GeneticD.r Chalavi

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Diagrammatic Representation Of The Scientific Method

Organisms Used For Genetic Studies

Viruses

Escherichia coli

Human

Drosophila

Maize

Arabidopsis thaliana

Cell Biology

Chromosomes

MeiosisProphase I:

Metaphase IAnaphase ITelophase I

Random Segregation

or

or

However, if a crossover event occurred

or

Meiosis: Summary of Key Points Meiosis is a basic and important process in sexually reproducing eukaryotic organisms, because it produces the haploid gametes that join to produce a new individual, and provides a mechanism for genetic variability, which is important for survival of the species. Meiosis can be broken down into three fundamental events:1- the pairing of homologous chromosomes 2- the exchange of genetic material between homologous chromosomes by crossing over 3- the random segregation of homologous chromosomes into daughter nuclei

Mendelian Genetics

In 1856, Gregor Mendel, a monk and teacher of natural sciences, performed a successful study of inheritance in the pea plants that he tended in his garden.

Mendel made two innovations to the science of genetics: - developed pure lines - counted his results and kept statistical notes

Results from Mendel's Experiments

Parental Cross F1 Phenotype F2 Phenotypic Ratio F2 Ratio

Round x Wrinkled Seed Round 5474 Round:1850 Wrinkled 2.96:1

Yellow x Green Seeds Yellow 6022 Yellow:2001 Green 3.01:1

Red x White Flowers Red 705 Red:224 White 3.15:1

Tall x Dwarf Plants Tall l787 Tall:227 Dwarf 2.84:1

Seed Color: Green and yellow seeds Seed Shape: Wrinkled and Round seeds

Tall X DwarfF1 generation: all plants were tall

F1 self-fertilization: 3/4 of F2's were tall, 1/4 were dwarf

Cross: Dd X Dd (F1)

The Monohybrid Cross

Mendel's Conclusions 1- The hereditary determinants are of a particulate nature. These determinants are called genes.

2- Each parent has a gene pair in each cell for each trait studied. The F1 from a cross of two pure lines contains one allele for the dominant phenotype and one for the recessive phenotype. These two alleles comprise the gene pair. 3- One member of the gene pair segregates into a gamete, thus each gamete only carries one member of the gene pair.

4-Gametes unite at random and irrespective of the other gene pairs involved.

Mendelian Genetics Definitions - Allele - one alternative form of a given allelic pair; tall and dwarf are the alleles for the height of a pea plant; more than two alleles can exist for any specific gene, but only two of them will be found within any individual - Allelic pair - the combination of two alleles which comprise the gene pair - Homozygote - an individual which contains only one allele at the allelic pair; for example DD is homozygous dominant and dd is homozygous recessive; pure lines are homozygous for the gene of interest - Heterozygote - an individual which contains one of each member of the gene pair; for example the Dd heterozygote - Genotype - the specific allelic combination for a certain gene or set of genes

Union of GametesAt Random

D d

PunnettSquare

D DD(Tall)

Dd(Tall)

d Dd(Tall)

dd(Short)

The F2 generation

Genotypic ratio of F2: 1 DD : 2 Dd : 1 dd (or 3 D_ : 1 dd) Phenotypic ratio of F2: 3 tall : 1 dwarf

Mendel's First Law - the law of segregation; during gamete formation each member of the allelic pair separates from the other member to form the genetic constitution of the gamete

Confirmation of Mendel's First Law Hypothesis With these observations, Mendel could form a hypothesis about segregation. To test this hypothesis, Mendel selfed the F2 plants. If his law was correct he could predict what the results would be. And indeed, the results occurred has he expected.

Phenotypes Genotypes Genetic Description

F2 Tall Plants 1/3 DD2/3 Dd

Pure line homozygote dominantHeterozygotes

F2 Dwarf Plants all dd Pure line homozygote recessive

Thus the F2 is genotypically 1/4 Dd : 1/2 Dd : 1/4 dd

-Backcross - the cross of an F1 hybrid to one of the homozygous parents; for pea plant height the cross would be Dd x DD or Dd x dd; most often, though a backcross is a cross to a fully recessive parent

- Testcross - the cross of any individual to a homozygous recessive parent; used to determine if the individual is homozygous dominant or heterozygous

-Monohybrid cross - a cross between parents that differ at a single gene pair (usually AA x aa)

- Monohybrid - the offspring of two parents that are homozygous for alternate alleles of a gene pair

The Test Cross

Genotype Phenotype

Parental: RR X rr

F1: Rr All purple

F2: RR, Rr, rR, rr 3 purple to 1 yellow

Variations to Mendel's First Law of Genetics

Trait: Flower color Pure line phenotypes: red or white flower Parental cross: Red x White F1: We would expect red or white flowers in this generation, depending upon which allele is dominant. But, the F1 plants produced pink flowers. As with any experiment of this sort, the F1 plants are selfed. The results that were obtained were: F2 phenotypic ratio: 1/4 Red : 1/2 Pink : 1/4 White

Mendel's Law of Independent Assortment

Dihybrid cross - a cross between two parents that differ by two pairs of alleles (AABB x aabb)

Dihybrid- an individual heterozygous for two pairs of alleles (AaBb) A dihybrid cross is not a cross between two dihybrids.

