What is Genetics?

Defined broadly, it’s the branch of biology that deals with heredity and variation.

What is Genetics?

A key question in genetics is what is the relationship between genotype and phenotype?

The question is simple, the answer is complex.

Unit 3 Genetics• Every living thing is made up of different traits

that make it unique.– i.e. Labrador retrievers. This type of dog and its traits

occur because of selective breeding.

• Traits can be passed on from generation to generation. This transmission of traits is called heredity and the traits that are passed on are said to be inherited

• Genetics is the branch of biology that deals with the principle of variation and inheritance.

Gregor Mendel

• Born 1822• Austrian monk• Attended the university of Vienna• Studied botany and mathematics• Conducted experiments on pea plants over a 8

year period from 1853-1861

• Mendel’s questions were not new – but his approach to understanding heredity was.

• Mendel’s experimental innovations included:– choice of model organism– study of discrete traits– use of pure-breeding lines– painstaking control of matings– rigorous mathematical analysis of large

numbers of offspring

Mendelian Inheritance

The Garden Pea Pisum sativum

Why did he choose pea plants?

• Commonly available in Europe• Were easy to grow and matured quickly• Sexual organs of the plant were entirely

enclosed in the flower which allowed him to control reproduction whether through cross pollination or self pollination

• Different varieties had different traits that could be observed from generation to generation. Each trait had two variations

Mendel's Experiments• How did he conduct his experiments?• His first step was to obtain purebred plants for the trait

that he wanted to study• A purebred organism is descended from ancestors of a

distinct type or breed. This trait results from previous generations through long term selective breeding

• He chose plants that were tall and bred these together. From these he obtained some plants that were tall and some that were short.

• He then proceed to breed only the tall plants together again, he did this until he got only tall offspring

• He did the same for the short plants as well as 6 other traits

Mendel's First experimentMonohybrid Cross

• Once he obtained purebred or true bred plants he named them the parental generation or P generation.

• He crossed pure tall with pure short the offspring from this cross he designated as the first filial generation or the F1

• The offspring from this cross were hybrids indicating that they were the result of a cross between two different purebreds

• This is called a monohybrid cross because it involves only one trait; plant height

• All offspring from this cross were tall ( there were no short plants)• What happened according to the theory of blended inheritance

he should get all medium plants, instead all plants were tall. This lead mendel to conclude that the trait for tall must be dominant and the trait for short must be recessive.

• A dominant trait is one that is always expressed .• A recessive trait is a characteristic that is present but inactive. It

is only expressed if it is the only trait present• He conducted the experiment many times with the same results

and therefore concluded that when plants with two contrasting plants are crossed one is always dominant over the other. This lead to his principle of dominance

Mendel Began with Monohybrid Crosses Between Parents Who Bred True for Antagonistic Pairs of Traits

Remove stamensfrom purple flower.

StamensTransfer pollen from stamens of white flower to the stigma of a purple flower.

Cross-pollinated flowerforms seeds.

Plant seedsfrom pod.

White

PurpleParentalgeneration

First-generationplants

Cross Pollination – One of Mendel’s Experimental Methods

One of Mendel’s Monohybrid Crosses

Regardless of the trait, the pattern remained the same.

Mendel Observed Consistent Patterns of Inheritance

Second experiment

• He bred the F1 generation through self pollination

• These produced the second filial generation or the F2 generation

• When crossed this way the F2 offspring 75% of the time resembled the dominant parent or P generation and 25% of the time resembled the recessive parent. This ratio of 3:1 is known as the mendelain ratio

Why did Mendel get these results from the F2?• He concluded that each parent in the F1

generation starts with 2 hereditary factors. One is dominant and the other is recessive

• The factors separate in the parent, only one factor from each parent is contributed to the offspring.

• Each offspring inherits one factor from each parent. If the dominant factor is present it will be expressed even if the recessive factor is also present.

