MENDELIAN GENETICS AND

INHERITANCE

VOCABULARY

characteristic – observable feature i.e. flower

color, height, hair color

trait – particular form of character i.e. white flower,

5’2”, blond

heritable – trait passed on from parent to offspring

genotype – the alleles for the trait

phenotype – the appearance of the genotype

REPRODUCTION

• asexual

a type of reproduction

where an organism

replicates itself, by

budding or dividing,

without the involvement

of other organisms

• sexual

production of new

generations involving the

exchange of

chromosomes from both

a male and female parent

INTRODUCTION

Before scientists discovered DNA they knew that plants

reproducing asexually would give offspring exactly the

same as the parents, but reproducing sexually can give

offspring very different from the parents.

Genes are expressed as physical characteristics.

PHENOTYPE

MORE SPECIFICALLY:

• There are different versions of the same gene (alleles) that account for various traits of inherited characteristics.

• An organism inherits 2 alleles, one from each parent, for every characteristic.

• A sperm or ovum carries only one allele for each inherited trait.

• If the 2 alleles differ, one is usually dominant and the other is usually recessive.

For Example:

There are different versions of the same gene.

For example of the gene for the characteristic of

tongue rolling,

there are two different

alleles that produce the

trait –

Tongue

rolling

can roll tongue cannot roll tongue

HOW SINGLE-GENE TRAITS ARE INHERITED

• An Austrian monk

Gregor Mendel

conducted experiments

with garden peas.

• Over 12 years he laid

the ground work for

what is now called the Chromosomal Theory of

Inheritance.

Mendel was interested in how

characteristics were passed on from one

generation to the next.

He observed that sometimes, offspring would

show traits from only one parent, both parents,

and sometimes neither!

He began a series of experiments

on garden peas to study this

phenomenon.

Mendel’s work was virtually ignored for 100 years.

His papers were re-discovered in the early

1900’s, and became the foundation of our

understanding of genetics.

THE SIGNIFICANCE OF HIS METHODS

• He concentrated on one trait at a time.

• He used large numbers so his data was statistically sound.

Mendel did 4 things that made his work

significant:

• He combined the results of many identical experiments.

• He use mathematics to analyze his results.

• Mendel found that each variety of garden pea he examined had its own recognizable traits that were passed on.

• He was able to study 7 individual traits.

parent

parent

F1

F2

100% one trait

75% one trait

25% other trait

DOMINANT AND RECESSIVE TRAITS

• Mendel found if you took a yellow seed plant variety and

a green seed plant variety, and bred them, the first

generation (F1) only showed either the yellow or green

color.

• But in the next generation (F2) the other seed color

returned.

• So, the trait is still there even if it didn’t show in the first

generation.

parent

parent

F1

F2

100% one trait

75% one trait

25% other trait

“F” stands for filial

SEGREGATION

When Mendel performed cross-pollination between a true-breeding yellow pod plant and a true-breeding green

pod plant, he noticed that all of the resulting offspring,

F1 generation, were green.

He then allowed all of the green F1 plants to self-pollinate. He referred to these offspring as the F2 generation. Mendel noticed a 3:1 ratio in pod color. About 3/4 of the F2 plants had green pods and about 1/4 had yellow pods.

• Mendel found that the green pods showed up in the first generation masking the yellow pod characteristic, but because the yellow pod showed up in the second generation, it still existed.

• Mendel named traits that take precedence –

DOMINANT and

• traits that are latent (not expressed) -

RECESSIVE

EVERY TRAIT

HE TESTED

GAVE THE

SAME RATIO

OF RESULTS

for all traits

P D x r

F1 100% D

F2 75% D and

25% r

Mendel correctly concluded there are different versions of the same gene (alleles) that account for variations in inherited traits

We now call those versions - alleles

An organism inherits 2 alleles, one from each parent, for every characteristic.

The location of the alleles is called

the gene loci

Gene for making earwax

Gene for widows peak

Gene for making the protein that

dissolves the webbing between your

fingers and toes before you’re born.

While the homologous

chromosomes carry the same

gene (for a characteristic) they

have different forms which

produce the specific trait. These

different forms are called alleles.

Gene. A portion of a

chromosome that serves as

the basic unit of heredity.

Genes control

characteristics that an

offspring inherits.

From these experiments Mendel formulated what is now

known as Mendel's Law of Segregation.

