannsen – Danish biologist who coined the term “gene” to define the physical units of heredity

• GENE: segment of DNA molecules that carries the instructions for producing a specific trait

• Besides straight dominant and recessive genes, two other possibilities for combinations were proven:

Codominance: when 2 alleles work together and BOTH are expressed without one masking the other

Multiple Alleles: when more than two possibilities for a trait are present.

• Example: Blood typeThere are 3 alleles for blood type -- A, B, O

Possible combinations:AA, AO -- Type A bloodBB, BO -- Type B bloodAB -- Type AB

bloodOO -- Type O blood

• Here, A and B are dominant over O, but if A and B are present together, neither dominates!!! This is codominance – they share the power of expression.

• Incomplete Dominance: when BOTH alleles in an individual affect the appearance of a trait and you get a brand new color that was not found in the original parents. Both traits are written in capitals and have different letters because BOTH control the appearance.

• Example: flower color in snapdragons

Pure red (RR) X Pure white (WW)

Humans are difficult to study…Why?1. # of human genes is extremely large (each cell

has ~100,000 different genes)2. Humans cannot be easily controlled by an

investigator3. Time span between generations is long4. Only a small # of offspring are produced by

each set of parents5. Environment has a HUGE effect on a person’s

development…

Have developed ways to approach the difficulties…

• Pedigree analysis – family history for a particular trait

• Study of Genetic diseases• Twin studies – Nature vs. nurture• Population Sampling• Genetic Technology

Pedigree analysis