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Human Genetics. Understanding Genetics with Family Trees. How do we use Mendel’s Observations about Dominant and Recessive Traits, Segregation and Independent Assortment to predict the inheritance of traits in humans?. Pedigree the family tree

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human genetics

Human Genetics

Understanding Genetics with Family Trees

slide2

How do we use Mendel’s Observations about Dominant and Recessive Traits, Segregation and Independent Assortment to predict the inheritance of traits in humans?

slide3

Pedigree

  • the family tree
  • representation of the ancestry of an individual’s family.
  • symbolic representations of family relationships and inheritance of a trait
slide4

Pedigree Analysis

Parents

Offspring

Unaffected Male

Unaffected Female

Affected male

Affected female

slide5

A numbering system is used to identify individuals

Individuals I1 and III1 express the unusual trait being studied

Rest have normal phenotypes

slide6

Proband - the first afflicted member of a family seeking medical attention.

- marked with the an arrow and the letter p

pedigree
Pedigree

The family tree

  • representation of the ancestry of an individual’s family.
  • symbolic representations of family relationships and inheritance of a trait
mode of inheritance determined by inheritance pattern of mutant phenotype

Most common

Mode of Inheritance determined by inheritance pattern of mutant phenotype
  • Autosomal recessive
  • Autosomal dominant
  • X-linked recessive
  • X-linked dominant
  • Y-linked
  • mitochondrial
criteria for an autosomal recessive trait
Criteria for an Autosomal Recessive Trait
  • Male and Female are both affected
  • Affected males and females can transmit the trait, unless it causes death before reproductive age
  • Trait can skip generations
  • Parents of affected are heterozygous or have the trait and are homozygous.
criteria for an autosomal dominant trait
Criteria for an Autosomal Dominant Trait
  • Male and Female are both affected. Can have male to male transmission.
  • Males and females transmit the trait with equal frequency
  • Successive generations are affected
  • Transmission stops if a generation arises where no individual is affected.
dominant trait
Dominant Trait

Distal symphalangism is an autosomal dominant disorders characterized by fusion of the proximal or distal interphalangeal joints.

slide16

Autosomal dominant

Autosomal recessive

Comparison of autosomal dominant and autosomal recessive inheritance

Males and females affected?

Yes

Yes

Males and females transmit the trait?

Yes

Yes

Trait skips generations?

No

Yes

At least one parent of affected child must be affected?

Yes

No

trick question alert
Trick Question Alert!
  • There is a tendency to believe that the dominant allele is more common than the recessive allele.
  • Sometimes that is true, but often it is not.
  • Dominance and Recessive traits BOTH can result from lack of expression of a gene and expression of an incorrect gene product. Recessive traits simply require two copies for the effect, while Dominance requires only one copy.
many genes have more then two alleles so the gene displays multiple phenotypes
Many genes have more then two alleles- so the gene displays multiple phenotypes

Gene for blood group in humans has 3 alleles

IA IB and IO

There are only two alleles for each gene present in a normal diploid individual

codominance
Codominance
  • Both alleles are expressed equally
  • Heterozygotes fully express both alleles
  • ABO Blood groups
    • 3 alleles
    • 6 genotypes
    • 4 phenotypes
  • Type AB blood - both the A and the B antigen are expressed on red blood cells
blood types
Blood Types

3 alleles -- IA , IB , and IO

IA and IB are codominant

IO is recessive

Type A blood can be IAIA or IAIO

Type B blood can be IBIB or IBIO

Type AB blood is IAIB - codominant state

Type O blood is IOIO

incomplete dominance
Incomplete dominance

Heterozygous phenotype is intermediate between the two parental homozygotes.

The heterozygous phenotype is typically intermediate to the homozygous phenotype.

slide24

Incomplete dominance

If you cross true breeding red and white snapdragons

F1 are all pink flowers

F2 - 1:2:1 red:pink:white

It takes 2 doses of the red allele to get red

RR- red Rr – pink rr - white

dimples1
Dimples

Autosomal dominant inheritance

brachydactyly
Brachydactyly

Autosomal dominant inheritance

Who could be carriers?

tay sach s disease
Tay Sach’s Disease
  • Autosomal Recessive Disorder
  • II1 and II2 must be carriers –Who else could be?

