Meiosis transmits chromosomes

Genes are the units of heredity, and are made up of segments of DNA

• Heredity is the transmission of traits from one generation to the next

• Genes are passed to the next generation via reproductive cells called gametes (sperm and eggs) 

• Each gene has a specific location called a locus on a certain chromosome

Most DNA is packaged into chromosomes

In asexual reproduction, a single individual passes genes to its offspring without the fusion of gametes

• A clone is a group of genetically identical individuals from the same parent

In sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents

• Sexual reproduction requires fertilization, the fusion of sperm and egg

  

Human somatic cells (any cell other than a gamete) have 23 pairs of chromosomes

• A karyotype is an ordered display of the pairs of chromosomes from a cell 

• The two chromosomes in each pair are called homologous chromosomes, or homologs

• Chromosomes in a homologous pair are the same length and shape and carry genes controlling the same inherited characters

• A karyotype is an image of a cell’s ordered, metaphase chromosomes

  

• Each pair of homologous chromosomes includes one chromosome from each parent

• The 46 chromosomes in a human somatic cell are two sets of 23: one set from the egg and one set from the sperm

• A diploid cell (2n) has two sets of chromosomes

  • two full sets (or pairs) of chromosomes

  • chromosome pairs differ in sizes, shape, genetic info, centromere location

  • cell contains one set from each parent

  • ex: body cells (skin cells, leaf cells, hypha cell)

• For humans, the diploid number is 46 (2n = 46)

  

The sex chromosomes, which determine the sex of the individual, are called X and Y (for humans)

• Human females have a homologous pair of X chromosomes (XX)

• Human males have one X and one Y chromosome

  • This is a non-homologous pair (or hemizygous)

The remaining 22 pairs of chromosomes are called autosomes

• A gamete (sperm or egg) contains a single set of chromosomes, and is haploid (n)

• haploid cells

  • represented by n

  • cells contain on set of chromosomes

  • ex. gametes, sex cells, (e.g. pollen, egg)

  • two haploid gamete cells come together in sexual reproduction to produce a diploid cell

• For humans, the haploid number is 23 (n = 23)

  

• Each gamete’s set of 23 consists of 22 autosomes and a single sex chromosome

• In an unfertilized egg (ovum), the sex chromosome is X

• In a sperm cell, the sex chromosome may be either X or Y

human homologous chromosomes

• In a cell in which DNA synthesis has occurred, each chromosome is replicated

• Each replicated chromosome consists of two identical sister chromatids

  

human reproductive cycle

• Fertilization is the union of gametes (the sperm and the egg)

  • The fertilized egg is called a zygote and has one set of chromosomes from each parent 

  • The zygote produces somatic cells by mitosis and develops into an adult

  At sexual maturity, the ovaries and testes produce haploid gametes

  • Gametes are the only types of human cells produced by meiosis, rather than mitosis

  • Meiosis results in one set of chromosomes in each gamete (half the number of chromosomes as the parent cell)

    • the diploid parent cell produces four haploid daughter cells, sex cells

    • Fertilization and meiosis alternate in sexual life cycles to maintain chromosome number

    
    

meiosis reduces chromosome number

• Like mitosis, meiosis is preceded by the replication of chromosomes

• Meiosis takes place in two sets of cell divisions, called meiosis I and meiosis II

• The two cell divisions result in four daughter cells, rather than the two daughter cells in mitosis

• Each daughter cell has only half as many chromosomes as the parent cell

  

• After chromosomes duplicate, two divisions follow

  • Meiosis I (reductional division) where homologs pair up and separate, resulting in two haploid daughter cells with replicated chromosomes

  • Meiosis II (equational division) sister chromatids separate

• The result is four haploid daughter cells with unreplicated chromosomes

• Meiosis I is preceded by interphase, when the chromosomes are duplicated to form sister chromatids

• The sister chromatids are genetically identical and joined at the centromere

• The single centrosome replicates, forming two centrosomes

• Division in meiosis I occurs in four phases

  • Prophase I

    • Prophase I typically occupies more than 90% of the time required for meiosis

    • Chromosomes begin to condense

      • DNA coils into visible duplicated (or double) chromosomes made up of sister chromatids

