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Genetics and cell division
10 MDCAT Biology questions with answers and explanations.
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In which phase of mitosis do the chromosomes line up at the equator of the cell?
- 1 Prophase
- 2 Metaphase
- 3 Anaphase
- 4 Telophase
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Correct answer: Metaphase
In metaphase the chromosomes line up on the metaphase plate. In anaphase the sister chromatids separate.
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Crossing over takes place during:
- 1 Prophase I of meiosis
- 2 Metaphase of mitosis
- 3 Anaphase II of meiosis
- 4 Interphase
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Correct answer: Prophase I of meiosis
Homologous chromosomes pair up in prophase I and exchange segments. This produces new combinations of alleles.
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How many chromosomes are present in a normal human gamete?
- 1 46
- 2 23
- 3 44
- 4 92
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Correct answer: 23
Human body cells have 46 chromosomes. Meiosis halves this, so each gamete has 23.
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A cross between two heterozygous tall plants (Tt x Tt) gives tall and short offspring in the ratio:
- 1 1 : 1
- 2 1 : 3
- 3 3 : 1
- 4 9 : 3
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Correct answer: 3 : 1
The offspring are TT, Tt, Tt and tt in the ratio 1:2:1. TT and Tt are tall, so tall to short is 3:1.
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Which base pairs with adenine in DNA?
- 1 Guanine
- 2 Cytosine
- 3 Uracil
- 4 Thymine
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Correct answer: Thymine
In DNA, adenine pairs with thymine by two hydrogen bonds, and guanine pairs with cytosine by three. Uracil replaces thymine in RNA.
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A test cross is used to find the genotype of an individual by crossing it with:
- 1 A homozygous dominant individual
- 2 A homozygous recessive individual
- 3 A heterozygous individual
- 4 Any unrelated individual
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Correct answer: A homozygous recessive individual
Crossing with a homozygous recessive shows the unknown parent's alleles in the offspring. Any recessive offspring means the parent carries a recessive allele.
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Down syndrome is caused by:
- 1 Monosomy of chromosome 21
- 2 Trisomy of chromosome 21
- 3 Loss of the Y chromosome
- 4 A missing X chromosome
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Correct answer: Trisomy of chromosome 21
Down syndrome results from three copies of chromosome 21, usually through non-disjunction in meiosis.
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Which experiment demonstrated that DNA replication is semi-conservative?
- 1 Griffith's experiment
- 2 Hershey and Chase
- 3 Meselson and Stahl
- 4 Mendel's pea crosses
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Correct answer: Meselson and Stahl
Meselson and Stahl used heavy nitrogen to show that each new DNA molecule keeps one old strand and gets one new strand.
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Haemophilia is inherited as:
- 1 An autosomal dominant trait
- 2 An X-linked recessive trait
- 3 A Y-linked trait
- 4 An autosomal recessive trait
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Correct answer: An X-linked recessive trait
The gene is on the X chromosome. Males have one X, so one recessive allele is enough to show the disease, which is why it is more common in males.
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The offspring of AaBb x AaBb (independent assortment, complete dominance) show a phenotypic ratio of:
- 1 9 : 3 : 3 : 1
- 2 3 : 1
- 3 1 : 2 : 1
- 4 1 : 1 : 1 : 1
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Correct answer: 9 : 3 : 3 : 1
This is Mendel's dihybrid cross. The four phenotypes appear in the ratio 9:3:3:1.