This law states that allele pairs separate (or segregate) during gamete formation, and randomly unite at fertilization.

• If the offspring

inherits 2 of the

same allele –

homozygous

If the offspring

inherits 2

different alleles -

heterozygous

But – you cannot tell by visual inspection the

difference between homo- and heterozygous

dominant

Which flower is PP and which is Pp?

Purple

flower

color is

dominant

the actual genetic combinations (the inherited alleles) are

known as genotypes*,

the appearance resulting from the genotype is the

phenotype*, of the organism.

*genotype: the

inherited alleles *phenotype: the

appearance

INHERITANCE IS DESCRIBED BY GENOTYPE

For example:

the yellow allele is dominant over the not-yellow allele, so if the organism is YY the organism is called “homozygous dominant”.

THE PUNNETT SQUARE

• We can “map” the offspring possibilities of any parental cross using the Punnett Square.

• The genotype of the parents are segregated and all possible offspring combinations are shown.

Punnett squares are used to figure out both genotype and phenotypes

genotypes and phenotypes can be expressed as ratios or %

HOW TO CONSTRUCT A PUNNETT

SQUARE

THE TEST CROSS

A way to find out

whether an individual

showing a dominant

trait is homozygous

or heterozygous.

The individual in question is crossed with an

individual known to be homozygous for the

recessive trait

tuck this away….

Natural selection only operates on an

organism’s phenotype (hidden recessive

traits are immune to selection).

Only when the homozygous recessive

trait expresses itself – is it open to

selection.

Justify with a punnett square

Based on what you know of dominant and recessive

inheritance and ratios of inheritance patterns in the

F1 – predict the genotype of the parents that produce

100% flat head top offspring.

MONOHYBRID CROSSES

• Cross that involves one pair of contrasting traits

• Short hair (L) is

dominant to long hair

(l) in mice. What is the

genotype and

phenotype ratio of a

heterozygous short-

haired mouse crossed

with a long-haired

mouse?

L l

l Ll ll

l Ll ll

Genotype ratio: 50% Ll: 50% ll Phenotype ratio:

50% short hair: 50% long hair

In humans, being a tongue roller (R) is dominant

over non-roller (r).

A true breeding man for non-rolling marries a

woman who is heterozygous for tongue rolling.

R R

r Rr Rr

r Rr Rr

Genotypes: 100% heterozygous

Phenotype ratio: 100% rolling

MAKING BABIES

WHAT COULD YOUR OFFSPR ING INHERI T?

A COUPLE OF

MISCONCEPTIONS:

The Relation Between Dominance and Phenotype

• Dominant and recessive alleles do not

“interact” (one doesn’t prevent the other)

• Lead to synthesis of different proteins that

produce a phenotype

Frequency of Dominant Alleles

Dominant alleles are not necessarily more common

in populations than recessive alleles

THE SPECTRUM OF DOMINANCE

• Complete dominance

• Occurs when the phenotypes of the heterozygote

and dominant homozygote are identical

• Incomplete Dominance – blending of

traits in heterozygote.

CRCR = red

CRCW = pink

CWCW = white

CODOMINANCE

Roan coats in horses

Some white hairs, some red hairs

• Codominance –both alleles are expressed

at the same time.

Gene: flower petal color

Possible (alleles) phenotype: Red (R)

White (W)

genotype RR RW WW

phenotype red petals Red and

white White petals

EXAMPLE: THE HUMAN BLOOD GROUP

SEX LINKED (X-LINKED) TRAITS

Genes that exist

on the X

chromosome but

are absent on

the Y.

Discovered in 1910, T.H. Morgan

EXAMPLE: HEMOPHILIA genetic blood disorder that impairs the body’s ability to control blood

clotting or coagulation.

hemophilia

Britain’s Queen Victoria (1819 – 1901) passed

the disease through her heirs into royal houses

across the continent. It is widely believed Queen

Victoria carried a spontaneous mutation.

Victoria Eugenia, granddaughter of

Queen Victoria married King Alfonso

XIII of Spain. Since she was carrying

the disease, she brought the

hemophilia gene into the Spanish royal

family.

The most famous case is that of Tsarevich Alexei of

Russia. One of Queen Victoria’s granddaughters married

Nicholas II of Russia. She became Empress Alexandra

Feodorovna. In 1904, a long-awaited male heir was born:

Tsarevich Alexei, but unfortunately he was a suffering

from Hemophilia B.

