Transcript Mendel and Meiosis
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Mendel and Meiosis
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Mendel: An Austrian Monk
Why offspring resemble their parents?
Pea plants to study inheritance of characteristics.
Traits transferred.
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Genetic Terms
Heredity
: The passing of characteristics from parents to offspring.
Genetics
: Branch of biology that studies heredity.
Traits
: Characteristics that are inherited.
Gametes
: Sex Cells (two distinct cells –male and female)
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Plant Fertilization
Male gamete is pollen (produced by anther) Female gamete is ovule (produced in the pistil)
Pollination
: The transfer of male pollen grains to the pistil of a flower.
Fertilization
: The uniting of male and female gametes. Occurs when male gamete in the pollen grain meets and fuses with the female gamete in the ovule.
After the ovule gamete is fertilized, it matures into a seed.
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Genetics Terms
Hybrid- Offspring of crosses between parents with different traits Purebred- Organisms that carry only one variation of a characteristic.
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Mendel’s Experiment: Why Pea Plants?
Reproduce sexually (sex cells) Male and female gametes are in same flower Reproductive parts are enclosed tightly together.
Reproduce by self-pollination (Male and Female coming from same plant) Mendel could manipulate
Cross-Pollination
: Breed-or-cross, one plant with another – Mendel opened petals and removed anthers – Dusted the pistil with pollen from plant he wanted to cross with – Covered with bag
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Mendel a careful researcher
Controlled experiments and peas One trait at a time Analyzed data mathematically Used plants that were tall for many generations (true breeding for tallness). Used plants that were short for many generations (true breeding for shortness.)
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Mendel’s Monohybrid Cross: Mendel crossed tall and short plants to produce new plants.
Hybrids
: Offspring of parents that have different forms of a trait. (short and tall).
Mono
(one): 1 st experiments monohybrids Each parent plants differed by one single trait (height).
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Mendel’s Monohybrid Cross
(P 1 =“parents”) first generation (F 1 =“filial”) Result: All tall pea plants F 1 (first generation) Self-pollinate – 3/4 th ’s Tall – 1/4 th Short F 2 (second generation)
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Crosses
P 1 generation “P” parent —original parents F 1 generation —Offspring of parents “F” filial—son or daughter F 2 generation —Cross 2 from F1 generation.
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Trait patterns Mendel Observed
In every case, he found that one trait of a pair seemed to disappear in the F 1 generation, only to reappear unchanged in ¼ of the F 2 plants.
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Role of Unit factors
Two factors control each traits Genes located on chromosomes.
Different forms of genes are called
ALLELES
.
Example: Alleles for height Two alleles for tallness Two alleles for shortness One allele for tallness and one for shortness Alleles are located on different copies of the chromosomes
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The Rule of Dominance
Dominant trait
: Observed trait – Masks recessive
(Example: Mendel’s F 1 —All tall plants/tall allele is dominant trait)
Recessive trait
: Trait hidden by dominant trait
(Example: Mendel’s F 1 —All tall plants/short allele that reappears in F 2 recessive trait) is
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Allele Shorthand
– Same letter for different alleles – Upper case for dominant allele – Lower case for recessive allele – Dominant written first Example: Allele for tallness = T Allele for shortness = t Tt
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Law of Segregation (Mendelian)
Each organism has two different alleles, it can produce two different types of gametes. During fertilization, male and female gametes randomly pair to produce four combinations of alleles.
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Phenotype
Greek words
phainein
, meaning “to show,” and
typos
, meaning “model.” The visible characteristics (appearance and behavior) of an organism makes up it phenotype.
Example: Round, Wrinkled Yellow, Green Brunette, Blonde Blue Eyes, Brown Eyes
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Genotype
From the Greek words
gen
or
geno
, meaning “race,” and
typos
, meaning “model.” The genetic characteristics of an organism make up its genotype.
Example:
Genotype TT
.
of a tall plant that has two alleles for tallness is
Genotype
of a tall plant that has one allele for tallness and one allele for shortness is
Tt
.
