Who was Gregor Mendel and what did he do? 3201/Biology 3201 Genetics... · A. Gregor Mendel Who was Gregor Mendel and what did he do? Name: Gregor Mendel – Lived in the 1800’s

Biology 3201 Unit 3 – Genetics Notes Topic 1 – Mendelian Genetics Page 1 of 23 Genetics: The scientific Study of Heredity. Heredity: The passing of traits from one generation to the next. Trait: Any observable characteristic on organism may have. Ex: eye colour, hair colour, height, tongue rolling Q. Who was the first person to study genetics? A. Gregor Mendel

Who was Gregor Mendel and what did he do? Name: Gregor Mendel – Lived in the 1800’s A monk who studied pea plants. What did he do? Mendel studies pea plants and the traits they passed on from one generation to the next. He studied 7 different traits in peas and he was able to discover several important laws about genetics and how traits are passed on. Why did Mendel study peas? There are 4 main reasons why Mendel studied peas.

o They reproduced quickly o Their traits were easily observable o They were readily available o They could self pollinate meaning he could control which plants pollinated.

Mendel’s Experiment Took the seeds of a purebred tall plant and the seeds of a purebred short plant and cross pollinated them and collected the seeds produced by this cross and planted them. Results: All the offspring of this cross were tall (No short plants) Did the trait for short disappear?? Next, Mendel self pollinated the plants from the F1 generation. These seeds were then planted and grown.

Biology 3201 Unit 3 – Genetics Notes Topic 1 – Mendelian Genetics Page 2 of 23 Results: Three quarters were tall and one quarter was short. To describe his findings Mendel said there was a dominant factor at play causing the plants to be all tall in the first cross and that a recessive factor was being expressed in the second cross. Principle of Dominance: When an organism is crossed for a pair of contrasting traits ONLY the dominant trait can be seen in the hybrid. The recessive factor was hidden.

Terminology Associated With Mendel’s Experiments and Genetics a. Unit Factors: These were the 2 factors that Mendel said affected the expression

of traits in organisms. Each organism has two factors for each trait. We now call them GENES.

b. Unit Theory of Inheritance: This was Mendel’s theory where he believed Unit factors

(now called genes) control the expression of traits in offspring.

c. Dominant Factor: This was a factor that was ALWAYS expressed in an organism. d. Recessive Factor: This was a factor that SOMETIMES was expressed in a organism. e. Allele This is the form a gene can be. It can be Dominant or recessive,

BUT NOT BOTH. f. F1 Generation Also called First Filial Generation. This is the first set of

offspring that are produced by parents. g. F2 Generation Also called the Second Filial Generation. This is the second set of

offspring created from the offspring of the parents. These offspring are the same thing as grandchildren.

h. Pure Breed/Strain A term that refers to an organism having EITHER two dominant

factors OR two recessive factors for a trait. Ex: A plant that is pure tall has two dominant factors for

tallness - One from the mother and one from the father. Ex: A plant that is pure short has two recessive factors for

tallness - One from the mother and one from the father. i. Hybrid A term meaning that an organism has one dominant and one

recessive factor for a trait.

Biology 3201 Unit 3 – Genetics Notes Topic 1 – Mendelian Genetics Page 3 of 23 j. Homozygous A term meaning that an organism has the SAME alleles for a gene.

Ex: both are recessive or both are dominant. Same as Pure breed. k. Heterozygous A term meaning that an organism has DIFFERENT alleles for a

gene. Ex: One is dominant and one is recessive. Same as hybrid. l. Genotype This is the genetic makeup of an organism. It is the combination of

alleles for an organism. For example a person might be homozygous dominant (TT) for

tallness, but at the same time be homozygous recessive (cc) for straight hair.

m Phenotype This is the physical characteristics displayed by an organism. It is

the appearance of a trait in an organism. For example: The organism above will be tall and have curly hair. n. Punnet Square A square that is used to help determine the possible/probable

outcomes of a cross between two individuals. See the diagram for a sample punnet square.

o. Cross This is another way of saying that a female has been mated with a

male.

Biology 3201 Unit 3 – Genetics Notes Topic 1 – Mendelian Genetics Page 4 of 23

Mendel’s Laws l. Principle of Dominance A principle proposed by Mendel that stated that the

Dominant form of a gene(trait) will ALWAYS be expressed.

