Plant Breeding and Genetics

Modes of reprocution

Sexual and asexual

Complete flower

includes all four floral organs

Incomplete flower

lack one or more floral organs.

Perfect flowers

bisexual flowers containing both the male and female parts.

Imperfect flower

unisexual

Monoecious plants

bear the pistillate and staminate flowers on the same plant

Dioecious plants

bear the pistillate and staminate flowers on different plants.

Autogamy

Self-pollination. Transfer of pollen from anther to stigma within the same flower

Why did self-fertilization evolve?

One possible reason is a lack of mates on the fringes of adaptation. Selfing provides reproductive assurance. About 10-15% of flowering plants predominantly self fertilize.

Cleistogamy

Flowers do not open and pollination occurs in closed bud

Chasmogamous

flowers may still shed pollen before flower opens

Mechanisms promoting autogamy

Cleistogamy

Allogamy

Cross-pollination. The union of sperm and eggs from different plants.

Mechanisms promoting allogamy

●Mechanical obstruction to self pollination
●Flower structures that promote cross pollination

Ploidy

refers to the number of copies of the entire chromosome set in a cell of an individual.

x

the number of chromosomes in a basic set

n

the haploid number. The number of chromosomes that occurs in gametes

2n

number of chromosomes that occurs in somatic cells.

Diploid example

●Humans: x \= 23
●Somatic cells are diploid

Allohexaploid example

●Wheat: x \= 7
●Somatic cells are hexaploid

male gametophyte development

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Microsporogenesis

formation of free microspores

Microgametogenesis

formation of male gametophytes

Mature pollen characteristics

•Surface consists of a complex of proteins

Pollination

•Transfer of pollen to stigma surface.

Success depends upon temperature

Megasporogenesis

process by which a single cell in megasporangium (area of tissue in ovule). The megasporangium undergoes meiosis to produce four megaspores. Only one of the four survives.

megagametogenesis

the surviving megaspore undergoes three rounds of mitosis to produce eight nuclei in inside the megagametophyte or embryo sac.

Endosperm in monocots

3n (triploid)
prominent in mature seeds. Mainly starch that feeds the growing embryo during germination.

Endosperm in dicots

Consumed by the embryo (2n

Seed coat

maternal tissue and forms from the ovule integuments

Vegetative Propagation

Form of asexual propagation
Stolons
Rhizomes
Bulbs
Tubers
Also

Clones

A group of plants that have been propagated from a single plant

Key points of vegetative reproduction

●A group of plants that have been propagated vegetatively from a single plant constitute a clone. They are genetically identical barring somatic mutations.
●A key advantage

Micropropagation

In vitro clonal propagation tissue culture of plants. It can also be called "tissue culture".

Apomixis

The asexual formation of a seed from the maternal tissues of the ovule

Pseudogamy

fertilization required for endosperm development.

Parthenogenesis

development of embryo without pollen.

Mechanisms of apomixis

1. Generation of a cell capable of forming an embryo without prior meiosis (apomeiosis)
2. Spontaneous

Sporophytic

Type of apomixis
Unreduced cell directly to embryo

Gametophytic

Type of apomixis
Unreduced cells become a megagametophyte
Two types: Diplospory and Apospory

Apospory

Type of gametophytic apomixis
from cell of the nucleus (sexual megagametophyte typically degenerates)
Example: Kentucky bluegrass

Diplospory

Type of gametophytic apomixis
from megaspore mother cell (no meiosis)
Example: Dandelion

Value of apomixis in agriculture

•Rapid multiplication of superior forms
•Uniformity
•Avoidance of some complications of sexual reproduction

Locus

The specific place on a chromosome where a gene is located.

Allele

One of the different forms of a gene that can exist at a single locus

Genotype

An allelic combination underlying a phenotype.
Y/Y

Monohybrid cross

A cross between two individuals identically heterozygous at one gene pair (e.g.

Dihybrid cross

A cross between two individuals identically heterozygous at two loci (e.g.

Law of equal segregation

during the production of gametes the two alleles at a locus segregate so that offspring receive one allele from each parent.

