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Segregation, Assortment, and Dominance Relationships. Genes and alleles Random segregation Independent assortment Assortment vs. Linkage Dominance relationships. A.Genes and Alleles. Gene Classical definition: A unit of inheritance - PowerPoint PPT Presentation
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Segregation, Assortment, and Dominance Relationships
A. Genes and allelesB. Random segregationC. Independent assortmentD. Assortment vs. LinkageE. Dominance relationships
A. Genes and Alleles
Gene Classical definition:
• A unit of inheritance• A factor transmitted during reproduction and responsible
for the appearance of a given trait Contemporary understanding:
• A segment on a DNA molecule• Usually at a specific location (locus) on a chromosome• Characterized by its nucleotide sequence
A. Genes and Alleles
Genes play three notable roles: To encode the amino acid sequences of proteins To encode the nucleotide sequences of tRNA or
rRNA To regulate the expression of other genes
A. Genes and Alleles
Alleles: Variant forms of a gene found within a population Alleles of a gene usually have small differences in
their nucleotide sequences The differences can affect the trait for which the
gene is responsible Most genes have more than one allele
A. Genes and Alleles
Homozygous and heterozygous: In a diploid species, each individual carries two
copies of each gene (with some exceptions) The two copies are located on different members of
a homologous chromosome pair If the two copies of the gene are identical alleles,
then the individual is homozygous for the gene If the two copies are different alleles, then the
individual is heterozygous for the gene
A. Genes and Alleles
Genotype: The genetic makeup of an individual with reference
to one or more specific traits A genotype is designated by using symbols to
represent the alleles of the gene
A. Genes and Alleles
Example: Consider a gene for plant height in the pea plant with
two alleles, “D” and “d” Each individual pea plant will carry two copies of the
plant height gene, on a homologous chromosome pair
An individual pea plant will be one of three possible genotypes:
• Homozygous “DD”• Homozygous “dd”• Heterozygous “Dd”
A. Genes and AllelesDominant and recessive: A dominant allele is expressed over a recessive allele in a
heterozygous individual This means that a heterozygous individual and a homozygous
dominant individual have identical phenotypes Often, a dominant allele encodes a functional protein, such as an
enzyme The recessive allele is a mutation that no longer has the information
for the correct amino acid sequence; Therefore, its protein product in nonfunctional
In the heterozygote, the dominant allele encodes sufficient production of the protein to produce the dominant phenotype. This is also called complete dominance
A. Genes and Alleles
Phenotype: The appearance or discernible characteristics of a
trait in an individual Phenotypes can be determined by a combination of
genetic and environmental factors
A. Genes and Alleles
Example: In the pea plant height gene, the dominant allele “D”
encodes a hormone that promotes tall growth The recessive allele “d” is a mutation that does not
produce functional hormone If an individual pea plant has at least “one good
copy” of the “D” allele, then it makes enough hormone to grow tall
Otherwise, the plant is dwarf in size
A. Genes and Alleles
Example (continued): Therefore, there are two possible phenotypes for
plant height in peas:• Genotype “DD” produces tall plants• Genotype “Dd” produces tall plants• Genotype “dd” produces dwarf plants
Note that “D” is completely dominant over “d” There is no observable difference in phenotype
between “DD” (homozygous dominant) and “Dd” (heterozygous) plants
B. Random Segregation
Mendel’s law of random segregation: Diploid germ-line cells of sexually reproducing
species contain two copies of almost every chromosomal gene
