Meiosis

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Learning Objectives

Compare and contrast sexual and asexual reproduction.
Compare and contrast mitosis and meiosis.
Understand the difference between homologous chromosomes and sister chromatids.
Understand the principle of segregation.
Understand the principle of independent assortment.
Understand why linked genes do not follow the principle of independent assortment.
Understand the origin of all genetic variability (mutations).
Compare and contrast dominance and incomplete dominance.

Solutions

Sexual reproduction (meiosis) produces gamete haploid cells. Here, a haploid egg and haploid sperm cell combine to form a zygote, a diploid cell. This cell has 2 sets of (homologous) chromosomes, one from each parent. Asexual reproduction (mitosis) forms 2 identical daughter cells.
Both go through prophase, metaphase, anaphase, and telophase. Mitosis cycles through this only once, and produced 2 identical daughter diploid cells. Meiosis cycles through this twice, and produces 4 individual/distinct daughter haploid cells.
Homologous chromosomes appears during the first round of PMAT. Here, crossing over can happen and chiasmata can form, which is the precursor to genetic variability in the end cells. The pre- and post-PMAT 1 cells are diploids. Sister chromotids appear during the second round of PMAT. When they are split into 2 cells, diploid cells turn into haploid cells.
The principle of segregation is demonstrated in monohybrid crosses. Alleles "segregate" from one another during the formation of daughter cells. 1 allele comes from each parent, and each allele can be homozygous dominant, heterozygous, or homozygous recessive.
The principle of independent assortment is demonstrated in dihybrid crosses. Genes controlling 2 or more traits "segregate" independently of each other.
Linked genes don't follow independent assortment because their loci are too close together, and thus are less likely to cross over when chiasmata form in PMAT 1.
The origin of all genetic variability are mutations. Mutations are changes in the DNA sequence. They can be caused by ultraviolet light, ionizing radiation, and certain chemicals. Somatic mutations occur in body cells, while germ line mutations occur in tissues that will produce sex cells. They can be benign, neutral, negative, or positive. Point mutation include deletions, duplication, inversions, or translocations. Changes in chromosome number often occur due to mistakes in chromosome pairing and separation. In plants, this often results in a higher yield. Aneuploid have 1 more or 1 less chromosome. Poluploid have more than 1 extra, complete pair of chromosomes, and are critical in the history of genetic diversity.
For complete dominance, if the dominant allele is present in the genotype, the phenotype will appear 100+ dominant, no matter if it's homozygous dominant or heterozygous. For incomplete dominance, if the genotype is heterozygous, the phenotype will appear somewhere between dominant and recessive. For instance, if the dominant phenotype is red and the recessive phenotype is white, the heterozygous phenotype will be some sort of pink.

Notes

Introduction

Asexual reproduction makes 2 genetically identical daughter cells
Sexual reproduction makes 4 genetically unique daughter gamete cells

Egg and sperm combine to form zygote

Cells have 2 sets of chromosomes

Called homologous chromosomes
Gets one set from each parent
They are identical in:

length
amount of DNA
types of genes
location of centromere

DNA sequence is NOT always identical

The Phases of Meiosis

During S-phase (interphase):

Homologous chromosomes held together by a centromere
2 homologous chromosomes, each chromosome has 2 chromatids, and each matching pair of a chromosome are called sister chromatids

Meiosis cycles through PMAT (prophase, metaphase, anaphase, and telophase) twice

Meiosis I: homologous chromosomes separate
Meiosis II: sister chromatids separate

PMAT

Prophase = condense
Metaphase = align
Anaphase = separate
Telophase = de-condense
This cycles twice in meiosis

Prophase I

Homologous chromosome condense (coil) and align in homologous pairs

Each has 4 chromatids
Technically the grandparents of the zygote

"Crossing Over": chiasmata form

DNA can break and be repaired with the chance of mixing up some of the gene
Exchange of DNA in homologous pairs possble because they are aligned in close homologous pairs

ONLY line in up meiosis I
Crossing over only between homologous pairs, otherwise it's a mutation

Note About Plant Life Cycle

Plants have a haplodiplontic life cycle

Gametes NOT the direct result of a meiotic devision
Diploid sporophyte cells undergo meiosis to produce haplod spores
Each spre goes through mitotic devisions to yield a multicellular, haploid gametophyte
Mitotic divisions within gametophyte required to produce the gametes

In meiosis, start with diploid and end with four haploid spores

Mendelian Genetics Part I

Principle of segregation

Allele: alternative forms of a gene
Two alleles of each gene

One from each parent

Locus: position of a gene on a chromosome

Gregor Mendel crossed tall and short pea plants in the 1860's
Three generations:

Parental (P)

Crossed a parent with itself (selfing)
All offspring were tall

First filial generation (F₁)

