THE LIFE & TIMES OF

GREGORMENDEL

GREGOR MENDEL

• Born in 1822 in Austria

• The son of a farmer• Had agricultural

experience

• Went to University of Vienna

• Studied Physics, Biology, & Chemistry

• He became a monk at a monastery in Brno

GREGOR MENDEL

• While living in the monastery he began to become interested in inheritance.

• He began to breed pea plants in order to study how traits were inherited

• Eventually he became known as the “Father of Genetics” for his studies on pea plants.

WHY DID HE CHOOSE PEA PLANTS?

• Easy to Grow.

• Variety of Traits to observe.

• Traits had an “either or” inheritance.

• Control of breeding.

• Relatively short life cycle

KNOWLEDGE OF FLOWERS

• Stamen & Anthers = male reproductive parts

• Carpel= female reproductive part

• Pea plants can self fertilize.• If the anther is not

removed, pollen may fertilize the carpel of the same plant

EXPERIMENT 1: MENDEL NOTICED THAT…

• Peas may have either white or purple flowers.

• Self-fertilized white flowers always gave white flowered plants.

• Self-fertilized purple flowers were sometimes all purple but sometime there were also white flowered plants.

STEP 1…

• Make sure plants were true breeding.

• Allow plants to self pollinate to see if their offspring are the same phenotype as the parent.

STEP 2… HYBRIDIZATION

• He called these true breeding plants the parent or the “P” generation.

• He called the offspring the “F1” (first) generation.

• He took 2 true breeding plants of opposite phenotypes and crossed them.

• All of the F1 were purple.

STEP 3…MONOHYBRID• When he did

this he noticed there was always the same ratio of white to purple flowered plants.

• He then allowed the F1 generation to self-pollinate.

F2

STEP 3…MONOHYBRID• When he did

this he noticed there was always the same ratio of white to purple flowered plants.

• He then allowed the F1 generation to self-pollinate.

• There was always a 3:1 ratio of purple to white flowered plants.

F2

STEP 3…MONOHYBRID• This made

him wonder why the white flowered plant had come back.

• What do you think?F2

• Mendel came up with 2 laws.

• Law of SegregationThere are two factors that separate during gamete formation and then come back together in different combinations to form the zygote.

• Law of DominanceThere are two factors responsible for deciding the trait and one is always dominant over the other.

EXPERIMENT 2: DIHYBRID CROSS

• He wanted to see whether traits were linked with other traits.

• Round or wrinkled seeds

• Yellow or green

• Followed same procedures as in experiment 1.

EXPERIMENT 2: DIHYBRID CROSS

• Had a true breeding round green and crossed it with a true breeding wrinkled yellow.

• All of the F1 was round and yellow.

• F1 Self pollinated and he got a 9:3:3:1 ratio in the F2

EXPERIMENT 2: DIHYBRID CROSS

Are traits inherited dependently or independently from each other?

He called his final law the Law of Independent AssortmentThe Law of Independent Assortment states that genes are inherited independently from each one another.

MENDEL’S 3 LAWS:Law of DominanceThere are 2 factors for every gene and one is more dominant than the other.

Law of SegregationDuring gamete formation the two factors will separate and come together in new combinations to form the zygote.

Law of Independent AssortmentTraits are inherited independently from one another.

EXCEPTIONS TO MENDEL’S PRINCIPLES

• Mendel studied inheritance in the 1800’s and therefore there was a lot of information that he was missing

• Little was known about genes and chromosomes and until Mendel, these things were not scientifically understood.

• As more studies occurred, scientists realized that though Mendel had laid a good foundation to the understanding of inheritance, there were still many limitations to Mendel’s principles

EXCEPTIONS TO MENDEL’S PRINCIPLES

• It was lucky that Mendel had chosen peas to study inheritance because of the simple nature of inheritance found within these plants

• As more complex organisms were studied many exceptions to his laws became evident

EXCEPTIONS TO LAW OF DOMINANCE

• Mendel originally stated that there are two factors for each gene and one factor can be dominant over the other– This was true for inheritance in pea

plants, however there are two modes of inheritance that are different than this

• The two exceptions are:–Incomplete Dominance•When two alleles are equally dominant•When the two different alleles combine they interact to produce a new phenotype– For example if a red and white individual

cross they produce a pink individual

CO-DOMINANCE

• This is another type of incomplete dominance

• The difference is that in this case both alleles are expressed at the same time– Ex) Blood type– Ex) White cow (HwHw) X Red bull (HrHr)• Will have offsrping with both hair

colors (roan)

PRACTISE PROBLEM

• In snap dragons there are white and red flowers. When a white and a red flower have children all offspring are pink– White snapdragon crossed with a red Snapdragon– WW X RR (complete punnett square)– What would the F2 be?