    • In synapsis, homologous chromosomes loosely pair up, aligned gene by gene, forming a tetrad

      • double chromosomes pair up based on size, shape, centromere location, genetic information

    • While closely aligned, crossing over occurs wherein nonsister chromatids exchange DNA segments

    • Each tetrad usually has one or more chiasmata, X-shaped regions where crossing over occurred

    • nuclear envelope begins to disappear

    • fibers begin to form

  • Metaphase I

    • In metaphase I, tetrads line up at the metaphase plate, with one chromosome facing each pole

      • double chromosomes remain in pairs

      • fibers align pairs across the center of the cell

    • Microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad

    • Microtubules from the other pole are attached to the kinetochore of the other chromosome

  • Anaphase I

    • In anaphase I, pairs of homologous chromosomes separate

      • fibers separate chromosome pairs

    • One chromosome moves toward each pole, guided by the spindle apparatus

      • each double chromosome, from the pair, migrates to opposite sides of the cell

    • Sister chromatids remain attached at the centromere and move as one unit toward the pole

  • Telophase I and cytokinesis

    • In the beginning of telophase I, each half of the cell has a haploid set of chromosomes; each chromosome still consists of two sister chromatids

    • Cytokinesis usually occurs simultaneously, forming two haploid daughter cells

    • nuclear envelope reappears and establishes two separate nuclei

    • each nucleus contains only one double chromosome from each pair

      • nucleus only contains half of the total information the parent nucleus contained

    • cytokinesis will separate the cell into two daughter cells

    

• Meiosis II

  Division in meiosis II also occurs in four phases

  • Prophase II: spindle forms and chromosomes migrate to the metaphase plate

    • nuclear envelope begins to disappear

    • fibers begin to form

  • Metaphase II: genetically distinct sister chromatids align on the metaphase plate, a spindle attaches to each sister chromatid

    • fibers align double chromosomes across the center of the cell

  • Anaphase II: sister chromatids separate and migrate toward opposite poles as spindle fibers shorten

    • fibers separate sister chromatids

    • chromatids (single chromosomes) migrate to opposite sides of the cell

  • Telophase II and cytokinesis: new haploid nuclei form as chromosomes decondense

    • nuclear envelope reappears and establishes separate nuclei

    • each nucleus contains single chromosomes

    • chromosomes begin to uncoil

    • cytokinesis will separate the two cells into four daughter cells

    • daughter cells are haploid and genetically different from each other and the parent cell

  Meiosis II is very similar to mitosis

  

products of meiosis and mitosis

• At the end of meiosis, there are four daughter cells, each with a haploid set of unreplicated chromosomes

• Each daughter cell is genetically distinct from the others and from the parent cell

• Mitosis conserves the number of chromosome sets, producing cells that are genetically identical to the parent cell

• Meiosis reduces the number of chromosomes sets from two (diploid) to one (haploid), producing cells that differ genetically from each other and from the parent cell

• both mitosis and meiosis…

  • involve…

    • nuclear envelope disappearing

    • DNA coiling into chromosomes

    • aligning chromosomes in the center of the cell

    • using fibers to separate chromosomes

    • nuclear envelope reappearing

    • chromosomes uncoiling

    • followed by cytokinesis and production of daughter cells

• mitosis and meiosis differ in the number of resulting cells and the genetic content of the cells

  • mitosis produces two daughter cells that are genetically identical to the parent cell

  • meiosis produces four haploid daughter cells that are genetically varied from each other and the parent cell

• 

unique properties of meiosis

• Three events are unique to meiosis, and all three occur in meiosis l

  • Synapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange genetic information

  • At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes

  • At anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate

increasing genetic variation

• The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation

• Three mechanisms contribute to genetic variation

  • Independent assortment of chromosomes

    • With independent assortment, chromosomes randomly align on the metaphase plate; Therefore, alleles on different chromosomes are inherited independently of one another

    • random assortment of chromosomes - the order of the homologous pairs during metaphase 1 affects which chromosomes end up in which gamete

      

  • Crossing over

    • Crossing over produces recombinant chromosomes, which combine DNA inherited from each parent

