Mendel and the Gene Idea

BASIC CONCEPTS IN GENETICS

WHAT IS HEREDITYThe passing on of characteristics from parents to offspring

Trait physical characteristicGenetics is the study of heredity

PATTERNS OF INHERITANCE Gregor Mendel

(1822-1884) is the called the “Father of Genetics”

Researched with garden peas

Developed the ideas that are the basis of genetics

MENDEL USED PEAS…Characters (inherited characteristic) are in two distinct forms (such as white and purple color) called traits.

Not many traitsEasy to keep track

The male and female gametes are enclosed within the same flower – He could control the fertilization process

Self-fertilizationCross-pollination

The garden pea is small, grows easily, matures quickly and produces many offspring.

MENDELIAN GENETICSMendel studied a number of characteristics in pea plants including:

• Height - short or TALL• Seed color - green or YELLOW• Seed shape - wrinkled or ROUND• Seed coat color - white or GRAY • Pod shape - constricted or SMOOTH• Pod color - yellow or GREEN• Flower position - terminal or AXIAL

QUESTIONS

What did Mendel cross?What are traits?What are gametes?What is fertilization?What is heredity?What is genetics?

CHROMOSOME LOGICAL STRUCTURE Gene – segments of DNA that code for the basic

units of heredity and are transmitted from one generation to the next

Allele – genes that reside at the same locus on homologous chromosomes

Alleles are the result of mutation!!!!!!

GENOTYPEGenotype

Gene(s) responsible for the traitThe alleles that are present on each homologous chromosome that code for the trait

PHENOTYPEPhenotype

Expression of the characteristicThe trait The way we “look”

Red hair or Brown hairThe expression of the gene

PREDICTING OUTCOMES-PROBABILITY The likelihood that a specific event

will occur Probability = # of one kind of possible

outcome total # of all possible

outcomes a coin lands on “heads”

1 outcome Total possible outcomes

= 2 heads or tails

Possibility that the coin will land on heads = 1/2

GENOTYPES PHENOTYPES

At each locus (except for sex chromosomes) there are 2 genes. These constitute the individual’s genotype at the locus. The alleles that are present.

The expression of a genotype is termed a phenotype. For example, hair color, weight, or the presence or absence of a disease.

MENDEL WAS A CAREFUL RESEARCHER Carefully controlled experiments Studied one trait at a time Kept detailed data Cross - combining gametes

from parents with different traits The offspring are called hybrids

offspring of parents with different traits

A monohybrid cross is one that looks at only one trait (let’s look at plant height – tall or short)

Cross fertilization Pollen from one plant to

fertilize another plant

MENDEL’S MONOHYBRID CROSSES Step One:

Mendel allowed the peas to self-pollinate for several generations.

What Did Mendel Find?

MENDEL’S MONOHYBRID CROSSESStep One:

Each variety was true-breeding for a particular character.

tall plants only produced tall plantsThese plants served as the parental generation. The P generation is the first two individuals that are crossed in a breeding experiment

MENDEL’S MONOHYBRID CROSSES Step Two:

Mendel cross-pollinated two P generation plants with different traits

The offspring were the first filial generation or F1 generation

Mendel recorded the traits of the offspring

What Did Mendel Find?

MENDEL’S MONOHYBRID CROSSES Tall plant crossed with

short plants produced all tall offspring

Purple flowers crossed with white flowers produced all purple offspring

MENDEL’S MONOHYBRID CROSSESStep Three:

Finally, Mendel allowed the F1 generation to self-pollinate.He called the offspring of the F1 generation, the second filial generation, or F2 generationAgain, Mendel recorded the traits of the offspring

What Did Mendel Find?

MENDEL’S MONOHYBRID CROSSES Mendel found that 3 out of 4 (¾) of

the offspring were tall & 1 out of 4 (¼) were short

(the short plants reappeared!!!!!!)

MENDEL’S MONOHYBRID CROSSES Mendel found the same 1:3 ratio

(1 out of 4) in the other traits as well!

