Fundamentals of Genetics

Fundamentals of Genetics

Chapter 9Chapter 9

Mendel’s LegacyMendel’s Legacy

Section 9.1Section 9.1

Genetics

The field of biology dedicated to understanding heredity…

Heredity- the passing of traits from parents to offspring

Was founded by the work of Gregor Johann Mendel, the “Father of Genetics”

Gregor Mendel

Austrian monk & science teacher

Studied heredity and statistics

Worked with pea plants in his garden

Mid 1800s

Mendel’s Peas:

Studied 7 characteristics of peas

Each characteristics had 2 different traits

ex) flower color

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Mendel’s Experiments:

1. Started off with 2 pure (true breeding) plant groups for a particular characteristic

P generation (parent)

2. Cross pollinated these plant groups (sexual reproduction) to produce the next generation of plants

F1 generation (first filial) he counted & recorded the traits of the offspring

3. Then he allowed the F1 generation plants self pollinate (sexual reproduction with self) to produce the next generation of plants

F2 generation (second filial) he counted & recorded the traits of the offspring.

Mendel’s Results:

The F1 generation always showed only 1 of the 2 traits for the characteristic

– Mendel named this the dominant factor

The F2 generation always showed a 3:1 (or a 75% to a 25%) ratio between the 2 traits

– Mendel named the second the recessive factor

Purple is dominantWhite is recessive

F1

F1

F2

Notice that the predicted ratios & the actual ratios are not exact!

Mendel’s Laws of Inheritance

•Mendel stated that… Mendel stated that… a a pair of factors is pair of factors is segregated, or separated, segregated, or separated, during the formation of during the formation of gametes.gametes.•So, What does this mean?So, What does this mean?

Each egg and sperm Each egg and sperm receives only one factor receives only one factor from each parent.from each parent.

1) Law of Segregation

2. Law of Independent Assortment

Mendel also stated that … factors for different characteristics are distributed to gametes independently.

So, What does this mean?– The factors for different characteristics are not

connected.

Molecular Genetics

The study of the structure & function

of chromosomes & genes

Allele: alternate form of a gene

– Mendel called them “factors”– Abbreviations:

Dominant allele = capital letter (B)Recessive allele = lower case letter (b)

Genetic Crosses

9.2

Vocab:

Genotype: gene combination for a trait (BB, Bb, bb)

Phenotype: the physical feature resulting from a genotype (Black, white)

Genotype Vocab:

Homozygous: “same genes”– When the organism has the same alleles for the characteristic

(also called pure)

Ex.) BB = homozygous dominant

bb = homozygous recessive

Heterozygous: “different genes”– When the organism has different alleles for the characteristic

(also called hybrid)

Ex.) Bb

Examples:

Genetic Probability: The likelihood (probability) of offspring of known

parents can be determined by Punnett squares

Types of Genetic Crosses:

1. Monohybrid Cross-

cross involving a single trait

ex.) flower color

2) Dihybrid Cross

Cross involving 2 traits

Let’s Practice Punnett Squares!

1. Monohybrid Cross2. Dihybrid Cross

How do you find an unknown genotype?

Testcross Example) Is the red flower a pure

(RR) or hybrid (Rr) ? Cross the unknown flower with a

pure recessive flower (rr). This will allow the recessives to show

up in the next generation.

The results:

If the flower is hybrid, the offspring will be 50% red and 50% white.

If the flower is pure, the offspring will be all red.

Result if flower is hybrid

Result if flower is pure

Complications!

Incomplete Dominance and Codominance

1) Incomplete Dominance: occurs when the F1 offspring has a phenotype between that of the parents.

neither allele is completely dominant

The flowers may be red or white in the purebred form, but the hybrid form of the flowers is pink.

Q- What would happen if you crossed a red four o’clock flower with a white four o’clock flower?

A- a pink flower!

2) Codominance

Occurs when both alleles are expressed in a heterozygous offspring.– Neither allele is dominant or recessive

Example)

Q- What would happen if you crossed a horse with a white coat (rr) with a horse with a red coat (RR)?

A- A horse with a roan coat (Rr) ~ both red and white hairs

Question:

In rabbits, the allele for black coat color (B) is dominant over the allele for brown coat color (b). Predict the results of a cross between a rabbit heterozygous for black coat color and a rabbit homozygous for brown coat color.

What is the genotypic ratio?

What is the phenotypic ratio?

Question:

Albinism is a recessive disorder (aa) that results in a lack of pigment in the eyes, skin, and hair.

What are the genotypes and phenotypes of the parents if they have 2 children with albinism and 2 children normally pigmented.

Use a Punnett square to complete this problem

Brown hair color is dominant over blonde hair, and brown eyes are dominant over blue eyes.

Q- If 2 heterozygous parents for both traits

mate, what is the phenotypic ratio of their offspring?

__________ Brown hair, Brown eyes

__________ Brown hair, Blue eyes

__________ Blonde hair, Brown eyes

__________ Blonde hair, Blue eyes

Incomplete Dominance Q:

Scottish fold cats are heterozygotes whose ears fold down onto their heads several days after birth. The homozygote genotype, rr, for this trait causes death. Construct a Punnett square to determine the genotypic and phenotypic ratios of a cross between 2 cats with folded ears.

B. Complications: Multiple alleles ABO blood groups

Dominant

Dominant

Codominant

Recessive

C. Complications: Pleiotropy

- One gene affects many characters - Sickling allele of hemoglobin

D. Complications: Polygenic Inheritance and Quantitative Characters

- One trait determined by multiple genes

- Converse of pleiotropy

- e.g., skin color: at least 3 genes

Complications: Epistasis

- Expression of one gene depends on another

- Mouse coat color: B - black coat b - brown coat C - pigment c - no pigment

What are my chances?

1. Obtain 2 pennies.2. Flip each coin simultaneously 50 times.

Record the results of each set of flips.3. Total the number of combinations of your

results:__ Heads-Heads__ Heads-Tails__ Tails-Tails

4. Record your results on the board.

5. Calculate your percent chance of each combination.

(# of combos) 50

6. Calculate the class average percent chance of each combination.