Symptoms:

• Spontaneous bleeding

• Bruising, especially a

large, lumpy bruise.

• Bleeding for no known

reason.

• Blood in the urine or

stool.

• Bleeding that does not

stop after getting a cut or injury

Hemophilia can be

mild, moderate or

severe based on the

levels of clotting

factors. (determined by

inheritance)

No cure.

Current treatment: IV replacements of clotting factors periodically

throughout the person’s life.

Hemophilia in females is rare. Why?

Breakthrough: Gene therapy

An experimental gene therapy improved symptoms for as long as

4 years in men with severe hemophilia.

The study shows the potential for gene therapy as a safe,

effective approach for treating this and other genetic disorders.

Using a virus as a

vector, human factor

IX gene is inserted into

the vectors. The

viruses deliver the

gene into the cells

they infect. The cells

then manufacture

functional protein.

UCL Cancer Institute

Example: red/green color

blindness

TRY THIS PROBLEM:

a. If a man and a woman, both with normal vision,

marry and have a colorblind son, draw the

Punnett square that illustrates this.

b. If the man dies and the woman remarries to a

colorblind man, draw a Punnett square showing

the type(s) of children could be expected from

her second marriage. How many/what

percentage of each could be expected?

PRACTICE! O O M P A L O O M P A G E N E T I C S

THE DIHYBRID CROSS

INHERI T ING MULT IPLE ALLELES

Law of independent Assortment

Each pair of alleles sort themselves

independently during gamete formation

LAW OF INDEPENDENT ASSORTMENT

• Mendel identified his second law of inheritance by

following two characters at the same time

• Crossing two, true-breeding parents differing in two

characters produces dihybrids in the F1 generation,

heterozygous for both characters

Involves two different

characteristics:

• Coat color

• Tail length

9:3:3:1

4 phenotypes

LET’S TRY ONE

WHAT ABOUT 3 DIFFERENT

CHARACTERISTICS?

That’s a 64 square Punnett square!!!

No thanks, can’t I just use math?

THE LAWS OF PROBABILITY GOVERN

MENDELIAN INHERITANCE

• Mendel’s laws of segregation and independent

assortment reflect the rules of probability

• The multiplication rule

• States that “the probability that two or more

independent events will occur together is the

product of their individual probabilities”

• A multicharacter cross

• Is equivalent to two or more independent monohybrid crosses

occurring simultaneously

• In calculating the chances for various genotypes from

such crosses Rr

Segregation of

alleles into eggs

Rr

Segregation of

alleles into sperm

R r

r R

R

R

R 1⁄2

1⁄2 1⁄2

1⁄4 1⁄4

1⁄4 1⁄4

1⁄2 r

r R r

r

Sperm

Eggs

Each character first is considered separately

and then the individual

probabilities are

multiplied together

25% RR: 50% Rr: 25% rr

FOR EXAMPLE:

• In guinea pigs, the allele for short hair (S) is

dominant to long hair (s), and the allele for black

hair (B) is dominant over the allele for brown hair

(b).

• What is the probable offspring phenotype ratio for a

cross involving two parents that are heterozygotes

for both traits? • Short hair = dominant = SS or Ss

Long Hair = recessive = ss Black coat = dominant = BB or Bb

Brown coat = recessive = bb

• SsBb x SsBb

• Gametes: SB, Sb, sB, sb

and SB, Sb, sB, sb

SB Sb sB sb

SB SSBB Short hair

SSBb Short hair

SsBB Short hair

SsBb Short hair

Sb SSBb Short hair

SSbb Short hair

SsBb Short hair

Ssbb Short hair

sB SsBB Short hair

SsBb Short hair

ssBB Long hair

ssBb Long hair

sb SsBb Short hair

Ssbb Short hair

ssBb Long hair

ssbb long hair

GUINEA PIG PUNNETT SQUARE

9:3:3:1

USING THE MULTIPLICATION RULE: Each character first is

considered separately

and then the individual

probabilities are

multiplied together What is the probable offspring that will

be brown with short hair from a cross

involving two heterozygous parents?

In a monohybrid cross for coat color alone (black dominant

to brown) there would be a ¼ chance of brown hair.

In a monohybrid cross for short hair alone (short dominant to long)

there would be a ¾ chance of short hair.

