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Which description correctly represents a normal human male karyotype?
- A 46, XX indicating 44 autosomes and two X chromosomes You picked ✕
- B 45, X indicating absence of one sex chromosome You picked ✕
- C 47, XY, +21 indicating an extra copy of chromosome 21 You picked ✕
- D 23, XY indicating the haploid chromosome number in somatic cells You picked ✕
- E 46, XY indicating 44 autosomes and two sex chromosomes You picked ✓
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The answer is E: 46, XY indicating 44 autosomes and two sex chromosomes
Explanation
Why E is correct
See Introduction and analysis technique list. Option E is correct because a normal human male has 46 chromosomes: 22 pairs of autosomes (the non-sex chromosomes, so 44 in total) plus two sex chromosomes, one X and one Y. A karyotype is written as the total number of chromosomes, then the sex chromosomes, so a normal male is 46,XY and a normal female is 46,XX. Cells in the body (somatic cells) are diploid, meaning they carry two copies of each chromosome, one from each parent. A karyotype is made by photographing chromosomes in a dividing cell and arranging them in order of size, which lets the number, size, shape and banding pattern of each chromosome be checked for abnormalities. Extra or missing chromosomes appear in the number: for example, 47,XY,+21 would mean an extra chromosome 21. Memory aid: the first number counts all chromosomes, and the letters after it are the sex chromosomes.
- Chromosome number: a normal human male has 46 chromosomes.
- Autosomes: 22 pairs of non-sex chromosomes, 44 in total.
- Sex chromosomes: one X and one Y.
- Notation: a karyotype gives the total number, then the sex chromosomes, so a normal male is 46,XY.
Why the other options are incorrect
- A. 46, XX indicating 44 autosomes and two X chromosomes: 46,XX is the normal female karyotype. It has the right number of chromosomes but two X chromosomes and no Y. Your answer
- B. 45, X indicating absence of one sex chromosome: 45,X means one X chromosome and no second sex chromosome, which is monosomy X, seen in Turner syndrome. It is not a normal male karyotype. Your answer
- C. 47, XY, +21 indicating an extra copy of chromosome 21: 47,XY,+21 means there are 47 chromosomes, with an extra copy of chromosome 21. That is trisomy 21, which causes Down syndrome. Your answer
- D. 23, XY indicating the haploid chromosome number in somatic cells: 23 is the haploid number, found in egg and sperm cells. Somatic cells used for a standard karyotype are diploid, and a haploid cell carries either an X or a Y, not both. Your answer
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1
Which laboratory technique is primarily used to amplify a specific DNA sequence from a very small starting sample?
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2
A laboratory is testing for a known single nucleotide polymorphism (SNP) in a gene using a restriction enzyme followed by gel electrophoresis. Which principle best explains how this technique distinguishes between different alleles?
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