Genetics: Lecture 3 Flashcards
Mendelian Genetics & Pedigrees
Multiple Alleles?
Possibility of having one of 3, 4, 5 or more alleles on two homologous chromosomes for the same gene.
Co-dominance?
both can be present and are at once
Example: cows with white and brown spots
Lethal Alleles?
Allele whose presence will lead to death. Typically won’t produce offspring if one is homozygous.
Ratio of offspring is not 3:1 but 2:1
ABO Blood groups: Multiple allele system with co-dominance?
There are three alleles for the ABO blood groups and their carbohydrates.
Multiple alleles because options are A, B and O
Co-dominance because they work together when having AB and AO etc.
Gene product of ABO group?
histo-blood group ABO system transferase (enzyme involved with transfers of carbohydrates specific to A or B but O have neither).
What is actually happening?
The A allele produces the enzyme A-transferase which catalyzes the transfer of GalNAc residues.
The B allele encodes the enzyme B-transferase, which catalyzes the transfer of galactose residues wikipedia
The i allele lacks both enzymatic activities
- antigens and antibodies are produced in each case which impact the compatibility.
Examples of lethal alleles?
Manx cats: short tail is desirable, two dominant (no tail) is lethal
Creeper chickens: Pp is desired form, PP is lethal
Human Karyotype?
A display of homologous chromosomes of a cell arranged by
size and type.
Giemsa Stain?
Giemsa stain band ≠ gene !
Giemsa stain is just a nucleic acid stain that attaches preferentially to adenine-thymine (A-T) bonds.
There are way more genes on a chromosome than there are Giemsa stain bands
Example: HUMAN CHROMOSOME 22 has 49 Million base pairs and between 417 - 496 genes!
Autosomes?
22 pairs of homologous chromosomes exactly the same in both male and female karyotypes.
Sex chromosomes?
One pair of chromosomes:
*Female XX
*Male XY
- in mammals
Multifactorial Disorders - Cancers?
Some forms; multiple causes, including genetic predisposition. Incidence: >1/3 (exclusive of skin cancer).
- very common
- genetic predisposition
- closer to 1/2 including skin cancer
Multifactorial Disorders - Congenital Malformations?
Cleft lip, heart defects, neural tube defects. Incidence: variable but
generally 1/250-600 with ethnic variation.
- varies by population and somewhat common
- congenital malformations are those that someone is born with
Multifactorial Disorders - Coronary Artery Disease?
Multiple causes, including genetic predisposition. Incidence: variable, but up to 1/15 in Western populations.
- common
Chromosome or Cytogenetic Disorder examples?
Down syndrome/ trisomy 21: incidence is 1/800
- Highly associated with the age of the mother at fertilization.
XYY Syndrome/ extra Y: incidence is around 1/1000 males
Nondisjunction?
Occurs during meiosis, causes trisomies and monosomies.
- occurs when chromosomes (sister chromatids or homologs) are pulled to the wrong side in meiosis I or II. We want an even split of the genetic material and nondisjunction means that that cannot occur.
Nondisjunction gives rise to?
Monosomics
- Lack a single copy of a chromosome (monosomy).
- Rare, typically lethal
- Happens with single X chromosomes
Trisomics
- Carry an additional copy of a chromosome (trisomy).
- Trisomy 21 is the most common (only viable)
Down syndrome?
- Trisomy 21, an additional chromosome 21 in the karyotype,
owing to nondisjunction during
meiosis. - Portion of chromosome 21 translocated to chromosome 14
- Incidence of Down Syndrome (and other trisomies) increases dramatically with the age of the mother
Klinefelter syndrome?
An individual with 47 chromosomes, XXY. They are born sterile and male with female body characteristics.
- Trisomic
Turner syndrome?
An individual with 45 chromosomes, X. Born sterile and females with short stature and immature sex organs.
- Monosomic
XXX syndrome?
Individual with 47 chromosomes, XXX. Fertile woman associated with learning difficulties.
- typically goes undiagnosed
- trisomic
- associated with ADHD
XYY syndrome?
Individual with 47 chromosomes, XYY. Fertile male associated with behavioural difficulties.
- typically goes undiagnosed
- trisomic
- associated with ADHD
Single gene disorders?
Caused by a mutant gene allele.
- The mutant allele may be present on only one chromosome of an homologous pair (dominant) or on
both chromosomes (recessive) to exhibit the disease.
Usually obvious pattern of inheritance in a pedigree.
- Most defects are rare.
Cystic Fibrosis?
Cystic Fibrosis
* common autosomal recessive
disease
* Incidence: ~1/2000 in some
Caucasian populations
* very rare in Asians
