Question
Huntington’s disease is a neurodegenerative genetic disorder caused by a dominant allele.
The pedigree chart for a family shows the inheritance of Huntington’s disease.
What can be concluded from this pedigree chart?
A. Huntington’s disease is sex-linked, since most affected individuals are male.
B. Huntington’s disease is not sex-linked, since individuals IV-1 and IV-3 are not affected.
C. Huntington’s disease is not sex-linked, since individual III-3 is affected.
D. Individual IV-1 is a carrier, since individual III-2 is affected.
▶️Answer/Explanation
Answer: C. Huntington’s disease is not sex-linked, since individual III-3 is affected.
Explanation:
- Huntington’s disease is caused by a dominant allele, which means only one copy of the defective allele is needed for an individual to be affected.
- The disease is autosomal (not sex-linked). If it were sex-linked (e.g., X-linked), it would follow a different inheritance pattern, typically showing more affected males or being passed from mother to son.
- III-3 is a male and is affected, which supports the idea that the disease is not sex-linked. If it were X-linked dominant, and III-3 inherited it from his affected father (II-2), that would be inconsistent with X-linked inheritance, since males inherit their X chromosome from their mother, not their father.
- In addition, both males and females are affected, which also supports autosomal dominant inheritance.
Why other options are incorrect:
A: Incorrect. While more males might be affected here, it’s not a consistent pattern to confirm sex-linkage, and the inheritance doesn’t follow typical sex-linked patterns.
B: Incorrect reasoning. IV-1 and IV-3 not being affected does not provide solid evidence against sex-linkage.
D: Incorrect. In dominant disorders like Huntington’s, there are no carriers — you either have the disease (one or two dominant alleles) or you don’t. Carrier status usually refers to recessive conditions.
Question
A couple have four children whose blood groups are $A, B$ and $A B$. What is the likely combination of the parents’ genotypes?
A. $\quad I^A i$ and $\mathrm{I}^{\mathrm{B}} \mathrm{i}$
B. ||$^A i$ and ||$^B$
C. $\left.\left.\quad\right|^A\right|^B$ and ii
D. $\left.||^A\right|^A$ and $\left|I^B\right|^B$
Answer/Explanation
Answer: A.
and
Explanation:
- The ABO blood group system is controlled by three alleles:
,
, and
.
- Blood groups
and
are co-dominant, meaning if both
and
are inherited, the phenotype is
. The allele
is recessive and produces blood group
when homozygous.
- For children to have blood groups
,
, and
, each parent must carry alleles for both
and
, but since the children don’t include
, each parent likely carries one dominant allele and one recessive allele,
.
- This makes
and
the most probable genotypes, allowing offspring with blood groups
,
, and
.
Answer Evaluation:
A. Correct – Parents with
and
can produce children with blood groups
,
, and
.
B. Incorrect – The notation is incomplete and unclear; genotypes must specify both alleles.
C. Incorrect – One parent with
and the other with
cannot produce children with blood group
and also both
and
separately without
.
D. Incorrect – Parents both homozygous for
and
would not produce the blood groups
or
separately; children would be either
or
, but not
.
Question
The pedigree chart shows the inheritance of hemochromatosis, a genetic disease which causes an excessive accumulation of iron in the body.
What can be deduced about this genetic disease from the pedigree chart?
A. It is sex-linked.
B. It is autosomal dominant.
C. It is autosomal co-dominant.
D. It is autosomal recessive.
Answer/Explanation
Answer: D. It is autosomal recessive.
Explanation:
Let’s analyze the pedigree and the nature of hemochromatosis:
- Affected individuals include both males and females, which indicates it is not sex-linked (eliminates option A).
- Two unaffected parents have an affected child (seen in the second generation). This pattern is consistent with recessive inheritance — both parents must be carriers of the recessive allele to pass it on to an affected child.
- If it were autosomal dominant (option B), then at least one affected parent would be required to pass the trait to an affected child, which is not the case here.
- There’s no evidence of co-dominance (option C), where both alleles would show effects in heterozygotes.
Question
Huntington’s disease is an autosomal dominant genetic disease. What are the chances of two parents that are heterozygous for the gene having a child with Huntington’s disease?
A. 25%
B. 50%
C. 75%
D. 100%
Answer/Explanation
Answer: C. 75%
Explanation:
- Huntington’s disease is an autosomal dominant disorder, which means only one copy of the faulty gene is needed for a person to have the disease.
- If both parents are heterozygous (Hh), each child has a chance of inheriting one of the following combinations:
– HH (homozygous dominant – has the disease)
– Hh (heterozygous – has the disease)
– hh (homozygous recessive – does not have the disease) - So, out of 4 possibilities, 3 result in the child having Huntington’s disease, giving a 75% chance.
Answer Evaluation:
A. Incorrect – 25% is the chance of the child being unaffected (hh), not of having the disease.
B. Incorrect – 50% would be the chance if only one parent were heterozygous.
C. Correct – Three out of four genotypes (HH and Hh) result in the disease.
D. Incorrect – 100% would be the chance if both parents were homozygous dominant, which is not the case here.
Question
What is a difference between two alleles of a gene?
