Mendel & Meiosis

Chapter 6

Mendelian Genetics

• What is this?

• Branch of genetics that deals with simple dominant/recessive traits based on the work of Gregor Mendel

• Ex: Height of a pea plant is tall (TT or Tt) or short (tt)

• Exceptions – We will get to this later…

R- a dominant allele that codes for muscles that help in tongue rolling

r- is a recessive allele that does not code for that muscle

This boy has at least one dominant allele in his 2 letter genotype.

His phenotype is that he is a tongue roller

Example Trait = Tongue Rolling

GeneticsPre-Mendel Theory = BLENDING

• Pre Mendel, theory of inheritance = qualities of the parents blended to form the qualities of the child

• Ex: tall and short parent = medium height child

Theory did NOT explain examples like:– two brown-eyed parents giving

birth to a blue-eyed baby• Because of Mendel's work

there became a consistent theory of heredity = GENETICS

Pre-Mendel Theory – Humunculous

The Origins of Genetics• Gregor Mendel (1822-1884)

– “Father of Genetics”– Czechoslovakian– cross pollinated pea plants to see the

outcomes– Noticed “atypical” characteristics

• Figure 10.3

Gregor's life story

Actual garden “sweet pea” flowers

Cross Pollination

Mendel mixed the pollen from a white flowered pea plant to a purple flowered pea plant pistil, and the results were…..

Mendel’s Experiment

• Generation 1– Purple x White = Purple (no white)

(P1 Parents) (F1 offspring)

Next, he mixed F1 plants with other F1 plants, and the results were…

What does this mean?

• Generation 2– F1 x F1 = Mostly purple, some white

(P2 Parents) (F2)

Out of 929 flowers, 705 = purple, 224 = whiteThat’s a 3:1 RATIO!

• Mendel called these “new” plants as hybrids

Genotype vs. PhenotypeWhat do these terms mean?

Mendel’s Conclusion• “Each organism has 2 factors that control each of

its traits” (now factors called alleles = types of a gene)

• Dominant Trait– A trait that dominates (masks/covers up) a recessive

one; written w/ Capital letter

• Recessive Trait– Can’t be expressed (seen) if a dominant trait is

present; written w/ lower case letter

Ex: Purple Flower = PWhite Flower = p

For every trait…• There are 2 alleles (types of genes)

– One from female parent, one from male parent– Process of meiosis allows for this

• Thus:– Purple flower = PP

• HOMOZYGOUS DOMINANT = pure dominant

– White flower = pp• HOMOZYGOUS RECESSIVE = pure recessive

• What about Pp?– HETEROZYGOUS

• “Hybrid”; carrier

Mendel’s Laws (Principles)• The Principle of Unit Characters states that individuals

pass information on as individual traits.

• The Principle of Dominance states that some unit characters (genes) can mask the expression of others.

• The Principle of Segregation states that each unit character (gene) separates into a different sex cell.

• The Principle of Independent Assortment** states that genes segregate according to chance; different genes separate INDEPENDENTLY of each other. *Dihybrids

Alleles = alternative versions of a gene

Let’s go back and look at the genes (alleles) in Mendel’s

Flowers• Parent Generation (P1) = PP x pp

– Each parent donate only 1 gene to F1

– Thus, all offspring here must be…• Pp

• Next (P2) he crossed F1 x F1 = Pp x Pp• In the F2 generation he received a ratio of 3:1

purple phenotype to white phenotype flowers• A punnett square predicts this ratio also

• Punnett Squares – using Mendel’s Work

• Gamete possibilities from the parents are written on the outside

• Each allele gets a separate box

•This is a MONOHYBRID cross

•It looks at only ONE trait

• Tt x Tt

• Possible genotypes are placed inside the boxes by mixing alleles from the parents

• All POSSIBLE combinations are shown here

G ratio - ____:____:____

HD Het HR

P ratio - _____ : _____

D R

You Try!

• A heterozygous purple flower pea plant is crossed with a white flower pea plant.

• What are the genotypic and phenotypic ratios of the F1 generation?

A Test Cross

• Used to determine an unknown genotype of parents

• (Works backwards)

• ALWAYS CROSS UNKNOWN WITH RECESSIVE PHENOTYPE – Why?

What Mendel Saw…

• How did he know which were dominant & which were recessive?

Dihybrid Crosses• The PROBABILITY of inheritance for 2

GENES for different traits can be calculated…

• Ex: Mendel crossed a round, yellow pea plant (RrYy) with another round, yellow pea plant (RrYy).

• How many of each gene can each parent give?

• What are the possible outcomes?

How to solve a dihybrid• 1. Draw a 16 box punnett square

• 2. Write out ALL possible gamete combinations - FOIL

• 3. Fill in the boxes

• 4. Interpret the phenotypic ratio*

• D/D: D/R: R/D: R/R

Meiosis vs. Mitosis WS

• Label the stages of Meiosis I and Meiosis II

Meiosis• Making haploid (n) gamete cells for

sexual reproduction 4 haploid gamete cells that are genetically different

• Pre-Meiosis – 1 round of Interphase– what happens here?

