Bio 178 Lecture 24 Genetics  J. Elson-Riggins

Reading Chapter 13 Quiz Material Questions on P Chapter 13 Quizzes on Text Website (

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). Locus Location of a gene on a chromosome.

Mendelian Genetics - Monohybrid Cross The inheritance of different traits of one character (alleles of one gene). Example : Flower Color Testcross A 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 F 1 (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 Cross The 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 F 1 ?

Dihybrid Cross (Cntd.) Mendel then self-fertilized the F 1 generation. What were the phenotypes of the F 2 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 Example You have performed Mendel’s dihybrid cross using the traits for seed shape and seed color. In the F 2 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 F 2 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.

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.) Pleiotropy When 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

Devon Rex