Crossed two different parents (one tall, one short)
All offspring were tall

Second filial generation (F₂)

Selfed an F₁ plant
Offspring in 3:1 tall to short ratio (3 tall to every 1 short)

Selfing: self-pollination

Many plants have male AND female parts
Inbreeding tolerated much better in plants than in animals

For any given pair of alleles, one (dominant) may mask the expression of the other (recessive)
Example: yellow and green pods

Green allele from both parents → green pods
Yellow allele from both parents → yellow pods
Yellow allele from one parent green allele from the other → yellow pods

Yellow allele (dominant) "hides" the green allele (recessive)

Phenotype: organism's physical appearance
Genotype: genetic information responsible for the phenotype

Homozygous: alleles identical
Heterozygous: alleles different

Monohybrid Cross

1:2:1 genotypic ration
3:1 phenotypic ratio
Talking about gametes, not spores
Punnet squares account for all possible combinations of egg and sperm and how often we would expect to see them based on starting material

	R	r
R	RR	Rr
r	Rr	rr

Mendelian Genetics Part II

Test cross: cross between a plant having a dominant phenotype with a homozygous recessive plant

Will determine whether plant with dominant phenotype is homozygous or heterozygous
G? × gg

If all are green, then original plant must have been GG (plants will all be Gg)
If half are green and half are yellow, then original plant must have been Gg (plants will be 1:1 Gg:gg)

Principle of independent assortment:

Genes controlling two or more traits segregate independently of each other
To calculate the number of possible gametes: 2ⁿ where n = number of homologous pairs
- Humans = 2²³ = 8,388,608
Unlinked genes: genes on different chromosomes
Linked genes: genes on the same chromosome; do not segregate independently

Dihybrid Cross: Tracking Two Traits

Start with parents differing in 2 traits

F₁ plants produce four types of gametes
Sixteen possible outcomes
3:1 ratio for each expression

Independent

	RD	Rd	rD	rd
RD	RRDD	RRDd	RrDD	RrDd
Rd	RRDd	RRdd	RrDd	Rrdd
rD	RrDD	RrDd	rrDD	rrDd
rd	RrDd	Rrdd	rrDd	rrdd

Linkage and Mapping

Linked genes: genes close to each other on a chromosome

The closer the genes are to one another, the more likely they are to be inherited together
Crossing-over is more likely between two genes located far apart on the chromosome than between two genes located closer together

Example: aB / D & Ab / D
A & B are more likely to be inherited together

Linkage means that ratios we just looked at on the dihybrid cross will get skewed if genes are inherited together

Mendelian Genetics Part III

Incomplete dominance (absense of dominance)

Heterozygote phenotype is in between the two homozygotes
Genotypic and phenotype ratios match in monohybrid

	B	b
B	BB	Bb
b	Bb	bb

Threshold Effet

Why are some species' pigments incompletely dominant?
Differences in the proteins that code for the enzyme
Differences between dominant and incomplete dominant is the threshold

Example: dominant codes for red, recessive codes for white
When the amount of pigment needed is low, homozygous dominant is red, heterozygous is red, and homozygous recessive is white
When the amount of pigment needed is higher, homozygous dominant is red, heterozygous is pink, and homozygous recessive is white

Alleles are for genes, and the genes code for protein. Sometimes that's sufficient for expression

Quantitative Traits

A range of phenotypes rather than discrete phenotypes
Traits are due to multiple genes
Environment also influences the expression

RRRR = dark red
RRRr = medium red
RRrr = light red
Rrrr = pink
rrrr = white

Note: location of R or r doesn't matter; only ratio of R:r does

Includes traits like fruit yield and days to flowering
Mendelian: one gene = one trait

Mutation

A change in DNA sequence
Mutagens can be:

UV light
Ionizing radiation
Certain chemicals

DNA repair enzymes can often find and correct damage
Somatic mutation: occurs in the body cell
Germ line mutation: occurs in the tissues that will produce sex cells

Passed on to future generations

All genetic variability in DNA due to mutations
Can be benign, neutral, or negative
Some greatly increase evolutionary fitness

Types of Mutations

Point mutations: change in a single base pair
Changes in chromosome structure

Deletions
Duplications
Inversions
Translocation

Piece of DNA breaks off and is moved to another chromosome

All of these can be bad for the cell, or could cause a change in the regulation of genes, duplication can lead to good or bad outcomes- source of new genetic diversity
Changes in chromosome number

Due to mistake during chromosome pairing and separation
Often larger or have higher yield
Aneuploid: extra of missing chromosomes
Polyploid: has at least one complete extra set of chromosomes

Meiosis fails to halve chromosome number, resulting in 2n gametes (as opposed to the usual 1n)
Happens often, and is critical in history for genetic diversity
Can be done artificially