• In cows a white cow and a red cow mate and have a calf with both white and red hairs (roan)– What type of inheritance is this?– If this calf mates with another roan cow what will

the phenotypes of their children be?

• In cows a white cow and a red cow mate and have a calf with both white and red hairs (roan)– What type of inheritance is this?– If this calf mates with another roan cow

what will the phenotypes of their children be?

SAMPLE PROBLEM

MULTIPLE ALLELES

•Multiple Alleles– The law of segregation states that there are

two factors for every trait (gene)– There are some genes that are more complex

than this and may have more than 2 possible alleles

MULTIPLE ALLELES CONTINUED…

– For example in humans there are 4 different possible blood types• Type A, B, AB, and O• There are however 3 alleles that determine

the type of blood that an individual will have• There are two dominant alleles, A and B• These are co-dominant (neither is more

dominant than the other)

BLOOD TYPES

• ABO blood group system:– A= IAIA or IAi– B= IBIB or IBi– AB= IAIB

– O= ii

PRACTISE PROBLEM

• A man with type O blood marries a woman with type AB blood. What will be the possible genotypes and phenotypes of their children? Show the punnett square for this cross

• If a baby with blood type B is produced from a mother of type A, could a type O man have fathered this child?

LETHAL GENES

• In 1905 Lucien Cuenot was studying inheritance of a coat colour gene in mice

• He crossed two heterozygous yellow mice– He never got the 3:1 ratio he should have– Instead he received a 2:1 ratio

• He determined that yellow was the dominant allele

• He did test crosses to show that all the yellow individuals were heterozygous

LETHAL GENES

• Later studies learned that homozygous yellow individuals experienced errors during embryonic development

• This resulted in the unusual 2:1 ratio

• In this experiment the lethal gene was in the form of a dominant allele, however they may also appear in recessive alleles

EXCEPTIONS TO THE LAW OF INDEPENDENT ASSORTMENT

• This law states that each pair of alleles segregates independently of other pairs of alleles during gamete formation

• There are two types of inheritance where this is not the case; Linked genes & Polygenic Traits

EXCEPTIONS TO THE LAW OF

INDEPENDENT ASSORTMENT

• Linkage– Linked genes are genes which are located

on the same chromosome• Genes (and alleles) are found in long sequences

along a chromosome• The area a gene is found on a chromosome is

called its locus

– Linked genes don’t segregate independently but are often inherited together

– They transmit together (ie. Red hair & Freckles/ Fair skin/freckles)

EXCEPTIONS TO THE LAW OF INDEPENDENT ASSORTMENT

• Linkage continued…

– New combinations do occur through crossing over that occurs when chromosome pair in meiosis (however this is more likely between genes found at further distances from each other on the chromosome)

– This means that a single chromosome can change as they pass from gen. to gen. but often genes found closer to each other are inherited together

EXCEPTIONS TO THE LAW OF

INDEPENDENT ASSORTMENT

• Polygenic traits– Mendel studied traits controlled by 1 gene

however some genes influence the display of the phenotype

– Polygenic Traits- traits that are affected by more than one gene• Ex. Skin colour, eye colour, height, weight, metabolic

rate

– Some traits are controlled by more than one gene– Characteristics are determined by several pairs of

independent genes – Take a look at page 476 in your text for

example.

POLYGENIC TRAITS…

• What’s the difference between polygenic traits and multiple alleles?–Multiple allele inheritance shows many

forms of a trait (there are more than 2 alleles)

– Polygenic inheritance is when genes interfere with the expression of each other and show a range of traits.

