Transcript Biol 178 Lecture 24

Bio 178 Lecture 24
Genetics
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).
• 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 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 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 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 Example
You 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
Eg. Human height, weight, skin color.
When multiple genes act jointly to influence a trait
(polygeny).
These genes segregate
independently of each
other in meiosis. There
is  a continuous
spectrum of possible
traits  bell-shaped
curve.
Polygeny
http://www.bio.miami.edu/dana/250/bellpeppers.jpg
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
http://www.bio.miami.edu/dana/250/manx.jpg
Devon Rex
http://www.catsincharge.co.uk/images/pudgy-devon-rex.jpg