A. Their positions on homologous chromosomes
B. Their amino acid sequence
C. The characteristic they influence
D. Their base sequence
▶️Answer/Explanation
Answer: D. Their base sequence
Explanation:
- Alleles are different versions of the same gene. They are found at the same position (locus) on homologous chromosomes, but they differ in their base sequence of DNA.
- These differences in DNA base sequence may lead to changes in the amino acid sequence of a protein or affect how much protein is made, which can influence the trait the gene controls.
Answer Evaluation:
A. Incorrect – Alleles of the same gene are found at the same position on homologous chromosomes, not different positions.
B. Incorrect – While alleles can lead to different amino acid sequences, this is a result, not the actual difference between the alleles themselves.
C. Incorrect – Different alleles usually affect the same characteristic (e.g., eye color), though they may cause variations.
D. Correct – The difference between alleles lies in their DNA base sequences, which may alter how the gene functions.
Question
Chromosome numbers vary between species. Which statement refers to humans?
A. An egg cell has 22 autosomes.
B. A sperm cell has 23 autosomes.
C. An egg cell has two X chromosomes.
D. A zygote has two autosomes
▶️Answer/Explanation
Answer: A. An egg cell has 22 autosomes
Explanation:
- Humans have 46 chromosomes in total — 44 autosomes and 2 sex chromosomes (XX or XY).
- Egg and sperm cells are haploid, meaning they contain half the number of chromosomes (23 in total).
- Each egg cell contains 22 autosomes and 1 sex chromosome (always an X).
Answer Evaluation:
A. Correct – An egg cell contains 22 autosomes and one X chromosome.
B. Incorrect – A sperm cell has 22 autosomes and 1 sex chromosome (either X or Y), not 23 autosomes.
C. Incorrect – An egg cell has only one X chromosome; having two would mean it is diploid, not haploid.
D. Incorrect – A zygote is diploid and has 44 autosomes (22 pairs), not just two.
Question
Scientists sequenced the genes in each chromosome of chimpanzees (Pan troglodytes) and humans (Homo sapiens). The graph shows the mean divergence between the genes of these species by chromosome.
What can be deduced from this data?
A. Autosomes are more similar than Y chromosomes.
B. There is the same number of chromosomes in humans and chimpanzees.
C. Humans are more closely related to chimpanzees than to other species.
D. Smaller chromosomes are more similar than larger chromosomes.
▶️Answer/Explanation
Answer: A. Autosomes are more similar than Y chromosomes
Explanation:
- The graph shows mean genetic divergence by chromosome between humans and chimpanzees. Lower divergence means greater similarity.
- The Y chromosome shows the highest divergence, meaning it is the least similar, while the autosomes (non-sex chromosomes) show lower divergence, meaning they are more genetically similar.
- This suggests that the Y chromosome has evolved faster or differently compared to autosomes, which are more conserved between species.
Answer Evaluation:
A. Correct – The graph shows that autosomes have lower divergence, meaning they are more similar than the Y chromosome.
B. Incorrect – The graph shows divergence, not the number of chromosomes, so no conclusion about chromosome count can be made.
C. Incorrect – While true, this conclusion cannot be drawn directly from this graph alone; it only compares humans and chimpanzees.
D. Incorrect – There is no clear trend in the graph linking chromosome size with genetic similarity.
Question
The pedigree chart shows the inheritance of three generations of Duane syndrome, a condition caused by a dominant allele that affects alignment of the eyes.
If individuals II:1 and II:2 had a third child, what is the probability that the child would have Duane syndrome?
A. 25 %
B. 50 %
C. 75 %
D. 100 %
▶️Answer/Explanation
Answer: B. 50%
Explanation:
- Duane syndrome is caused by a dominant allele, meaning only one copy of the allele is needed for the condition to appear.
- Individual II:2 is affected, so their genotype must be either Dd (heterozygous) or DD (homozygous dominant).
- However, if II:2 were DD, all children would have the syndrome. But II:1 and II:2 already have two children — one affected (III:1) and one unaffected (not shown, but if there is a chance of an unaffected child existing or the pattern requires explanation). Since we see both affected and unaffected children in general pedigree logic, it’s likely that II:2 is heterozygous (Dd).
- Individual II:1 is unaffected, so their genotype must be dd.
Punnett Square for Dd (affected parent) × dd (unaffected parent):
| d | d | |
|---|---|---|
| D | Dd | Dd |
| d | dd | dd |
- 50% chance of Dd (affected)
- 50% chance of dd (unaffected)
Question
Which statements are characteristics of alleles?
I. Alleles differ significantly in number of base pairs.
II. Alleles are specific forms of a gene.
III. New alleles are formed by mutation.
A. I and II only
B. I and III only
C. II and III only
D. I, II and III
▶️Answer/Explanation
Answer: C. II and III only
Explanation:
- Alleles are different versions of the same gene, found at the same locus on a chromosome. They may vary by a few base pairs, but not significantly in size.
- Mutations (such as base substitutions, insertions, or deletions) can create new alleles, leading to genetic variation in a population.
- While alleles may differ in base sequence, they usually do not differ significantly in number of base pairs — this makes statement I incorrect.
Answer Evaluation:
I. Incorrect – Alleles differ by small changes in base sequence, not by a large number of base pairs.
II. Correct – Alleles are specific forms of a gene with slight differences in DNA sequence.
III. Correct – Mutations are the source of new alleles.