Chapter 11

The first scientists to study the laws of heredity

had some difficult initial problems to solve

Two parents have to contribute equally to make one

child

Sometimes offspring show similar traits to their

parents while in other ways they show traits that don’t

appear related to their parents in any way

Mixed breeds: two different species can sometimes

produce offspring

Laws of heredity must explain how two parents

can allow their traits to mix with each other and

make a child (the process is stable, with chaos).

When Charles Darwin wrote Origin of Species he was attempting to explain how species evolve from one generation to the next He had to explain how traits are passed for this to make

sense

Darwin proposed a “blending” theory, which says that parent genes (called particles at the time) blend traits to produce an offspring

In the meantime an Austrian monk named GregorMendel attempted to mathematically explain the change from one generation to the next (Mendel was able to explain “how” genes pass from

parent to offspring even though “genes” as we know them wouldn’t be officially discovered for another 100 years)

Mendel was a mathematician, so he attempted to

explain heredity using statistics and data

He used the common garden pea for his experiments

The pea is easy to cultivate, has a short generation time, and

can be self-pollinated or cross-pollinated

The first experiment involved crossing (mating) gametes

of a tall pea plant with gametes of a short pea plant.

Gametes are male and female sex cells (egg/sperm, etc)

This first generation is called the P generation

Hypothesis: “If the blending theory is correct, then all

offspring should be medium length because each

receive an equal share of genes from their parent”

The first offspring generation is called the F1 generation

Mendel crossed plants thousands of times to ensure

the accuracy of his data

When the F1 generation grew, his results were

contrary to his hypothesis: 100% of the plants were

tall plants. Only one parental trait was passed on.

Did the “short” parental genes disappear?

Mendel then crossed members of the F1 generation

with each other to produce the F2 generation.

The results: 787 tall plants and 277 short plants

The ratio was pretty close to 3:1 (74% : 26%)

The “short” trait had disappeared for a generation,

but reappeared later

Perhaps, however, this was just a trait with the

height of plants

Mendel repeated the same experiments over the

next few months with the following traits: pea pod

shape, seed shape, pod color, flower color, seed

color, flower position.

Each time, the same results: 100% of one trait in

the F1 generation, a 3:1 ratio in the F2.

*Note: this ratio represents the simplest type of gene.

Only a small percentage of all genes in all organisms are

actually this simple and easy to calculate. And Mendel

happened to pick those genes.

In other words…Mendel got really lucky.

After analyzing all possible mathematical

explanations for his results, Mendel wrote his first

of two laws: The Law of Segregation

Each organism has two factors for each trait

When gametes form in the organism, each gamete

contains only one of the two factors

When gametes fertilize, each new organism contains

one factor from each parent for each trait

We now know that these “factors” are the strands

of DNA that contain our genes

Each gene has a minimum of two possible alleles

An “allele” is an alternate form of the same gene

Gene: plant size. Alleles: tall and short

One of these genes is a dominant allele and the

other is recessive

Dominant alleles means the trait they code for will

always appear in an organism

Recessive alleles can be masked (covered, but not

absent) by the dominant allele

Because you receive a set of genes from each parent, eukaryotic organisms all have two alleles for each gene (one from mom, one from dad)

The combination of alleles an organism has is called their genotype Each allele in a genotype is given a single-letter label

Capital letters are dominant, lower-case are recessive

The actual trait that appears in an organism is called a phenotype T=tall plants, t=short plants

TT or Tt genotypes = tall phenotypes

tt is only genotype that codes for a short phenotype

Genotypes with the same allele are called homozygous. Different alleles are called heterozygous

Mendel noticed that in his crosses different

combinations of genes always occurred

You never ALWAYS had green peas with tall plants. You could also

have yellow/tall, green/short, yellow/short

This must mean that genes are not connected to each

other.

This led to Mendel’s second law, the Law of Independent

Assortment

Each gene separates independently from itself

Every theoretical combination of alleles is possible within an

individual organism

Mendelian, or Simple Heredity traits, are traits with the

following three rules: the trait is found on only one

gene, has only two alleles, and one allele is dominant

over the other

There are many traits that break one of the three

Mendelian rules

Incomplete dominance

In incomplete dominance, the heterozygote phenotype

actually is a blending of the two alleles

Snapdragon flower color is an incomplete dominance

trait

R=red flowers

R’=white flowers

RR= red flowers

R’R’= white flowers

RR’= pink flowers

Hair style (straight, wavy, curly) is an incomplete

dominance trait in humans

Codominance

Codominant traits are traits that show both alleles

equally in the genotype

Dairy cow fur has is a codominant trait

B = Black fur

W = White fur

BB = All black

WW = All white

BW = Black and White spotted

A human example of codominance is sickle blood

cells

The two traits are round blood cells and sickle-shaped

blood cells

Multiple Allele Traits

Multiple allele traits are traits that have three or more

alleles

The alleles all have an order of dominance

Labrador fur shows multiple alleles

Y=Yellow

Y1=Black

Y2=Chocolate

YY; YY1; YY2= Yellow

Y1Y1; Y1Y2 = Black

Y2Y2 = Chocolate

For humans, a multiple allele trait is blood type

Your immune system must recognize the difference between foreign substances and your own blood

To do this, your blood has specific proteins called antigens on its plasma membrane. Antigens are glycolipids

Your immune system recognizes these proteins and knows that the blood cell belongs to you and isn’t an intruder

The different antigens are labeled A and B

Alleles: IA=A-Type Blood; IB=B-Type Blood;

i=Neither type

There are 4 possible phenotypes of blood, arising

from 6 possible genotypes

Genotype Antigens Present Phenotype (Blood Type)

IAIA (AA); IAi (AO) A-Type only Type A

IBIB (BB); IBi (BO) B-Type only Type B

IAIB (AB) Both A and B Types Type AB

ii (O) Neither A or B Types Type O

It is important for you to know what your blood type is BEFORE you get a blood transfusion

If you get blood with a different protein than what your immune system is used to, it will attack the blood

This results in blood clots and, usually, is deadly

(Because “O” type blood has NO proteins on it, your cells won’t recognize the WRONG proteins.)

