FUNDAMENTALS FUNDAMENTALS OF GENETICSOF GENETICS

CHAPTER 8CHAPTER 8

GENETICSGENETICS

GeneticsGenetics is a field of Biology is a field of Biology that is devoted to understanding that is devoted to understanding HOWHOW characteristics are passed characteristics are passed on from parents to offspring.on from parents to offspring.

Gregor Johann MendelGregor Johann Mendel – the – the father of geneticsfather of genetics

GREGOR MENDALGREGOR MENDAL

18431843 – Gregor Mendal joined the monastery at – Gregor Mendal joined the monastery at the age of 21. His task was to tend to the the age of 21. His task was to tend to the garden, which allowed him to observe and think garden, which allowed him to observe and think about the growth of many generations of plants.about the growth of many generations of plants.

18511851 – He entered the U. of Vienna to study math – He entered the U. of Vienna to study math and science, where he studied statisticsand science, where he studied statistics

Statistics helped him in the field of Statistics helped him in the field of HeredityHeredity – the – the transmission (passing on) of characteristics from transmission (passing on) of characteristics from parents to offspring.parents to offspring.

GARDEN PEASGARDEN PEAS

Mendel is remembered most for his work Mendel is remembered most for his work with Pisum sativum, Garden Peas.with Pisum sativum, Garden Peas.

Seven Characteristics of garden peas Seven Characteristics of garden peas were observed.were observed.

For each characteristic, two contrasting For each characteristic, two contrasting traits were observedtraits were observed

A A traittrait is a genetically determined variant is a genetically determined variant of a characteristic.of a characteristic.

GARDEN PEASGARDEN PEAS

The Seven CharacteristicsThe Seven Characteristics:: Plant Height (traits: Long & Short)Plant Height (traits: Long & Short) Flower Position along stem (traits: axial & Flower Position along stem (traits: axial &

terminal)terminal) Pod Color (traits: green & yellow)Pod Color (traits: green & yellow) Pod appearance (traits: inflated and Pod appearance (traits: inflated and

constricted)constricted) Seed Texture (traits: round and wrinkled)Seed Texture (traits: round and wrinkled) Seed Color (traits: yellow & green)Seed Color (traits: yellow & green) Flower Color (traits: purple & white)Flower Color (traits: purple & white)

GARDEN PEASGARDEN PEAS

Mendel collected the seeds of his pea plants Mendel collected the seeds of his pea plants and recorded each plant’s traits and seedsand recorded each plant’s traits and seeds

He planted the seedsHe planted the seeds FLOWER COLORFLOWER COLOR: He observed that purple : He observed that purple

flowering plants came from most of the flowering plants came from most of the seeds that he had collected from purple seeds that he had collected from purple plantsplants

But there where some white flowering plants But there where some white flowering plants that came from purple plant seeds.that came from purple plant seeds.

GARDEN PEASGARDEN PEAS

PLANT HEIGHTPLANT HEIGHT: : Mendel observed that while tall plants Mendel observed that while tall plants

grew from most of the seeds that were grew from most of the seeds that were obtained from tall plantsobtained from tall plants

But Mendel also observed some short But Mendel also observed some short plants grow from seeds that were also plants grow from seeds that were also obtained from tall plants.obtained from tall plants.

Mendel wanted to find an explanation for Mendel wanted to find an explanation for such variation.such variation.

MENDEL’S METHODSMENDEL’S METHODS Mendel observed how traits were passed from one Mendel observed how traits were passed from one

generation to the next by controlling how the generation to the next by controlling how the plants were pollenated.plants were pollenated.

Pollination – when the pollen grains from the male Pollination – when the pollen grains from the male reproductive part of the flower (anthers) is reproductive part of the flower (anthers) is transferred to the female reproduction part of the transferred to the female reproduction part of the flower (stigma).flower (stigma).