    • Crossing over begins very early in prophase I, as homologous chromosomes pair up gene by gene

    • In crossing over, homologous portions of two nonsister chromatids trade places

      • nonsister chromatids of double homologous chromosomes exchange segments

      • homologous chromosomes - carry info for the same genes, one from each parent

    • Crossing over contributes to genetic variation by combining DNA from two parents into a single chromosome

      

  • Random fertilization

    • Random fertilization adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg)

      • fertilization is random in that any gamete can contribute to the diploid nature of the genomes in offspring; this increases the potential for genetic diversity

    • The fusion of two gametes (each with 8.4 million possible chromosome combinations from independent assortment) produces a zygote with any of about 70 trillion diploid combinations

    • Crossing over adds even more variation

    • Each zygote has a unique genetic identity

connection through common ancestry

• Several lines of genetic evidence support common ancestry of all life on Earth

  • all organisms use nucleic acids to store and transmit genetic information

• All cells on Earth utilize ribosomes for protein synthesis

• Several different genes are widely conserved access all domains of life

  • Examples include:

    • Active-transport proteins

    • The small ribosomal subunit

    • Glycolysis is an almost universal process

  

introduction to genetics

• Gregor Mendel’s breeding experiments on garden peas was summarized in his paper in 1865, Experiments in Plant Hybridization

• His experiments tracked the inheritance of seven traits that appear in an either/or fashion

• By following offspring of these plants for several generations, he was able to establish principles of inheritance

  

• mendel’s procedure

1. Mendel removed one flower’s stamen (the pollen/sperm producing part) leaving behind the carpel (the ovule/egg containing part)

2. Mendel transferred pollen of the other phenotype to the first flower

3. Hybrid seeds were produced by the ovule-containing flower

4. Pea seeds containing hybrid DNA matured in pods and were planted

The next generation of pea plants grew, displaying only purple flowers

useful genetic vocabulary

• By mating plants of a certain type repeatedly, he created lineages that were true-breeding 

  • Plants that produce offspring of the same variety when they self-pollinate

• In a typical experiment, Mendel mated two contrasting, true-breeding varieties, a process called hybridization

  • The true-breeding parents are the P generation

• The hybrid offspring of the P generation are called the F₁ generation

• When F₁ individuals self-pollinate or cross-pollinate with other F₁ hybrids, the F₂ generation is produced

  

• An allele is a version of a gene and typically represented with an uppercase or lowercase letter

  • Diploid organisms have two alleles for a trait at a particular locus on a homologous pair

• An organism’s phenotype is its appearance, which is due to its genotype, or genetic composition

  • Dominant alleles typically present themselves in the phenotype when in combination with the other allele and are symbolized with an uppercase letter

  • Recessive alleles are only phenotypically observable when an organism has two recessive alleles at that locus, and are symbolized with a lowercase letter

    

• A homozygous genotype has two of the same alleles while a heterozygous genotype has two different alleles

  

principal’s of inheritance

1. The Law of Segregation

   Mendel substantiated that when hybrid individuals were crossed, their alleles segregated, or separated, into different gametes

   This consistently produced a 3:1phenotypic ratio in the F₂ generation

   1. explains the separation of alleles during gamete formation

      1. each gamete carries only one allele for each gene therefore each gamete receives only one parental allele

      2. segregation of parental alleles into gametes provides opportunity for more varied combinations of alleles when fertilization occurs

2. The Law of Independent Assortment

   Mendel derived the law of segregation by following a single character

   • The F₁ offspring produced in this cross were monohybrids, individuals that are heterozygous for one character

     • A cross between such heterozygotes is called a monohybrid cross

   Mendel identified his second law of inheritance by following two characters at the same time

   • Crossing two true-breeding parents differing in two characters produces dihybrids in the F₁ generation, heterozygous for both characters

   A dihybrid cross, a cross between F₁ dihybrids, can determine whether two characters are transmitted to offspring as a package or independently

   1. independent assortment suggests that genes for two or more traits will be sorted into gametes independently; genes are not linked

      1. inheritance of each gene is random and not connected to inheritance of any other gene

      2. assortment of genes independently into gametes provides more possible gene combinations when fertilization occurs

         

using a testcross

• How can we tell the genotype of an individual with the dominant phenotype?

