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FUNDAMENTALS OF GENETICS Modern Biology Chapter 9 Pages 164 - 182
FUNDAMENTALS OF GENETICSModern Biology
Chapter 9Pages 164 - 182
Objectives:•Describe how Mendel’s results can be explained by the scientific knowledge of genes and chromosomes.•Differentiate between a monohybrid cross and a dihybrid cross.•Predict & perform results of monohybrid and dihybrid crosses
Fundamentals of Genetics
SOOKIE
Do Now• What is the genetic code?• What molecule carries the genetic code?• What is genetics?
K – W - LWhat do you know about inheritance?
What do you want to know about inheritance?
What have you learned about inheritance?
Fundamentals of GeneticsAll of your characteristics or traits are unique to you.Parents may pass many of their own traits to their children, or offspring.For example, the color of your hair, the size of your feet and the shape of your nose are some ofyour traits.
The passing of these traits from parents to offspring is called heredity. The study of heredity is called genetics. Biologists who study heredity are calledGeneticists.
Fundamentals of Genetics
Fundamentals of Genetics
• Look at the photographs to the right.
• What traits have these babies inherited from their parent?
Gregor MendelThe Father of Genetics
•Genetics was founded with the works of an Austrian Monk, scientists and mathematician Gregor Johann Mendel.•He experimented with garden pea plants.
rpee Seeds and Plants Home> Vegetables > Peas > Pea, Easy Peasy
Gregor Mendel•His task of tending the garden gave him time to observe the passing of traits from parentpea plants to their offspring.•He became interested in whycertain patterns oftraits showed upin living things.
• Mendel began his experiments by collecting seeds from his pea plants, carefully recording the traits of each plant.
• Seeds from tall plants usually produced tall plants but sometimes produced short plants.
• Seeds from short plants only produced short plants.
• …but, “WHY?”
He studied 7 different characteristics in his pea plants, each with 2 contrasting traits.–CHARACTERISTIC-a distinguishing quality that an organism exhibits.•Ex: height, hair color, eye color, skin color.–TRAIT- specific hereditary options available for each characteristic.•Ex: tall height/short height, smooth/ blonde hair, brown/blue eyes, Dark/light skin.
Mendel’s Experiments
CHARACTERISTICS
TRAIT
1. Plant Height Tall vs. Short2. Seed Color Yellow vs. Green 3. Seed Shape Round vs. Wrinkled4. Pod Color Green vs. Yellow5. Pod/Flower Location
Axial vs. Terminal
6. Pod Shape Inflated vs. Constricted
7. Flower Color Purple vs. White
• He decided to grow plants that were purebred - having a trait that will always be passed to the next generation
• The term strain denotes all plants that are pure for a trait.
Mendel’s Methods
• He produced 14 strains (one for each of the 14 traits he observed) by allowing the plants to self-pollinate for several generations
• This became his Parent generation or his
“P1 Generation”
Mendel Controlled Pollination
• Pollination-transfer of pollen from anther (male flower part) to stigma (female flower part)
• Self–Pollination – occurs on same plant
• Cross Pollination – occurs between different plants
Mendel Controlled Pollination
•Then, Mendel cross pollinated plants that had contrasting traits to see what the offspring would look like. (P1 X P1- i.e. pure tall x pure short)•Would the offspring (F1 Generation – offspring of P1) be tall, short, or medium ?
Mendel’s Methods
•In his first crosses, Mendel found that only one of the two traits appeared in the offspring plants – (F1 generation).•For example, when he crossbred tall pea plants with short pea plants, the offspring (F1) were always tall.
Mendel’s Results
•After his first crosses, Mendel took those offspring plants (F1) and crossed them.•In these second crosses, both traits showed up again in the F2 generation.(F2 GENERATION-offspring of crosses between the F1 generation).•He observed that ¾ of the plants had the same trait as the F1 generation.
•The same results happened in every experiment. One trait, like being tall, was always there in the first generation (F1).•The other trait, like being short, seemed to go away; only to reappear again in the second generation (F2).
This happened with every set of traits that Mendel studied.