Self pollination occurs when pollen is transferred Self pollination occurs when pollen is transferred from anthers to stigma of the same plantfrom anthers to stigma of the same plant

Cross-pollination occurs between two different Cross-pollination occurs between two different plantsplants

MENDEL’S METHODSMENDEL’S METHODS To Control his ResultsTo Control his Results:: Mendel removed the anthers of a Mendel removed the anthers of a

plant and cross pollinated the plants plant and cross pollinated the plants by manually transferring pollen from by manually transferring pollen from the flower of a second plant to the the flower of a second plant to the stigma of the antherless plant.stigma of the antherless plant.

This way he prevented self pollination This way he prevented self pollination and controlled the specific traits of and controlled the specific traits of the parents.the parents.

MENDEL’S EXPERIMENTSMENDEL’S EXPERIMENTS Mendel’s Experiments: he first studied Mendel’s Experiments: he first studied

each characteristic and it’s each characteristic and it’s contrasting traits individually.contrasting traits individually.

He began growing plants that were true-He began growing plants that were true-breeding, or pure for each trait. True-breeding, or pure for each trait. True-breeding plants always produce offspring breeding plants always produce offspring with that trait when they self pollinate.with that trait when they self pollinate.

Mendel produced true-breeding plants, by Mendel produced true-breeding plants, by self pollination, for each of the self pollination, for each of the characteristics and for each trait. So in characteristics and for each trait. So in the end he had 14 true breeding plants.the end he had 14 true breeding plants.

MENDEL’S EXPERIMENTSMENDEL’S EXPERIMENTS

He began to cross-pollinate pairs of plants w/ opposite traits of a characteristic. He called the first true-breeding parents the P generation. He cross-pollinated a plant true-breeding for yellow pods with another plant true-breeding for green pods

MENDEL’S EXPERIMENTSMENDEL’S EXPERIMENTS Mendel recorded the number of each type of Mendel recorded the number of each type of

offspring that resulted from the cross-pollination offspring that resulted from the cross-pollination of the P generation. He called the offspring of of the P generation. He called the offspring of the P generation the F1 generation – F for Filial the P generation the F1 generation – F for Filial which has its roots in latin for son or daughter.which has its roots in latin for son or daughter.

He then allowed the flowers of the F1 generation He then allowed the flowers of the F1 generation to self- pollinate and then collected the seeds.to self- pollinate and then collected the seeds.

He called the offspring of the F1 generation the He called the offspring of the F1 generation the F2 generation.F2 generation.

Mendel performed hundreds of crossings and Mendel performed hundreds of crossings and documented the results of each generation.documented the results of each generation.

Mendel’s Results & ConclusionsMendel’s Results & Conclusions The resulting F1 generation from the cross of the green The resulting F1 generation from the cross of the green

pod plant and the yellow pod plant resulted in only green pod plant and the yellow pod plant resulted in only green pod plantspod plants

Mendel saw that no yellow pod plants developed even Mendel saw that no yellow pod plants developed even though one parent had been true-breeding for yellow though one parent had been true-breeding for yellow pods.pods.

Mendel then allowed the F1 generation to self pollinate: Mendel then allowed the F1 generation to self pollinate: Three – Fourths (75%) of the F2 generation produced Three – Fourths (75%) of the F2 generation produced green pods and One-Fourth (25%) produced yellow podsgreen pods and One-Fourth (25%) produced yellow pods

He concluded and hypothesized that something inside He concluded and hypothesized that something inside the pea plants controlled the characteristics and traits the pea plants controlled the characteristics and traits that were observed. He reasoned that a pair of control that were observed. He reasoned that a pair of control factors must control each trait.factors must control each trait.

