Transcript Document

Genetics
Chapter 16
Traits, Genes, Alleles
• Genetics
– Study of genes and ways they are inherited
• Genes
– Internal factors
– Provide instructions to plant cells
• How to grow and develop
• How to respond to environment
• Characters
– Traits such as flower shape and color, stem length,
leaf shape and arrangement, fruit type, seed shape
Gregor Mendel
• Discovered basic principles of genetics
• Used garden peas as experimental
organism
• Published report in 1866
– Work rediscovered around 1900
Gregor Mendel
• Reasoned that factors for characters came
in more than one form
• Trait
– Variant
• Genes
– Factors
• Alleles
– Alternate forms of a gene
DNA Code Reflected in Traits
• Genes
– Sequences of nucleotides in DNA
– Watson and Crick provided valuable
information when they discovered helical
structure of DNA
• Recognized how sequence of bases in DNA
molecule act as code to specify sequence of amino
acids in protein
DNA Code Reflected in Traits
• Mutation
– Changes in base sequence of DNA
– Raw material for evolution through natural
selection
• Altered DNA passed from mutant organism to its
progeny
• Mutation spreads through population
• Population evolves
Comparison of Wild-type Form and
Mutant Form in Corn (Zea mays)
Wild-type Form
Alternate (Mutant) Form
Name of Gene
Filled endosperm
Shrunken endosperm
(lacks sucrose synthase)
sh
Yellow endosperm
White endosperm
y
Colored (red) endosperm Yellow endosperm
R
Normal endosperm
Waxy endosperm
(altered starchsynthesizing enzyme)
wx
Dormant seed
Viviparous (germinates
on cob)
Vp
Has isocitrate
dehydrogenase
Lacks isocitrate
dehydrogenase
idh
Comparison of Wild-type Form and
Mutant Form in Garden Peas
(Pisum sativum)
Wild-type Form
Alternate (Mutant) Form
Name of Gene
Yellow cotyledons
Green cotyledons
I
Red flower petals
White flower petals
A1
Smooth seed surface
Wrinkled seed surface
R
Tall (more than 20
internodes)
Short (10-20 internodes)
T
Green foliage
Yellow-green foliage
O
Axillary flowers
Terminal flowers
Fa
Straight pod
Curved pod
Cp
Tendrils
No tendrils
N
Chromosomes
• DNA in nucleus combined with proteins to form
chromatin
• Chromatin divided into chromosomes
– Each chromosome is single linear strand of DNA
• Gene
– Has particular position (locus) on chromosome
– Portion of DNA composed of between 300 and 3,000
bases
– Separated from adjacent genes by stretches of DNA
thought to be nonfunctional
Chromosomes
• Genome
– All the genes in an organism
• Number of chromosomes in plant varies
with species
– Smallest number in plant is four
– Coast redwood and some ferns have several
hundred
Chromosomes
• Mitosis
– Every allele found in original cell will also be
present in all the cells of plant
• Vegetative reproduction
– Also mitotic division
– Progeny have same alleles as parent plants
Meiosis
• Mechanism that offers greater genetic
variety than mutation alone
• Union of haploid egg and sperm result in
diploid zygote
Meiosis
• Carries out two rounds of cell division
– Meiosis I
• Converts original 2n cell to two 1n cells with
different combinations of parental genes
– Meiosis II
• Mitotic division that separates sister chromatids
and converts two 1n (haploid) cells to four 1n
(haploid) cells
Meiosis I
Stage
Description
Prophase I
Chromatids condense, synapsis occurs  each homologous
pair of chromosomes comes together, pairing makes it easy
for cell to divide in a way that it produces haploid cells,
crossing over  allow homologous chromosomes to trade
segments, synapsis and crossing over give chromosomes
new combinations of parental genes, spindle forms
Metaphase I
Pause for checking for missing links between chromosomes
and spindle, chromosome pairs move to cell’s equator
Anaphase I
Spindle pulls each chromosome with its two sister chromatids
to one of the poles
