Bio 178 Lecture 24Genetics

J. Elson-Riggins

Reading

• Chapter 13

Quiz Material

• Questions on P 276-278

• Chapter 13 Quizzes on Text Website (www.mhhe.com/raven7)

Outline

• Genetics

Mendelian Patterns of Inheritance

Non-Mendelian Patterns of Inheritance

Principles of Genetics (Cntd.)• Dominant and Recessive Alleles

Dominant alleles mask the effects of recessive alleles in heterozygotes (*for typical Mendelian traits).

• Genotype and Phenotype

Genotype - The alleles an individual possesses.

Phenotype - The physical manifestation of the genotype (eg. Appearance of an individual).

• LocusLocation of a gene on a chromosome.

Mendelian Genetics - Monohybrid CrossThe inheritance of different traits of one character (alleles of one gene).

Example : Flower Color

• TestcrossA cross between an individual with a dominant phenotype and a recessive homozygote (often parental) to determine genotype.

Mendel’s Question

What is the genotype of a plant with the dominant phenotype?Experiment

Cross the F1 (purple) plants with the white flowered P generation.

Testcross (Cntd.)Testcross a phenotypically dominant plant.

Monohybrid Cross Example

The presence of freckles on human skin is a dominant trait controlled by a single gene.

(a) What is/are the possible genotypes of an individual with freckles?

(b) What is the probability that 2 parents without freckles will produce a child with freckles?

(c) If 2 parents with freckles produce a child without freckles, what is the probability that their next child will:

(i) also be without freckles?

(ii) have freckles?

Dihybrid CrossThe inheritance of pairs of genes.

Mendel’s Question

Do pairs of genes segregate independently?

Experiment

Examined the traits for seed shape (round (R) vs wrinkled (r)) and seed color (yellow (Y) vs green(y)).

P: Mendel crossed true breeding individuals that were homozygous dominant for both traits with individuals that were homozygous recessive for both traits.

What would be the expected phenotypes of the F1?

Dihybrid Cross (Cntd.)Mendel then self-fertilized the F1 generation.What were the phenotypes of the F2 generation?

Mendel’s Laws(In modern biological terms)

• Mendel’s First Law - Segregation

Alternative alleles for a trait (heterozygotes) segregate from each other into gametes.

In terms of meiosis:

In anaphase I, homologous chromosomes separate from one another. Each gamete will therefore receive 1 allele, not both.

Mendel’s Laws (Cntd.)

• Mendel’s Second Law - Independent Assortment

Genes located on different chromosomes are inherited independently of one another.

In terms of meiosis:

Dihybrid Cross ExampleYou have performed Mendel’s dihybrid cross using the

traits for seed shape and seed color. In the F2 generation you obtain plants with round yellow seeds. You decide to determine the genotype of ONE of these plants. In your cross you obtain progeny with the following phenotypes:

25% produce round yellow seeds, 25% produce round green seeds, 25% produce wrinkled yellow seeds, & 25% produce wrinkled green seeds.

(a)Describe the cross you performed to determine genotype.

(b)What is the genotype of the F2 plant you tested?

Non-Mendelian patterns of Inheritance

• Mendelian Traits

Have a simple genetic basis. Each character:

(a) is controlled by only 1 gene for which there are only 2 alleles.

(b) has 1 allele that is completely dominant to the other.

• Non-Mendelian Traits

Heritable traits that do not have the above simple genetic basis.

Non-Mendelian patterns of Inheritance

• Quantitative Traits (Polygeny)

• Pleiotropy

• Incomplete Dominance

• Environmental Effects

• Multiple Alleles and Codominance

• (Epistasis)

Non-Mendelian patterns of Inheritance• Quantitative Traits - Continuous Variation

When multiple genes act jointly to influence a trait (polygeny).

Eg. Human height, weight, skin color.

These genes segregate independently of each other in meiosis. There is a continuous spectrum of possible traits bell-shaped curve.

http://www.bio.miami.edu/dana/250/bellpeppers.jpg

Polygeny

Polygeny in Bell Peppers

* Y - timing of chlorophyll elimination (Y - early; y - normal) * R - color of carotenoids (R - red; r - yellow) * C - regulation of carotenoid deposition (C - normal; c1, c2 - lowered concentration) * This leads to a few possible genotypes producing interesting phenotypes:* Y- rr c1c2 - pale yellow * Y- rr Cc2 - darker yellow * yy rr CC - green * Y- R- CC - red * yy Rr CC - purple * Y- Rr Cc2 - pale yellow

Non-Mendelian patterns of Inheritance (Cntd.)• PleiotropyWhen a single gene can have more than one effect.

Dominance - can be dominant with respect to 1 phenotype but recessive wrt another phenotype.

Example - Yellow Fur Color in Mice

Yellow allele is responsible for fur color & lethality.

Dominance:

Dominant wrt fur color.

Recessive wrt lethality - homozygous yellow individuals die but heterozygous individuals live.

Manx Cat

http://www.bio.miami.edu/dana/250/manx.jpg

Devon Rex

http://www.catsincharge.co.uk/images/pudgy-devon-rex.jpg