Mendel’s Results & ConclusionsMendel’s Results & Conclusions Recessive and Dominant Traits : Whenever Recessive and Dominant Traits : Whenever

Mendel crossed traits – one of the P traits failed to Mendel crossed traits – one of the P traits failed to appear in the F1 plants. An every case, the trait appear in the F1 plants. An every case, the trait reappeared in the F2 generation at a ratio of 3:1. reappeared in the F2 generation at a ratio of 3:1. This pattern lead Mendel to hypothesize that one This pattern lead Mendel to hypothesize that one factor in the pair may prevent the other from factor in the pair may prevent the other from having an effect.having an effect.

• Dominant Trait masks or dominates the Dominant Trait masks or dominates the appearing characteristicappearing characteristic

• Recessive Traits is only expressed when paired Recessive Traits is only expressed when paired with another plant or animal displaying that with another plant or animal displaying that recessive traitrecessive trait

Results and ConclusionsResults and Conclusions

The Law of SegregationThe Law of Segregation: :

Mendel concluded that each reproductive Mendel concluded that each reproductive cell or gamete, receives one factor from cell or gamete, receives one factor from each parent. When the gametes combine each parent. When the gametes combine during fertilization, the offspring have two during fertilization, the offspring have two factors for each characteristic. The factors for each characteristic. The Law Law of Segregationof Segregation states that a pair of factors states that a pair of factors is segregated, or separated, during the is segregated, or separated, during the formation of gametes.formation of gametes.

Results and ConclusionsResults and Conclusions The Law of Independent AssortmentThe Law of Independent Assortment::

Mendel also crossed plants that differed in Mendel also crossed plants that differed in two characteristics, such as flower color two characteristics, such as flower color and seed color. The results from these and seed color. The results from these crosses showed that the traits produced by crosses showed that the traits produced by dominate factors do not necessarily appear dominate factors do not necessarily appear together. A white flowering plant can together. A white flowering plant can produce green pods. Mendel hypothesized produce green pods. Mendel hypothesized that the factors for individual characteristics that the factors for individual characteristics are not connected.are not connected.

RESULTS & CONCLUSIONSRESULTS & CONCLUSIONS

REMEMBERREMEMBER that in meiosis, the random that in meiosis, the random separation of homologous chromosomes separation of homologous chromosomes is called is called independent assortment.independent assortment.

The Law of Independent AssortmentThe Law of Independent Assortment states that characteristics separate states that characteristics separate independently of one another during the independently of one another during the formation of gametes.formation of gametes.

SUPPORT FOR CONCLUSIONS…SUPPORT FOR CONCLUSIONS…

Support for Mendel’s ConclusionsSupport for Mendel’s Conclusions: Most of : Most of what Mendel found is supported by what what Mendel found is supported by what biologists now know about molecular biologists now know about molecular genetics. genetics. Molecular geneticsMolecular genetics is the study is the study of the structure and function of of the structure and function of chromosomes and genes.chromosomes and genes.

A A genegene is the segment of DNA on a is the segment of DNA on a chromosome that controls a certain chromosome that controls a certain hereditary trait.hereditary trait.

Remember that chromosomes occur in pairs Remember that chromosomes occur in pairs so genes also occur in pairs. An so genes also occur in pairs. An alleleallele is the is the name given to one of the alternate forms of name given to one of the alternate forms of a gene that governs a characteristic.a gene that governs a characteristic.

Section 2: Genetic CrossesSection 2: Genetic Crosses

1.1. DifferentiateDifferentiate between the genotype and between the genotype and phenotype of an organismphenotype of an organism

2.2. ExplainExplain how probability is used to predict the how probability is used to predict the results of genetic crossesresults of genetic crosses

3.3. UseUse a Punnett square to predict results of a Punnett square to predict results of monohybrid and dihybrid genetic crossesmonohybrid and dihybrid genetic crosses

4.4. ExplainExplain how a testcross is used to show the how a testcross is used to show the genotype of an individual whose phenotype genotype of an individual whose phenotype expresses the dominant traitexpresses the dominant trait