EXAMPLES:Plant
Height Cross
Seed Color Cross
Seed Shape Cross
Parent P1 x P1
Tall x Short
All Tall Plants
Tall x Tall
¾ Tall¼ Short
Yellow x Green
All Yellow Plants
Yellow x Yellow
¾ Yellow¼ Green
Round vs. Wrinkled
All Round Plants
Round X Round
¾ Round¼ Wrinkled
First Generation
F1 x F1
Second Generation
F2
• Mendel hypothesized that something in the pea plants was controlling the characteristics that came through
• He called these controls “factors”
(We now know that these factors are really traits controlled by Genes)
• Because each characteristic had two forms, he said there must be a pair of “factors” controlling each trait.
• Each pair consists of alternate forms (we now call alleles) of the same trait; one from mother and one from father.
MENDEL’S 3 CONCLUSIONS:
Based on his findings, Mendel formulated three laws or principles of heredity:1. Principle of Dominant and Recessiveness2. Principle of Segregation3. Principle of Independent Assortment
Through crossing thousands of pea plants, he was able to conclude that that both of these factors (alleles) together controlled the expression of a trait.Dominant traits were controlled by dominant alleles and recessive traits were controlled byrecessive alleles.
Principle of Dominant & Recessiveness
DOMINANT-can mask or dominate the other ‘factor’ and is displayed most often.
RECESSIVE-the ‘factor’ that can be covered up; is displayed less often.• Ex: the ‘factor’ (allele) for tall is
dominant over the ‘factor’ (allele) for short, so the short allele would be the recessive allele.
Principle of Dominant & Recessiveness
•Letters are used to represent the alleles that carry the trait found on genes
•If the gene that controls the trait is dominant, the letter is written in uppercase.If the gene is recessive, the letter is written in lowercase.
–i.e. T- represents a dominant trait for tallness; t – represents a recessive trait for lack of tallness, or shortness
Principle of Dominant & Recessiveness
•T – dominant allele for tallness•t – recessive allele for lack of tallness or shortness.•W – dominant allele for round or smooth seeds•w – recessive allele for wrinkled seeds•P – dominant for flower color (purple)•p – recessive allele for white flower
GENE- a segment of DNA that codes for a specific characteristic.• Ex: height
ALLELE-the different forms of a gene (Mendel’s “factor”)Ex: allele for brown eyes is B/ allele for
blue eyes is bSO…if BB is a brown eyed person
and bb is a blue eyed person, what color eyes does someone with Bb have?
Vocabulary Review
Principle of Segregation• Each parent has two factors
(copies of each trait) and they segregate, or separate into different sex cells (gametes)
• Each gamete gets only 1 factor (allele)of each trait
Principle of Independent Assortment
• Mendel also crossed plants that differed in 2 characteristics, such as flower height and flower color.
• The data from these crosses showed that dominant traits do not always appear together
ttP?
Principle of Independent Assortment
• The alleles for different genes on different chromosomes are not connected.
• The alleles for different traits are distributed into gametes independently (randomly) from each other.
Principle of Independent Assortment
Gregor Mendel and his pea plants experiments (1857-1865)
Do Now• Who is the father of genetics?• What type of organism did he
work with?• What are dominant and
recessive traits?
Vocabulary Review• Chromosomes – made of DNA• Gene – segment of DNA that
controls a specific hereditary trait.
• Because chromosomes occur in pairs, genes occur in pairs
• Allele - (Mendel’s “factor”) – contrasting form of a gene– Dominant allele – capital letter– Recessive allele – lowercase
letter
AFTER MENDEL• Today, Geneticists rely on
Mendel’s work to predict the likely outcome of genetic crosses.
• Why would geneticists want to predict the probable genetic make up and appearance of offspring resulting from specified crosses?
GENOTYPE & PHENOTYPE• GENOTYPE-the genetic makeup
of an organism (the combination of alleles an organism inherits).– Use 2 letters together to represent
genotype.• PHENOTYPE-the trait displayed
based on the genotype.Ex: BB – Brown eyes– bb – Blue eyes– Bb – Brown eyes
GENETIC CROSSES
b b
Blue alleles
Genotype
Phenotype
•Organisms with different genotypes may have the same phenotype.