2. Law of Segregation A law proposed by Mendel that stated that the pair of

factors for a trait separated (segregated) during the formation of gametes (sperm and egg) and then recombined during fertilization. See diagram below. (Copy this diagram)

Either of these sperm can join with either of the eggs. The letters are used to show the different forms of the traits. T stands for the dominant form of the trait for TALLNESS. t stands for the recessive form of the trait for tallness. 3. Law of Independent Assortment: A law stating that during meiosis, genes for

different traits are separated and distributed to gametes independently of one another.

Male Parent Tt

Female Parent TT

Gametes segregate

T sperm t sperm T egg T egg

Biology 3201 Unit 3 – Genetics Notes Topic 1 – Mendelian Genetics Page 5 of 23 THE SINGLE FACTOR CROSS Single Factor Cross: A cross that is done for ONE TRAIT. Also called a “Single Factor

Cross” Sample: Mary is Homozygous dominant for Straight Hair (SS). Steve is Heterozygous for

Straight hair (Ss). What are the possible ratios of offspring for these two individuals?

Answer: Parents: Gametes: S S S s Punnet Square:

S s

S

SS

Ss

S SS

Ss

Ratios:

Genotypic: 1:1 Homozygous Straight : Heterozygous Straight 50% homozygous straight: 50% heterozygous straight

Phenotypic: 4:0 100% straight hair All will have straight hair

SS SsX


The Monohybrid Cross Monohybrid Cross: A single factor cross where the parents are Heterozygous or

Hybrid for the trait in question. Ex: Crossing two plants that are heterozygous for tallness Tt Parents Gametes F1 Generation:

T t

T

TT

Tt

T

Tt

tt

Phenotypic Ratio: 3: 1 Tall: Short Genotypic Ratio: 1: 2: 1 Homozygous Dominant: Heterozygous Dominant: Homozygous Recessive NOTE: In a MONOHYBRID Cross, the Ratios are ALWAYS as above.

Genotypic and Phenotypic Ratios for a Monohybrid Cross Phenotypic: 3: 1 (Dominant to recessive) Genotypic: 1: 2 : 1 (homozygous dominant : heterozygous dominant : homozygous recessive

Tt TtX

T Tt t


The Product Rule A rule that uses the principles of probability to determine the possible outcomes of a genetic cross. Product Rule: The probability, or chance, that two or more independent events will occur

together is the product of their individual probabilities of occurring alone. Example: Tossing coins Toss a single coin: Chances of getting heads = ½ Chances of getting tails = ½ Toss two coins together: Chance of getting two heads = ½ x ½ = ¼ Chance of getting two tails = ½ x ½ = ¼ Chance of getting heads and tails: ½ x ½ + ½ x ½ = ½ The same can be applied to Genetics crosses. Let’s assume you have a monohybrid cross that you are performing. You can use the product rule to predict your outcomes. Parents: TT X Tt Gametes: T T T t Use the FOIL rule to get your possibilities: First (T and T) ½ T x ½ T = ½ TT Outside (T and t) ½ T x ½ t= ¼ Tt Inside (T and T) ½ T x ½ T = ¼ TT

(You add this one to the one above for firsts because they are the same) ¼ TT + ¼ TT = ½ TT

Last: (T and t) ½ T x ½ t = ¼ Tt (You add this to the one above for Tt because they are the same) ¼ Tt + ¼ Tt = ½ Tt Overall Results: ½ TT and ½ Tt (50% chance of TT and 50% Tt)


Test Cross Test Cross: A cross in which an individual of unknown genotype is crossed with a

homozygous recessive individual. This is done to determine the genotype of the unknown individual. Two crosses are performed.

a. Cross a Homozygous dominant individual with the homozygous recessive individual. b. Cross a Heterozygous dominant individual with the homozygous recessive individual.

Examine the results to tell the genotype of the unknown individual. TEST CROSS A -- Assume the unknown genotype is Homozygous Dominant. (We will use the letters TT for homozygous dominant and tt for homozygous recessive). Parents: Gametes: F1 Generation:

T

T

t Tt

Tt

t Tt

Tt

TT ttX

T T t t

Phenotypic Ratio: 4:0 All Tall Genotypic Ratio: 4:0 All Heterozygous Dominant

Biology 3201 Unit 3 – Genetics Notes Topic 1 – Mendelian Genetics Page 9 of 23 TEST CROSS B: Assume Unknown Genotype is Heterozygous Dominant. Parents: Gametes: F1 Generation:

T t

t Tt

tt

t Tt

tt

Phenotypic Ratio: 1:1 ½ tall: ½ short Genotypic Ratio: 1:1 ½ heterozygous dominant: ½ homozygous recessive

