AP Biology : Understanding Autosomal and Sex-Linked Inheritance

Study concepts, example questions & explanations for AP Biology

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Example Question #1 : Understanding Autosomal And Sex Linked Inheritance

Which of the following is true about alleles?

Possible Answers:

Alleles are only inherited from the mother

Every organism of a given species has a different set of alleles from every other individual of that species

Alleles are different forms of a given gene

All genes have two alleles

Alleles are always dominant or recessive

Correct answer:

Alleles are different forms of a given gene

Explanation:

Alleles are defined as "alternative forms of a given gene."  Though Mendelian genetics tells us that the ideal model of a gene has only two alleles, dominant and recessive, we know this is not always the case, from things like codominance (blood type) and others. Some characteristics are defined by a combination of several alleles with varying weight of expression. Alleles on autosomes are inherited from both parents, but alleles in mitochondrial DNA are inherited from the mother only. Twins are an example of organisms with identical alleles, so the answers claiming that all organisms have different alleles is false.

Example Question #1 : Understanding Autosomal And Sex Linked Inheritance

 

Autosomal_dominant

In this pedigree, affected individuals have a disease causing the inability to walk forward, called Disease J. Assume all individuals whose alleles cannot be determined do not carry the allele for the disease (are not heterozygous).

How is Disease J inherited?

Possible Answers:

Autosomal dominant

Autosomal recessive

X-linked recessive

X-linked dominant

Correct answer:

Autosomal dominant

Explanation:

The first generation shows an affected father and an unaffected mother. They produce both affected and unaffected children in the second generation, meaning that the disease cannot be recessive; if it were recessive, none of the second generation could be affected due to dominant alleles inherited from the mother. We can also conclude that the affected father is heterozygous.

Knowing that the trait is dominant, we must determine if it is autosomal or sex-linked. The trait can affect females, so it cannot be on the Y chromosome. The female in the second generation is affected, even though her mother is not, meaning she must be heterozygous. If the trait is on the X chromosome, it will be passed from the affected father to all female offspring, meaning that both females in the second generation would be affected. Because one female is not affected, she must have inherited an unaffected autosomal allele from the heterozygous father.

As such, the allele for the disease must be autosomal dominant.

Example Question #1 : Understanding Autosomal And Sex Linked Inheritance

If an autosomal trait skips a generation, it ___________; however, if an autosomal trait does not skip a generation, it ___________.

Possible Answers:

must be dominant . . . can be either recessive or dominant

must be recessive . . . can be either recessive or dominant

can be either recessive or dominant . . . must be dominant

can be either recessive or dominant . . . must be recessive

Correct answer:

must be recessive . . . can be either recessive or dominant

Explanation:

If an autosomal trait skips a generation, it must be recessive; however, if an autosomal trait does not skip a generation, it can be either recessive or dominant.

These concepts can be easily seen when outlined via a pedigree analysis. A dominant trait cannot skip a generation; any presence of the allele will lead to expression, thus if the trait is not expressed in a given generation, it cannot be passed down (cannot skip). A recessive allele can be masked by carriers and reappear in a later generation.

Example Question #2014 : Ap Biology

Which of the following statements about X-linked traits is true?

Possible Answers:

There are more genetic diseases carried by the Y chromosome than the X chromosome

They can be passed from father to son

None of the other choices are correct

They generally affect more males than females

Correct answer:

They generally affect more males than females

Explanation:

Because males only have one X chromosome, while females have two, they are more likely to be affected by a problematic X chromosome. Females can mask recessive X-linked alleles as carriers; males will express all alleles on their singular X chromosome.

Males only pass on a Y chromosome to their sons, so it is impossible for them to pass an X-linked trait to a son. Furthermore, Y chromosomes are virtually free of contributing to inheritance-linked diseases.

Example Question #3 : Understanding Autosomal And Sex Linked Inheritance

A man with a sex-linked disorder for color-blindness and a woman with a normal genotype have two children. What is the probability that both of them will be carriers?

Possible Answers:

25%

0%

100%

50%

75%

Correct answer:

25%

Explanation:

The man's genotype can be written as , where  represents the allele with the disorder. The woman, who has a normal genotype, is therefore .

Their possible children are represented by the Punnett square shown here:

                     

          

               

Any sons will inherit the chromosome from the father and an unaffected from their moth. Any daughters will inherit chromosomes from both parents. By necessity, all daughters will inherit an normal from the mother and an affected  from the father. All of the couple's daughters will be carriers, while all of their sons will be phenotypically normal; therefore, there is a 50% chance that their child will be a carrier.

For two children, the probability is .

Example Question #2 : Understanding Autosomal And Sex Linked Inheritance

Scientists have characterized a new genetic disorder that only affects males. What is the most likely explanation of how this disorder is passed from generation to generation?

Possible Answers:

Inheritance of the mother's Y-chromosome

Epigenetic inheritance

Abnormal testosterone levels

Inheritance of the father's Y-chromosome

Correct answer:

Inheritance of the father's Y-chromosome

Explanation:

If only males display the disorder, it is most likely a Y-linked genetic disorder. The only possible way to inherit this disease, then, would be through the inheritance of the father's Y-chromosome.