Parental Cross: Yellow, Round Seed x Green, Wrinkled SeedF1 Generation: All yellow, round F2 Generation: 9 Yellow, Round, 3 Yellow, Wrinkled, 3 Green, Round, 1 Green, Wrinkled

Female Gametes

GW Gw gW gw

GWGGWW (Yellow,round)

GGWw (Yellow,round)

GgWW (Yellow,round)

GgWw (Yellow,round)

MaleGw

GGWw (Yellow,round)

GGww (Yellow,wrinkled)

GgWw (Yellow,round)

Ggww (Yellow,wrinkled)

GametesgW

GgWW (Yellow,round)

GgWw (Yellow,round)

ggWW (Green,round)

ggWw (Green)

gwGgWw (Yellow,round)

Ggww (Yellow,wrinkled)

ggWw (Green,round)

ggww (Green,

wrinkled)

Punnett square

forked line method

Phenotype General Genotype

9 Yellow, Round Seed G_W_

3 Yellow, Wrinkled Seed G_ww

3 Green, Round Seed ggW_

1 Green, Wrinkled Seed ggww

Mendel's Second Law - the law of independent assortment; during gamete formation the segregation of the alleles of one allelic pair is independent of the segregation of the alleles of another allelic pair

The Dihybrid Cross

Punnett Square for the Backcross

Female Gametes

GW Gw gW gw

MaleGametes

gw GgWw

(Yellow, round)

Ggww(Yellow,

wrinkled)

ggWw(Green, round)

ggww(Green,

wrinkled)

The phenotypic ratio of the test cross is: 1 Yellow, Round Seed 1 Yellow, Wrinkled Seed 1 Green, Round Seed 1 Green, Wrinkled Seed

Pedigree Analysis

Pedigree Analysis

Example pedigree showing the inheritance of Huntington disease in one family.Circles indicate females and squares indicate males. Affected individuals are shown in red.

The Chi-Square Test

Chi-Square Formula

Degrees of freedom (df) = n-1 where n is the number of classes

df P =

0.05 P =

0.01 P =

0.001

1 3.84 6.64 10.83

2 5.99 9.21 13.82

3 7.82 11.35 16.27

4 9.49 13.28 18.47

5 11.07 15.09 20.52

6 12.59 16.81 22.46

7 14.07 18.48 24.32

8 15.51 20.09 26.13

9 16.92 21.67 27.88

10 18.31 23.21 29.59

Chi-Square Table

Let's test the following data to determine if it fits a 9:3:3:1 ratio.

Observed Values Expected Values

315 Round, Yellow Seed (9/16)(556) = 312.75 Round, Yellow Seed

108 Round, Green Seed (3/16)(556) = 104.25 Round, Green Seed

101 Wrinkled, Yellow Seed

(3/16)(556) = 104.25 Wrinkled, Yellow

  32 Wrinkled, Green (1/16)(556) =   34.75 Wrinkled, Green

556 Total Seeds                        556.00 Total Seeds

Number of classes (n) = 4 df = n-1 + 4-1 = 3

Chi-square value = 0.47

A Chi-Square Table

Probability

Degrees ofFreedom

0.9 0.5 0.1 0.05 0.01

1 0.02 0.46 2.71 3.84 6.64

2 0.21 1.39 4.61 5.99 9.21

3 0.58 2.37 6.25 7.82 11.35

4 1.06 3.36 7.78 9.49 13.28

5 1.61 4.35 9.24 11.07 15.09

Pleiotropic Effects and Lethal Genes

Gene Symbols: gray = yyellow = Y

Expected Punnett Square

Female Gametes

Y y

MaleGametes

Y YY

(yellow) Yy

(yellow)

y Yy(yellow)

yy(gray)

Lethal Gene - a gene that leads to the death of an individual; these can be either dominant or recessive in nature Pleiotropic gene - a gene that affects more than one phenotype

Chromosome Number in Various Species

Species: Chromosome #

Human 46

Cat 38

Dog 78

Goldfish 94

Maize 20

Sunflower 34

The Chromosomal Basis of Mendel's Postulates

Did you know?In humans, there are around 30,000-40,00 different genes in the haploid genome, and 23 chromosomes, for an average of about 1300 to 1700 genes per chromosome.

Sex Chromosomes

There is one example where a somatic cell may not have two copies of a particular chromosome under normal circumstances. This involves the sex chromosomes

Because the sex chromosomes are somewhat unique in their properties, they are classified separately from the other chromosomes. This is indicated by their name: the sex chromosomes. The remaining chromosomes are called autosomes.