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Bell Work
Independent variable?Dependent variable?
Mendel and Meiosis
Chapter 10
What do you see?
Mendel’s Laws of HeredityHow Genetics Began – Gregor Mendel
considered the founder of Genetics• Heredity – the passing on of characteristics from
parents to offspring• Traits – characteristics that are inherited• Genetics - the branch of biology that studies heredity
Link
Mendel’s Laws of Heredity• Sexual Reproduction – male and female sex
cells called gametes fuse (fertilization) to produce a fertilized cell called a zygote which then develops into a seed.
The transfer of pollen from a male reproductive structure to a female structure in a plant is called pollination
What is happening here?
Making a “cross” Cross pollination: transferring pollen from one plant to another with different traits.
The Inheritance of Traits• Mendel used true-breeding pea plants to study 7 traits• Original plants = Parent (P)• Offspring = F1 generation• Offspring of parents with
different forms of a trait = hybrid• Offspring of F1 = F2 generation
What is the F2 ratio?
How many traits being studied?
Crossing yellow seeded andGreen seeded plants:
Monohybrid Cross – the two parents of a cross differed by a single trait.What would a dihybrid cross be?
Bell Work
Identify the independent variables and dependent variables of an experiment based on this diagram.
Flower lab
Bell Work
How did he explain his results?• Different forms of a trait are called alleles• Each trait controlled by two alleles– Dominant form – appears in F1. Represented as a
capital letter.– Recessive form – hidden in F1. Represented as a
lower case letter.– In F2 generation, the ratio is 3:1
dominant to recessive
How does dominance work?
• Homozygous – both alleles for a trait are the same.
• Heterozygous – alleles for the trait are different. Only dominant trait can be seen.
Y – yellow y- greenYY Yy yY yy
Dominant or Recessive?? ?
Genotype versus Phenotype
• Genotype – an organism’s actual allele pair.• Phenotype – expression of an allele pair, or
the way it looks or behaves.
Y – yellow y- greenYY Yy yY yy
Homozygous or HeterozygousAA, Bb, cC, dd, EE, Ff,
gg, HH, Ii, KK, mm
Dominant or Recessive
A, b, c, D, E, f, G, h, J, K, m, n
B – brown, b blue eyesBB
Genotype Phenotype
B – brown, b blue eyesBb
Genotype Phenotype
B – brown, b blue eyesbb
Genotype Phenotype
B – brown, b blue eyes4 Offspring: BB, Bb, Bb, bb
Genotype Phenotype
B – brown, b blue eyes4 Offspring: Bb, Bb, bb, bb
Genotype Phenotype
Bell Work
• Bell Work
Law of Segregation• The law of segregation states that different alleles
for the same trait separate when gametes are formed. Thus, a mother that is heterozygous for brown eyes (Bb) could pass either a dominant brown allele (B) or a recessive blue allele (b) for eye color to her offspring.T t
Sperm (gamete)
Egg (gamete)
Law of Independent Assortment• The law of independent assortment states that
when pairs of alleles separate, they do so independently of each other. Thus, the alleles for hair color and the alleles for eye color in humans are not inherited together.
Punnett Squares
• Developed to predict possible offspring of a cross between two known genotypes.
• Predict genotype and phenotype of offspring.
Genotypic ratio?Phenotypic ratio?
Monohybrid Cross• When parents with different forms of a gene (trait) are
crossed, the offspring (F1) are heterozygous organisms called hybrids.
• A cross involving hybrids for a single trait is called a monohybrid cross. Their offspring (F2) will have a phenotypic ratio of 3:1
Bell Work
Bell Work
How many chromosomes does each body cell of a fruit fly have?
How many do you think a sex cell would have?
Bell work
Chapter 10
Section 2: Meiosis
Meiosis
• Organisms have tens of thousands of genes that determine individual traits.
• How are they organized? Where?
Diploid and Haploid
• In body cells (somatic), chromosomes occur in pairs
– one from each parent.• Diploid: a cell with two
of each kind of chromosome– All body cells (somatic cells)– Called 2n
• Gametes (sex cells) contain oneof each kind of chromosome– Called haploid– 1n
Fertilization• The fusion of gametes (sperm and egg) forms a zygote
(fertilized egg). This is called fertilization.
n=23egg
sperm n=23
2n=46zygote
Chromosomes – Bell Work• If an organism has the Diploid number (2n) it has two matching
homologues per set. One of the homologues comes from the mother (and has the mother’s DNA).… the other homologue comes from the father (and has the father’s DNA).
• Most organisms are diploid. Humans have 23 sets (pairs) of chromosomes.
1. What is the diploid number of chromosomes for humans?2. What is the haploid number of chromosomes for humans?
Bell Work
Haploid-Diploid Practice
Homologous Chromosomes
23
Homologue
Homologue
Eye color locus
• Pair of chromosomes (maternal and paternal) that are similar in shape and size and normally contain identical arrangements of genes.
• Each locus (position of a gene) is in the same position on homologues.