• The recessive factor will be expressed if only recessive factors are present.

Law of segregation• The results from Mendel F2 generation gave rise

to the law of segregation• A pair of factors for a trait separate ( segregate )

during the formation of gametes(sperm and egg) and then recombine during fertilization

• We know today that these factors that Mendel referred to are genes, the part of the chromosome that governs the expression of a particular trait.

• Gene has two forms called alleles. • these factors are sometimes referred to as unit

characters.

• When using letters to designate traits or unit characters a purebred tall plant will have 2 uppercase letter i.e., TT this means the plant is homozygous for tall.

• The pure breed short plant has 2 alleles for short designated in lowercase letters as tt.

• The product of the cross between a tall and short plant would have the alleles T and t. It would have a dominant and a recessive trait is said to be heterozygous for tall.

Principle of Segregation

… and these gametes unite at random in fertilization to create a new individual.

Principle of Segregation

Principle of Segregation: Genes come in pairs that segregate (separate) in the formation of gametes…

The Principle of Segregation Coupled with the Postulate of Dominant and Recessive Alleles Explains the Outcome of Monohybrid Crosses

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• Cross a homozygous dominant individual with the

homozygous recessive individual• Cross a heterozygous dominant individual with a

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 letters TT for homozygous

dominant and tt for homozygous recessive• Test cross B- assume unknown genotype is

heterozygous dominant– We will use letters Tt for heterozygous dominant and tt

for homozygous recessive

A Test Cross Reveals the Genotype of a Dominant Individual

Test crosses are the most important form of progeny testing.

Mendel used test crosses to buttress his proposal of segregation.

More on Mendel

• Punnett Squares• Probability• Product Rule• Test Cross• Dihybrid and Trihybrid Crosses

• Incomplete Dominance—Neither gene is completely dominant over the other. Snapdragons.

• Codominance—Both alleles are expressed simultaneously.

• Human ABO blood type

Single Factor/ Monohybrid Cross

• Single factor cross: A cross that is done for one trait. • Example: 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

• Solution: • Ratios:• Genotypic: 1:1 -- 50% homozygous straight-50% heterozygous

straight• Phenotypic :100% straight hair

• Monohybrid cross is where the parents are heterozygous or hybrid for the trait in question

• Ex. Crossing two plants that are heterozygous for tallness (Tt)

Results of monohybrid cross

• Phenotypic ratio: 3:1• Tall : short • Genotypic Ratio: 1:2:1 , 1 homozygous

dominant, 2 heterozygous dominant: 1 homozygous recessive.

• NOTE: In a monohybrid cross the ratios are always as above

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

• Monohybrid Cross Example #1:• In a particular type of fish, a "G" allele produces green

scales, which is dominant to the "g" allele which produces blue scales. What genotype and phenotype ratios do we get if we cross two Gg fish?

• To solve this problem,we must first determine the possible gametes each fish can produce. Recall that during meiosis the chromosome number is reduced by half. Each gamete will have only HALF the number of chromosomes as the individual.

• Two green fish, each with a genotype of Gg, can produce gametes that are either G or g.

G g

G GG Gg

g Gg gg

Then we set up a Punnett square like this

sperm

eggs

Then we combine the gametes to fill in the rest of the chart:

Results:genotype ratio: 1 GG : 2 Gg : 1 ggphenotype ratio: 3 green fish : 1 blue fishBe sure to write the genotype and phenotype ratios like this!

Genotype ratios must have numbers, letters (for the alleles), and colons between them to show it is a ratio. (It is also OK to use percentages for the numbers: 25% GG : 50% Gg : 25% gg)Phenotype ratios must have numbers, words (that describe the trait), and colons between them to show it is a ratio. (It is also OK to use percentages for the numbers: 75% green fish : 25% blue fish)

Let's try another example

• Let's cross a male heterozygous green fish with a female homozygous recessive blue fish. What genotype and phenotype ratios do we get if we cross these two fish?