5.5. DifferentiateDifferentiate a monohybrid cross from a a monohybrid cross from a dihybrid crossdihybrid cross

Genotype and PhenotypeGenotype and Phenotype

Genotype and PhenotypeGenotype and Phenotype GenotypeGenotype is the organism’s genetic make is the organism’s genetic make

up. It is the alleles that the organism inherits up. It is the alleles that the organism inherits from the parents. When you think of from the parents. When you think of Genotype – Think Genotype – Think GENESGENES

Example: The genotype for a white-flowering Example: The genotype for a white-flowering plant would be pp. The genotype of a purple plant would be pp. The genotype of a purple flowering plant would be Pp or PP.flowering plant would be Pp or PP.

Genotype & PhenotypeGenotype & Phenotype

PhenotypePhenotype is the organism’s is the organism’s appearance. When you hear appearance. When you hear phenotype – think phenotype – think PHYSICALPHYSICAL..

Example: The phenotype of PP or Example: The phenotype of PP or Pp would be purple-flowers. The Pp would be purple-flowers. The phenotype of pp would be white phenotype of pp would be white flowers.flowers.

GENOTYPE & PHENOTYPEGENOTYPE & PHENOTYPE

Genotype doesn’t necessarily mean that Genotype doesn’t necessarily mean that phenotype and visa versa. A plant with the phenotype and visa versa. A plant with the genotype to be tall can be short because of genotype to be tall can be short because of environmental factors.environmental factors.

When both alleles of a pair are alike, the When both alleles of a pair are alike, the organism is said to be homozygous. This organism is said to be homozygous. This means that they can be homozygous means that they can be homozygous dominant (PP) or homozygous recessive (pp).dominant (PP) or homozygous recessive (pp).

When the alleles are different, they are said to When the alleles are different, they are said to be heterozygous. An example of be heterozygous. An example of heterozygous is Pp. heterozygous is Pp.

ProbabilityProbability

ProbabilityProbability is the likelihood that a specific is the likelihood that a specific event will occur. (the chances that event will occur. (the chances that “something” will happen). Probability may “something” will happen). Probability may be expressed as a decimal number, a be expressed as a decimal number, a percentage, or a fraction.percentage, or a fraction.

Probability is determined by the following Probability is determined by the following equation:equation:

Probability = Probability = the # of times an event is expected to the # of times an event is expected to happenhappen

The # of times an event could happenThe # of times an event could happen

Predicting Results..Predicting Results.. A monohybrid crossA monohybrid cross is a cross in which only one is a cross in which only one

characteristic is tracked.characteristic is tracked. MonohybridsMonohybrids are the offspring of the monohybrid are the offspring of the monohybrid

cross.cross. A Punnett squareA Punnett square is an aid in the prediction of the is an aid in the prediction of the

probable distribution of inherited traits in the probable distribution of inherited traits in the offspring.offspring.

Homozygous x HomozygousHomozygous x Homozygous We are going to cross a Pea Plant that is We are going to cross a Pea Plant that is

homozygous for Purple flowers (the alleles are PP) homozygous for Purple flowers (the alleles are PP) and a pea plant that is homozygous for White and a pea plant that is homozygous for White flowers (the alleles are pp). The alleles that are flowers (the alleles are pp). The alleles that are carried by each parent’s gamete are represented carried by each parent’s gamete are represented by the letters on the outside of the boxes:by the letters on the outside of the boxes:

Predicting ResultsPredicting Results

The combinations within the four boxes The combinations within the four boxes represent the possible genotypes that can represent the possible genotypes that can result from the cross of the homozygous pea result from the cross of the homozygous pea plants. The outcome was Pp in every case, plants. The outcome was Pp in every case, so the probability of the offspring having Pp so the probability of the offspring having Pp is 100%. The probability of the flower being is 100%. The probability of the flower being purple is 100% as wellpurple is 100% as well

PREDICTING RESULTS..PREDICTING RESULTS..

