Chapter 9 - Introduction to Genetics

Genetics – the branch of Biology that studies heredity – how traits are passed

on.

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

Early Ideas on HeredityUntil the 19th century it was thought that the reason people look like their parents is because they were a blend of both parents.The work of Gregor Mendel changed the views of how characteristics are passed on from 1 generation to the next.

Gregor MendelBorn in 1822 in AustriaStudied at the University of ViennaBecame a priest and lived in a monasteryHe was in charge of the garden and he studied pea plants

Gregor MendelPea flowers have both male and female partsThey normally produce seeds through self-pollinationThe pollen from the male flower part fertilizes the egg from the female flower part of the same flowerThese seeds would inherit all their characteristics from the single plant that bore them

Gregor MendelMendel figured out that self-pollination could be preventedHe cut the male parts off of 1 plant and the female parts off another plantThen he pollinated the 2 plants by dusting the pollen from one onto the flower of the otherThis is known as cross-pollinationProduces seeds with the characteristics of both plants

Gregor MendelMendel had a stock of peas that were purebredThey would only produce offspring that were identical to themHe decided to cross plants with different characteristics to produce hybridsHe studied a few isolated characteristics to simplify his experiments

Gregor MendelPea Traits Studied

Seed Shape

Seed Color

Seed Coat Color

Pod Shape

Pod Color

Flower Position

Plant Height

Round Yellow

Gray Smooth

Green Axial Tall

Wrinkled

Green White Constric-ted

Yellow Terminal Short

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 crosses with a homozygous recessive individualB? x bb