Starter quiz
- Which is the best definition of genotype?
- An individual’s genetic make-up.
- All of an individual’s genes.
- An individual’s physical characteristics.
- An individual’s combination of alleles. ✓
- An individual’s combination of genes.
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- Horses can breed with donkeys to produce offspring called mules. Horse body cells have 64 chromosomes and those of a donkey have 62. How many chromosomes will the zygote that grows into a mule have?
- 126
- 64
- 63 ✓
- 62
- 2
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Tay-Sachs is a single gene disorder caused by a recessive allele. Here is a family tree. It shows three generations. The shaded male has Tay-Sachs. Which statement is true?- All the grandparents must be carriers of Tay-Sachs.
- One grandparent is a carrier of Tay-Sachs.
- One parent is a carrier of Tay-Sachs
- Both parents are carriers of Tay-Sachs. ✓
- All the females are carriers of Tay-Sachs.
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Albinism is caused by a recessive allele. The Punnett square shows the possible genotypes resulting from two carriers having a child. What is the probability that a child will be a carrier?- 0.00
- 0.25
- 0.50 ✓
- 0.75
- 1.00
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- A student thinks you can tell the genotype of an individual by looking at its phenotype. When is this statement true?
- Always.
- Only if the individual is heterozygous.
- Only if the individual is homozygous dominant.
- Only if the individual is homozygous recessive. ✓
- Never.
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- Although it wasn’t recognised at the time, Mendel made a significant contribution to our modern understanding of inheritance. What was this contribution?
- He worked out that proteins are made from amino acids.
- He isolated DNA from a nucleus.
- He worked out that three DNA bases code for each amino acid.
- He discovered chromosomes contain DNA and proteins.
- He worked out rules of inheritance of some traits. ✓
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Exit quiz
- Match each term to its correct meaning.
- Sexual reproduction⇔Production of offspring by the fusion of gametes. ✓
- Biological sex⇔In humans, features determined by the 23rd chromosomes. ✓
- Sex chromosomes⇔In humans these can be X or Y. ✓
- Sex hormones⇔Chemical messengers that control reproduction and some characteristics ✓
- What percentage of the chromosomes in the nucleus of a zygote comes from the father?
- 0%
- 25%
- 50% ✓
- 75%
- 100%
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- Put the statements in order to describe how sex chromosomes cause the development of male biological sex.
- 1⇔A Y chromosome is inherited.
- 2⇔A gene on this chromosome codes for a protein which triggers the testes' growth.
- 3⇔Male sex hormones are produced and released into the blood.
- 4⇔Testosterone causes the development of male sex characteristics.
- In humans, biological sex is determined by ...
- all 46 chromosomes.
- only the Y chromosome.
- only the X chromosome.
- only the 23rd chromosome pair. ✓
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- What is a sex-linked genetic disorder?
- A disorder caused only by alleles on the X chromosome.
- A disorder caused only by the alleles on the Y chromosome.
- A disorder caused by alleles on the X or Y chromosomes.
- A disorder caused by the combination of alleles on the X and Y chromosomes. ✓
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- Duchenne Muscular Dystrophy (DMD) is a sex-linked genetic disorder causing muscle weakness. A recessive gene variant (d) on the X chromosome can trigger the disorder. Which statement is true for DMD?
- Only biological males can have DMD.
- Only biological females can have DMD.
- Only biological males can be carriers of DMD.
- Only biological females can be carriers of DMD. ✓
- Only biological females can pass on the DMD allele.
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Worksheet
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Lesson Details
Key learning points
- During sexual reproduction, an individual’s biological sex is determined by the chromosomes they inherit.
- In humans, the 23rd pair of chromosomes are sex chromosomes; XX in females and XY in males.
- A gene on the Y chromosome triggers the development of testes, which make male sex hormones (androgens).
- Interpreting, completing and constructing Punnett squares showing the inheritance of human sex chromosomes.
- Some disorders are caused by alleles on the sex chromosomes, so have sex-linked inheritance.
Common misconception
Males only have Y chromosomes, only males inherit sex-linked conditions.
Males have one Y and one X chromosome, males only need one recessive allele to suffer from a sex-linked genetic disorder, whereas females need two and therefore the condition is less common in females.
Keywords
Sexual reproduction - The process of producing offspring where a male and a female provide half the genetic material via gametes.
Biological sex - Male or female as determined by the combination of sex chromosomes inherited (i.e. XX female, XY male).
Sex chromosome - The chromosomes that determine the reproductive and secondary sexual characteristics (i.e. X and Y).
Sex hormone - Chemical messengers, secreted by glands, that help to control the reproductive and secondary sexual characteristics.
Sex-linked disorder - An inherited condition determined by alleles carried by the X or Y chromosome.
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