The two copies are located on members of a homologous chromosome pair
During meiosis, the two copies separate, so that a gamete receives only one copy of each gene
B. Random Segregation
Random segregation can be demonstrated with a monohybrid cross experimentMonohybrid cross: A parental cross between two individuals that differ in the
genotype of one gene The offspring of the parental generation is called the F1 (first
filial) generation The F1 generation can be allowed to interbreed or self-fertilize
(inter se cross, or “selfing”) to produce the F2 (second filial) generation
B. Random Segregation
Example of a monohybrid cross:
P generation: Homozygous tall pea plants (pollen)X
Homozygous dwarf pea plants (ovules)F1 generation: All tall pea plants
F1 tall X F1 tall
F2 generation: About ¾ of the F2 plants will be tall
About ¼ of the F2 plants will be dwarf
B. Random SegregationGenotypic explanation of the monohybrid cross: Parental generation:
Pollen from a DD plant X ovules from a dd plantPollen genotype: DOvule genotype: d
Therefore, in the F1 generation:Genotype of all F1 plants: Dd
F1 pollen: ½ D and ½ dF1 ovules: ½ D and ½ d
B. Random Segregation
Genotypic explanation (continued): When the F1 plants self-fertilize:
F1 pollen X F1 ovule F2 genotype F2 phenotype
½ D ½ D ½ x ½ = ¼ DD
¼ DD+ ½ Dd = ¾ Tall
½ D ½ d or½ d ½ D
(½ x ½ ) +(½ x ½) = ½ Dd
½ d ½ d ½ x ½ = ¼ dd = ¼ Dwarf
B. Random Segregation
Random segregation can also be demonstrated with a testcrossTestcross: Cross heterozygous F1 individuals with homozygous
recessive
Pollen from Dd X Ovules from dd Testcross progeny
½ D All d ½ x 1 = ½ Dd Tall
½ d All d ½ x 1 = ½ dd Dwarf
C. Independent Assortment
Mendel’s law of independent assortment When the alleles of two different genes separate
during meiosis They do so independently of one another Unless the genes are located on the same
chromosome (linked)
C. Independent Assortment
Independent assortment is demonstrated by a dihybrid crossDihybrid cross: A parental cross between two individuals that differ
in the genotype of two different genes
C. Independent Assortment
Example: Consider genes for vestigial wing shape and ebony body color in Drosophila melanogaster Vestigial wing shape gene:
vg+ allele: normal “wild type” wing shape; dominantvg allele: vestigial wing; recessive
Ebony body color gene:e+ allele: tan-colored “wild type” body; dominante allele: ebony body; recessive
C. Independent Assortment
As usual with complete dominance, there are three possible genotypes for wing shape, and three for body color:
vg+ vg+ = homozygous wild type wingvg+ vg = heterozygous wild type wingvg vg = vestigial winge+ e+ = homozygous wild type body colore+ e = heterozygous wild type body colore e = ebony body color
C. Independent Assortment
P: Homozygous wild type males X Vestigial ebony females
F1: All wild type phenotypes, males & femalesF1 X F1
F2: 9/16 wild type phenotypes3/16 wild type wings, ebony body
3/16 vestigial wings, wild type body1/16 vestigial ebony
C. Independent AssortmentGenotypic explanation for the dihybrid cross P generation:
vg+ vg+ e+ e+ males X vg vg e e females F1 generation:
All heterozygous vg+ vg e+ e , males and femalesF1 sperm F1 ova¼ vg+ e+ ¼ vg+ e+
¼ vg+ e ¼ vg+ e
¼ vg e+ ¼ vg e+
¼ vg e ¼ vg e
C. Independent Assortment
How many different ways can we make wild type wing, wild type body color in the F2?
F1 sperm F1 ova
¼ vg+ e+ ¼ vg+ e+
¼ vg+ e ¼ vg+ e
¼ vg e+ ¼ vg e+
¼ vg e ¼ vg eAnswer: 9 different ways
C. Independent Assortment
How many different ways can we make wild type wing, ebony body color in the F2?
F1 sperm F1 ova
¼ vg+ e+ ¼ vg+ e+
¼ vg+ e ¼ vg+ e
¼ vg e+ ¼ vg e+
¼ vg e ¼ vg eAnswer: 3 different ways
C. Independent Assortment
How many different ways can we make vestigial wing, wild type body color in the F2?
F1 sperm F1 ova
¼ vg+ e+ ¼ vg+ e+
¼ vg+ e ¼ vg+ e
¼ vg e+ ¼ vg e+
¼ vg e ¼ vg eAnswer: 3 different ways
C. Independent Assortment
How many different ways can we make vestigial wing, ebony body color in the F2?