Tt ttX

T t t t


BS BS BS BS

bs BbSs Black /Split hoof

BbSs Black /Split hoof















Two Factor Crosses Two Factor Cross: A cross performed where two traits are involved at the same time. Sample Two Factor Cross A cow that is homozygous dominant for Brown Hair coat (BB) and Homozygous dominant for split Hoof (SS) is crossed with a cow that is homozygous recessive for brown hair (bb) and homozygous recessive for split hoof (ss). What are the possible genotypic and phenotypic ratios for the offspring? Parents: Gametes: F1 Generation: Phenotypic Ratio: 16:0 (All black coat and with split hoof) Genotypic Ratio: 16: 0 (All Heterozygous Black and heterozygous Split hoof)

BBSS X bbss

BS BS BS BS bs bs bs bs


BS Bs bS bs

BS BBSS Black /Split hoof

BBSs Black /Split hoof

BbSS Black /Split hoof


Bs BBSs Black /Split hoof

BBss Black /Solid hoof


Bbss Black /Solid hoof

bS BbSS Black /Split hoof


bbSS White /Split hoof

bbSs White /Split hoof


Bbss Black /Solid hoof

bbSs White /Split hoof

bbss White /Solid hoof

DIHYBRID CROSS Dihybrid Cross: A two factor cross where both parents are Heterozygous for BOTH traits. Sample Dihybrid Cross In cows, the gene for black coat colour (B) is dominant to the gene for white coat colour (b). The gene for split hoof(S) is dominant to the gene for solid hoof (s). What are the expected genotypic and phenotypic ratios for a Dihybrid cross between two cows? Answer: Recall that Dihybrid means that both parents are heterozygous for the two traits. Parents: Gametes: F1 Generation: Phenotypic Ratio: 9:3:3:1 Black/Split Hoof: Black/ Solid Hoof: White/Split Hoof: White/Solid Hoof Genotypic Ratio: 1:2:2:4:1:2:1:2:1 Homozygous Black/Homozygous Split hoof 1 Homozygous Black/Heterozygous Split hoof 2 Heterozygous Black/Homozygous Split hoof 2 Heterozygous Black/Heterozygous Split hoof 4

BbSs X BbSs

BS Bs bS bs BS Bs bS bs

Biology 3201 Unit 3 – Genetics Notes Topic 1 – Mendelian Genetics Page 12 of 23 Dihybrid cross continued … Homozygous Black/Homozygous solid hoof 1 Heterozygous Black/Homozygous solid hoof 2 Homozygous white/Homozygous split hoof 1 Homozygous White/Heterozygous split hoof 2 Homozygous white/Homozygous solid hoof 1 NOTE: THESE RATIOS REMAIN CONSTANT FOR ANY

DIHYBRID CROSS!!!!


INCOMPLETE DOMINANCE This is a situation where neither of the two alleles for a trait is dominant. Examples of incomplete Dominance:

a. Snapdragon Flowers (Heterozygous = pink) b. Four O’ Clock Flowers (heterozygous = pink)

Let’s Look at a cross involving Incomplete Dominance Snapdragon Flowers The gene for White petal colour is incompletely dominant while the gene for Red petal colour is also incompletely dominant. When you cross a white flower with a red flower you get all PINK flowers. Let’s see how this occurs. Let’s make the allele for Red petal colour (R). Let’s make the allele for White petal colour (R’). Because both of these genes are incompletely dominant, they both have a capital letter. We use R’ because both are dominant, but not one over the other. Now, let’s say we cross a white flower (Homozygous - the only thing it can be) with a Red flower (again, homozygous). Here are the results. Parents: Gametes: F1 Generation:

Phenotypic Ratio = 4:0 for all PINK flowers. Genotypic Ratio = 4:0 for Heterozygous Pink flowers (R R’).

R’ R’

R R R’ R R’

R R R’ R R’

RR X R’R’

R R R’ R’

Biology 3201 Unit 3 – Genetics Notes Topic 1 – Mendelian Genetics Page 14 of 23 What would be the result when two pink Flowers were crossed? Let’s see. Remember: Pink Snapdragons are Heterozygous (R R’) Parents: - Gametes: F1 Generation: Phenotypic Ratio = 1:2:1 for Red: Pink: White Genotypic Ratio = 1:2:1 for Homozygous Red: Heterozygous Pink: Homozygous White. Note: Other ways of showing the alleles for incomplete dominance

i)

R – red

R’ = white

ii)

FR - red

Fw - white

iii)

R – red

W - white

R R’

R RR R R’

R’ R R’ R’ R’

RR’ x R R’

R R’ R R’


CO-DOMINANCE Co-Dominance

This is a situation where both alleles for a trait may be dominant. The alleles are said to be co-dominant. BOTH alleles are expressed in the heterozygous individual.