SEX DETERMINATION

• Thomas Hunt Morgan (1866-1945) conducted experiments on fruit flies (drosophila malanogaster)–Wanted to study Mendel’s principles– Led him to discover sex chromosomes

and linkage of traits

SEX DETERMINATION

• We have 46 chromosomes – 23 pairs of chromosomes

– 1 pair of these chromosomes are sex chromosomes• Determines the sex of a child

– 22 of these 23 are autosomal chromosomes – All chromosomes other than the sex

chromosomes

There are two copies of each autosome (chromosomes 1-22) in both females and males. The sex chromosomes are different: There are two copies of the X-chromosome in females, but males have a single X-chromosome and a Y-chromosome.

THE SEXES…

• There are two different sex chromosomes (X & Y chromosomes)– Females have two X chromosomes (XX)– Males have one X and one Y chromosome (XY)

• The X chromosome carries more information than just sex determination – They carry anywhere between 100-200 other

genetic traits

• The Y chromosome is much smaller and only contains information related to sex determination– It carries the SRY gene which carries

information that creates maleness

SEX LINKED TRAITS

• Sex-linked traits are those whose genes are found on the X chromosome but not on the Y chromosome

• Females would have 2 alleles for genes that are located on the X chromosome– Females are XX (so one allele would be on

each X chromosome)– Females are more likely to show the

dominant forms of genes because they have two alleles for the genes • They are able to be HH, Hh, or hh which

means there is a greater chance of females having a dominant allele

SEX LINKED TRAITS…• Males are more susceptible to receiving recessive

forms of genes because they only have one allele for genes on the X chromosome because the gene may not be located on the Y chromosome (therefore they have a 50% chance)

• In humans, different sex-linked genes have a defective recessive allele that causes a disease– Hemophilia (blood cannot clot), colour blindness,

sickle cell anemia

• Lethal recessive traits- sex-linked recessive traits that cause lethal diseases/conditions (Duchenne muscular dystrophy)

SAMPLE PROBLEM…

• A man with hemophilia marries a homozygous normal woman. Predict the genotypes and phenotypes of their children.

• Step 1: determine the chromosomes for each parent– Dad: X Y Mom: X X

• Step 2: determine possible alleles that attach to the X chromosome of each parent– Xn and Y XN

• Step 3: set up a punnett square and combine gametes

• Step 4: state genoytpic and phenotypic ratio of the F1 generation

PEDIGREE CHARTS• A pedigree chart is a graphic representation of

inheritance• It is an organized method of recording the history

of a certain trait or condition throughout a family– Essentially it is a family tree displaying the

inheritance of a specific trait

PEDIGREE CHARTS

• Pedigree charts show only the phenotype of a specific trait (they do not display genotypes)– Estimating genotypes is possible by observing

a pedigree chart• Pedigree charts specific symbols to represent

different types of individuals and relationships•

PEDIGREE CHARTS

• The symbols used in pedigree charts include the following:

= A Male

= A Female

PEDIGREE CHARTS

• The symbols used in pedigree charts include the following:

= Individuals with a trait/condition

= indicates marriage/mating

PEDIGREE CHARTS

• The symbols used in pedigree charts include the following:

• =offspring in birth order • (I & II represents generations)• (individuals are always placed

from left to right according to birth order)

I

II

PEDIGREE CHARTS

• Inheritance Pattern 1

• If Parents without the trait have a child with and without the trait this indicates that the parents are heterozygous.• Because they are heterozygous

and (therefore have a dominant trait) but are not shaded in, we know that the trait is a recessive condition

I

II

PEDIGREE CHARTS

• Inheritance Pattern 2

• If Parents with the trait have a child with and without the trait this indicates that the parents are heterozygous.• Because they are heterozygous

and (therefore have a dominant trait) and are shaded in, we know that the trait is a dominant condition

I

II

PEDIGREE CHARTS

• Inheritance Pattern 3

• If Parents (I) with and without the trait have children with and without the traits this indicates that the parents are homozygous and heterozygous.• This is the only way they can have

children (II) both with and without the trait.

• By looking at the parents from generation (II), we can tell that they both have to be heterozygous, otherwise neither of the children in generation (III) would have the trait.

I

II

III