MENDEL’S RESULTS He discovered different laws

and rules that explain factors affecting heredity

THEORY OF HEREDITY Before Mendel, people thought offspring were a

blend of traits Tall x short = medium

Mendel’s experiments did not support this theory Mendel’s work led him to the understanding that

traits are carried in pairs (one from each parent)

MENDEL’S 4 HYPOTHESES

Hypotheses 1 For each inherited character, an

individual has two copies of each gene

One on each chromosome Alternative versions of genes

account for variations in inherited characters.

Alleles are different versions of genes that impart the same characteristic.

MENDEL’S 4 HYPOTHESES

Hypotheses 2 Offspring inherit 2 alleles, 1 from each parent

for each characteristic (i.e. height, color, etc.) Gametes (sperm or egg) carry only one allele

as a result of pair separation during meiosis

MENDEL’S 4 HYPOTHESES Hypotheses 3 The Rule of Dominance

If the two alleles differ one, the dominant

allele, is fully expressed in the organism's appearance

the other, the recessive allele, has no noticeable effect on the organism's appearance.

MENDEL’S 4 HYPOTHESES Hypotheses 4

The two genes for each character segregate during gamete production.

The chromosome count is changed from the diploid number to the haploid number.

The genes are sorted into separate gametes, ensuring variation. 

This sorting process depends on genetic recombination

MENDEL’S FIRST LAW OF HEREDITY

Law of Random Segregation A parent randomly passes only one allele for each trait to each offspring

MENDEL’S SECOND LAW OF HEREDITY Law of Independent Assortment

Different gene pairs assort independently in gamete formation.

This Law is only true for genes on separate chromosomes!

LAW OF INDEPENDENT ASSORTMENT

• P GENERATION -True Breeding Parent Plants– All purple (PP)– All white (pp)

• Gametes will be either P or p

• FIRST FILIAL GENERATION F1• F1 are all purple because

of dominance (Pp)• SECOND FILIAL

GENERATION F2• F2 results in a

mathematically predictable 3:1 ratio

PATTERNS OF INHERITANCE A dominant

allele is always expressed

A recessive allele is only visible when paired with another recessive allele.

ALLELES – PATTERNS OF INHERITANCE

To test for patterns of inheritance, Mendel crossed two organisms with different traitsEach had two distinct formsEach allele is represented by a letter

CAPITAL = dominantlower = recessive

Homozygous - if the two alleles for a trait are the same (AA or aa)

• Heterozygous - if the two alleles for a trait are different (Aa)

TT Tt tt

Dominant Trait Recessive Trait

MENDELIAN INHERITANCE

Mendelian inheritance patterns Involve genes directly influencing traits Obey Mendel’s laws

Law of segregation Law of independent assortment

Include Dominant / recessive relationships Gene interactions Phenotype-influencing roles of sex and environment

Most genes of eukaryotes follow a Mendelian inheritance pattern

PREDICTING INHERITANCE Mendel’s Principle of Dominance

Some genes (alleles) are dominant and others are recessive.

The phenotype (trait) of a dominant gene will be seen when it is paired with a recessive gene.

The phenotype of the recessive gene will be “hidden” (not exhibited)

PREDICTING INHERITANCE To determine the chances of inheriting a

given trait, scientists use Punnett squares and symbols to represent the genes.

UPPERCASE letters are used to represent dominant genes.

lowercase letters are used to represent recessive genes.

PREDICTING INHERITANCE For example:

T = represents the gene for TALL in pea plants

t = represents the gene for short in pea plants

So: TT & Tt both result in a TALL plant,

because T is dominant over t. t is recessive.

tt will result in a short plant.

Remember there are two alleles for every trait!

A diagram that predicts the outcome of a genetic crossConsiders all the possible

combinations of gametes1ST DRAW A BIG SQUARE AND DIVIDE IT

IN 4’S

PUNNETT SQUARES

The genes from one parent go here.

The genes from the other parent go here.