• To solve this problem, we must first determine the possible gametes each fish can produce. A male heterozygous green fish has a genotype of Gg, and can produce gametes that are either G or g. A female homozygous recessive blue fish has a genotype of gg, and can only produce eggs that are g. Then we set up a Punnett square:

Mendel’s Results and Conclusions

• Mendel concluded that:• 1. Each trait is controlled by 2 genes or alleles.• 2. Some alleles are dominant, while other

alleles are recessive.• Law of Segregation—During meiosis, each

allele separates in an orderly manner from its partner, the two always go to different gametes.

• 4. Law of Independent Assortment—During Metaphase and Anaphase I of meiosis, the alleles for different traits segregate independently. This mixes chromosomes from your father and mother

• www.siskiyous.edu/class/bio1/genetics/monohybrid_v2.html

Mendels Crosses

• www.sumanasinc.com/webcontent/animations/content/mendel/mendel.html

Practice

• Recessive male (wrinkled) and Heterozygous Female (round)

• P. 533 #’s 1,2, & 3• P.531 #’s 1 &2• Assignment 1

Does the Segregation of Alleles of One Gene Occur Independently of the Segregation of Alleles of a

Second Gene?

This was Mendel’s next question, and he addressed it using dihybrid crosses.

A dihybrid cross

Dihybrid Cross• Mendel wanted to know if the inheritance of one trait

influenced the inheritance of another trait• He approached this problem in the same manner as he

approached the others• First he produced plants that were purebred for the traits

he wanted to examine. He selectively bred plants until the offspring were round and yellow. These plants were homozygous dominant for both seed shape and colour

• He breed them with plants that were homozygous recessive for seed colour and shape.(wrinkled and green)

• He performed a dihybrid cross. All offspring were heterozygous for both traits. RrYy making them round and yellow.

• He allowed the F1`s of his dihybrids to self pollinate. Of the 551 plants in the F2– 320 round yellow– 104 round green– 101 wrinkled yellow– 26 wrinkled green

• these results show a phenotypic ratio of 9:3:3:1.

Alternative Hypotheses About the Inheritance of Two Traits

A Dihybrid Cross Between Smooth, Yellow and Wrinkled, Green Strains

The result

The Dihybrid Cross and Its Explanation

Regardless of the particular traits, the pattern remained the same.

These results led Mendel to postulate the Principle of Independent Assortment.

Example

• In pea plants tall (T) is dominant over short plants (t). Green pods (G) is dominant over yellow pods(g). Mendel crossed a plant heterozygous for both tall and green with a plant that was heterozygous for both tall and green.

• What results did he obtain?

Mendel's Hypothesis

• He realized that the ratio 9:3:3:1 could be explained if the alleles from one trait were inherited independently of the alleles for another trait. This he called the Law of Independent assortment.

• According to this law different pairs of alleles are passed to the offspring independently of each other

The Principle of Independent Assortment

Segregation of pairs of alleles of one gene occurs independently of segregation of pairs of alleles of another gene.

The Principle of Independent Assortment

• http://www.dnaftb.org/5/problem.html

Instructions: How to complete a Dihybrid cross

1.Determine what letters you will use for each trait. Capital letters are used for dominant alleles, or traits, lower case letters are used

for recessive alleles. If green is dominant, the green allele would be G, the yellow allele would be g. If

rough pods are dominant, the rough allele would be R, the smooth allele would be r.

2. Heterozygous crosses have one dominant and one recessive allele for each trait. The genotype for each parent would be GgRr. Each plant has two alleles because it receives one copy from its parents (one from mom and one from dad).

3. Draw a square. Divide the square into 16 boxes. Each square gets one parent at the top (two letter in each box) and one parent on the left side (again two letter in each box). Each possibility is given a square.