- at least 2 carriers in generation I

- but all could be carriers

or is it incomplete dominance
Or, is it Incomplete Dominance?
  • The condition is caused by insufficient activity of an enzyme called hexosaminidase A that catalyzes the biodegradation of acidic fatty materials known as gangliosides.
  • Gangliosides are made and biodegraded rapidly in early life as the brain develops.
  • Patients and carriers of Tay-Sachs disease can be identified by a simple blood test that measures hexosaminidase A activity.
albinism
Albinism

- Autosomal recessive

- Heterozygotes carry the recessive allele but exhibit the normal skin color (sometimes called wild type phenotype)

pedigree analysis
Pedigree Analysis
  • How do you evaluate a pedigree to determine the mode of inheritance of a particular trait?
    • form a hypothesis as to how the trait is inherited
    • test the pedigree for consistency with that hypothesized mode of inheritance
    • if consistent with the hypothesized mode of inheritance, accept your hypothesis only when all other modes of inheritance can be been ruled out.
what type of inheritance does this trait have
What type of inheritance does this trait have?
  • Dominance
  • Recessive
  • Dominance or recessive- insufficient data to tell
what type of inheritance does this trait have1
What type of inheritance does this trait have?
  • Dominance
  • Recessive
  • Dominance or recessive- insufficient data to tell
predict inheritance of a dominant vs recessive
Predict inheritance of a dominant vs recessive?

Offspring 2 marries someone homozygous for the trait. What will their children be if the trait is recessive? Dominant- Heterozygote? Homozygote?

statistical genetic predictions

A a

A

a

AA

Aa

aa

Aa

Ellen is not affected and

cannot carry aa genotype

Ellen

Michael

?

Statistical Genetic Predictions

Ellen and Michael’s parents must be carriers.

Ellen’s brother Michael has

sickle cell anemia, an autosomal recessive disease.

What is the chance that Ellen’s child has a sickle cell anemia allele (a)?

chance Ellen is a carrier = 2/3

chance child inherits sickle cell allele = 1/2

Overall chance child carries sickle cell allele from Ellen = 2/3 x 1/2 = 1/3

consanguinity
Consanguinity

most common – first cousins marriages

- share grandparents - risk passing on the same recessive alleles to offspring

first cousin marriages
First Cousin Marriages
  • Over a billion people worldwide live in regions where 20%-50% of marriages are consanguineous - that is where the partners are descended from the same ancestor.
what is the risk really
What is the risk really?
  • Dr. Bittles, Director for the Centre for Human Genetics in Perth, Australia
  • Collated data on infant mortality in children born within first-cousin marriages from around the world and found that the extra increased risk of death is 1.2%.
  • In terms of birth defects, he says, the risks rise from about 2% in the general population to 4% when the parents are closely related.
data quality isn t good
Data Quality isn’t Good
  • Born in Bradford Study
    • 50% of births are Pakistani, and 70% of Pakistani births in the study so far are from consanguineous marriages
    • 150 genetic diseases identified, more than expected from other cities
  • British Paediatric Surveillance Unit
    • Since 1997, 902 British children born with neurodegenerative conditions
    • 8% of those were in Bradford which only has 1% of the population.
ptolemy dynasty 323 bc to 30 bc
Ptolemy Dynasty 323 BC to 30 BC

Preferred marriage was brother/sister

Why is so much inbreeding not fatal?

It could have an effect, but the early miscarriages and diminished fertility and infant deaths weren’t recorded.

what are the proposed solutions and ethical considerations
What are the proposed solutions and ethical considerations?
  • Make first cousin marriage illegal?
    • This is a strong cultural preference in many cultures and could be discriminatory
  • Test pregnancies for genetic defects and have selective abortions?
    • This requires legal abortions and screening that is as early as possible
  • Provide parental screening and counseling?
    • This could lead to prejudice against carriers
there are very few life guards at the gene pool in wv
There are very few life guards at the gene pool in WV
  • Human Genetics Testing Staff are certified by:
    • The American Board of Medical Genetics
    • The American Board of Genetic Counseling
    • The American Board of Pathology: Molecular Genetic Pathology
  • Sharon L Wenger, PhD

West Virginia University

Dept Pathology