Mendelian Genetics

Question 1

What is Mendel’s First Law: The law of dominance?

Answer 1

A heterozygous genotype favors the expression of the dominant gene

Question 2

What is Mendel’s Second Law: The Law of independent assortment?

Answer 2

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

Question 3

What are linked alleles

Answer 3

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.

Question 4

What is a test cross (backcross)?

Answer 4

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.

Question 5

What is a dihybrid cross, and why was it significant to Mendel’s findings?

Answer 5

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.

Question 6

Incomplete dominance

Answer 6

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.

Question 7

What is codominance?

Answer 7

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

Question 8

What are epistatic genes?

Answer 8

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

Question 9

What is Pleiotropy?

Answer 9

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.

Question 10

How can two heterozygote genes produce a 2:1 genotypic ratio?

Answer 10

If the homozygous recessive gene is lethal, we would only see a 2:1 genotypic ratio from the offspring.

Question 11

What is polygenic inheritance?

Answer 11

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.

Question 12

What are multiple alleles?

Answer 12

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

Question 13

What are the bloodtypes of the universal donors and the universal recipients?

Answer 13

Universal donor: O

Universal recipient: AB

Question 14

What happens when an individual with heterozygous A bloodtype has a child with a heterozygous B individual.

Answer 14

AOxBO
All four blood types could be possible in a 1:1:1:1 ratio
AB, AO, OB, OO

Question 15

What are the surface level antigens and plasma antibodies for all the different blood types?

Answer 15

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

Question 16

What is holandric inheritance?

Answer 16

Y-linked traits. Generally we don’t see many of these, and we won’t be tested on any specific ones on the DAT

Question 17

What are some more well known X linked diseases?

Answer 17

• Color Blindness
• Hemophilia
• Duchenne Muscular Dystrophy

Question 18

What is a barr body

Answer 18

In females, one egg is always inactivated in embryonic development by methylation

Question 19

What is another term for an XY genotype?

Answer 19

hemizygous

Question 20

A hemophiliac carrier woman and a hemophiliac male have a child. What is the probability to have a normal boy?

Answer 20

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%

Question 21

A hemophiliac carrier woman and a hemophiliac male have two boys. What is the probability that both boys have the disease?

Answer 21

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.

Question 22

What are some common examples of autosomal recessive diseases?

Answer 22

• PKU
• Cystic Fibrosis
• Albinism

Question 23

What are some common examples of autosomal dominant diseases?

Answer 23

• Achondroplasia (dwarfism)

- Marfan syndrome (connective tissue disorder)

Question 24

What are the common traits of autosomal recessive inheritance?

Answer 24

Only if you have two recessive alleles do you inherit the recessive phenotype.
These affect both sexes equally, and can appear to skip generations.

Question 25

What are the common traits of autosomal dominant inheritance?

Answer 25

If you have one dominant gene, you inherit the dominant phenotype.
These affect both sexes equally and do not skip generations.

Question 26

What is crossing over?

Answer 26

This occurs during Meiosis I. Parts of the genome swap spots with analogous spots on homologous chromosomes.

Question 27

What is recombination?

Answer 27

It is the results of crossing over.

Question 28

What is a linkage map?

Answer 28

A linkage map is formed by analyzing the % of crossovers that occur between genes. The more crossing over that occurs, the further the genes are from each other and vice versa

Question 29

What is nondisunction?

Answer 29

It is a chromosomal abnormality that results from the failure of homologous chromosomes to separate from one another during Anaphase I or II of Meiosis.

This is 95% of the cause for down syndrome.

This means the offspring will have the wrong number of chromosomes. This is called aneuploidy

Most of the time this is lethal, but not always.

Question 30

What is aneuploidy?

Answer 30

It’s when an organism doesn’t have the right number of chromosomes.

Question 31

What is Turner Syndrome?

Answer 31

It is when a female only has one X chromosome.

The female is sterile, has poorly developed ovaries, incompletely developed secondary sex characteristics, and no barr bodies.

Question 32

What is Kleinfelter Syndrome?

Answer 32

XXY Sterile Male with feminization

Question 33

What is Down Syndrome

Answer 33

A third 21st chromosome. Also known as trisomy 21

Mental retardation
Heart defects
More prone to alzheimers and leukemia

Question 34

What is polyploidy?

Answer 34

Occasionally liver cells will be 3n or 4n.

This is very common in plants. We see 3N or 4N endosperm of plant seeds in many weeds, dandelions, and wild oats.

Question 35

What is a chromosomal deletion?

Answer 35

This occurs when a break occurs and we lose a fragment of DNA containing genes.

These are associated with some cancers.

Question 36

What is Cri du Chat syndrome?

Answer 36

This occurs when the short arm of Chromosome #5 is deleted.

Question 37

What is duplication in chromosomes?

Answer 37

An addition is added onto a chromosome from one that is fragmented. Partial trisomy occurs

Question 38

What is Inversion?

Answer 38

This occurs when a segment of a chromosome is flipped 180 degrees and reattached to the chromosome.

Question 39

What is translocation?

Answer 39

This occurs when a deleted portion of the gene is attached to another nonhomologous chromosome.

5% of the time, this is the cause of down syndrome. A translocation of the chromosome long arm of 21 is attached to the long arm of chromosome 14.

Question 40

amniocentesis

Answer 40

Amniotic Fluid of the fetus is sampled. Fetal secretions and epidermal cells from the respiratory and GI tract can be cultured and subected to karyotypic analysis

Question 41

Chorion Villus Sampling (CVS)

Answer 41

A sample of the placenta is physically removed. Chorionic tissue is rich in cells, therefore the karyotype can be done in a timely fashion. Slightly higher risk than a amniocentesis.

Question 42

How many different gametes can be formed with the following genotype? Xx Yy Zz WW BB

Answer 42

Take 2^n, where n = the number of heterozygotes. Since N = 3 then 2^3 = 8 different gametes.