Women have two X-chromosomes, one from each parent, and could not possibly pass down the disorder.

Epigenetic inheritance could potentially explain a genetic disorder, but, if this were the case, it should not differentiate between males and females. Abnormal testosterone levels may be a result of the disorder, but they do no explain how the disorder is inherited.

Example Question #1 : Understanding Autosomal And Sex Linked Inheritance

Individuals with Klinefelter syndrome are phenotypically male, but experience reduced sperm production and breast development in adolescence. Klinefelter individuals have two X-chromosomes and one Y-chromosome (they are XXY instead of XY). What meiotic error gives rise to this condition?

Possible Answers:

Monosomy

Aneuploidy

Translocation

Duplication

Polyploidy

Correct answer:

Aneuploidy

Explanation:

Aneuploidy is a chromosomal condition in which there are an abnormal number of chromosomes in the cells of the body. Aneuploidy typically refers to monosomy (one chromosome copy) or trisomy (three chromosome copies), and arises due to nondisjunction during meiosis and gametogenesis. Nondisjuction causes one daughter cell to receive three or four chromatids, and the other to receive one or zero. If this gamete is used to form a zygote, all cells in the resulting offspring will carry the abnormal chromosome number.

Translocation occurs when chromosomal fragments join non-homologous chromosomes. Polyploidy is a condition in which a cell has more than two complete chromosomal sets; in this example, only one set of chromosomes carries three copies. Duplication is the presence of additional segments within a single chromosome.

Example Question #2 : Understanding Autosomal And Sex Linked Inheritance

Red-green colorblindness is an X-linked recessive disorder. Which of the following scenarios is not a possible method by which this disorder can be inherited?

Possible Answers:

A genotypically normal mother and a colorblind father have a son who is also colorblind

A carrier mother and a normal father have a son who is born with colorblindness

A carrier mother and a colorblind father have a daughter who is born colorblind

A colorblind mother and normal father have a daughter who is a carrier for the condition

Correct answer:

A genotypically normal mother and a colorblind father have a son who is also colorblind

Explanation:

X-linked disorders are inherited when a parent passes on his or her X-chromosome. Since females have two X-chromosomes, they are less likely to exhibit symptoms of a recessive disorder than males, who have only one. Females are capable of carrying a recessive X-linked trait without expressing it, while males are not. A male must inherit his Y-chromosome from the father and an X-chromosome from the mother, while a female must inherit X-chromosomes from both parents.

If a genotypically healthy mother and a colorblind father have a son, then this child must inherit an X-chromosome from the mother and a Y-chromosome from the father. The mother's chromosome are both genotypically normal, and do not possess the colorblind allele. This means that the son cannot possibly inherit a colorblind allele if the mother is genotypically normal.

All other presented answer represent scenarios that are possible.

Example Question #2 : Understanding Autosomal And Sex Linked Inheritance

Colorblindness is a recessive X-linked disorder. A genotypically normal man and a colorblind woman have two sons and one daughter. What is the probability that one son is colorblind and the other two children are phenotypically normal?

 

Possible Answers:

Correct answer:

Explanation:

The Punnett square below represents the couple's possible offspring, with the mother having genotype  and the father having genotype .

                         

                

                 

Since the disorder is X-linked, we know that any sons will necessarily inherit an affected allele from the mother. Any daughters will inherit an chromosome from each parent; by necessity, any daughters will be heterozygous carriers. The probability of any daughters being phenotypically normal is 100%, and the probability of any sons being colorblind is also 100%.

The question states that the couple had two sons and one daughter, and asks the probability that one son is colorblind, one son is normal, and one daughter is colorblind. These probabilities are 100%, 0%, and 100%, respectively.

There is a 0% chance that this combination of children is possible.

Example Question #1 : Understanding Autosomal And Sex Linked Inheritance

 

X_linked

In this pedigree, affected individuals have a disease causing the person to be born with feathers instead of hair, called Disease P. Assume all individuals whose alleles cannot be determined do not carry the allele for the disease (are not heterozygous).

Disease P in the figure above is inherited in what manner?

Possible Answers:

The mode of inheritance cannot be determined

Autosomal recessive

Autosomal dominant

X-linked dominant

X-linked recessive

Correct answer:

X-linked recessive

Explanation:

The first generation shows us a father with the disease and a mother without the disease. They produce three children, none of whom have the disease. Knowing that they do not have the disease allows us to eliminate dominant from consideration. In order for the third generation to be affected, the mother from the second generation must be a carrier. In the third generation, we see that the carrier mother has a male child with the disease with a father who does not have the disease. The male child will inherit the Y chromosome from his father, but must receive an X chromosome from the mother. He inherits the disease on this X chromosome.

Were the disease autosomal recessive, the father of the third generation child would need to be affected in order for him to inherit the trait. The disease must be X-linked recessive.

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