Factors at a Single Locus Can Affect the Results of Genetic Crosses

Slides:



Advertisements
Similar presentations
GENETICS. Mendel and the Gene Idea Genetics The study of heredity. The study of heredity. Gregor Mendel (1860’s) discovered the fundamental principles.
Advertisements

Patterns of Inheritance Chapter Early Ideas of Heredity Before the 20 th century, 2 concepts were the basis for ideas about heredity: -heredity.
Mendelian Genetics.
Incomplete & Codominance
Lesson Four: Extending Mendelian Genetics. Incomplete Dominance In Mendel’s experiments, each trait observed exhibited complete dominance of one allele.
Chapter 4: Modification of Mendelian Ratios Allele *Wild-type allele *Mutant allele Conventional symbols for alleles: recessive allele - initial letter.
1/30/15 Objective: Who was Gregor Mendel and what were his contributions to genetics? Do Now: What is a trait, name a few.
Ch. 10.3: Exceptions to Mendel’s Rules of Inheritance Objectives: 1.Describe how alleles interact in intermediate inheritance. 2.Describe inheritance patterns.
Patterns of Inheritance Chapter 10. Blending Hypothesis of Inheritance Trait Trait A variation of a particular characteristic A variation of a particular.
4/11/ 12 Bell Ringer 1.What does incomplete dominance mean? 2.If I had a Red Homozygous Flower and a White Homozygous Flower, what would the Heterozygous.
LECTURE CONNECTIONS 5 | Extensions and Modifications of Basic © 2009 W. H. Freeman and Company Principles.
BEYOND MENDEL’S LAWS Heredity. Incomplete Dominance Review:  Inherited traits were either dominant or recessive  The dominant allele in an individual.
 Genetics – the scientific study of heredity  Why the garden pea?  Easy to grow  Produce large numbers  Mature quickly  Reproductive organs in same.
Chapter 4: Modification of Mendelian Ratios Allele *Wild-type allele *Mutant allele Conventional symbols for alleles: recessive allele- initial letter.
04/10/12 1.What does heterozygous mean? 2.What does homozygous mean? 3.What is a phenotype? 4.What is a genotype? 5.What three genotypes can an organism.
Intro to Genetics What is an allele? –one of the alternative forms of a gene that governs a characteristic, such as hair color.
Chapter 10 HOW INHERITED TRAITS ARE TRANSMITTED. Genetics is the science of heredity.
2. MULTIPLE ALLELES 3. PENETRANCE AND EXPRESSIVITY
Incomplete Dominance and Codominance
Mendelian Genetics Patterns of inheritance chp 12
Punnett Squares: Codominance and Incomplete Dominance.
Inheritance Patterns Simple dominance, incomplete dominance, codominance, sex-linkage polygenic inheritance.
Chapter 14: Mendel & The Gene Idea
Mystery of Heredity Before the 20th century, 2 concepts were the basis for ideas about heredity heredity occurs within species traits are transmitted directly.
Extensions and Modifications of
Beyond Mendelian Genetics
Patterns of Inheritance
Genes and Alleles Scientists call the factors that control traits genes They call the different forms of a gene alleles (uh leelz). The factors that.
copyright cmassengale
6.1-Beyond Mendel’s Observations of Inheritance
Genes and Inheritance Review
General Genetics Ayesha M. Khan Spring 2013.
Non-Mendelian Genetics
Incomplete Dominance & Codominance
Incomplete Dominance Codominance Multiple Alleles
Incomplete Dominance & Codominance
Factors at a Single Locus Can Affect the Results of Genetic Crosses Part - II Genetics.
HW: Co-dominance Practice
Codominance and Incomplete Dominance.
KEY CONCEPT Phenotype is affected by many different factors.
KEY CONCEPT Phenotype is affected by many different factors.
Punnett Squares: Part 2.
MENDEL AND THE GENE IDEA
Patterns of Inheritance
NON-MENDELIAN GENETICS
Linkage, Recombination, and Eukaryotic Gene Mapping
Dihybrid Crosses Reveal the Principle of Independent Assortment
Sex-Linked Characteristics Determined by Genes on the Sex Chromosomes
Genetics: Mendel & The Gene Idea.
Lecture # 6 Date _________
12.2 Section Objectives – page 315
Genetics (10.2, 10.3, Ch.11) SB2. Students will analyze how biological traits are passed on to successive generations. (c) Using Mendel’s laws, explain.
Incomplete Dominance Codominance Multiple Alleles
Incomplete dominance and co-dominance
Incomplete Dominance Incomplete Dominance: The heterozygous genotype produces a phenoytype that falls in between the dominant trait and the recessive trait.
4/11/ 12 Bell Ringer What does incomplete dominance mean?
Incomplete Dominance & Codominance.
Other types of Inheritance
Genetics Genetics The study of how traits are passed from parents to offspring.
Mendelian Genetics Biology 30.
Incomplete Dominance Incomplete Dominance: The heterozygous genotype produces a phenotype that falls in between the dominant trait and the recessive trait.
Incomplete Dominance Codominance Multiple Alleles
Genetics Test Review.
Codominance and Incomplete Dominance.
Patterns of Inheritance
Genes and Heredity.
Incomplete Dominance Codominance Multiple Alleles
Dominance and Multiple Alleles
KEY CONCEPT Phenotype is affected by many different factors.
Incomplete Dominance Codominance Multiple Alleles
Presentation transcript:

Factors at a Single Locus Can Affect the Results of Genetic Crosses Genetics

Cuenot’ found that, when he crossed pure breeding gray mice with pure breeding white mice, all of the F1 progeny were gray, and interbreeding the F1 produced 3 : 1 ratio of gray and white mice in the F2, as would be expected if gray were dominant over white.

