Genetics Chapter 13. Traits, Genes, Alleles Genetics –Study of genes and ways they are inherited Genes –Internal factors –Provide instructions to plant.

Genetics

Chapter 13

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 endospermShrunken endosperm (lacks sucrose synthase)

sh

Yellow endosperm White endosperm y

Colored (red) endosperm Yellow endosperm R

Normal endospermWaxy endosperm (altered starch-synthesizing enzyme)

wx

Dormant seedViviparous (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 IPause for checking for missing links between chromosomes and spindle, chromosome pairs move to cell’s equator

Anaphase ISpindle pulls each chromosome with its two sister chromatids to one of the poles

Telophase I and Cytokinesis

Creates new nuclear envelopes, cells divide into 2 haploid cells, each cell has different combination of parental genes, each chromosome still has two sister chromatids

Meiosis II

Stage Description

Prophase IINo synapsis, no crossing over, each cell forms new spindle that links each sister chromatid of each chromosome

Metaphase IIChromosome moved separately to equator, cell pauses to check for spindle linkage

Anaphase IIIn 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

T T

t Tt Tt

t Tt Tt

Egg

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

T t

T TT Tt

t Tt tt

Egg

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

R R

r Rr Rr

r Rr Rr

Egg

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

R r

R RR Rr

r Rr rr

Egg

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

TR

tr TtRr

Egg

Pollen

TTRR = tall plant with round seeds 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

TR Tr tR tr

TR TTRR TTRr TtRR TtRr

Tr TTRr TTrr TtRr Ttrr

tR TtRR TtRr ttRR ttRr

tr TtRr Ttrr ttRr ttrr

Egg

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

T T

t Tt Tt

t Tt Tt

T t

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.

Egg Egg

Pollen Pollen

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