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Homozygous Trait for two alleles is the same
Example: Two alleles for tallness (TT) homozygous dominant Two alleles for shortness (tt) — homozygous recessive.
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Heterozygous
Two different alleles for one trait.
Tt
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Punnett Squares.
(1905) Reginald Punnett, an English biologist, created way to expected proportions of possible genotypes in the offspring of cross-Punnett Square.
Know the genotypes of the parents, you can use a Punnett square to predict the possible genotypes of their offspring.
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Making a Punnett Square
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Monohybrid Crosses
Mendel’s cross Tt x Tt Half the gametes of each parent would contain the T allele, and the other half would contain the t allele.
Gametes that each parent forms Tt x Tt Other Parent
It doesn’t matter which set of gametes are on top and which are on the side.
One parent
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Determining Phenotypes (characteristics)
If organism has at least one dominant allele, dominant trait will be expressed.
(TT, BB, Tt, Bb ) For the recessive trait to be expressed, organism must lack dominant allele and have two recessive alleles. (bb or tt) Of the offspring ¼ will be homozygous dominant (TT/BB) 2/4 or ½ will be heterozygous (Tt, tT, Bb, bB) and ¼ will be homozygous recessive.
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Mendel’s Dihybrid Crosses
Di
means “two.” Dihybrid cross: A cross involving two different traits.
Mendel did true-breed dihybrid cross between round yellow seeds (RRYY) and wrinkled green seeds (rryy) Mendel’s results of dihybrid cross: – P 1 round yellow x wrinkled green – F 1 – F 2 All round yellow (round yellow dominant) 9 round yellow, 3 round green, 3 wrinkled yellow, 1 wrinkled green. Mendel found 2 dominate traits for round and yellow seeds.
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Mendel’s dihybrid cross
Dihybrid cross led to Mendel’s Law of Independent Assortment.
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Dihybrid Crosses
Think of the RRYY x rryy cross (Round Yellow x Wrinkle Green seeds) Mendel found that seed shape and seed color would be inherited independently of each other.
Punnett square you will need four boxes on each side.
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Dihybrid Cross
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Law of Independent Assortment
Mendelian principle stating that genes for different traits are inherited independently of each other.
Example: Genotype RrYy produces gametes: Rr will separate Yy will separate Recombine in 4 different ways.
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Dihybrid Cross
Punnett squares are good for showing all the possible combinations of gametes and the likelihood that each will occur. However, you don’t get the exact ratio of results shown in the square.
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Probability
Genetics follows the rules of chance.
Probability or chance that an event will occur can be determined by dividing the number of desired outcomes by the total number of possible outcomes.
desired # of outcomes / total # of possible outcomes (Example: Toss a coin the probability of getting heads would be one in two chances, written 1:2 or ½. A Punnetts square can be used to determine the probability of the event.
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Probability
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Meiosis
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Genes
Tens of thousands of genes Lined up on chromosomes
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Chromosomes
Occur in pairs (Male, Female)
DIPLOID
—A cell with two of each kind of chromosome is said to be diploid, or 2
n
, number of chromosomes
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Gametes
Male (sperm) and Female (egg) Contain one of each kind of chromosomes.
A cell with one of each kind of chromosome is called a
HAPLOID
and is said to contain a haploid, or
n
, number of chromosomes.
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Chromosome Numbers of Some Common Organisms
Organism Fruit Fly Garden Pea Corn Tomato Leopard Frog Apple Human Chimpanzee Dog Adder’s tongue fern Body Cell (2n) 8 14 20 24 26 34 46 48 78 1260 Gamete (n) 4 7 10 12 13 17 23 24 39 630
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Homologous Chromosomes
Pair chromosomes are called
homologous chromosomes — determine phenotype
.
Gene for same trait – same order, – chromosomes in a homologous pair are not always identical.
(Chromosome 4 contains 3 traits Mendel Studied)
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Meiosis
From the Greek word
meioun
, meaning “to diminish”. Cell division that results in a gamete containing half the number of chromosomes of its parents.