•For example, a brown-eyed organism (BB) and a brown eyed organism (Bb) have different genotypes.
•However, they have the same phenotype, which is brown eyes
GENOTYPE & PHENOTYPE
b
Brown AllelesBrown Alleles
One pair of chromosomesfor eye color
One pair of chromosomesfor eye color
BBB
AFTER MENDEL
•HOMOZYGOUS- organism has 2 of the same alleles for a trait.•Homozygous Dominant-has 2 dominant alleles; dominant trait is displayed
–Ex: BB = Brown-eyed organism•Homozygous Recessive-has 2 recessive alleles; recessive trait is displayed
–Ex: bb = blue-eyed•HETEROZYGOUS-organism has 1 dominant and 1 recessive allele; the dominant trait is displayed.
–Ex: Bb = brown eyes
GENOTYPE & PHENOTYPE
b b b B
Blue Allele Brown AlleleBlue alleles
Homozygous – alleles are the same
Heterozygous – alleles are different
Do Now• What are Mendel’s Laws of
Inheritance? • What is an allele? What is
homozygous vs. heterozygous?• What is genotype vs.
phenotype?
Probability• In order to understand genetics
you need to have some basic concepts concerning probability.
• Probability – the likelihood that a specific event will occur
• Can be expressed as a decimal, percentage, ratio or fraction.
P= number of times an event is expected to happen
number of opportunities for an event to happen
ProbabilityIf you flip a coin once, what is the probability that it will land on heads?P(Event)= 1 (Heads)
2 (Heads or Tails)P= 1 2 ; .5; or 50%; 1:2If you flip a coin twice, what is the probability that it will land on heads twice?
P = 1 (Heads) 4 (Heads, Tails; Tails, Heads; Tails, Tails; Heads, Heads)P = ??¼
•Remember, probability is the likelihood that a chance event will occur.•The value of studying genetics is in understanding how we can predict the likelihood of inheriting particular trait.
Predicting the Results of Genetic Crosses
•MONOHYBRID CROSS – a genetic cross between 2 individuals involving 1 pair of contrasting traits.
Predicting the Results of Genetic Crosses
•One of the easiest ways to calculate the mathematical probability of inheriting aspecific trait wasinvented by an early 20th
century English geneticist, Reginald Punnett .
Predicting the Results of Genetic Crosses
Predicting the Results of Genetic Crosses
His technique employs what we now call a Punnett square.
A Punnett square is a chart that shows possible gene combinations of offspring of two parents whose genotypes are known.
HOW TO DRAW A PUNNETT SQUARE
• 1. Write what each allele means.
• 2. Write the genotypes of the parents.
• 3. Draw a grid.• 4. Put the alleles for one parent
along the top; put the alleles for the other parent along the side.
• 5. Fill in the grid.
EXAMPLE 1:HOMOZYGOUS X HOMOZYGOUS
T= tall plant TALL X SHORT t = short plant (TT x tt)
Genotype = 4 TtPhenotype = 4 tall plants
Probability = number of times an event(tall) is expected to happen
number of opportunities (total) for an event to happen
Probability Ratio : 4/4 Probability percent: 100%
TTt
Ttt
Tt
TtT
tTt
EXAMPLE 2:HOMOZYGOUS X HETEROZYGOUS
T= tall plant TALL X SHORT t = short plant (Tt x tt)
Genotype = 2 Tt, 2 ttPhenotype = 2 tall, 2 short
Probability = number of times an event(tall/short) is expected to happen number of opportunities (total) for an event to happen
Probability: 2/4 tall plants; 50% tall plants; 2:4 2/4 short plants; 50% short plants; 2:4
TTt
ttt
Tt
ttTt
tt
EXAMPLE 2:HOMOZYGOUS X HETEROZYGOUS
T= tall plant TALL X TALL t = short plant (TT x Tt)
Genotype = 2 TT, 2 TtPhenotype = 4 Tall
Probability = number of times an event(tall/short) is expected to happen number of opportunities (total) for an event to happen
Probability: 4/4 tall plants; 100% tall plants; 4:4 0% short
TTt
TTt
TTTt
TTTt
EXAMPLE 2:HETEROZYGOUS X HETEROZYGOUS
T= tall plant TALL X TALL t = short plant (Tt x Tt)
Genotype = 1 TT, 2 Tt, 1 ttPhenotype = 3 Tall, 1 short
Probability = number of times an event(tall/short) is expected to happen number of opportunities (total) for an event to happen
Probability: ¾ tall plants; 75%; 3:4 ¼ short plants; 25%; 1:4
TTt
tTt
TTTt
Tttt
TESTCROSS• TESTCROSS - Cross to
determine genotype of parent with dominant phenotype.
