AQA · GCSE · Combined Science

    Course version: 8464 · Version from Sept 2018

    Sex determination: Students should to be able to carry out a genetic cross to show sex inheritance.

    In humans, sex is determined by the sex chromosomes inherited at fertilisation.

      Study notes

      From the checked teaching notes for this specification point.

      The idea

      In humans, sex is determined by the sex chromosomes inherited at fertilisation. Females have two X chromosomes (XX) and males have one X and one Y (XY). A genetic cross is a diagram that tracks how alleles or chromosomes separate in gametes and recombine at fertilisation. To show sex inheritance, write the parental genotypes, XX for the mother and XY for the father. Each parent produces gametes containing one sex chromosome: the mother makes only X eggs, while the father makes X and Y sperm in equal numbers. Draw a Punnett square with the mother's gametes along one side and the father's along the other. The four boxes give XX, XX, XY and XY. This shows a 1:1 ratio of female to male offspring, so each fertilisation has a 50% chance of producing a female and a 50% chance of producing a male.

      What earns the marks

      • Parental genotypes correctly stated as XX for the female and XY for the male.
      • Gametes correctly identified: eggs all carry X; sperm carry either X or Y in equal proportions.
      • A completed Punnett square or equivalent cross showing the four possible offspring combinations XX, XX, XY and XY.
      • Offspring genotypes interpreted correctly as two XX (female) and two XY (male).
      • A correct conclusion that the expected ratio of female to male offspring is 1:1, giving a 50% probability of each sex.
      • Use of correct genetic terminology such as gamete, fertilisation, genotype and chromosome.

      Common mistakes and how to put them right

      • Writing the father's gametes as XY rather than separating them into X and Y sperm.

        Put it right: Gametes carry only one chromosome from each pair, so each sperm carries either X or Y.

      • Showing the mother producing X and Y eggs.

        Put it right: The mother is XX, so all her eggs carry an X chromosome.

      • Claiming that a particular family must have equal numbers of boys and girls.

        Put it right: The 1:1 ratio is a probability for each fertilisation, and small samples can differ by chance.

      Exam tips

      • Always show the parental genotypes, the gametes and the completed cross, because working can earn credit even if the final ratio is wrong.
      • Label the Punnett square clearly with female and male gametes so the examiner can follow your reasoning.
      • State the outcome as both a ratio and a probability, for example 1:1 and 50%, to cover the wording of the question.

      Try the lesson

      The start of the Sex determination: Students should to be able to carry out a genetic cross to show sex inheritance. lesson, made from checked teaching notes for this specification point.

      Have a go first

      In humans, sex is determined by the sex chromosomes X and Y. A female has the genotype XX and a male has the genotype XY. Which sex chromosomes does a sperm cell carry?

      Listen

      In humans, sex is determined by the sex chromosomes inherited at fertilisation. Females have two X chromosomes (XX) and males have one X and one Y (XY). A genetic cross is a diagram that tracks how alleles or chromosomes separate in gametes and recombine at fertilisation. To show sex inheritance, write the parental genotypes, XX for the mother and XY for the father. Each parent produces gametes containing one sex chromosome: the mother makes only X eggs, while the father makes X and Y sperm in equal numbers. Draw a Punnett square with the mother's gametes along one side and the father's along the other. The four boxes give XX, XX, XY and XY. This shows a 1:1 ratio of female to male offspring, so each fertilisation has a 50% chance of producing a female and a 50% chance of producing a male. A strong answer earns marks for these points. Parental genotypes correctly stated as XX for the female and XY for the male. Gametes correctly identified: eggs all carry X; sperm carry either X or Y in equal proportions. A completed Punnett square or equivalent cross showing the four possible offspring combinations XX, XX, XY and XY. Offspring genotypes interpreted correctly as two XX (female) and two XY (male). A common slip: Writing the father's gametes as XY rather than separating them into X and Y sperm. Instead, gametes carry only one chromosome from each pair, so each sperm carries either X or Y. Exam tip: Always show the parental genotypes, the gametes and the completed cross, because working can earn credit even if the final ratio is wrong.

      See it

      A top-down flowchart. The first box states the parental genotypes: mother XX and father XY. From it, two boxes branch out: one says the mother's gametes are all X eggs, the other says the father's gametes are X and Y sperm in equal numbers. Both lead into a box describing the Punnett square, with the mother's gametes along one side and the father's along the other. This leads to a box giving the offspring combinations XX, XX, XY and XY, then to a box saying two XX are female and two XY are male, and finally to a box stating the expected ratio is 1:1, giving a 50% probability of each sex.
      It shows the steps of a sex inheritance cross from parental genotypes through gametes and the Punnett square to the 1:1 ratio of female to male offspring.

      Label the diagram

      Some boxes have lost their names. Pick the right name for each number.

      The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. The first box states the parental genotypes: mother XX and father XY. From it, two boxes branch out: one says the mother's gametes are all X eggs, the other says the father's gametes are X and Y sperm in equal numbers. Both lead into a box describing the Punnett square, with the mother's gametes along one side and the father's along the other. This leads to a box giving the offspring combinations XX, XX, XY and XY, then to a box saying two XX are female and two XY are male, and finally to a box stating the expected ratio is 1:1, giving a 50% probability of each sex.

      • 1Box 1

      • 2Box 2

      • 3Box 3

      0 of 3 labelled

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