• Humans have 23 pairs of homologous chromosomes.– 22 pairs of autosomes– 1 pair of sex chromosomes
Why Meiosis?• What if mitosis was the only means of cell division? • Division of a cell results in 2 identical daughter cells. If cell is
2n, what are daughter cells? • If two 2n cells join, what is the resulting zygote?
2n=46egg
sperm 2n=46
4n=?zygote
Why Meiosis?• Need a way to produce offspring with the same number of
chromosomes as the parents.• Requires cell division that produces ½ the number of
chromosomes as a parent’s body (somatic) cell.
n=23egg
sperm n=23
2n=46zygote
Sperm + Ovum (egg) Zygotefertilization
Sexual Reproduction-production and fusion of haploid cells
The Zygote then develops by mitosis into a multicellular organism
Meiosis is the process by which ”gametes” (sex cells) , with half the number of chromosomes, are
produced.
During Meiosis a diploid cell is reduced to 4 haploid cells called sex cells (eggs (ova) and sperm). These are the gametes.
1 Diploid (2n) cell 4 Haploid (n) cells
Meiosis is two cell divisions (called meiosis I and meiosis II)
with only one duplication of chromosomes.
Tetrads
Paternal Maternal
eye color locus
eye color locus
hair color locus
hair color locus
In meiosis, homologous chromosomes line up, gene by gene forming a four-part structure called a tetrad. Each tetrad consists of two homologous chromosomes, each made up of two sister chromatids.
Bell Work
H is dominant for hairy and h is recessive for nonhairy knuckles.
1. HH Is this homozygous dominant, homozygous recessive or heterozygous?
2. A cross between two heterozygous individuals will produce what genotype? What phenotype?
2n = 4
2n = 4
2n = 4
Sister chromatids
tetrad
Sister chromatids
n=2
Interphase I
• Similar to mitosis interphase.
• Chromosomes replicate (S phase).
• Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres.
Meiosis I (four phases)
• Cell division that reduces the chromosome number by one-half.
• four phases:a. prophase Ib. metaphase Ic. anaphase Id. telophase I
We need another division, each chromosome is still doubled!
Prophase I - Synapsis
Homologous chromosomes
sister chromatids sister chromatidsTetrad
Prophase I
• Longest and most complex phase.• 90% of the meiotic process is spent in Prophase I
• Chromosomes condense.
• Synapsis occurs: homologous chromosomes come together to form a tetrad.
• Tetrad is two chromosomes or four chromatids (sister and nonsister chromatids).
During Prophase I “Crossing Over” occurs.
• During Crossing over segments of nonsister chromatids break and reattach to the other chromatid. This creates
variation (diversity) in the offspring’s traits.
nonsister chromatids
chiasmata: site of crossing over variation
Tetrad
Metaphase I• Can line up differently, results in diversity
metaphase plate
OR
metaphase plate
Telophase I• Each pole now has haploid set of chromosomes.• Cytokinesis occurs and two haploid daughter cells
are formed.
Question:
• A cell containing 20 chromosomes (diploid) at the beginning of meiosis would, at its completion,
produce cells containing how many chromosomes?
Answer:
• 10 chromosomes (haploid)
Question:
• A cell containing 40 chromatids at the beginning of meiosis would, at its completion, produce cells
containing how many chromosomes?
Answer:
• 10 chromosomes
Meiosis II
• No interphase II (or very short - no more DNA replication)
• Remember: Meiosis II is similar to mitosis
Prophase II
• same as prophase in mitosis
Metaphase II
• same as metaphase in mitosis
metaphase platemetaphase plate
Anaphase II
• same as anaphase in mitosis• sister chromatids separate
Telophase II
• Same as telophase in mitosis.
• Nuclei form.
• Cytokinesis occurs.
• Remember: four haploid daughter cells produced.
gametes = sperm or egg
Telophase II
Bell Work
Non-disjunction• Non-disjunction is the failure of homologous chromosomes, or sister chromatids, to separate
during meiosis.• Non-disjunction results with the production of
zygotes with abnormal chromosome numbers
Non-disjunctions usually occur in one of two fashions.
• The first is called Monosomy, with only one chromosome. The second is called Trisomy,
with 3 chromosomes.
Non-disjunction
Common Non-disjunction Disorders
• Down’s Syndrome – Trisomy 21• Turner’s Syndrome – Monosomy 23 (X)• Kleinfelter’s Syndrome – Trisomy 23 (XXY)• Edward’s Syndrome – Trisomy 18
Amniocentesis• An Amniocentesis is a procedure a pregnant woman can have
in order to detect some genetics disorders…..such as non-disjunction.
Karyotype(picture of an individual’s chromosomes)
One of the ways to analyze the
amniocentesis is to make a Karyotype
What genetic disorder does this karyotype
show?Trisomy 21….Down’s
Syndrome
13
4
5
2
Gene linkage and Maps• Linked genes: those located close together on the
same chromosome that are usually inherited together.– How could genes on the same chromosome become
separated?
• Crossing over produces new gene combinations and can be used to make chromosome maps
Thomas Hunt Morgan