segregation, assortment, and dominance relationships

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


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


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


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


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


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


Phenotype: The appearance or discernible characteristics of a

trait in an individual Phenotypes can be determined by a combination of

genetic and environmental factors


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


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


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


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


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


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)


Independent assortment is demonstrated by a dihybrid crossDihybrid cross: A parental cross between two individuals that differ

in the genotype of two different genes


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


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


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


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


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+



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+



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


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


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


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


Because each genotype has a unique phenotype, the F2 phenotypic ratio in codominance or incomplete dominance is 1:2:1


segregation, assortment, and dominance relationships

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pea plant height gene

d allele

homozygous dominant

deach individual pea

responsiblemost genes

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enzymethe recessive