¼ X ¾ = 3/16

COMPLEX PATTERNS OF INHERITANCE

EP I S TAS IS , POLYGE NIC , S E X - INF LU E NCED, L INKED

GENES , EP IGENET ICS

INHERITANCE PATTERNS ARE OFTEN MORE

COMPLEX THAN PREDICTED BY SIMPLE

MENDELIAN GENETICS

The expression of inherited genes (phenotype) are

frequently influenced by a variety of external

influences such as other genes and the environment.

A gene is not a

hermit!!!

It will often interact

with other genes.

POLYGENIC INHERITANCE

AaBbCc AaBbCc

aabbcc Aabbcc AaBbcc AaBbCc AABbCc AABBCc AABBCC

20⁄64

15⁄64

6⁄64

1⁄64

• A single characteristic controlled by more

than one gene.

Ex. Skin color

Polygenic inheritance

often results in a bell

shaped curve when

you analyze the

population

TED talk – Skin Color is an

illusion

Often these traits are in fact

controlled by many genes

on many chromosomes.

Each dominant allele has

an additive effect, so the

resulting offspring can have

a variety of genotypes, from

no dominant alleles to

several dominant alleles.

An additive effect of two or more genes on a

single phenotype

height

• Many human characters are controlled by several

genes

The specific degree of a complex trait is very difficult to predict

from one generation to the next because the precise

combination of genes contributing to the trait cannot be

predicted either (or, perhaps, even known).

FOR COMPLEX TRAITS THERE IS NO OBVIOUS PATTERN FROM ONE GENERATION TO THE NEXT.

Most polygenetic traits are partially influenced by the environment.

Examples of environmentally influenced traits are

susceptibility to heart disease, certain types of cancer

and mental illnesses like schizophrenia and depression.

A person may be genetically predisposed to have

depression, so when that person's environment

contributes major stresses like losing a job or losing a

close relative, the person is more likely to become

depressed.

IS AUTISM INHERITED?

Autism spectrum disorders (ASD) are

among the most heritable of all

neuropsychiatric conditions. Yet, most

genetic links to ASD found in recent

years have involved de novo mutations,

which are not passed from parent to

child, but instead arise spontaneously.

Moreover, researchers found that many of these

partially-disabling mutations occurred in genes in

which a complete disruption of the gene has been

known to cause more severe or even fatal inherited

diseases.

Recent studies attribute ASD to a homozygous recessive

condition . Researchers found that autism risk could be

attributed to inherited mutations that result in a partial

loss of gene function.

EPISTASIS

• One gene alters the phenotypic expression

of a totally different gene at a second locus.

Gene “A” blocks the effect of gene “B”

EPIGENETICS

ENVIRONMENTAL INFLUENCES

NATURE AND NURTURE: THE ENVIRONMENTAL IMPACT ON PHENOTYPE

• When the phenotype for a character

depends on environment as well as on

genotype.

• the phenotypic range of a particular genotype is

influenced by the environment

Figure 14.13

THE PEDIGREE

TRACKING AND PREDICT ING INHERI TANCE

When geneticists look for evidence of genetic influence on a

disease, such as heart disease or mental illness, they look for

families that have many affected over several generations.

• A pedigree • Is a family tree that shows the interrelationships of parents and

children across generations

• Inheritance patterns of particular traits can be traced

and described using pedigrees

Ww ww ww Ww

ww Ww Ww ww ww Ww

WW

or

Ww

ww

First generation

(grandparents)

Second generation

(parents plus aunts

and uncles)

Third

generation

(two sisters)

Ff Ff ff Ff

ff Ff Ff ff Ff FF or Ff

ff FF

or

Ff

Widow’s peak No Widow’s peak Attached earlobe Free earlobe

(a) Dominant trait (widow’s peak) (b) Recessive trait (attached earlobe)

UNDERSTANDING PEDIGREE SYMBOLS

It is important to be able to interpret pedigree charts in order to

predict the pattern of a disease or condition.

Specifically, using a pedigree chart, you can tell if the disease or

condition is autosomal, X-linked, dominant, or recessive and how

likely it is to show up in a given generation.

Symbols are used to represent people and their relationships

deceased

UNDERSTANDING PEDIGREE SYMBOLS

Shown in birth order

READING PEDIGREES

How to solve 2 How to solve 1

How to solve example