Examples of Co-dominance

a. Feather colour in chickens (black and white feathers expressed at same time – Barred plumage)

b. Roan horses (Red and white hair expressed at same time) c. Blood type (AB – Type A and B blood expressed at same time)

Sample of Co-Dominance Cross In chickens, the gene for black feather colour is co-dominant to the gene for white feather colour. What are the expected ratios for a cross between a black feathered rooster and a white feathered hen? Answer: Recall that both of these traits are dominant so both will be expressed. Both traits

receive a different capital letter. Parents: Gametes: F1 generation: B B

W BW BW W BW BW

Phenotypic Ratio: 4:0 All Barred feathers (Note: Barred = Black and white mix) Genotypic Ratio: 4:0 All heterozygous black/white (Barred) NOTE: If you cross two Barred coloured chickens, it would be possible to get black

and white chickens produced.

BB X WW

B B W W


MULTIPLE ALLELES Some genetic traits are expressed by multiple (many) alleles. Such a trait is blood type in Humans. The blood type of humans is controlled by many alleles and not just one or two as is the norm. The human blood types/groups are as follows: 1. Type A 2. Type B 3. Type AB 4. Type O To have one of these blood types you must of course have received the genes/alleles from your parents. The alleles that you could have received are as follows:

Genotype Phenotype (Blood type)

I A IA or IAi A IB IB or IBi B

IAIB AB (A & B are co-dominant) Most common blood type.

ii O (recessive) Least common blood type

Biology 3201 Unit 3 – Genetics Notes Topic 1 – Mendelian Genetics Page 17 of 23 MIXING BLOOD We all know that only certain types of blood cannot be mixed with other types of blood. This is because if you mix the wrong types of blood agglutination or clumping will occur. This will causes the blood cells in the person being affected to clump together and the person would die from a blood clot in the brain or heart etc. Q. What types of blood can be mixed together? A. To answer this question we should look at the alleles for each blood type.

Blood Type/Allele Blood it can mix with- safely A - I A IA or IAi A or AB (Must have the A allele) B - IB IB or IBi B or AB (Must have the B allele) AB - IAIB AB only!!!! (Must have A and B alleles) O - ii Any of the blood types (Recessive, so it can mix

with any blood type) (Universal donor)!!! NOTE: AB is the universal ACCEPTOR (it can accept all blood types)

O is the universal DONOR (can donate to all blood types because it is recessive) Sample Cross involving Multiple Alleles (Blood types) A woman with Type A blood marries a man with type AB blood. What are the possible blood types of their offspring? Answer: Recall, Type A blood results from both a homozygous condition (I A IA) or a heterozygous condition (IAi). A type AB person will have only one genotype (IAIB). To figure out the results, we will have to complete two crosses. Cross # 1: We will assume the mother is homozygous for A blood. I A IA Parents: Gametes: F1 generation:

I A IA x IAIB

I A IA x IA IB

IA IA

IA IA IA IA IA

IB IA IB IA IB

Phenotypic Ratio: 1:1 (Type A and Type AB blood) Genotypic Ratio: 1:1 Homozygous A: Heterozygous AB

Biology 3201 Unit 3 – Genetics Notes Topic 1 – Mendelian Genetics Page 18 of 23 Cross # 2 We will assume the mother is heterozygous for A blood. IAi Parents: Gametes: F1 generation:

I A i x IAIB

I A i x IA IB

IA i

IA IA IA IA i

IB IA IB IB i

Phenotypic Ratio: 2:1: 1 (Type A: Type AB: Type B) Genotypic Ratio: 1:1:1:1 Homozygous A: Heterozygous: HeterozygousAB: Heterozygous B

Biology 3201 Unit 3 – Genetics Notes Topic 2 – Modern Ideas Page 19 of 23

Chromosome Theory of Inheritance Proposed by: Walter Sutton and Thoeodor Boveri. (1902) What they did

Studied chromosomes during the various phases of meiosis. What they found

Chromosomes occur in pairs Chromosome pairs separate (segregate) during anaphase 1 (This backs up Mendel’s

claims of the Law of Segregation) Chromosomes align themselves independently along the equator of a cell. (This backs up

Mendel’s ideas of independent assortment. What the theory says

Mendel’s factors or genes are carried on chromosomes. The segregation and independent assortment of chromosomes during meiosis is what

accounts for inheritance. Note: The work of Sutton and Boveri confirmed Mendel’s ideas about genes (Factors) and how

they are inherited (segregation and independent assortment). THOMAS MORGAN

Studied Fruit flies (Drosophila melanogaster) Did crosses involving eye colour.