We are going to cross a Vegasaurous We are going to cross a Vegasaurous Rex that is homozygous dominant (allele Rex that is homozygous dominant (allele is CC) for crazy curly hair and another is CC) for crazy curly hair and another Vegasaurous Rex (allele is Cc) that is Vegasaurous Rex (allele is Cc) that is heterozygous for crazy curly hair. heterozygous for crazy curly hair.

Predicting Results…Predicting Results… The two possible genotypes from this The two possible genotypes from this

cross are CC and Cc. The probability of cross are CC and Cc. The probability of an offspring having the genotype CC is an offspring having the genotype CC is 2/4 or 50%. 2/4 or 50%.

You could expect about 2/4 or 50% of the You could expect about 2/4 or 50% of the offspring to have Cc. The probable offspring to have Cc. The probable phenotype of this cross is crazy curly phenotype of this cross is crazy curly hair. Thus, there is a 100% probability hair. Thus, there is a 100% probability that the offspring will have crazy curly that the offspring will have crazy curly hair.hair.

Predicting Results…Predicting Results…

If it were the other way around: cc x Cc , If it were the other way around: cc x Cc , then the probability of an offspring having then the probability of an offspring having the genotype cc is 2/4 or 50%. You can the genotype cc is 2/4 or 50%. You can expect the other half, 50% or 2/4 to have expect the other half, 50% or 2/4 to have Cc. So the phenotype results would change Cc. So the phenotype results would change – the genotype Cc would exhibit crazy curly – the genotype Cc would exhibit crazy curly hair and the cc would not. hair and the cc would not.

Predicting results…Predicting results…

Heterozygous x HeterozygousHeterozygous x Heterozygous In rabbits the allele for brown coat color In rabbits the allele for brown coat color

(B) is dominant over white coat color (b). (B) is dominant over white coat color (b). Now, we are going to cross to rabbits Now, we are going to cross to rabbits that are heterozygous Bb. They are both that are heterozygous Bb. They are both brown rabbits.brown rabbits.

Predicting results…Predicting results…

As you can see, ¼ or 25% of the offspring As you can see, ¼ or 25% of the offspring predicted will have the genotype BBpredicted will have the genotype BB

Another 25% or ¼ of the offspring Another 25% or ¼ of the offspring predicted will have the genotype bb and predicted will have the genotype bb and the rest, 50% or ½ will have the genotype the rest, 50% or ½ will have the genotype Bb. Bb.

The phenotype results in 75% or ¾ being The phenotype results in 75% or ¾ being brown rabbits while the rest, 25% or ¼ will brown rabbits while the rest, 25% or ¼ will have white fur.have white fur.

Predicting Ratios…Predicting Ratios…

RatioRatio: : Genotype RatioGenotype Ratio: the ratio of the genotype : the ratio of the genotype

that appear in the offspringthat appear in the offspring ExampleExample: 1 BB: 2Bb: 1bb: 1 BB: 2Bb: 1bb Phenotype RatioPhenotype Ratio: the ratio of the phenotype : the ratio of the phenotype

that appear in the offspringthat appear in the offspring ExampleExample: 3 brown: 1 white: 3 brown: 1 white

TESTCROSSTESTCROSS Is performed when the genotype of an individual is Is performed when the genotype of an individual is

unknown.unknown. In a test cross the individual with the unknown In a test cross the individual with the unknown

genotype is crossed with a homozygous recessive genotype is crossed with a homozygous recessive individual.individual.