  • Such an individual could be either homozygous dominant or heterozygous

• The answer is to carry out a testcross: breeding the mystery individual with a homozygous recessive individual

  • If any offspring display the recessive phenotype, the mystery parent must be heterozygous

rules of probability

• The multiplication rule states that the probability that two or more independent events will occur together is the product of their individual probabilities

• The addition rule states that the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities

• The rule of addition can be used to figure out the probability that an F₂ plant from a monohybrid cross will be heterozygous rather than homozygous

• Example: If two trihybrid organisms are crossed (AaBbCc x AaBbCc), what is the probability of having AAbbCc offspring?

Fertilization restores the diploid chromosome number of the species as two haploid gametes fuse

• Human sperm are n = 23 and human eggs are n = 23, there fertilization results in a 46-chromosome human zygote

• A zygote is a fertilized egg, prior to any cell division

• Fertilization results in increased genetic variation, as the products of meiosis possess genetic diversity, and fertilization of any egg by a sperm cell is due to chance

  

Many patterns of inheritance do not follow simple Mendelian predictions, or complete dominance

• In incomplete dominance, the phenotype of F₁ hybrids is somewhere between the phenotypes of the two parental varieties

• In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways

  

• Some traits have more than two allelic forms, known as multiple alleles

• For example, the  four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: I^A, I^B, and i.

• The enzyme encoded by the I^A allele adds the A carbohydrate, whereas the enzyme encoded by the I^B allele adds the B carbohydrate; the enzyme encoded by the i allele adds neither

  

• Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype

• Skin color in humans is an example of polygenic inheritance

  

observing inheritance over generations

• A pedigree chart is a family tree that describes the interrelationships of parents and children across generations

• Inheritance patterns of particular traits can be traced and described using pedigrees

  • Squares represent males, circles represent females and shading represents “affected”

• If an individual possesses a mutated allele but is not affected, they are a carrier

  

patterns of inheritance

• Alleles occur on autosomes or sex chromosomes and will affect how they manifest in phenotypes

• Traits may be

  • Autosomal, X-linked, or Y-linked

  • Dominant or recessive

• Traits that are autosomal recessive occur on autosomes and require two mutated alleles to appear in the phenotype

  

• Traits that are autosomal dominant occur on autosomes and require one mutated allele to appear in the phenotype

  

• Traits that occur on sex chromosomes are sex-linked and can be on either the X or Y chromosome

• X-linkage describes an allele on the X chromosome

  • X-linked recessive traits require two mutated X chromosomes in females, or one mutated X chromosome in the male, to appear in the phenotype (X^aX^a, X^aY are affected)

  • X-linked dominant traits require at least one mutated X to appear in the phenotype (X^AX^a, X^AX^A, X^AY are affected)  

• Y-linkage describes an allele on the Y chromosome. 

  • A mutated Y confers the affected phenotype (XY^a)

  

non-nuclear inheritance

• Mitochondrial and chloroplast inheritance are due to mutated alleles in mitochondrial or chloroplast DNA

  • chloroplast and mitochondria contain their own non-nuclear genome

  • chloroplast and mitochondria are randomly assorted to gametes and daughter cells during cell division

  • mitochondria are transmitted to the egg and not sperm in animals

    • such traits are maternally inherited

• The egg in animals supplies all the mitochondria of the zygote - any sperm mitochondria are destroyed during fertilization

• The ovule in plants supplies all the mitochondria and chloroplasts of the zygote - any mitochondria and chloroplasts in the pollen’s sperm are destroyed during fertilization

  
  

exceptions to mendelian inheritance (non-mendelian genetics)

• The behavior of chromosomes during meiosis can account for Mendel’s laws of segregation and independent assortment

  

• The first solid evidence associating a specific gene with a specific chromosome came from Thomas Hunt Morgan, an embryologist

• Morgan’s experiments with fruit flies provided convincing evidence that chromosomes are the location of Mendel’s heritable factors

• Morgan noted wild type, or normal, phenotypes that were common in the fly populations

• Traits alternative to the wild type are called mutant phenotypes

  