F1 sperm F1 ova
¼ vg+ e+ ¼ vg+ e+
¼ vg+ e ¼ vg+ e
¼ vg e+ ¼ vg e+
¼ vg e ¼ vg eAnswer: 1 way
Summary of All Possible F2 genotypes
¼ x ¼ = 1/16 vg+ vg+ e+ e+
9/16
Wild Wing, Wild Body
(¼ x ¼) + (¼ x ¼ ) = 2/16 vg+ vg e+ e+
(¼ x ¼) + (¼ x ¼ ) = 2/16 vg+ vg+ e+ e
(¼ x ¼) + (¼ x ¼ ) + (¼ x ¼) + (¼ x ¼ ) = 4/16 vg+ vg e+ e
¼ x ¼ = 1/16 vg+ vg+ e e 3/16
Wild Wing, Ebony
(¼ x ¼) + (¼ x ¼ ) = 2/16 vg+ vg e e
¼ x ¼ = 1/16 vg vg e+ e+ 3/16
Vestigial, Wild Body
(¼ x ¼) + (¼ x ¼ ) = 2/16 vg vg e+ e
¼ x ¼ = 1/16 vg vg e e 1/16 Vestigial,
Ebony
C. Independent AssortmentHere is a “shortcut” for dihybrid cross ratios: combine the monohybrid cross ratios!
F2 wing phenotypes: F2 body phenotypes:¾ wild type wings ¾ wild type body¼ vestigial wings ¼ ebony body
¾ x ¾ = 9/16 wild wings, wild body¾ x ¼ = 3/16 wild wings, ebony body¼ x ¾ = 3/16 vestigial wings, wild body¼ x ¼ = 1/16 vestigial wings, ebony body
C. Independent Assortment
The testcross can also be applied to independent assortment:
vg+ vg e+ e X vg vg e e
¼ vg+ vg e+ e (wild wing, wild body)¼ vg+ vg e e (wild wing, ebony body)
¼ vg vg e+ e (vestigial wing, wild body)¼ vg vg e e (vestigial wing, ebony body)
D. Assortment vs. LinkageIndependent assortment works because the two genes are located on separate homologous chromosomes pairsTheir alleles assort independently during meiosis
D. Assortment vs. Linkage
D. Assortment vs. LinkageIf two genes are located on the same chromosome, their alleles can recombine only when there is crossing over during meiosisThe probability that crossover will occur is proportional to the distance between the genesTypically, there are fewer recombinant (crossover) gametes than nonrecombinant gametes
D. Assortment vs. Linkage
E. Dominance Relationships
Codominance Two alleles are codominant if each encodes a
different but functional protein product In the heterozygote, the presence of two different
functional proteins means that the phenotype of the heterozygote is different from either homozygous dominant or homozygous recessive
Example: M-N blood groups
E. Dominance Relationships
Example of codiminance: M-N blood group gene in humans Two alleles, LM & LN
Each produces a “functional” blood cell antigen (capable of causing an immunological reaction)
Three possible genotypes & phenotypes• LM LM: Produces group “M” blood• LM LN: Produces group “MN” blood• LN LN: Produces group “N” blood
Incomplete dominance An incompletely dominant allele produces a
functional protein product However, in the heterozygote, there is insufficient
protein production from the allele to produce the same phenotype as homozygous dominant
Therefore, the phenotype of the heterozygote is different from either homozygous dominant or homozygous recessive
Example: snapdragon flower color
E. Dominance Relationships
Example of incomplete dominance: snapdragon flower color Two alleles, “R” and “r” “R” produces red pigment; “r” produces no pigment Three possible genotypes & phenotypes
• RR: Red flowers• Rr: Pink flowers (One copy of “R” produces
less red pigment than two copies of “R”)• rr: White flowers
E. Dominance Relationships
Because each genotype has a unique phenotype, the F2 phenotypic ratio in codominance or incomplete dominance is 1:2:1
E. Dominance Relationships