BEYOND MENDEL’S LAWS Mendel studied traits that were purely dominant or recessive. Unfortunately, in nature, there exists more than simply two choices.

BEYOND MENDEL’S LAWS

Mendel studied traits that were purely dominant or recessive. Unfortunately, in nature, there exists more than simply two choices for some traits.

Snapdragons can be white, red, pink, yellow, or orange.

• Incomplete dominance• Codominance• Multiple alleles• Polygenic inheritance• Linked genes• Sex-linked inheritance

Lesson 2.

INCOMPLETE DOMINANCENeither trait is dominant or recessive.

A heterozygous individual is a blend of the two traits.

Ex. Snapdragons

Instead of using R and r, we use FR and FW to show

incomplete dominance.

Red = FR FR

Pink = FR FW

White = FW FW

INCOMPLETE DOMINANCE

Cross a Red Flower with a White flower:

If we crossed two plants from F1, we would get:

CO-DOMINANCE

BOTH alleles for a trait are dominant, a heterozygous individual expresses both traits.

Ex. Black rooster FBFB x white hen FWFW.

The F1 are all ‘barred’ (FBFw

black and white feathers)

EXAMPLE

In four o’clock plants, red flowers are incompletely dominant over white flowers. Heterozygous flowers are pink. If a red flower is crossed with a white flower what is the colour of:

a)The F1 generation?

b)The F1 generation crossed with the red parent?

c)The F1 generation crossed with the white parent?

EXAMPLE

A cross between a yellow snake & a blue snake produces offspring that are all green: 

a)What are the genotypes of the parent generation (P)?

b)What are the genotypes of the F1 generation?

c)What would be the phenotypic ratios of offspring produced by two green snakes?

What is Sickle Cell Anemia (SCA)?• First described in Chicago in

1910 by James Herrick as an inherited condition that results in a decrease in the ability of red blood cells to carry oxygen throughout the body

– Sickle red blood cells become hard and irregularly shaped (resembling a sickle)

– Become clogged in the small blood vessels and therefore do not deliver oxygen to the tissues.

– Lack of tissue oxygenation can cause excruciating pain, damage to body organs and even death.

Some Genetic History The error in the hemoglobin gene results from a genetic

mutation that occurred many thousands of years ago in people in parts of Africa, the Mediterranean basin, the Middle East, and India.

A deadly form of malaria was very common at that time

Malaria epidemics caused the death of many

In areas where malaria was a problem, children who inherited one sickle hemoglobin gene and who, therefore, carried the sickle cell trait - had a survival advantage.

Unlike the children who had normal hemoglobin genes, they survived the malaria epidemics they grew up, had their own children, and passed on the gene- for sickle hemoglobin.

Genetics 2 copies of the

gene for Hb (each parent)

HbS –Recessive S=Sickle A=Normal

Multiple Alleles: Human Blood Groups

• In humans a single gene controls a person’s ABO blood type

• This gene determines what type of an antigen protein is attached to membrane of a red blood cell

• Three alleles A,B and O• Combination of three alleles make 4 different

blood types ___________

BLOOD TYPES• A human blood group or “type” is a group

of red blood cells that have the same antigen protein markers on the cell surface

• There are four human blood groups:

• A, B, AB, O

Type A produces antibodies against Type B antigens

Type AB produces no antibodies against blood antigens

Type B produces antibodies against Type A antigens

Type O produces antibodies against both Type A & Type B antigens

BLOOD TYPES

• AB = Universal recipient (can receive from anyone)• O = Universal donor (can donate to anyone)

• Each blood type has + and – versions (Rh factor)• + = Universal recipient (can receive from anyone)• – = Universal donor (can donate to anyone)

Blood Type % Amongst Canadians

Can Receive

From

Can Donate To

A 42 A, O A, AB

B 9 B, O B, AB

AB 3 A, B, AB, O AB

O 46 O A, B, AB, O

EXAMPLE

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

Sickle Cell Trait

Sickle haemoglobin (S) + Normal haemoglobin (A) in RBC Adequate amount of normal Hb (A) in red blood cells

RBC remain flexible Carrier Do Not have the symptoms of the sickle cell Heterozygous advantage (HbA HbS)

Polygenic Inheritance

• Display continuous variation, in which phenotypes vary gradually from one extreme to another– Height, skin colour, eye

colour

• Variation is controlled by more than one gene; polygenic trait

Polygenic inheritance explains this…

Or this…

Inheritance of linked genes

Lack of independent assortment with linked genes

Only way for Pl and pL is crossing over – 3% of the time

SEX-LINKED INHERITANCE

• Using fruit flies astest subjects, ThomasMorgan studied eyecolour using simplemonohybrid crosses.

Red eyes (R) aredominant over whiteeyes (r).

SEX-LINKED INHERITANCE

• When he crossed

purebred white-eyed

males with red-eyed

females, he was unable to

produce a female with

white eyes.

• He concluded that the

gene must be located on

the X chromosome.

SEX-LINKED INHERITANCE

• Some traits are located on the sex chromosomes, so the inheritance of these traits depends on the sex of the parent carrying the trait.

SEX-LINKED INHERITANCE

• Most known sex-linked traits are X-linked (carried on the X chromosome). This is probably because the X chromosome is much larger than the Y chromosome

SEX-LINKED DISORDERS

• Some sex-linked traits are associated with disorders.

• Most are found on the X chromosome, Y-linked disorders are rare.

• Males are at a much greater risk for inheriting sex-disorders because they only inherit one X, so if the X has the allele for the disorder, they will suffer from the disorder.

• Recessive lethal X-linked traits result in death.

EXAMPLES OF SEX-LINKED TRAITS and DISORDERS

• Red-green colour vision deficiency (CVD) , hemophilia, X-linked severe combined immunodeficiency (SCID)

Hemophilia

• Condition that affects body’s ability to produce proteins involved in blood clotting

• X-linked recessive

• “Royal Disease”

Symptoms

• Uncontrolled bleeding “bleeds”

• Most bleeding is internal and often into joint spaces

• Prior to 1960, life expectancy was 11 years.

• Today, life expectancy only 10 years shorter than an unaffected male

SEX-LINKED INHERITANCE• Punnett squares are used to

predict the outcome of sex-linked inheritance.

• Assume the trait is X-linked unless told otherwise!

• Most disorders are recessive, some are dominant, the question will tell you.

• A “carrier” is a female who is heterozygous for the trait.

EXAMPLE

• Hemophilia is a recessive X-linked trait. What is the probability of a couple having a hemophiliac child if the man does not have hemophilia and the woman is a carrier?

Determining sex-link patterns in a pedigree

• This pedigree shows the inheritance of red-green CVD in a family. Identify the genotype of each family member.

• How does the inheritance pattern support X-linked inheritance?

• Hint Start with a legend

Barr Bodies: Inactive X Chromosomes• In females every cell only has one

functioning X chromosome

• Early in embryo development one X chromosome is condensed tightly into a Barr body

• Occurs randomly in each cell

• Explains calico and tortoiseshell cats (50% of cells have active X with allele for black, and 50% have and active X allele for orange)

• All calicos are female, male calicos are sterile. Why?