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

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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

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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

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How many chromosomes does each body cell of a fruit fly have?

How many do you think a sex cell would have?

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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?

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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.

Cell Cycle and Mitosis Review

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

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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

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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