Mendelian Genetics

Question 1

Test Cross

Answer 1

Cross an unknown genotype with a known homozygous recessive to determine the unknown genotype

Question 2

Backcross

Answer 2

Crossing an hybrid with a parent or same make up as parent

Question 3

Dihybrid cross

Answer 3

crossing two organisms with 2 genes of interest

Question 4

A particular organism is both female and carries an Xx genotype with parents of Xx and xx genotypes. What is the likelihood of this?

Answer 4

This question can be approached by using a Punnett Square. A typical cross between an Xx and an xx individual yields a 50% chance of any particular offspring organism being Xx (while the remaining 50% corresponds to xx offspring). However, in humans, only 50% of offspring are female, while the other 50% are male. For this reason, the probability of having a particular child being both female and Xx is (0.5)(0.5) = 0.25, or 25%.

Question 5

An orchid breeder crosses two types of orchids, one that is tall with a petite bloom and one that is short with a large bloom, with the goal of obtaining a tall orchid with a large bloom. The first cross yields all short flowers with petite blooms. What is the expected yield of tall orchids with large blooms if bred from the first generation?

Answer 5

The first cross yields only short flowers with petite blooms, indicating that these are the dominant traits. The parent flower that was tall with petite blooms must have been homozygous recessive for height and homozygous dominant for bloom size. The parent flower that was short with large blooms must have been homozygous dominant for height and homozygous recessive for bloom size. All of the first generation flowers are heterozygotes for both traits, so if you were to cross two of these progeny, you could expect a result that follows normal Mendelian ratios for a dihybrid cross: 9:3:3:1. Here, the “1” represents the 1/16, or 6.25%, that will display both recessive traits, tall orchids with large blooms, which is this orchid breeder’s goal.

Question 6

The ABO bloodtyping antigen is known to have blood cells with type A, B, AB, or O types. This pattern of dominance is known as…

Answer 6

Codominance, one does not mask the other

Question 7

Snap dragon flowers can be either red, white, or pink. The red is homozygous dominant while the white is homozygous recessive. Pink is heterozygous. What kind of dominance is this?

Answer 7

Incomplete dominance

Question 8

Penetrance

Answer 8

Percent of individuals with a given genotype who display the associated phenotype.

Question 9

Women with the BRCA1 gene have an 80% likelihood of developing breast cancer. This likelihood is referred to as…

Answer 9

Penetrance

Question 10

Expressivity

Answer 10

Severity of a phenotype

Question 11

Congestive heart failure is shown to have a genetic basis but individuals with it show varied severity. THis is attributed to…

Answer 11

Expressivity

Question 12

What is the difference between penetrance and expressivity

Answer 12

Penetrance is a yes or no, Expressivity is how much

Question 13

True or False: in pedigrees, circles refer to females, squares are used to refer to males, and shading reflects individuals with the phenotype in question

Answer 13

True

Question 14

The carrier frequency for cystic fibrosis, an autosomal recessive disorder, in the Ashkenazi Jewish population is 1 out of every 24 individuals. A baby born to a couple from this demographic is tested for this mutation. A family history reveals that the mother’s father was afflicted with cystic fibrosis. What is the approximate probability that the father is a carrier and the child will also be a carrier of this disease?

Answer 14

Since the mother’s father expresses the autosomal recessive trait, he must have two copies of the mutation and will pass one on to his daughter, making her an obligate carrier. The husband has a 1/24 chance of also being a heterozygote. If both parents are carriers, there is a 50% chance that the child will be one as well (along with a 25% chance that the child will have cystic fibrosis, and a 25% probability that the child will be homozygous dominant). Since we want the probability of the father being a carrier AND the child being a carrier, multiply (1/24)(1/2) = 1/48, which is closest to choice C.

Question 15

Law of segregation

Answer 15

allel pairs segregate randomly into gametes

Question 16

Law of independent assortment

Answer 16

separate traits are independently inherited. When they are on different chromosomes this law holds true

Question 17

Law of dominance

Answer 17

it is what it sounds like

Question 18

When is the law of independent assortment iffy

Answer 18

linked inheritance and crossing over (genetic recombination)

Question 19

Recombination frequency

Answer 19

The likelihood of recombination between two alleles/genes on a chromosome. Formation of chiasmata

Question 20

Centimorgan

Answer 20

A centimorgan is the unit of measure for recombination frequency with 1 centimorgan corresponding to 1% change in recombination frequency

Question 21

True or False: genes with a 50% or more recombination frequency are unlinked or located on different chromosomes

Answer 21

true

Question 22

What is an exception to the law of segregation?

Answer 22

Mendel’s first law (the law of segregation) does not apply in nondisjunction cases (the incorrect partition of chromosomes in meiosis). In nondisjunction, gametes may incorrectly gain or lose a chromosome copy. This changes the chromosome number in the gamete.

Question 23

When is the law of independent assortment broken?

Answer 23

Gene linkage violates the law of independent assortment (Mendel’s second law). Gene linkage occurs when two genes are located on the same chromosome. Such genes are “linked” and have a higher likelihood of assorting together into a particular gamete, than genes located on different chromosomes.

Question 24

Of 2000 total offspring in a particular cross, 180 are recombinants. The genes in question are known to be linked. In units of centimorgans, what distance would you expect to separate these two genes on the chromosome on which they are located?

Answer 24

First, calculate the recombination frequency:

(180 recombinants)/(2000 total offspring) = 9/100 = 9%

Each percent of a particular recombination frequency is measured as one centimorgan of distance along a chromosome, making our answer 9 cM.

Question 25

A man with blonde hair, blue eyes, and fair skin has a daughter with a woman with dark hair, brown eyes, and dark skin. (Assume that hair, eye, and skin color are each dependent on a single gene.) Based on their differences in these three traits, how many possible phenotypic combinations are possible for their child?

Answer 25

With no information regarding dominance or recessivity, all potential allelic combinations must be taken into account. With three genes each with two alleles, use 2^3 to find that there are eight different combinations.

Question 26

A family has a history of inherited female sterility. A woman who is concerned about whether or not she has this phenotype is concerned about the trait’s:

Answer 26

Penetrance describes whether an allele’s phenotype will or will not be expressed. If a woman is concerned about whether she will or will not have a trait, she is concerned about the trait’s penetrance.

Question 27

Scientists measure the distance between gene loci A and B on a germ cell chromosome to be 15 centimorgans. After meiosis, what will be the percent frequency of each kind of gamete that is produced?

Answer 27

The frequency with which recombination of two genes occurs is equal to the distance between the two genes in centimorgans.

Therefore, the genes A and B will recombine 15% of the time. If 15% of the time 2 chromosomes recombine, 30% of all gametes produced will have altered or unexpected combinations of alleles. In this case, AB and ab chromosomes recombined such that 15% of all gametes receive an Ab chromosome, 15% receive an aB chromosome, and 70% receive a non-recombined chromosome.