The only exception was!! His crosses of yellow mice suggested that yellow coat color was dominant over gray, but he was never able to obtain true-breeding (homozygous) yellow mice. Whenever Cuenot crossed two yellow mice, he obtained yellow and gray mice in approximately a 3 : 1 ratio, suggesting that the yellow mice were heterozygous (Yy × Yy → 3/4 Y_ and 1/4 yy). If yellow were indeed dominant over gray, some of the yellow progeny from this cross should have been homozygous for yellow (YY) and crossing two of these mice should have yielded all yellow offspring (YY × YY → YY). However, he never obtained all yellow progeny in his crosses.

The ratio that Cuenot observed, was not 3:1 but actually was 2 : 1 ratio of yellow and non-yellow. How? Castle and Little recognized that the allele for yellow was lethal when homozygous , and thus all the yellow mice were heterozygous (Yy). A cross between two yellow heterozygous mice produces an initial genotypic ratio of 1/4 YY, 1/2 Yy, and 1/4 yy, But the homozygous YY mice die early in development and do not appear among the progeny, resulting in a 2 : 1 ratio of Yy (yellow) to yy (nonyellow) in offspring.

Because only mice homozygous for the Y allele die, the yellow allele is a recessive lethal. The yellow allele in mice is unusual?? in that it acts as a recessive allele in its effect on development but acts as a dominant allele in its effect on coat color.

Types of Dominance -------------------------- Complete Dominance incomplete dominance Codominance Dominance

Types of Dominance One of Mendel’s important contributions to the study of heredity is the concept of dominance—the idea that an individual organism possesses two different alleles for a characteristic but the trait encoded by only one of the alleles is observed in the phenotype. With dominance, the heterozygote possesses the same phenotype as one of the homozygotes.

Types of Dominance Complete and incomplete dominance can be understood in regard to how the phenotype of the heterozygote relates to the phenotypes of the homozygotes. Example: Flower color potentially ranges from red to white. One homozygous genotype, A1A1 , produce red pigments, resulting in red flowers; another, A2A2, produce no-pigment, resulting in white flowers. Where the heterozygote falls in the range of phenotypes determines the type of dominance. If the heterozygote (A1A2) produces the same amount of pigment as the A1A1 homozygote, resulting in red, then the A1 allele displays complete dominance over the A2 allele; that is, red is dominant over white. If, on the other hand, the heterozygote (A1A2) produces no pigment resulting in flowers with the same color as the A2A2 homozygote (white), then the A2 allele is completely dominant, and white is dominant over red.

Incomplete dominance Incomplete dominance, the heterozygote need not be exactly intermediate between the two homozygotes, it might be a slightly lighter shade of red or a slightly pink shade of white.

Example of incomplete Dominance When the F1 are crossed with each other, 1/4 of the F2 are purple (PP), 1/2 are violet (Pp), and 1/4 are white (pp). Note that this 1 : 2 : 1 ratio is different from the 3 : 1 ratio that we would observe if eggplant fruit color exhibited complete dominance.

Codominance Another type of interaction between alleles is codominance, in which the phenotype of the heterozygote is not intermediate between the phenotypes of the homozygotes; rather, the heterozygote simultaneously expresses the phenotypes of both homozygotes. An example of codominance is seen in the MN blood types. Example: At the MN locus, there are two alleles: the LM allele, which encodes the M antigen; and the LN allele, which encodes the N antigen. Homozygotes with genotype LMLM express the M antigen on their red blood cells and have the M blood type. Homozygotes with genotype LNLN express the N antigen and have the N blood type. Heterozygotes with genotype LMLN exhibit codominance and express both the M and the N antigens; they have blood-type MN.

i, which encodes no antigen (O). The ABO blood group Another multiple-allele system is at the locus for the ABO blood group. This locus determines your ABO blood type. The three common alleles for the ABO blood group locus are: IA, which encodes the A antigen; IB, which encodes the B antigen; i, which encodes no antigen (O). We can represent the dominance relations among the ABO alleles as follows: IA > i, IB > i IA = IB. Both the IA and the IB alleles are dominant over i and are co-dominant with each other; The AB phenotype is due to the presence of an IA allele and an IB allele, which results in the production of A and B antigens on red blood cells.

why the pink flowers exhibit in-complete dominance—that is, why is this outcome not an example of codominance? The flowers would exhibit codominance only if the heterozygote produced both red and white pigments, which then combined to produce a pink phenotype. However, in our example, the heterozygote produces only red pigment. The pink phenotype comes about because the amount of pigment produced by the heterozygote is less than the amount produced by the A1A1 homozygote.

References Benjamin A. Pierce, 2010. Genetics: A Conceptual Approach, 4th Edition. 4th Edition. W. H. Freeman.

The End