4. Multiply the letters together and place four letters in each box, capital letters first. The offspring would have a genotype of one GGRR, two GGRr, one GGrr, two GgRR, four GgRr, two Ggrr, one ggRR, two ggRr, and one ggrr.

• In plants round seeds are dominant to wrinkled seeds and green pods are dominant to yellow pods. A plant heterozygous for round and yellow is crossed with a plant that is heterozygous for both traits.

• Step one– Assign letter– R round, r wrinkled, G green and, g yellow

• Step 2– P generation– RrGg x RrGg

• Step 3– Assortment of gametes– RG, Rg, rG, rg for both parents

• Step 4– Draw 16 punnett square

RG Rg rG rg

RG

Rg

rG

rg

Eggs

sperm

• Step 5– Complete table as if multiplying

• Step 6– Calculate the genotypic and phenotypic ratios

of the F1 offspring– Once table is complete should reveal for 2

heterozygous parents a ratio of 9:3:3:1 for the phenotype

Textbook practise: p. 540 #1 and p. 544 #’s1 and 2Assignment 2

Beyond Mendel’s laws

• some organisms show different patterns of inheritance where the dominant or recessive allele are not expressed as expected.

• These include• Incomplete dominance• Co-dominance • Multiple alleles

Incomplete Dominance• This occurs when neither of the two alleles expressed are dominant

for a trait• Examples include four o’clock flowers and Snap dragon flowers • Some examples would include

– Snap dragon flowers RR red, RR’ would be pink, and R’R’ would be white.

– The heterozygous condition presents with a colour that is in between the dominant and recessive colors

– To show the red colour you would need both RR to be present. Those with just one R are unable to make enough red pigment to produce red flowers and therefore they appear pink. Those with the R’R’ have no red pigment and are therefore white.

Snap Dragon Flowers• The gene for white petal is incompletely

dominant while the gene for red petal is also incompletely dominant. When you cross a white flower with a red flower you get all pink flowers. How is this possible?

• Lets make the allele for red petal color (R). Lets 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.

• Cross a white snap dragon with a red snapdragon. What are the results of such a cross.

• This cross is setup exactly like the monohybrid cross we have already studied.– Obtain genotypes of parents– Separate the alleles– Place them in correct places in punnett square– Complete the cross – Calculate the ratios

R’ R’

R RR’ RR’

R RR’ RR’

sperm

egg

Red snap dragon RRPink snap Dragon RR’White snap dragon R’R’

Here are the results:Genotypic ratio= 4:0 for heterozygous Pink flowers (RR’)Phenotypic ratio = 4:0 all pink flowers

What would be the results if we crossed two pink snapdragon flowers?

• Pink snap dragons (RR’)

R R’

R RR RR’

R’ RR’ R’R’

sperm

egg

Genotypic ratio= 1:2:1 for homozygous red; heterozygous pink; homozygous whitePhenotypic ratio = 1:2;1 for red: white: pink

• In Japanese four o’clock plants incomplete dominance occurs. Red is incompletely dominant to white. Red and white crossed will produce pink.

• Cross a red with a pink. Show the punnett square and the genotypic and phenotypic ratios

Co-Dominance

• 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

• example of co-dominance feather colour in chickens, cattle

• What happens when a black Rooster is crossed with a white hen?

• Black –B• White – W

• Black BB and white WW

• All offspring will be speckled black and white

B B

W BW BW

W BW BW

• If the colours blended you would expect the offspring to have grey plumage. If black was dominant, then they young would be black

• However you ended with offspring that are checkered black and white

Multiple Alleles• Many genes have more than two alleles• One of these genes is human blood types• In this case 3 allele are involved A, B, O• Each person has two of the three alleles• These are expressed as IA, IB ,i• IA and IB are dominant over i.• IA and IB are co-dominant and expressed

equally• Multiple allele inheritance is often

influenced by environmental factors, such as diet and climate

Phenotype (Blood Type) Genotypes

A IAIA or IA i

B IBIB or I Bi

AB IAIB

O ii

Blood Types

• AB is the universal recipient (it can accept all blood types)