Example: Brown Bunnies. Is it BB or Bb? Cross it Example: Brown Bunnies. Is it BB or Bb? Cross it with Homozygous Recessive Bunnywith Homozygous Recessive Bunny

If all the offspring appear Brown then the bunny is If all the offspring appear Brown then the bunny is Homozygous Dominant (BB)Homozygous Dominant (BB)

If half the bunnies are white, then the bunny is If half the bunnies are white, then the bunny is Heterozygous Dominant (Bb)Heterozygous Dominant (Bb)

INCOMPLETE DOMINANCEINCOMPLETE DOMINANCE When the dominant allele completely masks When the dominant allele completely masks

recessive allele is called complete dominance.recessive allele is called complete dominance. But there are some traits, where the recessive But there are some traits, where the recessive

allele comes through a little bit. (Incomplete allele comes through a little bit. (Incomplete dominance)dominance)

For Example: When crossing a certain type of For Example: When crossing a certain type of Flower: Red Flowers (RR) and White Flowers Flower: Red Flowers (RR) and White Flowers (rr) The F1 generation are all (Rr) but the (rr) The F1 generation are all (Rr) but the phenotype is Pink.phenotype is Pink.

In humans, skin color, eye color and hair In humans, skin color, eye color and hair shades are an example of incomplete shades are an example of incomplete dominancedominance

CodominanceCodominance Occurs when BOTH alleles for a gene are Occurs when BOTH alleles for a gene are

expressed in a heterozygous offspringexpressed in a heterozygous offspring In codominance – NIETHER allele is In codominance – NIETHER allele is

dominant or recessive, and they don’t blend dominant or recessive, and they don’t blend as in incomplete dominance.as in incomplete dominance.

Example: is in Blood TypesExample: is in Blood Types

Predicting Results of Predicting Results of Dihybrid CrossesDihybrid Crosses

A dihybrid crossA dihybrid cross is a cross in which two is a cross in which two characteristics are tracked.characteristics are tracked.

DihybridsDihybrids are the offspring of these are the offspring of these crossescrosses

Naturally, with the addition of a trait, more Naturally, with the addition of a trait, more combinations are possible.combinations are possible.

Homozygous x HomozygousHomozygous x Homozygous

By using a Punnett square, we are going By using a Punnett square, we are going to predict the possible offspring of a cross to predict the possible offspring of a cross between two Pea plants with are between two Pea plants with are Homozygous for Homozygous for Round, Yellow Seeds (RRYY)Round, Yellow Seeds (RRYY)

&&

Wrinkled, Green Seeds (rryy)Wrinkled, Green Seeds (rryy)

Homozygous x HomozygousHomozygous x Homozygous

Assort the alleles Assort the alleles RY, RY, RY, RYRY, RY, RY, RY ry, ry, ry, ryry, ry, ry, ry Each box gets filled by the letters above it Each box gets filled by the letters above it

and to the leftand to the left In this case each box has RrYy, so ach In this case each box has RrYy, so ach

plant would have round yellow seeds.plant would have round yellow seeds.

Predicting Results…Predicting Results…

Heterozygous x HeterozygousHeterozygous x Heterozygous So, now lets take two pea plants from the So, now lets take two pea plants from the

F1 generation that is heterozygous yellow F1 generation that is heterozygous yellow round (RrYy)round (RrYy)

This cross would result in NINE different This cross would result in NINE different genotypes genotypes

This cross would result in FOUR different This cross would result in FOUR different phenotypesphenotypes

Predicting ResultsPredicting Results

9/16 would have the phenotype of round, 9/16 would have the phenotype of round, yellow seeds (genotypes: RRYY, RRYy, yellow seeds (genotypes: RRYY, RRYy, RrYY, RrYy)RrYY, RrYy)

3/16 would have the phenotype of round, 3/16 would have the phenotype of round, green seeds (genotypes: RRyy & Rryy)green seeds (genotypes: RRyy & Rryy)

3/16 would have the phenotype of wrinkled, 3/16 would have the phenotype of wrinkled, yellow seeds (genotypes: rrYY, rrYy)yellow seeds (genotypes: rrYY, rrYy)

1/16 would have the phenotype of wrinkled, 1/16 would have the phenotype of wrinkled, green seeds (genotype: rryy)green seeds (genotype: rryy)

Heterozygous x HeterozygousHeterozygous x Heterozygous