• Use to determine if the unknown is heterozygous or homozygous dominant genotype.– Ex: A plant with green seed pods
could have a genotype of GG or Gg.
• Cross the unknown parent with a homozygous recessive.
EXAMPLE 2: ? ? X HOMOZYGOUS
T= tall plant TALL X SHORT t = short plant (T? x tt)
If Phenotype = 4 TallGenotype of Unknown = TT
TTt
?tt
TtTt
? t? t
TT tT t
EXAMPLE 2: ? ? X HOMOZYGOUS
T= tall plant TALL X SHORT t = short plant (T? x tt)
If Phenotype = 3 Tall, 1 shortGenotype of Unknown = Tt
TTt
?tt
TtTt
? t? t
t t tt t
The way genes control traits can be complex and interact in different ways.
More Complex Patterns of Heredity
•All of these crosses we just did were examples of COMPLETE DOMINANCE.•COMPLETE DOMINANCE-one allele is totally dominant over the other allele.–EXAMPLE: PP and Pp = purple flower plants
When one gene for a certain trait is not completely dominant over the other gene, a blending effect occurs.INCOMPLETE DOMINANCE is a type of inheritance in which one allele (dominant) for a specific trait is not completely dominant over the other (recessive) allele. This results in a combined phenotype (expressed physical trait).
Incomplete Dominance
Incomplete Dominance
R R
r
r R r
R r
R r
R r
EXAMPLE:Four o’clocks (flowers)RR = red rr =
white Rr = pink
RED (RR) X WHITE (rr)
Genotype = 4RrPhenotype = 4 pink
Incomplete Dominance
R r
r
R R R
r rR r
R r
EXAMPLE:Four o’clocks (flowers)RR = red rr =
white Rr = pink
PINK (Rr) X PINK (Rr)
Genotype = 1 RR;2Rr;1rr
Phenotype = 1 red, 2 pink, 1 white
Codominance•Another pattern of heredity can occur when two dominant genes are present for a certain trait.•This pattern of heredity is called co-dominance (both variations of the gene appearing at the same time).•Neither allele is dominant or recessive, nor do they blend.
CodominanceR R
R’
R’ R R’
R R’
R R’
R R’
EXAMPLE:roan horse:RR – red coat colorR’R’ – white coat colorRR’ – roan coat – both
red and white hairs
Genotype = 4 RR’Phenotype = 4 Roan
•Many traits are controlled by one gene that has more than two possible variations. •These traits are controlled by multiple alleles.• Human blood groups are controlled by multiple alleles.
Codominance & Multiple Alleles
•There are 3 alleles for the gene that determines blood type. A, B, O•(Remember: You have just 2 of the 3 in your genotype - 1 from mom & 1 from dad). •With three alleles, we have a higher number ofpossible combinations in creating a genotype.There are 6 differentgenotypes and four differentphenotypes blood type.
Codominance & Multiple Alleles
Blood Type A (IAIA) X Blood Type B (IBIB)
Genotype = 4 IAIB
Phenotype = 4 Blood Type AB
IA
IBIB
IAIAIB IAIB
IAIB IAIB
Possible Blood Type Combinations
The A and B alleles are equally dominant.A child who inherits and A allele from one parent and a B allele from the other parent will have type AB blood.