• O is the universal Donor ( can donate to all blood types because it is recessive)

Practise problems

• If a woman has blood type AB and a man has blood type A what are the possible blood types of their offspring

• Find the genotype of each parent first• Mother is IA IB the father is IA I A or I A i• Complete each of the crosses as if you were

completing a monohybrid cross• Pg. 542 # 1 and 544 #5

IA IB

IA IAIA IAIB

i IA i IB i

Mother

Father

IA IB

IA IAIA IAIB

IA IAIA IAIB

Mother

Father

Chromosome Theory of Inheritance• Proposed by: Walter Sutton and Thoeodor Boveri

(1902)• They studied chromosomes during the various

phases of meiosis.• They made 3 key observations– Chromosomes occur in pairs– Chromosomes 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

Chromosome Theory• 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• Was an American who studied fruit flies in 1910.• He performed crosses involving eye colour• Crossed 2 red eyed parents and produced a white

eyed male• He crossed the red eyed female from the white

eyed male, with a normal red eyed male, all females had red eyes, while in the males half were red and half were white

• Found that chromosomes in Fruit flies are the same except for one pair

• He called the dissimilar pair sex chromosomes because he believed they determined the sex of the fly.

• 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

• Sex linked trait; A trait that is carried on one of the sex chromosomes (X or Y)

Linked Genes

• Morgan also found certain genes do not follow the law of independent assortment, but instead tend to be inherited together. He reasoned that genes located on the same chromosome would likely not segregate. These is called Linked genes.

• He did find that some of these genes do sometimes segregate. How is this possible?

Gene Chromosome Theory• Genes exit at specific sites arranged in a

linear fashion along chromosomes. • Morgan found that the closer the genes are

located together on the chromosome they will almost always be inherited together, while genes located further apart are more likely to be separated by a crossing over event.

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

Law of Independent Assortment in Modern Terms

• Mendel’s law of independent assortment based on Morgan's work should be restated as: – If crossing over does not take place, genes that

are located on different chromosomes will assort independently, while genes that are located on the same chromosome will be inherited together.

Sex- Linked Traits• Some traits are passed from one generation to the next depend on

the sex of the parent carrying the trait. This is because it is located on the sex chromosomes

• Sex Linked traits are traits that are carried on the sex chromosomes(X,Y).

• Most often, the traits are carried on the X chromosome and referred to as X-Linked

• Some are carried on the Y and are called Y –linked. These are rare do to the small size of the Y 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

Sex linked traits• Things to keep in mind about sex-linked traits• Sex-linked traits are recessive(small letters)• Sex-linked traits are carried on the X chromosome

normally• If a person has a big allele (N) and a small allele

(n) they are a carrier of the trait• When using punnett squares to predict outcomes

involving genes that are sex-linked assume the trait is located on the X chromosome unless stated otherwise.

• When you write the genotypes and phenotypes you have to include the sex within the ratios.

• Sex-linked traits are traits that are carried on the sex chromosomes, either X or Y.

• Hemophilia is a recessive, sex-linked disorder on the X-chromosome. Therefore, a male with a recessive allele on the X-chromosome will always express it phenotypically.

• In order for a female to express a recessive trait, it must be in a homozygous state.

Morgan and His White-Eye Mutant

T. H. Morgan

Wild type and white-eye mutant Drosophila

Morgan Knew Male and Female Drosophila Had Distinct Karyotypes

(Note: sex in Drosophila is determined by the ratio of X chromosomes to autosomes; the Y chromosome is dispensable.)