PUNNETT SQUARES

T T

t

t

PUNNETT SQUARES

T T

t t

t t

PUNNETT SQUARES

T T

t t t

t t t

PUNNETT SQUARES

T T

t Tt Tt

t t t

PUNNETT SQUARES

T T

t Tt Tt

t Tt Tt

PUNNETT SQUARES

T Tt Tt Tt

t Tt TtF1 generation

PUNNETT SQUARES

The genotype for all the offspring is Tt.The genotype

ratio is: Tt – 4:4 = 100%

heterozygous

The phenotype for all the offspring is tall.The phenotype

ratio is: tall – 4:4 =

100% tall

INTERPRETING THE RESULTS

T Tt Tt Tt

t Tt Tt

T t

T TT Tt

t Tt ttF2 generation

PUNNETT SQUARES

This time the ratios are different! Tt.

The genotype ratio is: TT – 1:4 Tt – 2:4 tt – 1:4 1:2:1 2 – homozygous 2 – heterozygous

The phenotype ratio is: TT, Tt, Tt = 3 tall tt = 1 short 3:1 – tall : short

INTERPRETING THE RESULTS

T tT TT Ttt Tt tt

PUNNETT SQUARES

A. Simple dominance: one allele is completely dominant

P=Aa x Aa A=red, a=clear A a

 A

a

AA Aa

Aa aa

F1 Genotype= AA: Aa: aa

1: 2: 1F1 Phenotype= A (red): a (clear) 3: 1

LAWS OF PROBABILITY HELP EXPLAIN GENETIC EVENTS

Genetic ratios are most properly expressed as probabilities: 3/4 tall: 1/4 dwarf The probability of each zygote having the

genetic potential for becoming tall is ¾ The probability of each zygote having the

genetic potential for becoming small is ¼

Probabilities range from 0 (an event is certain NOT to happen) to 1.0 (an event is certain to happen).

How do we calculate the probability of 2 or more events happening at the same time?

PRODUCT LAW

For simultaneous outcomes (this AND that)

What is the chance that you will roll snake eyes with two dice? (1 and 1) Chance of rolling 1 with first die = 1/6 Chance of rolling 1 with second die =

1/6 Chance of rolling two 1’s = 1/6 X 1/6

= 1/36

PROBABILITIES

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Multiplication RuleThe probability that two independent events, A and B, are realized simultaneously is given by the product of their separate probabilities

What fraction would we expect to be

Round AND Green

3/4 x 1/4 = 3/16

SUM LAW

For outcomes that can occur more than one way (this OR that)

What is the chance that you will roll either a 1 or a 6 with one die? Chance of rolling 1 = 1/6 Chance of rolling 6 = 1/6 Chance of rolling 1 or 6 = 1/6 + 1/6

= 2/6 = 1/3

PROBABILITIES

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Addition Rule The probability that one or the other of two mutually exclusive events, A or B, is the sum of their separate probabilities

What fraction would we expect to be

(Round and Green) OR

(wrinkled and yellow)

3/16 + 3/16 = 6/16

DIHYBRID CROSS F1 produces equal

amounts of 4 possible genotypes

F2 reveals even more genotypic possibilities (9:3:3:1)

Dihybrid cross is equivalent to two monohybrid crosses (12:4 or 3:1)

Illustrates the Law of Independent Assortment

NON-MENDELIAN INHERITANCEMany genes do not follow a Mendelian

inheritance patternIncomplete DominanceCo-dominanceMultiple allelesPleiotropyPolygenic InheritanceEpigenetic inheritance (epistasis)ComplementationExtra-nuclear inheritance

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INCOMPLETE DOMINANCE

The phenotype of the heterozygous genotype is intermediate between the phenotypes of the homozygous genotypes E.g. snapdragons Heterozygotes differ from

homozygotes Predictable 1:2:1 ratio

Different than “blending” hypothesis

No testcross necessary

CODOMINANCEBoth alleles are dominant and affect the phenotype in two

different but equal waysAndalusian chickens show this pattern of inheritance.If you cross a black (BB) chickenWith a white (WW) chickenYou get black+white speckled (BW) chicken

MULTIPLE ALLELES More than two possible alleles controlling one trait Example3 alleles in blood type – OAB

4 possible phenotypes = O, A, B, AB 6 possible genotypes

Note: this is also an example of co-dominance

MULTIPLE ALLELESGenes that have more than two alleles

More than two alleles exist in a given population

However, any one individual only has two of these allelesExample - Coat color in rabbits (4 alleles)

Himalayan Rabbit Full Color Rabbit Albino Rabbit Chinchilla Rabbit

PLEIOTROPY Gene influences

multiple characteristics

A singe gene influences more than one phenotypic trait.

