Most Repeated MDCAT Biology: Variation and Genetics MCQs

Explanation

Hypophosphatemic rickets is an X-linked dominant disorder. X-linked dominant disorders are genetic conditions caused by mutations in genes located on the X chromosome.

Explanation

ADA, or Adenosine Deaminase Deficiency, is a genetic disorder that affects the immune system. Specifically, it is a type of primary immunodeficiency disorder. This condition is caused by mutations in the ADA gene, which leads to a deficiency in the enzyme adenosine deaminase. This enzyme plays a crucial role in the development and function of certain immune cells, particularly lymphocytes.

Explanation

Pattern baldness is a sex-influenced trait, meaning its expression differs between males and females. In men, it is an autosomal dominant trait, allowing baldness to occur with just one dominant allele. However, in women, it is an autosomal recessive trait, which means baldness occurs only if the woman has two recessive alleles (homozygous recessive). This is why a woman can only be bald if she is homozygous recessive. The other genotypes, homozygous dominant and heterozygous, result in the dominant allele masking the recessive allele, preventing baldness in females.

Explanation

The Law of Dominance states that in a heterozygous individual, the dominant allele will determine the phenotype while the recessive allele is masked. In this case, the cross between a homozygous tall plant (TT) and a pure short plant (tt) clearly illustrates this law, as all offspring (Tt) will be tall, demonstrating that the dominant allele 'T' expresses itself over the recessive 't'.The other options do not adequately illustrate the Law of Dominance: the first option shows both parents expressing the tall phenotype, while the second option leads to a mix of phenotypes.

Explanation

Males have 1 X chromosome and 1 Y chromosome, and females have 2 X chromosomes. The genes that can give you red-green colour blindness are passed down on the X chromosome.The woman’s father is red-green colour blind. Since the woman got one of her X chromosomes from her father this means that she is a carrier. As it can be seen from the table the probability of having a colourblind child is 0.

Explanation

To understand and solve this, you can divide your thinking into two. For character 1. When genes are passed down to offspring, when the genes of this character pass down, the Dominant gene will always be present, so only one phenotype is possible from this character. For character 2. Three genotypes are possible, RR, Rr, and rr. RR and Rr are one phenotype and rr another.

Explanation

This is the correct answer. Codominance occurs when both alleles of a heterozygous individual are expressed simultaneously in the phenotype. Neither allele is dominant over the other, and both contribute equally to the observed trait. A classic example of codominance is the ABO blood type system, where the IA and IB alleles are codominant.

Explanation

A karyotype is an individual's complete set of chromosomes. The term also refers to a laboratory-produced image of a person's chromosomes isolated from an individual cell and arranged in numerical order.

Explanation

Haemophilia is an X-linked recessive disorder, which means it is more commonly seen in males than females. This is because the gene responsible for haemophilia is located on the X chromosome. Males have one X chromosome and one Y chromosome, while females have two X chromosomes. If a male inherits the faulty gene on his X chromosome, he will have haemophilia because he does not have another X chromosome to compensate for the faulty gene. Females, on the other hand, have two X chromosomes, so even if one X chromosome carries the faulty gene, the other X chromosome can often compensate for it. That's why haemophilia is more prevalent in males.

Explanation

The locus of a gene on a chromosome refers to its specific location or position on the chromosome. It is the precise point on the chromosome where the gene is situated, and it is usually described by its chromosomal coordinates (e.g., 3q21, meaning the gene is located on the long arm of chromosome 3, at position 21).

Explanation

C) Rhesus monkey: While the Rh antigen was named after the Rhesus monkey due to its initial discovery in Rhesus monkey blood, the discovery of its significance in human blood transfusions came first. The Rh factor was identified in humans in 1937, followed by its discovery in Rhesus monkey blood in 1940. So, while it was discovered in Rhesus monkeys after humans, the term "Rh" itself came from the Rhesus monkey.

Explanation

If the black coat color in horses is the dominant phenotype, and both parental horses are black, then they must have at least one allele for the black coat color. Because the white coat color is the recessive phenotype, it can be caused only by a homozygous recessive genotype. The probability of these horses producing a foal with a black coat is 75%, or 3 out of 4.

Explanation

The correct answer is that both green and wrinkled are recessive characters. This conclusion is based on the fact that recessive traits appear only when both alleles are recessive (homozygous recessive). The observation of 17 green and wrinkled seeds out of 254 suggests these traits follow Mendel's laws of inheritance for recessive traits. Other options are incorrect because they either suggest genetic phenomena not supported by the data (crossing-over, linkage) or propose incorrect inheritance patterns for the traits involved.

Explanation

The correct sequence of genes on the chromosome is PSRQ. This arrangement makes sense given the crossover values provided:The low crossover value of 12% between R and S indicates they are closely linked, which is consistent with having them next to each other in this sequence.The higher crossover value of 59% between P and Q suggests they are further apart on the chromosome, which is also supported in this arrangement.The other options (PRSQ, QSPR, RQSP, SPRQ) do not appropriately reflect the crossover data, leading to contradictions with the provided percentages.

Explanation

Dominance is the physiological effect of an allele over its partner allele occuping the same locus on respective homologue. Therefore, option C is correct.