Question 1

In Mendel?s experiments, if the gene for tall (T₁) plants was incompletely dominant over the gene for short (T₂) plants, what would be the result of crossing two T₁T₂ plants?

Accepted Answer

In the case of incomplete dominance, crossing two heterozygous (T1T2) plants would result in a 1:2:1 phenotypic ratio. This means 25% would be homozygous dominant (tall), 50% would be heterozygous (intermediate), and 25% would be homozygous recessive (short).

Question 2

A recessive allele on the X chromosome causes color deficiency. A color-deficient boy has a mother with normal vision and a color-deficient father. From which parent did he get the color-deficiency gene?

Accepted Answer

The boy must have inherited the X chromosome carrying the color deficiency allele from his mother, as males inherit their single X chromosome from their mothers and the Y chromosome from their fathers. The father cannot pass an X chromosome to his sons, only to his daughters.

Question 3

A type of muscular dystrophy shows sex-linked recessive inheritance. Affected persons usually die by the age of 20. Suppose that a boy with the disease lives long enough to marry a woman heterozygous for the trait. If they have a son, what is the probability that he will have the disease?

Accepted Answer

The disease is sex-linked recessive, meaning it is carried on the X chromosome. The affected father (X^dY) will pass his Y chromosome to his son, while the heterozygous mother (X^DX^d) has a 50% chance of passing the X^d chromosome. Thus, the probability the son will have the disease is 50%.

Question 4

A recessive allele on the X chromosome causes color deficiency. A noncolor-deficient woman (whose father is color deficient) marries a color-deficient man. If they have a son, what is the chance that he will be color deficient?

Accepted Answer

The son inherits his X chromosome from his mother and his Y chromosome from his father. Since the mother is not color deficient but her father is, she must be a carrier of the recessive allele (X^CX^c). The father, being color deficient, has the genotype X^cY. Therefore, the son can either inherit the X^C (normal vision) or X^c (color deficient) allele from his mother. This results in a 50% chance of being color deficient.

Question 5

If an individual who is homozygous for type B blood marries a heterozygous type A individual, what is the chance that their first child will have type AB blood?

Accepted Answer

A homozygous type B individual has the genotype BB, and a heterozygous type A individual has the genotype AO. The possible offspring genotypes are AB and BO, so there is a 50% chance of having a child with type AB blood.

Question 6

Blood typing is often used as evidence in paternity cases in court. In one case, the mother had blood type B and the child had blood type O. Which of the following blood types could the father NOT have?

Accepted Answer

Blood type O is recessive, meaning both parents must contribute an O allele for a child to have blood type O. A parent with blood type AB can only contribute A or B alleles, making it impossible for a child to have blood type O if one parent is AB.

Question 7

In humans, the sex of the offspring is determined by the

Accepted Answer

The sex of the offspring in humans is determined by the sex chromosome carried by the sperm cell. The sperm can carry either an X or a Y chromosome, while the egg always carries an X chromosome. If the sperm carrying an X chromosome fertilizes the egg, the offspring will be female (XX). If the sperm carrying a Y chromosome fertilizes the egg, the offspring will be male (XY).

Question 8

Codominance occurs when

Accepted Answer

Codominance is a form of inheritance where both alleles in a heterozygous individual are fully expressed, leading to a phenotype that simultaneously displays the traits of both alleles.

Question 9

A human sperm cell possesses autosomes and

Accepted Answer

Human sperm cells carry either an X or a Y chromosome in addition to autosomes, determining the sex of the offspring.

Question 10

Human eye color is the result of

Accepted Answer

Human eye color is determined by multiple genes, making it an example of polygenic inheritance.

Question 11

Hemophilia is a sex-linked recessive gene that causes a blood disorder. What are the chances that the daughter of a normal man and a heterozygous woman will have hemophilia?

Accepted Answer

Since hemophilia is a sex-linked recessive trait, a daughter would need two copies of the gene to express the disorder. A normal man cannot pass on the hemophilia gene, and a heterozygous woman has only a 50% chance of passing on the affected X chromosome. However, since the daughter can only inherit her father's normal X chromosome, she cannot be affected, making her chances of having hemophilia 0%.

Question 12

Imagine that smurfs live in widely separated groups and rarely interbreed, so geneticists know very little about them. On one occasion, two smurfs from different groups did mate. A big -footed, white smurf mated with a small-footed, blue smurf. Three offspring resulted: one big-footed and blue and two small-footed and blue. Which statement about the inheritance of color in smurfs is most likely to be correct?

Accepted Answer

Since all offspring are blue, despite one parent being white, this suggests that blue is the dominant color trait over white.

Question 13

A man who carries a harmful sex-linked (on the X chromosome) gene will pass the gene on to

Accepted Answer

A man has one X chromosome and one Y chromosome. Any gene on his X chromosome will be passed to all of his daughters (since they receive his X chromosome), but not to his sons (since they receive his Y chromosome).

Question 14

Traits controlled by sex-linked recessive genes are expressed more often in males because

Accepted Answer

Males have only one X chromosome (and one Y chromosome), so they have only one copy of genes that are located on the X chromosome. If a male inherits a recessive gene on the X chromosome, he will express the trait because there is no corresponding gene on the Y chromosome to mask the recessive gene. This is why sex-linked recessive traits are expressed more often in males.

Question 15

Two parents with normal vision have two sons, one color deficient and one with normal vision. If this couple then has six daughters, can you predict what percentage of the daughters will have normal color vision?

Accepted Answer

Color blindness is a sex-linked recessive trait carried on the X chromosome. Since the mother must be a carrier (XcX) for the son to be color deficient, and the father has normal vision (XY), all daughters will inherit one normal X from the father and either a normal or carrier X from the mother, resulting in normal vision.

Question 16

When the expression of a trait is influenced by the action of many genes, the pattern of inheritance is called

Accepted Answer

Polygenic inheritance refers to the phenomenon where a single trait is controlled by two or more genes, each of which may have two or more alleles. This results in a continuous range of possible phenotypes, such as skin color, height, and eye color, rather than a simple 'either-or' phenotype.

Question 17

An individual who is a carrier for a sex-linked trait such as hemophilia

Accepted Answer

In sex-linked traits like hemophilia, carriers are typically females who have one normal allele and one affected allele on their X chromosomes. Males cannot be carriers because they have only one X chromosome.

Question 18

Human skin color is the result of

Accepted Answer

Human skin color is the result of polygenic inheritance, meaning it is determined by the interaction of multiple genes, rather than a single gene or a simple dominant-recessive relationship. This complex genetic basis accounts for the wide range of human skin colors.

Question 19

Which of the following is a sex-linked recessive condition?

Accepted Answer

Hemophilia is a sex-linked recessive condition, meaning it is often carried on the X chromosome. Males are more frequently affected because they have only one X chromosome, so the presence of the recessive gene on their single X chromosome results in the condition manifesting.

Question 20

Color deficiency is more common in men than in women because

Accepted Answer

Color deficiency is more common in men than in women because the genes responsible for the most common forms of color vision deficiency are located on the X chromosome. Men have only one X chromosome (and one Y chromosome), so a single defective gene on their X chromosome will result in color deficiency. Women have two X chromosomes, so a defective gene on one of their X chromosomes can often be compensated for by a normal gene on the other X chromosome, making them less likely to exhibit color deficiency.

Quiz 11: Patterns of Inheritance

In Mendel?s experiments, if the gene for tall (T1) plants was incompletely dominant over the gene for short (T2) plants, what would be the result of crossing two T1T2 plants?