Sex determinationSpec 4.6.1.8
Ordinary human body cells contain 23 pairs of chromosomes. 22 pairs control characteristics only, but one of the pairs carries the genes that determine sex.
- In females the sex chromosomes are the same: XX.
- In males the sex chromosomes are different: XY.
All eggs contain an X chromosome. Half of the sperm contain an X and half contain a Y. The sperm that fertilises the egg decides the sex of the baby.
Sex inheritance cross
Show the cross between a female (XX) and a male (XY) and state the ratio of female to male offspring.
- Gametes from the mother: X and X. Gametes from the father: X and Y.
- Punnett square: XX, XX, XY, XY.
- Two of the four boxes are XX (female) and two are XY (male).
Answer: Ratio of female : male = 1 : 1. The probability of each child being a girl, or a boy, is 50%.
Quick check
Why are the offspring of asexual reproduction genetically identical to the parent?
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There is only one parent and no fusion of gametes, and only mitosis is involved.
How does meiosis halve the number of chromosomes?
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The cell divides twice to form four gametes, each with a single set of chromosomes.
Define the genome of an organism.
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The entire genetic material of that organism.
What is the difference between genotype and phenotype?
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Genotype is the alleles present; phenotype is how the characteristic is expressed.
What are the sex chromosomes of a human female and male?
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Female XX, male XY.