Complex Inheritance

Review

Genes are carried on chromosomes

Humans have 22 pairs of autosomesand 1 pair of sex chromosomes

Autosomal Inheritance

• 50/ 50 chance amongst males and females

Autosomal Recessive: Cystic Fibrosis • 7q31.2• Causes mucus build-up in the lungs and

digestive organs such as the pancreas

Autosomal Recessive: Tay-Sachs Disease• 15q23• Nervous system disorder

that causes mental and physical deterioration

• Most patients die before the age of four

• Ashkenazi Jewish population from Central/ Eastern Europe

Autosomal Dominant: Huntington’s Disease

• 4p16.3• Neurodegenerative disease that

presents in mid-life and causes loss of coordination and muscle control

• Most patients die around 20 years after symptoms begin to appear

• Most common amongst Western Europeaners

Autosomal Dominant: Polydactyl

• 13q and 7p• Extra “digit” on either the hands or feet often

resulting in a splitting of the pinky finger or thumb

Autosomal Dominant: Achondroplasia• 4p16.3

• Disorder in bone growth that causes most forms of dwarfism

• Lethality: Having two copies of the gene usually causes death very early in life

Sex-Linked Traits

• First discovered in Drosophila among White Eyed Males

• Plants that Mendel studied were dioecious

X-Linked Dominant: Rett Syndrome

• Disorder of the grey matter in the brain• Patients often have no verbal skills and are not

ambulatory

X-Linked Recessive: Hemophilia

• Defect in the clotting factor found in blood• Most patients have problems controlling blood

loss

X-Linked Recessive: Color Blindness

• Individuals are missing or have defects in parts of the eye that distinguish colors

• Many different types of colorblindness, but the most common is inherited on the X-chromosome

X-Linked Recessive: Male Pattern Baldness

• Androgenic Alopecia• Shrinkage of the fair follicle eventually results

in a follicle that produces no hair

Y-Chromosome

• SRY gene in mammals codes for male embryonic development

• Why is the Y-Chromosome so small?

Bellwork!1. What are differences between autosomal and sex-

linked traits? (You should be able to list a few)2. If a hemophiliac woman has children with a non-

hemophiliac male, what percent of their offspring will have the disease? (Be specific about boys and girls)

3. If a white-eyed Drosophila male mates with a heterozygous red-eyed female Drosophila, give the percentage of offspring with the mutant phenotype? (Again, be specific about boys and girls)

X-Chromosome Inactivation

• Dosage Compensation• Occurs randomly and early in development

X-Chromosome Inactivation & Barr Bodies

How do we get pink flowers?

Incomplete dominance is a form of intermediate inheritance in which one allele for a specific trait is not completely dominant over the other allele. This results in a combined phenotype.

Incomplete Dominance

Species: SnapdragonsTrait: Flower colorPure line phenotypes: red or white flowerParental cross: Red x WhiteF1: We would expect red or white flowers in this generation, depending upon

which allele is dominant. But, the F1 plants produced pink flowers. As with any experiment of this sort, the F1 plants are selfed. The results that were obtained were:

F2 phenotypic ratio: 1/4 Red : 1/2 Pink : 1/4 White

It appears as if the red and white alleles are interacting in theheterozygote to generate the pink flowers.

Incomplete DominanceThe most well-studied example of incomplete dominance in humans occurs in the genes for curly hair. Inheriting a gene for curly hair from one parent and a gene for straight hair from the other parent will give a hair texture that is a blend of the two, wavy hair.

Codominance

How do some organisms get two colors?

ROAN COWSThese cows have possibilites of several colors ofhairs that aren't a blend of the dominant andrecessive phenotypes because the traits for eachphenotype are dominant. RR = red cow hairsRW = red and white cow hairsWW = white cow hairs

ERMINETTE CHICKENS These chickens have feathers that are bothwhite and black, but not grey. BB = black feathersBW = black and white feathersWW = white feathers

Blood Typing

A codominant genetic trait in humans occurs with blood types. There are three different alleles for blood type: A, B and O. While the O type is actually the absence of either A or B proteins in the blood, the A and B blood types are codominant. If someone inherits one A allele and one B allele, they will not have a blood type that is in between those two, but will instead have a blood type AB, which expresses both A and B proteins in the blood.

Blood Typing

Blood Type Genotypes