Introduction to Genetics

Chapter 9

HeredityTransmission of characteristics form parents to offspring

Gregor MendelAustrian MonkStudied heredity in pea plantsStudied 7 characteristics of peas

Traitscharacteristic occur in two forms

7 Characteristics of Pea Plants

Plant height (long or short)Flower Position along the stem (axial or terminal)Pod Color (green or yellow)Pod appearance (inflated or constricted)

Seed texture (smooth or wrinkled)Seed color (yellow or green)Flower color (purple or white)

ChromosomesHumans have 46 chromosomes arranged in 23 pairs (44 autosomes and 2 sex chromosomes)Chromosomes are made up of DNA and ProteinsChromosomes and therefore the DNA can be divided into genesGenes are short segments of DNA

Chromosome with genes

Allele – alternate form of a gene

Example: Brown/blue, tall/shortcan be represented by a letter

Example: Brown – B Blue - b

Homologous Chromosomes

GenesDetermines eye color

Determines hair color

b b b B

Blue Allele Brown AlleleBlue Alleles

One pair of your chromosomesfor eye color

One pair of your chromosomesfor eye color

b b b B

Blue Allele Brown AlleleBlue alleles

Homozygous – alleles are the same Heterozygous – alleles are different

Homozygous (pure)The two alleles are the sameThe two letters are the sameBB or bbHomozygous Dominant – BBHomozygous Recessive - bb

Heterozygous (hybrid)The two alleles are differentThe two letters are differentBb

Dominant GeneA gene that will be expressed if it is presentExamples – brown, tall, green, purpleWritten as a capital letter

Recessive GeneA gene that will only be expressed if there are two alleles presentRecessive genes are not expressed if there is a dominant gene presentShort, blue, yellow, whiteWritten as a lower case letter

GenotypeThe two alleles an organism hasWhat the genes “say”Ex. BB, Bb, bb

PhenotypePhysical appearance of an organismThe result of the expression of the genesExample: tall, short, brown, blue

The GoalThe goal of genetics is to determine the possible offspring that can be produced from two parentsThe likelihood of an outcome is determined through the laws of probability

ProbabilityThe likelihood that a specific event will occurCan be expressed as a decimal, percent or a fraction

Equation for Probability

Probability =

Number of times an eventis expected to happen

The number of opportunitiesfor an event to happen

Probability ExampleYou flip a coin onceThe probability of getting tails is ½

You flip a coin twiceThe probability of getting tails twice is 1/41

2

1

2X

= 1

4

H TFirst Flip or

Second Flip

If I got Tails on the first flip I can either get heads or tailsOn the second flip

TH

H

T

T

H

H

T

FirstSecond Probability of getting:

1/4

1/4

1/4

1/4

GametesEgg or spermHaploidContains half the number of chromosomesOne gene from each homologous pair

Genetic CrossesWhen doing genetic crosses it is important to keep tract of the generationsP1 – parentsF1 – children of the parents (P1)F2 – Children of the F1’s

GG gg

Gg

GG

gg

Gg

A Genetic Cross

Punnett Square – used to predict the possible outcomes of a cross

AA aa

                 

aa

Male produces onetype of sperm with the gene A

Female produces onetype of egg with the gene a

The Baby Elephants will be..

Aa Aa

AaAa

Monohybrid Cross

Aa Aa

Aa Aa

A

A

a a

How to fill in the Punnett Square

Monohybrid CrossCross where one trait is looked at Dark elephant vs. light elephant

Aa

                 

Male produces twotypes of sperm with the gene A or a

Female produces twotype of eggs with the gene A or a

Aa

The Baby Elephants will be..

AA Aa

Aa aa

Genotypic RatioRatio of the offspring's genotypes

Genotypic Ratio: 1:2:1

EE – 1Ee – 2ee – 1

Phenotypic RatioRatio of the offspring's phenotypes

Phenotypic ratio: 3:1

3 – smooth1- wrinkled

Genotypic Ratio?

Phenotypic Ratio?

Phenotypes?

Genotypes?

Law of SegregationA pair of genes is segregated or separated during the formation of gametes

Law of Independent Assortment

Genes for different characteristics are not connectedThey are inherited independently

Dihybrid CrossA cross between individuals that involves two pairs of contrasting traitsLook at hair color and eye color at the same timeLook at height and color at the same time

Dihybrid Cross Punnett Square

AABB aabb

AB AB

AB ABabab

abab

AB ab

AB ab

AaBbGenes in the fertilized egg

Fertilization

Egg Sperm

AaBb AaBb AaBb AaBb

AaBb AaBb AaBb AaBb

AaBb AaBb AaBb AaBb

AaBb AaBb AaBb AaBb

AB

AB

AB

AB

ab ab ab ab

AaBb

Genotypic Ratio: 1Phenotypic Ratio: 1

What if the Dihybrid cross has heterozygous parents?

How do you find the gametes?

A

a

B

b

B

b

AB

Ab

aB

ab

How to find genes for the gametes of a dihybrid cross

AaBb AaBb

AB Ab

aB ababaB

AbAB

Dihybrid Heterozygous Cross Genotypic Ratio

1:2:1:2:4:2:1:2:1

TestcrossWhen an individual of unknown genotype is crossed with a homozygous recessive individualB? x bb