• Phases - 2 sets of PMAT-C– Meiosis I = PMAT-C I– Meiosis II = PMAT-C II

Meiosis OverviewI-PMAT+ CI + PMAT+CII

http://www.youtube.com/watch?v=eaf4j19_3Zg

• http://www.youtube.com/watch?v=2aVnN4RePyI

Meiosis I

Interphase I Prophase I Metaphase I Anaphase I

Cells undergo a round of DNA replication, forming duplicate Chromosomes.

Each chromosome pairs with its corresponding homologous chromosome to form a tetrad.

Spindle fibers attach to the chromosomes.

The fibers pull the homologous chromosomes toward the opposite ends of the cell.

Section 11-4

Figure 11-15 Meiosis

Go to Section:

Meiosis II

Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.

Prophase II Metaphase II Anaphase II Telophase IIThe chromosomes line up in a similar way to the metaphase stage of mitosis.

The sister chromatids separate and move toward opposite ends of the cell.

Meiosis II results in four haploid (N) daughter cells.

Section 11-4

Figure 11-17 Meiosis II

Go to Section:

Meiosis vs. Mitosis• Prophase I:

– Homologous chromosomes form a tetrad– Tetrad synapsis can occur– Crossing over can occur

Meiosis – Increasing genetic variation in offspring

• Genetic Recombination Crossing over– Occurs: late Prophase 1

& Metaphase 1– Provides new variation

Meiosis vs. Mitosis• Metaphase I:

– Homologous pairs line up on the equator– Crossing over can occur – increase

variation

Meiosis Mitosis

Meiosis vs. Mitosis

• Anaphase I:– Homologous pairs

separate• Each with its 2 chromatids

– No separation of centromeres/sisters

Meiosis vs. Mitosis

• Telophase I/Cytokinesis I:– Spindle breaks– RESULT: 2 genetically different cells with

diploid # of chromosomes

Meiosis I

Interphase I Prophase I Metaphase I Anaphase I

Cells undergo a round of DNA replication, forming duplicate Chromosomes.

Each chromosome pairs with its corresponding homologous chromosome to form a tetrad.

Spindle fibers attach to the chromosomes.

The fibers pull the homologous chromosomes toward the opposite ends of the cell.

Section 11-4

Figure 11-15 Meiosis

Go to Section:

Meiosis vs. Mitosis

• PMAT-C II:– Equivalent of mitosis– RESULT: 4 genetically different cells with

a haploid # of chromosomes

Meiosis II

Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.

Prophase II Metaphase II Anaphase II Telophase IIThe chromosomes line up in a similar way to the metaphase stage of mitosis.

The sister chromatids separate and move toward opposite ends of the cell.

Meiosis II results in four haploid (N) daughter cells.

Section 11-4

Figure 11-17 Meiosis II

Go to Section:

Meiosis shows how Mendel’s Principle of Independent Assortment works

2 genes – 4 different combinations…

30,000 human genes x 2 parents…

Means you are 1 in 43 trillion!

Summary - Meiosis

• Sexual reproduction • IPMAT+C I & PMAT + C II• Gametes (n) produced by both male and

female organisms recombine in fertilization• Fertilization 2n zygote genetically

DIFFERENT to parents• WHY must gametes be haploid (n) & not

diploid like those produced in mitosis?

Spermatogenesis vs. Oogenesis

• Sperm Formation– 4 sperm/round

survive– Occurs lifelong

• Egg Formation– Only 1 survives– All eggs formed at

birth– Egg contains

organelles

Why the difference?

Spermatogenesis vs. OogenesisWhere is the mistake?

Figure 13.8 A comparison of mitosis and meiosis: summary

IPMAT + cytokinesis IPMAT + PMAT + cytokinesis

Mistakes in Meiosis

• What went wrong here?

• What might the result be?

• Nondisjunction– Failure of homologues to separate properly

Mistakes in Meiosis

ENTIRE CHROMOSOME MISTAKES– Trisomy

– Monosomy

– Polyploidy

• We’ll address individual GENE problems later…

Mistakes in Meiosis

• Trisomy – ONE EXTRA chromosome for just ONE chromosome number (3) and not a pair– Ex: Downs’ Syndrome (TRISOMY 21),

XYY males

Monosomy – ONE LESS CHROMOSOME for just ONE chromosome number

– Ex: Turner’s SyndromePolyploidy – a whole extra SET of

chromosomes

Mistakes in Meiosis

Trisomy

Trisomy 21 Karyotype

XYY Male Karyotype

XXY – may appear male OR female (NOT both)

Karyotype

MonosomyTurner’s Syndrome

• Monosomy X (Turners Syndrome)– Missing 1

chromosome = missing 1,000’s of genes!!!

– Short neck– short stature– sterile,– Heart, kidney, bone,

thyroid problems

Monosomy

• Myelodysplasia– Bone marrow

disease

• Spontaneous abortions

Polypolid• In plants

– tri & tetra

Triploidy – Fatal in Animals