Chapter 9 - Introduction to Genetics
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Transcript of Chapter 9 - Introduction to Genetics
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