Creates new nuclear envelopes, cells divide into 2 haploid
Telophase I and
cells, each cell has different combination of parental genes,
Cytokinesis
each chromosome still has two sister chromatids
Meiosis II
Stage
Description
Prophase II
No synapsis, no crossing over, each cell forms new
spindle that links each sister chromatid of each
chromosome
Metaphase II
Chromosome moved separately to equator, cell pauses
to check for spindle linkage
Anaphase II
In each cell, spindle pulls the two sister chromatids of
each chromosome to opposite poles
Telophase II and
Cytokinesis
Each cell divides into two cells, each cell is haploid with
different combinations of parental genes
Meiosis
• Special events of prophase I
– Synapsis
• Homologous chromosomes come together to form
pairs
– Crossing over
• Chromatids of homologous chromosomes may
exchange corresponding pieces with each other
• Cross formed by chromatids during exchange 
chiasma
• Results in rearranged chromatids with fragments
from both of the homologous chromosomes
Meiosis
– Recombination
• New combinations of alleles resulting from
crossing over
Key Terms for Understanding
Genetics
• Phenotype
– Visible traits of an organism
• Genotype
– Collection of alleles of an organism
• Homozygous
– Two copies of the same allele
– Example: TT or tt
• Heterozygous
– Different alleles of a gene
– Example: Tt
Key Terms for Understanding
Genetics
• Dominant
– Will be expressed (Tt or TT) condition
– Overshadows recessive allele
• Recessive
– Expressed only in homozygous condition (tt)
• Codominant or incompletely dominant
– Plant shows trait that is intermediate between those
of parents
– Example: cross between red and white flowers yields
progeny with pink flowers
Punnett Square
• Named for professor who popularized it
• Way to keep track of combinations of
alleles formed during fertilization
• Shows expected genotypes of progeny
• Also shows probability of expected
genotypes
Gamete Possibilities
•
•
•
•
•
•
•
TT  T
Tt  T, t
tt  t
TTRR  TR
TtRR  TR, tR
TTRr  TR, Tr
TtRr  TR, Tr, tR, tr
TT
T
T
TtRr
TR
Tr
tR
tr
Cross Involving Single Gene
Egg
T
T
t
Tt
Tt
t
Tt
Tt
Pollen
T = tall t = dwarf
In a cross between a homozygous tall plant and a homozygous dwarf
plant, all the progeny will be heterozygous tall plants.
Cross Involving Single Gene
Egg
T
t
T
TT
Tt
t
Tt
tt
Pollen
T = tall t = dwarf
When two heterozygous tall plants are crossed, the expected progeny are as
follows:  homozygous tall,  heterozygous tall,  dwarf. The genotypic ratio
is 1:2:1 and the phenotypic ratio is 3:1.
Codominance or Incomplete
Dominance
Egg
R
R
r
Rr
Rr
r
Rr
Rr
Pollen
RR = red flower
rr = white flower Rr = pink flower
Crossing a homozygous red flower and a white flower results in progeny with
all pink flowers (Rr).
Codominance or Incomplete
Dominance
Egg
R
r
R
RR
Rr
r
Rr
rr
Pollen
RR = red flower
rr = white flower Rr = pink flower
A cross between two pink flowers yields expected progeny as follows:  red
flowers (RR),  pink flowers (Rr),  white flowers (rr). The expected
genotypic ratio is 1:2:1 and the expected phenotypic ratio is 1:2:1.
Cross Involving Two Genes
• Overall pattern determined by
combinations of alleles of several genes
• Cross involving two genes
– Dihybrid cross
Dihybrid Cross
Egg
TR
Pollen
tr
TTRR = tall plant with round seeds
TtRr
ttrr = dwarf plant with wrinkled seeds
A cross between a homozygous tall plant with round seeds and a dwarf
plant with wrinkled seeds yields progeny that are predicted to all be tall
plants with round seeds.
Dihybrid Cross
Egg
TR
Tr
tR
tr
TR
TTRR
TTRr
TtRR
TtRr
Tr
TTRr
TTrr
TtRr
Ttrr
tR
TtRR
TtRr
ttRR
ttRr
tr
TtRr
Ttrr
ttRr
ttrr
Pollen
T = tall plant t = dwarf plant
R = round seeds r = wrinkled seeds
In a cross between two heterozygous tall plants with round seeds (TtRr x TtRr), the
expected phenotypic ration of the progeny is 9:3:3:1.