Morgan’s Curious Results

Morgan’s Curious Results

(female is from the F1 generation of original cross)

Morgan’s Explanation – the Eye Coloration Gene is Physically Linked to the X Chromosome

P

F1

F2

Morgan’s Explanation – the Eye Coloration Gene is Physically Linked to the X Chromosome

The X–Y system in mammals

44 +XY

44 +XX

The X–O system in certain insects

6 +X

6 +XX

The Z–W system in birds

16 +ZZ

16 +ZW

The haplo-diploid system in bees

16haploid

32diploid

There are Many Sex Determination Systems

Examples• Mate a carrier female (a female with the

mutant gene, but who is unafflicted because she has the normal gene on her other X-chromosome) with a normal male.

• Step 1: Define the symbols:– XH - normal– X h - hemophilia– Y - does not have the allele

• Step 2: Determine the genotypes of the parents:– X H X h x X HY

• Step 3: Draw your Punnett square.• F1 Generation• Genotype Phenotype• XHXH Normal female• XH X h Carrier female• XH Y Normal male• XhY Hemophiliac male• Due to the male total dependence on their X chromosome,

males have a much greater chance of getting hemophilia than females.

• For a female to get hemophilia, she has to have either a hemophiliac father, carrier mother or bad luck, or hemophiliac mother and hemophiliac father.

• Males and females produce the same amounts of proteins coded by genes located on the X chromosomes

• Females have 2 copies of the X chromosomes and males have one.

• This does not mean that females have more in fact one of the X chromosomes is inactive. This inactive X chromosome is called a Barr body. Which one is inactive is random.

• What is the probability that two parents with normal color vision will have color blind sons and daughters if the mother’s father is color blind?

• A recessive sex-linked gene (h) located on the X chromosome increases the blood clotting. This causes the disease hemophilia.– Explain how a hemophiliac can be born to two

normal parents?– Can any of the female offspring develop

hemophilia? Textbook p.544 # 8 Assignment # 4

Polygenic inheritance

• This is the idea that traits are affected by more than one gene

• Results: A range of phenotypes ie. not just short and tall, but some in the middle etc.) This is known as continuous variation

• Continuous Variation: Variation among individuals in a population where there is a gradient of phenotypes for one trait.

• Ear length in corn– Shortest (homozygous recessive for both alleles)– short, medium, long, ( these 3 are the least popular because

the are obtained by a combination of alleles– longest ( both alleles are homozygous dominant).– Medium is the most popular because it can be obtained

through a different combination of alleles• Note: either of these phenotypes may occur• This type of inheritance is used by plant and animal breeders

to improve livestock and crops, as well as human characteristics such as susceptibility to cardiovascular disease and athletic ability

• In humans height and skin color are examples of polygenic inheritance

• Polygenic inheritance is the same as multiple gene inheritance

Crosses involving autosomal traits and Sex-linked traits

• Many traits are inherited together some as a result of gender, others not related to gender.

• Example• In humans, normal color vision (R) is dominant to color

blindness (r). This is a sex-linked trait (X-linked). Brown eye color (B) is dominant to blue (b) eye color. If a color blind, heterozygous brown-eyed female mated with a normal, blue-eyed male, what percentage of the offspring would be color blind, blue eyed males?

ResultsParents:

Female (BbXrXr)• Male (bbXrY) bXR bXR bY bY

BXr BbXRXr BbXRXr BbXrY BbXrY

BXr BbXRXr BbXRXr BbXrY BbXrY

bXr bbXRXr bbXRXr bbXrY bbXrY

bXr bbXRXr bbXRXr bbXrY bbXrY

• Genotypic Ratio 1:1:1:1BbXRXr

bbXRXr

bbXrYBbXrY• Phenotypic ratio1:1:1:1• Brown and normal; female 1• Blue and normal female 1• Blue and colorblind male 1• Brown and colorblind male 1

• Fruit Flies• Traits Dominant Recessive• Wing shape Normal (N) vestigial (n)• Eye colour Red (XR) white (Xr)• • Cross a red eyed male hybrid for normal

wings with a white-eyed female with heterozygous for normal wings

•