What type of dominance is this?co-dominance
The O allele is recessive to both A and B alleles. A child who inherits an A allele from one parent and an O allele from the other parent will have a genotype of AO and a phenotype of Type A blood.
A child who inherits on O allele from one parentand an O allele from the other will have:Genotype?Phenotype?
FYI
Predicting the Probability of a Dihybrid Crosses
• Cross between individuals studying one trait is Monohybrid Cross
• Cross between individuals studying two traits is Dihybrid Cross
Predicting the Probability of a Dihybrid Crosses
• A DIHYBRID CROSS is more complicated than monohybrid because there are more possible combinations.
• MONOHYBRID CROSS = 2 traits/4 possible offspring
• DIHYBRID CROSS = 4 Traits/ 16 possible offspring
Predicting the Probability of a Dihybrid Crosses
• Example: AA or Aa = purple; aa = whiteBB or Bb = tall; bb = short
• AaBb x AaBb (Purple Flower, Short Plant x Purple Flower, Short Plant)
FYI• One really important thing that Mendel
noticed from this type of cross was that traits (like flower color, height) are inherited independently - not together as a unit.
• This is type of cross helped Mendel develop the Law of Independent Assortment.
• REMEMBER - Law of Independent Assortment - Genes for various traits assort into gametes independently (due to homolouges lining up randomly at the metaphase plate.)
Dihybrid Cross Example:Homozygous x Homozygous
Tall, Round Plant (TT RR) X Tall, Round Plant (TT RR)
• First, we need to determine what alleles each parent could possibly give - all possible combinations of the alleles from each trait.
• TTRR
• TTRR
TR, TR, TR, TR
TR, TR, TR, TR
Tall, Round Plant (TT RR) X Tall, Round Plant (TT RR)
TR TR TR TRTR
TRTR
TR
TTRR
TTRR
TTRRTTRR
TTRR
TTRR
TTRR
TTRR
TTRR
TTRR
TTRR
TTRR TTRR
GENOTYPE: 16 TTRR
Dihybrid Cross Example:Homozygous x Homozygous
PHENOTYPE: 16 Tall, Round Plants
TTRR
TTRR
TTRR
Dihybrid Cross Example:Heterozygous x Homozygous
LET’S TRY IT !!Cross Tall, Round Plant (TtRr) X Short, Wrinkled Plant (ttrr)• Determine what alleles each
parent could possibly give - all possible combinations of the alleles from each trait.
• TtRr
• ttrr
TR, Tr, tR, tr
tr tr tr tr
LET’S TRY IT !! Cross Tall, Round Plant (TtRr) X Short, Wrinkled Plant (ttrr)
tr tr tr trTRTrtRtr
TtRr
TtrrTtRrTtrr
ttRrttRr
ttrrttrr ttrrttRr
ttrrttRr
Ttrr Ttrr
TtRr TtRr
GENOTYPE: 4TtRr, 4 Ttrr, 4 ttRr, 4 ttrr
PHENOTYPE:4 Tall, Round4 Tall, Wrinkled4 Short, Round4 Short, Wrinkled
Dihybrid Cross Example:Heterozygous x Homozygous
Dihybrid Cross Example:Heterozygous x Heterozygous
Cross Tall, Round Plant (TtRr) X Tall, Round Plant (TtRr) • Determine what alleles each
parent could possibly give - all possible combinations of the alleles from each trait.
• TtRr
• TtRr
TR, Tr, tR, tr
TR Tr tR tr
Tall, Round Plant (Tt Rr) X Tall, Round Plant (Tt Rr)
TR Tr tR trTRTrtRtr
TTRRTTRr
TTRrTTrr
TtRrTtRR
TtrrTtRr ttRrttRR
ttrrttRr
TtRr Ttrr
TtRR
TtRrPHENOTYPE:9 tall, round3 tall, wrinkled3 short, round1 short, wrinkled
Phenotypic Ratio= 9:3:3:1
Dihybrid Cross Example:Heterozygous x Heterozygous
GENOTYPE: 1 TTRR, 2TTRr, 2TtRR, 4TtRr, 1 TTrr, 2 Ttrr, 1ttRR, 2ttRr, 1 ttrr
