Mendelian Genetics Flashcards
What is Mendel’s First Law: The law of dominance?
A heterozygous genotype favors the expression of the dominant gene
What is Mendel’s Second Law: The Law of independent assortment?
Genes located on different chromosomes assort independently of one another. During gamete formation, the alleles for height segregate independently form the alleles for a trait such as color
What are linked alleles
The closer genes are to each other on a chromosome, the more likely they are to be linked. Genes on the same chromosome are generally considered to be linked.
What is a test cross (backcross)?
If you are unsure if a dominant trait is due to a homozygous dominant genotype or a heterozygous genotype, you can cross it with a homozygous recessive genotype. If all of them show the dominant phenotype, then the parent dominant genotype was a homozygous dominant. If there is a 50:50 mix of recessive and dominant phenotype, then you know the parent was a heterozygous.
What is a dihybrid cross, and why was it significant to Mendel’s findings?
A dihybrid cross shows the crossing of two separate alleles at the same time.
Mendel did a dihybrid of color and height crossing two plants with both genotypes heterozygous. The resulting phenotypes came out to a 9:3:3:1 ratio, showing that for these genes there was independent assortment, meaning that the genes were located on different chromosomes.
Incomplete dominance
This occurs when neither two alleles exerts dominance. This can be seen as the color of snapdragons. One form of the allele codes for red pigment and the other codes for white pigment, so when there is a heterozygous one, it creates a pink flower.
What is codominance?
Codominance is when a pair of nonidentical alleles code for two separate phenotypes at the same time.
Blood is an example of this. AB gives the blood type of AB, the alleles are said to be codominant
What are epistatic genes?
An epistatic gene is a gene that covers up the expression of another gene in the phenotype. This is similar to normal dominance, except that two separate genes are involved. In other words, two alleles of one gene, mask the expression of another gene’s allelles.
In labradors, a specific gene codes for what color difference in the melanin that will be inserted into their hair. Another gene, determines how much of that melanin is actually inserted into the hair. If the insertion gene is coded for no insertion of melanin, then an albino dog is produced despite having the enzymes to create melanin being produced as well. The two genes have an epistatic relationship, and the insertion gene is the epistatic gene
What is Pleiotropy?
Pleiotropy is when a single gene affects the organism in various ways.
In sickle cell anemia, the single mutation affects O2 utilization and can cause tissue damage / organ damage
In PKU, there are elevated levels of phenylalanine that result in mental retardation, larger heads, lighter hair color, all due to a single gene.
How can two heterozygote genes produce a 2:1 genotypic ratio?
If the homozygous recessive gene is lethal, we would only see a 2:1 genotypic ratio from the offspring.
What is polygenic inheritance?
Two or more genes contribute to a single trait.
For example, hair color, height, and skin color result from a lot of interactions of genes. Height for instance shows a bell curve of different heights amongst humans, rather than the tall or short phenotype of pea plants.
What are multiple alleles?
This occurs when a gene has more than two alleles possible.
For example, the ABO blood type in humans. There could be four different possible phenotypes. A, B, AB, or O. All produced from three different combinations of i^A, i^B, i^O
A: AA, AO
B: BB, BO
AB: AB
O: OO
What are the bloodtypes of the universal donors and the universal recipients?
Universal donor: O
Universal recipient: AB
What happens when an individual with heterozygous A bloodtype has a child with a heterozygous B individual.
AOxBO
All four blood types could be possible in a 1:1:1:1 ratio
AB, AO, OB, OO
What are the surface level antigens and plasma antibodies for all the different blood types?
Type A: A antigens on surface, B antibodies in plasma
Type B: B antigens on surface, A antibodies in plasma
TypeAB: A&B antigens on surface, no antibodies in plasma
Type O: No antigens on surface, A&B antibodies in plasma
What is holandric inheritance?
Y-linked traits. Generally we don’t see many of these, and we won’t be tested on any specific ones on the DAT
What are some more well known X linked diseases?
- Color Blindness
- Hemophilia
- Duchenne Muscular Dystrophy
What is a barr body
In females, one egg is always inactivated in embryonic development by methylation
What is another term for an XY genotype?
hemizygous
A hemophiliac carrier woman and a hemophiliac male have a child. What is the probability to have a normal boy?
Xh X * Xh Y
XhXh, XhX, XY, XhY
25% chance of a normal boy.
NOTE: THIS IS NOT 50%. Since we aren’t sure if boys will be born, we have to take into account the probability of a boy vs a girl being born. If the question stated that a boy was born, then it would be 50%
A hemophiliac carrier woman and a hemophiliac male have two boys. What is the probability that both boys have the disease?
Xh X * Xh Y
XhXh, XhX, XY, XhY
50% chance that a boy has the disease. 50% * 50% = 25% chance in having two boys with the disease.
NOTE: Since the question states that the boys already have been born, we can eliminate the probability of boys being born from the equation. We are only concerned with wether or not they have the disease.
What are some common examples of autosomal recessive diseases?
- PKU
- Cystic Fibrosis
- Albinism
What are some common examples of autosomal dominant diseases?
- Achondroplasia (dwarfism)
- Marfan syndrome (connective tissue disorder)
What are the common traits of autosomal recessive inheritance?
Only if you have two recessive alleles do you inherit the recessive phenotype.
These affect both sexes equally, and can appear to skip generations.