What he found o Found that chromosomes in Fruit flies are the same except for one pair. o He called the dissimilar pair Sex chromosomes because he believed they determined the

sex of the fly. o Found that certain traits such as eye colour in Fruit Flies are found on the X gene. This is

what he called “sex-linkage”. Today, we call this “Sex-linked” genes or traits. Sex-Linked trait: A trait that is carried on one of the sex Chromosomes (X or Y)

o Morgan also found that certain genes on the same chromosome are called “Linked Genes”.


o He said that Linked genes get inherited together and not separately as Mendel had proposed (They do not obey Mendel’s Law of Independent assortment.)

o This would account for some differences in ratios of crosses. Instead of getting a 9:3:3:1 expected ratio in a Dihybrid cross, the ratio may be different.

o Morgan also found that genes on the same chromosome that are separated by a great distance will separate as a result of Crossing Over.

Morgan Helped to restate Mendel’s Law of Independent Assortment Law of Independent Assortment in Modern terms If crossing over does not occur, genes that are located on the same chromosomes will be inherited together while those on separate chromosomes will assort independently.


SEX-LINKED TRAITS Sex-Linked traits are traits that are carried on the sex Chromosomes (X, Y).

Most often, the traits are carried on the X- Chromosome.

Most of these traits are recessive and often lethal (deadly).

Sex-linked traits affect males more often than females

Examples of Sex-Linked Traits

Red-Green Colour blindness. Male Pattern Baldness Hemophilia Duchenne Muscular Dystrophy

To see how these traits are passed on, let’s do a sample cross. Sample Sex-linked cross. A woman who has normal vision marries a man that is colourblind. What are the possible ratios for their offspring? Answer Because this is a sex-linked trait we must use the X and Y chromosomes in our cross. Here is how it goes. Parents: Gametes: The results will be as follows: Females: All females (XX) will be carriers of the gene, but they will be Normal. Males: All males will be Normal.

X n

Y

XN XN X n XN Y

XN XN X n XN Y

XN XN x Xn Y (the N is normal, the n is colour blind)

XN XN X n Y

Biology 3201 Unit 3 – Genetics Notes Topic 2 – Modern Ideas Page 22 of 23 Let’s do another cross. This time we will cross one of the females with one of the males from above and see what happens. Parents: Gametes In this case the results are these: Females: All females will be normal; however, one of the females will be a carrier of the

gene for colour blindness (XNXn). Males: One male (50%) of the males will be normal and 50% will be colourblind. Things to keep in mind about Sex-linked traits: 1. Sex-linked traits are recessive (Small letters). 2. Sex-linked traits are carried on the X chromosome, normally. 3. If a person has a big allele (N) and a small allele (n), they are a carrier of the trait.

XN Y

XN XN XN XNY

Xn XN Xn XnY

XN X n x XN Y

XN Xn XN Y


POLYGENIC INHERITANCE This is the idea that traits are affected by more than one gene. Result: A range of phenotypes (i.e. not just short and tall, but some in the middle etc.).

This is known as continuous variation. Continuous Variation: Variations among individuals in a population where there is a

gradient of phenotypes. Shortest short medium long longest Note: Either of these phenotypes may occur. Examples of Polygenic Inheritance

Length of Ear corn Human skin colour Human height

Q. How does polygenic inheritance operate? A. Two genes work at the same time for the same trait. This leads to variations in the

expression of the trait. Example: In corn, the shortest length of corn occurs when the alleles for both genes are

homozygous recessive. In corn, the longest length of corn occurs when the alleles for both genes are

homozygous dominant. All other lengths of corn ears are caused when the alleles are something other than

homozygous dominant and/or recessive. NOTE: Polygenic Inheritance is the same as Multiple-gene inheritance.

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Who was Gregor Mendel and what did he do? 3201/Biology 3201 Genetics... · A. Gregor Mendel Who was Gregor Mendel and what did he do? Name: Gregor Mendel – Lived in the 1800’s