Law of independent assortment

individual pairs of alleles on different chromosomes assort independently during meiosis.
Later modified due to discovery of linkage

Chi-square test

A statistical test to help determine if observed results agree with those expected (predicted) based on a hypothesis).

Linked chromosomes

The situation which two genes are on the same chromosome as deduced by recombination frequencies less than 50 percent.
A deviation from independent assortment
Parental types high %
Recombinant types low %

Genes on chromosomes

Genes on different chromosomes and genes far apart on the same chromosomes generally produce an equal (1/4) progeny of each genotype
Linked genes are generally close together on chromosomes. Lower cM \= more likely to be linked

Crossing over

When homologous chromosomes pair at meiosis

Homolog

A member of a pair of homologous chromosomes.

Cis confirmation

cis \= adjacent
the two dominant/wild type alleles are present on the same homolog
AKA coupling phase linkage
AB/ab

Trans confirmation

trans \= opposite
the two dominant alleles are on opposite homologs
AKA repulsion phase linkage
Ab/aB

Linkage map

maps of loci on chromosomes based on recombination frequencies

Recombination frequency

frequency of recombinants of two loci produced by crossing over

Genetic map unit/centimorgans (cM)

distance between loci for which one product of meiosis in 100 recomiants
E.g. 10% RF \= 10 genetic map units

Double recombinants

can result in a deviation between sums of distances between close intervening loci and the total distance among more distant loci
Gamete restored to parental type and not counted among recombinants

Interference

a measure of the independence of crossovers from each other
1- coc \= interference

Coefficient of coincidence (coc)

ratio of the observed number of double crossovers to expected number of double crossovers

Genetic markers

represent genetic differences between individual organisms

Molecular markers

reveal variation at the level of DNA
Most widely used in plant breeding applications
SNPs and microsatelites (SSR)

Simple sequence repeats (SSRs)

Large stretches of DNA space between genes
These regions are polymorphic \= lots of differences
Not present at phenotypic level

Intra-locus allelic interactions

variations on dominance
I.e. complete dominance is a form of interaction among alleles at a single locus

Inter-locus allelic interaction

epistasis
Interactions among alleles at different loci to produce phenotype not predictable y average effect of individual alleles

Complete dominance

dominant alelle will be expressed in phenotype when one copy is present (heterozygote). Recessive allele is masked

Incomplete dominance

results in the phenotype of the heterozygote being intermediate to those of the two homozygotes
I.e. White and purple flower \= pink offspring

Codominance

The expression of both alleles of a heterozygote
Product of each allele are different from one another though both are functional
Ex. blood type

Pleiotropy

a single gene influence more than one phenotypic trait
Some traits are negatively correlated

Epistasis

One gene influences the phenotypic expression of another gene

Patterns of different forms of epistasis

Complementary gene action: 9:7
Duplicate gene action: 15:1
Dominant supression: 13:3
Dominant epistasis: 12:3:1
Recessive epistasis: 9:3:4

Complementary gene action

type of epistasis in which homozygosity for recessive alleles at either loci causes the same phenotype
9:7 ratio in F2

Duplicate gene action

A epistatic pattern in which dominant allele at either of two loci produces dominant phenotype
15:1 ratio
aabb only recessive phenotype

Dominant suppression

form of epistasis in which dominant allele at one locus suppresses the effect of a dominant allele at another
13:3 ratio in F2

Dominant epistasis

form of epistasis where one allele may mask the effect of other alleles at two loci
12:3:1 ratio in F2
aabb is mix of two parent phenotype

Recessive epistasis

type of epistasis in which the homozygous recessive genotype at one locus masks the effect of alleles at another
9:3:4 ratio in F2
doggos

Penetrance

genotype fails to produce the phenotype expected from the genotype (nonpenetrant)
Penetrance \= genotype produces expected phenotype

Expressivity

degree of variation of how much an allele is being expressed
Caused be genetic background

Dichogamy

flowering system in which male and female reproductive organs mature at different times within a single flower or across multiple flowers

Protandry

The condition in which the male reproductive organs (stamens) of a flower mature before the female ones (carpels)

Protogyny

The condition of flowers whose female reproductive organs (carpels) mature before the male ones (stamens)