• In one experiment, Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type)

  • The F₁ generation all had red eyes

  • The F₂ generation showed the 3:1 red:white eye ratio, but only males had white eyes

• Morgan determined that the white-eyed mutant allele must be located on the X chromosome

• Morgan’s finding supported the chromosome theory of inheritance and substantiated sex-linked genes

  • traits that are determined by genes located on sex chromosomes are known as sex-linked traits

  • sex chromosomes are nonhomologous

    

chromosomal basis of sex

• In humans and some other animals, there is a chromosomal basis of sex determination

  • In humans and other mammals, there are two varieties of sex chromosomes: a larger X chromosome and a smaller Y chromosome

  • Only the ends of the Y chromosome have regions that are homologous with corresponding regions of the X chromosome

  • The SRY gene on the Y chromosome codes for a protein that directs the development of male anatomical features

    • Females are XX, and males are XY

      

    • Each ovum contains an X chromosome, while a sperm may contain either an X or a Y chromosome

    • Other animals have different methods of sex determination

      

• Morgan performed further research and showed that genes located on the same chromosome that tend to be inherited together 

  • These are called linked genes

    • genes that are adjacent and close to one another on the same chromosome and that are inherited together

    • less likely to be separated during crossing over in meiosis

• Gene linkage is an exception to Mendelian independent assortment

  • Independent assortment asserts that different pairs of alleles are inherited independently of each other

• Morgan crossed a dihybrid fruit fly with a double mutant

  • The expected ratio was 1:1:1:1 or four phenotypes in equal proportion, based on independent assortment

  • The observed data did not match the predicted data

    

gene linkage

• Morgan found that body color and wing size are usually inherited together in specific combinations (parental phenotypes) 

  • He noted that these genes do not assort independently, and reasoned that they were on the same chromosome

  • However, nonparental or recombinant phenotypes were also produced

    • A 50% frequency of recombination is observed for any two genes on different chromosomes

  • Morgan discovered that genes can be linked, but the linkage was incomplete, because some recombinant phenotypes were observed

  • He proposed that some process must occasionally break the physical connection between genes on the same chromosome

    • That mechanism was the crossing over of homologous chromosomes

    

• Recombinant chromosomes bring alleles together in new combinations in gametes

• Random fertilization increases even further the number of variant combinations that can be produced

  • This abundance of genetic variation is the raw material upon which natural selection works

gene mapping EMAILLL

map distance

• tells you how close together a pair of linked genes is

• determined by how frequently a pair of genes participates in a single crossover event

• linked genes have a recombination frequency of less than 50%

• 

• Alfred Sturtevant, one of Morgan’s students, constructed a genetic map, an ordered list of the genetic loci along a particular chromosome

  • Sturtevant predicted that the farther apart two genes are, the higher the probability that a crossover/chiasma will occur between them and therefore the higher the recombination frequency

  • A linkage map is a genetic map of a chromosome based on recombination frequencies

  • Distances between genes can be expressed as map units; one map unit, or centimorgan, represents a 1% recombination frequency

  • Map units indicate relative distance and order, not precise locations of genes

    

• Genes that are far apart on the same chromosome can have a recombination frequency near 50%

• Such genes are physically linked, but genetically unlinked, and behave as if found on different chromosomes

  

alternatives to mendelian inheritance

• Incomplete Dominance

• Codominance

• Multiple alleles

• Polygenic inheritance

• Gene linkage

• Sex-linkage

• Non-nuclear traits

• Phenotypic plasticity

Phenotypic plasticity is the ability of individual genotypes to produce different phenotypes when exposed to different environmental conditions

chromosomal effects

• Chromosomal inheritance is a source of genetic variation

  • Segregation, independent assortment and random fertilization create new combinations of alleles

  • Nondisjunction between homologs during meiosis can cause mutation

    

• the chromosomal basis of inheritance provides an understanding of gene transmission

  • certain genetic disorders can be caused by a single mutated allele or a specific chromosomal change that is passed from parents to offspring

  • parent to offspring inheritance can be analyzed to determine patterns of gene transmission

  • mutations or mis-formations in gametes can result in disorders being present in offspring that were not present in parents