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Meiosis
Divisions: Meiosis I and Meiosis II Begins with one diploid (2
n
) cell four haploid (
n
) cells. Sex cells (gametes) haploid.
Sperm fertilizes an egg-results in zygote (diploid) Zygote develops by MITOSIS into a multi cellular organism.
Reproduction
—Production and subsequent fusion of haploid sex cells.
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Interphase
Chromosomes replicate Chromosome – two identical sister chromatids held together by a centromere
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Prophase I
Chromosomes coil up and a spindle forms.
Homologous chromosomes comes together, matched gene by gene, to form a four-part structure called a tetrad.
Chromatid pair so tight that sometimes non-sister chromatids from homologous chromosomes sometimes exchange genetic material in a process known as crossing over.
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Crossing Over
Exchange of genetic material Any location Several locations at once Humans-Two to three crossovers for each pair of homologous chromosomes.
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Metaphase I
Centromere attaches to a spindle fiber Spindle fibers pull the tetrads into the middle, or equator, of the spindle.
Chromosomes are lined up side by side as tetrads.
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Anaphase I
Chromosomes separate and move to opposite ends of the cell. Centromeres holding the sister chromatids together do not split like they do in anaphase of mitosis.
Ensures that each new cell have only one chromosome from each homologous pair.
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Telophase I
Spindle is broken down Chromosomes uncoil Cytoplasm divides
new cells.
2 Half of genetic information of original cell (one chromosome from each homologous pair) Another cell division needed
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Meiosis II
Newly formed cells go through short interphase (chromosomes don’t replicate) Prophase II —Spindle forms in each of the two new cells and the spindle fibers attach to the chromosomes.
Metaphase II —The chromosomes, still made up of sister chromatids, are pulled to the center of the cell and line up randomly at the equator.
Anaphase II —Centromere of each chromosome splits, allowing sister chromatids to separate and move to opposite poles.
Telophase II —Nuclei reform, spindles break down, and cytoplasm divides.
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Meiosis Results
Four haploid sex cells have been formed from one original diploid cell.
Each haploid cell contains one chromosome from each homologous pair.
Haploid cells become gametes, transmitting the genes they contain to offspring.
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Genetic Recombination
Gene combinations vary based on how chromosomes lines up during metaphase I (random).
As number of chromosome increase the number of gene combinations increase.
Example: Pea Plants (7
n
) 2 7 = 128 x 128 =16,384 different offspring Sperm Egg Human (23
n
) 2 23 = 8 million x 8 million =70 trillion possible zygotes.
Reassortment of chromosomes and the genetic information they carry, either by crossing over or by independent segregation of homologous chromosomes is called
GENETIC RECOMBINATION
.
Provides genetic variation
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Mistakes in Meiosis
Nondisjunction (failure of homologous chromosomes to separate) results in gametes with either an extra chromosome or a missing chromosome.
Extra chromosomes often survive; those lacking one or more usually do not.
TRISOMY —Extra Chromosome e.g.
—Extra chromosome 21—Down’s Syndrome
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Mistakes in Meiosis Continued…
MONOSOMY
—When a gamete with a missing chromosomes fuses with a normal gamete during fertilization, resulting zygote lacking chromosome.
Most zygotes don’t survive; if do organisms generally does not.
Turner syndrome —human females with only one X chromosome.
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Mistakes in Meiosis
Lack of separation chromosomes- Gamete inherits a complete diploid set of chromosomes.
Triploid
—gamete with extra set of chromosome is fertilized by a normal haploid gamete, resulting offspring has a set of three chromosomes.
Tetraploid
—Fusion of two gametes, each with an extra set of chromosomes, produces offspring with four sets of chromosomes.
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Polyploidy
Organisms with more than the usual number of chromosome sets are called
POLYPLOIDY
.
Animals almost always cause death in zygotes Plants happens often; Larger and healthier. Great commercial value.
Plant breeders artificially produce polyploidy plants using chemicals that cause non disjunction.