If you have this blood type… …You can receive these blood types

Type A Type A or Type O

Type B Type B or Type O

Type AB Type A or Type B or Type O

Type O Type O only

Polygenic Traits

Polygenic traits follow all normal Mendelian rules, but

are combinations of multiple different genes

Seed color in wheat: three different genes

Human height and skin color: an unknown number of

genes, but we think its between 5 and 12.

Polygenic traits tend to result in what appears to be

multiple or infinite different phenotypes

This is Dolly and Wally. Dolly and Wally have brown hair.

Each of them have six genes for hair color, a total of 12

alleles. Each of them also has three alleles for each gene

(black, brown, red and blonde).

If they have three children, is it possible for these children to

have black, red, or blonde hair?

WallyDolly

Dolly Wally

Their first child is Holly. She has black

hair, because she received a total of six

black alleles, four brown alleles, and one

each of red and blonde.Holly

WallyDolly

HollyOllie

Their second child is

Ollie. He’s a redhead,

because his parents gave

him a total of six red

alleles, four blonde

alleles, and two brown

alleles

WallyDolly

HollyOllie

Molly

Their third child is Molly. She has blonde hair because

her parents gave her six blonde alleles, three brown

alleles, two red alleles and only one black allele.

Oh what a diverse family of hair color!

Epistasis

Epistasis is when one gene masks the effect of another

gene.

Albinism in humans is an example of epistasis

Humans have multiple genes for what their skin color

will be.

The different tones of color are controlled by how much

melanin is produced by skin cells

The more melanin, the darker the skin

They have another gene elsewhere that controls whether

or not melanin will be produced

If no melanin is produced, then the organism will be an albino

and their skin color genes no longer matter

These diseases are recessive alleles that can be

passed from parents to offspring

Tay-Sachs disease

Neurological impairment at 7-8 months old

Blindness, seizures, paralysis possible

Cystic Fibrosis

Mucus forms in the bronchial tubes and prevents lungs

from working properly

CF children develop more slowly and only live to 20-30

years

Phenylketonuria

Cannot digest the amino acid phenylalanine

Results in severe mental disability

Neurofibromatosis

Tumors covering the nerve endings may cause

deformations in bone and tissue structure

Huntington’s Disease

Brain cells begin to deteriorate at age 40

Victims lose motor and cognitive function as well

You can be tested to see if you have the gene for

Huntington’s, but there is no cure

Question: If these diseases are dominant, how come

hardly anyone ever get’s them?

Humans have a total of 46 chromosomes (23 from

mom, 23 from dad

Homologous chromosomes #1-22 are all called

autosomes. These chromosomes contain the same

genes no matter which parent they came from

Not necessarily the same alleles.

There is one set of homologous chromosomes that

may be different. These are the sex chromosomes

Two “X” chromosomes (Female—XX)

One “X” chromosome, one “Y” chromosome (Male—XY)

These chromosomes obviously contain gender-

determination genes, but they have other genes as well

that don’t relate to gender.

Any trait controlled by one of these chromosomes is

called a sex-linked trait.

Females have two X chromosomes. Therefore,

mothers can only donate an X chromosome to

their offspring

Males have both an X and Y chromosome.

If Dad donates an X chromosome, the offspring will be

female.

If Dad donates a Y chromosome, the offspring will be

male.

A father cannot pass a sex-linked Y-chromosome

trait to his daughter OR an X-chromosome trait to

his son.

A female will never show a phenotype from a Y-

chromosome’s gene

X-chromosome sex-linked traits are harder to track

Males only have one X chromosome. They will only have

one allele for any X-linked trait

Dominance/recessiveness doesn’t apply.

Females have two X chromosomes, so they will have two

alleles for the trait.

Dominance/recessiveness still applies

It is harder for females to have a recessive X-linked

phenotype than it is for males. She needs 2, he only

needs one

Phenotype ratios for sex-linked traits are

different depending on the gender of the

offspring

Muscular Dystrophy

Muscles are weak, to the point where the victim loses

almost all use of their muscles

Death usually results by age 20

Color Blindness

Color-blind people have difficulty distinguishing colors,

particularly in the red/green spectrum

Hemophilia

Hemophilia is an absence of the ability to clot blood.

Fragile X syndrome

A form of mental disability, but victims are able to live

to become a grandfather

Sometimes organisms have a gene for a specific

trait, but the trait is not expressed because of

the organism’s gender.

A sex-determined trait is when a trait only

appears in a certain gender

Hormones produced by other genes block sex-

determined genes from expressing in an

organism

Reason: Parents of one gender may not express the

same traits as children of the opposite gender.

However, parents still need to pass all necessary

genes to their child regardless of age.

Both men and women produce testosterone and estrogen.

Around puberty, the body begins to produce higher amounts of one or the other

Your gender determines which structures your body will form (testes or ovaries), and these structures produce high quantities of testosterone and estrogen, respectively

Therefore, even though you have the gene to produce both hormones, your gender decides which you will produce more of.