Genes that exert effects on multiple aspects of physiology or anatomy are pleiotropic

POLYGENIC INHERITANCE Multiple genes

have an additive effect on a single character in the phenotype Example: Skin

Color or height Usually is

described by a bell-shaped curve with majority clustered in the middle

EXAMPLES OF POLYGENIC INHERITANCE

LETHAL DOMINANCET/t x T/t =T/T T/t t/t1 : 2 : 1 ratio at conception0 : 2 : 1 ratio at birth

CHARACTERS INFLUENCED BY THE ENVIRONMENT

pH of the soil will change the color of hydrangea flowers from blue to pink

CHARACTERS INFLUENCED BY THE ENVIRONMENT

Temperature will affect color change in fur

CHARACTERS INFLUENCED BY THE ENVIRONMENT

Body temperature affects color in Siamese cats

Height is affected by nutrition

HOW DO WE DETERMINE INHERITANCE OF HUMAN TRAITS

Pedigree Analysis In humans, pedigree analysis is used to determine

individual genotypes and to predict the mode of transmission of single gene traits

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SOME EXAMPLES OF DOMINANT AND RECESSIVE TRAITS IN HUMANS (AT ONE GENE LOCUS)

Huntington disease is a progressive nerve degeneration, usually beginning about middle age, that results in severe physical and mental disability and ultimately in death

Every affected person has an affected parent Two unaffected parents will not produce affected children.

(aa x aa) Both males and females are affected with equal frequency. Pedigrees show no Carriers.

AUTOSOMAL DOMINANT TRAITS

EXAMPLES OF AUTOSOMAL DOMINANT DISORDERS

Dwarfism Polydactyly and Syndactyly Hypertension Hereditary Edema Chronic Simple Glaucoma – Drainage system for fluid in the eye does not work and

pressure builds up, leading to damage of the optic nerve which can result in blindness. Huntington’s Disease – Nervous system degeneration resulting in certain and early

death. Onset in middle age. Neurofibromatosis – Benign tumors in skin or deeper Familial Hypercholesterolemia – High blood cholesterol and propensity for heart disease Progeria – Drastic premature aging, rare, die by age 13. Symptoms include limited

growth, alopecia, small face and jaw, wrinkled skin, atherosclerosis, and cardiovascular problems but mental development not affected.

AUTOSOMAL RECESSIVE TRAITS Albinism = absence of pigment in the skin, hair, and iris of the eyes Most affected persons have parents who are “normal” (Aa x Aa) The parents are heterozygous for the recessive allele and are called carriers

(Aa) Approximately 1/4 of the children of carriers are affected (aa) Close relatives who reproduce are more likely to have affected children. Both males and females are affected with equal frequency. Pedigrees show both male and female carriers.

EXAMPLES OF AUTOSOMAL RECESSIVE TRAITS

Congenital Deafness Diabetes Mellitus Sickle Cell anemia Albinism Phenylketoneuria (PKU) – Inability to break

down the amino acid phenylalanine. Requires elimination of this amino acid from the diet or results in serious mental retardation.

Galactosemia – enlarged liver, kidney failure, brain and eye damage because can’t digest milk sugar

Cystic Fibrosis – affects mucus and sweat glands, thick mucus in lungs and digestive tract that interferes with gas exchange, lethal.

Tay Sachs Disease – Nervous system destruction due to lack of enzyme needed to break down lipids necessary for normal brain function. Early onset and common in Ashkenazi Jews; results in blindness, seizures, paralysis, and early death.

MENDEL AND HEREDITY

Why we look the way we look...