9 tall plants with round seeds
3 tall plants with wrinkled seeds
3 dwarf plants with round seeds
1dwarf plant with wrinkled seeds
Test Cross
• Used to determine genotype of organism
with dominant phenotype
• For example, tall plant could have
genotype TT or Tt
• Cross plant in question with recessive
plant in order to determine genotype
• Examine phenotypes of progeny to
determine genotype of parent with
dominant phenotype.
Test Cross
Egg
Egg
T
T
Pollen
T
t
Pollen
t
Tt
Tt
t
Tt
tt
t
Tt
Tt
t
Tt
tt
If all the progeny from the cross are
tall plants, then the organism in
question was homozygous (TT).
If there are any dwarf plants that
result from the cross, then the
organism in question was
heterozygous (Tt). The expected
genotypic ratio in this case is 1:1 and
the phenotypic ratio is 1:1.
Linked Traits
• If genes migrate as a unit rather than
independently during gamete formation, then
genes are described as linked
• Crossing over may result in formation of
recombinant combinations of linked alleles
– Farther apart two genes are, the more likely crossing
over will occur between them
– No matter how far apart the two genes are, the
fraction of gametes with recombinant combinations is
never greater than the fraction of gametes with
parental combinations of alleles (never greater than
50%)
Maternal Inheritance Involving
Organellar Chromosomes
• Most genes in plant cell located in nuclear
chromosomes
• Also chromosomes of DNA in plastids and
mitochondria
– Organellar chromosomes
• Smaller than nuclear chromosomes
• Do contain genes which can mutate
Maternal Inheritance Involving
Organellar Chromosomes
• During fertilization, only chloroplasts and
mitochondria from egg are incorporated
into zygote
• Chloroplasts and mitochondria from sperm
cells either do not enter egg or degenerate
during fertilization
• All chloroplast and mitochondria genes in
zygote come from egg and all alleles of
these genes show maternal inheritance
Plant Breeding
• Earth’s population keeps growing
• Acres of land under cultivation has decreased
• Food production/person is at least as great as it
was in the 1950s
• Results mainly due to breeding of new, more
productive plants
– Easier to grow or harvest
– Resistance to disease or stress
– Edible parts that are more attractive or nutritious
Mating Plants Combines Useful
Traits
• Example: want to breed tomato variety
that is fungus resistant
– Mate successful but fungus-susceptible
commercial variety with wild variety that
shows fungus resistance
– Progeny will be resistant but probably have
inedible fruit
– Mate progeny with the commercial variety
(back cross) and test progeny of mating for
resistance
Mating Plants Combines Useful
Traits
– By chance, some of resistant progeny will
have acquired genes needed for edible fruit
– Most resistant progeny mated again with
commercial variety and most resistant
progeny again selected
– After several cycles, strain of commercial fruit
with resistance to fungus results
Multiple Genes
• Some traits vary continuously within a
certain range
– Size of harvested organ, sugar content,
firmness of fruit
• Factors that lead to continuous variation
– Involvement of multiple genes
• Individually have small effect on phenotype
• Collectively combine to provide wide range of
variation
Multiple Alleles
– Sometimes gene has multiple alleles
• Each has different degree of activity
• Increases the number of possible phenotypic forms
– Environmental effects may alter form of
phenotype
• Randomness of environmental effects tends to blur
distinction among genotypes
Heterosis
• Hybrid vigor
– Progeny from mating two inbred (highly
homozygous) strains
• Larger and healthier than parents
– Special problem
• Hybrids do not breed true  produce both
heterozygous and homozygous progeny
Heterosis
– Solution
• Produce new hybrid plants through vegetative
reproduction
• Produce hybrid seed by mating two homozygous
strains
– Develop strains that are male-sterile (do not produce
anthers or viable pollen)
Polyploidy
• Refers to having more than two sets of
chromosomes
• May occur spontaneously
– Cell replicates DNA and separates chromatids
but fails to complete cell division
• Commonly occurs in last stages of
development of tracheary elements and
storage tissues
• Less common in meristem
Polyploidy
• If polyploid plant fertilizes itself, progeny
will also be polyploid
• Polyploid plants often larger and more
vigorous than parental types
• Polyploids seem more tolerant of
environmental stresses such as short, cool
growing seasons, aridity, or high
temperatures
Polyploidy
• Polyploidy can also result from
interbreeding between different species of
plants
– Original progeny sterile
– Can become fertile if cells become polyploid