Question 1 Medium
Give the percentage chance of a baby being a girl.
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Answer: 50%
- Half the sperm carry X.
- So 50% are girls.
Inheritance: genetic crosses, sex and inherited disorders: GCSE Combined Science Biology practice questions
A second set of questions on inheritance: genetic crosses, sex and inherited disorders. They are different from the ones on the topic page. Try each, then open the answer and compare your working.
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Question 2 Medium
A person has the genotype Ff for a recessive disorder. What word describes this person?
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Answer: carrier
- They have one copy of the allele.
- They are a carrier.
Question 3 Medium
Cystic fibrosis is caused by the recessive allele f. What is the genotype of a person who has cystic fibrosis?
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Answer: ff
- A recessive disorder needs two copies.
- So the genotype is ff.
Question 4 Medium
Two Bb parents have a child. What is the chance that the child shows the dominant feature? Give a percentage.
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Answer: 75%
- The boxes are BB, Bb, Bb and bb.
- Three out of four show the dominant feature, which is 75%.
Question 5 Hard
Give the ratio of dominant to recessive offspring from the cross Bb × bb.
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Answer: 1:1
- Two are Bb and two are bb.
- The ratio is 1 : 1.
Question 6 Hard
Two carriers of cystic fibrosis (Ff) have a child. What is the chance the child is a carrier? Give a percentage.
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Answer: 50%
- Two boxes in four are Ff.
- So the chance is 50%.
Question 7 Hard
A parent has genotype BB and the other has bb. What is the genotype of all the offspring?
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Answer: Bb
- Each gamete from BB carries B.
- Each gamete from bb carries b, so all are Bb.
Question 8 Hard
Polydactyly is caused by a dominant allele (P). One parent is Pp and the other is pp. What percentage of children are expected to have polydactyly?
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Answer: 50%
- The cross Pp × pp gives Pp, Pp, pp, pp.
- Half have polydactyly.
Question 9 Hard
Two parents without a condition have a child who has it. Is the condition dominant or recessive?
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Answer: recessive
- The parents must carry a hidden allele.
- So the condition is recessive.
Quick checks
Short questions with options. Choose one before you open the answer.
Check 1
What is an allele?
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A different version of a gene.
Check 2
What is a genotype?
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The combination of alleles a person has.
Check 3
What is a phenotype?
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The characteristic that shows from the genotype and environment.
Check 4
What is a dominant allele?
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An allele that shows even if there is only one copy.
Check 5
What is a recessive allele?
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An allele that only shows if two copies are present.
Check 6
What does homozygous mean?
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Having two identical alleles for a gene.
Check 7
What does heterozygous mean?
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Having two different alleles for a gene.
Check 8
What is the ratio from Bb × Bb?
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3 dominant to 1 recessive.
Check 9
What is the ratio from Bb × bb?
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1 dominant to 1 recessive.
Check 10
What are the sex chromosomes of a human female?
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XX.
Check 11
What are the sex chromosomes of a human male?
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XY.
Check 12
What is the chance of a baby being a boy?
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50%.
Check 13
What is polydactyly?
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Having extra fingers or toes, caused by a dominant allele.
Check 14
What is cystic fibrosis?
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An inherited disorder that causes thick, sticky mucus, caused by a recessive allele.
Check 15
What is a carrier?
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A person who has one copy of a recessive allele and does not show the disorder.
Check 16
What is embryo screening?
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Testing embryos for genetic disorders before they are placed in the womb.
Check 17
What is a gene?
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A section of DNA that codes for a protein.
Check 18
How many alleles of a gene does a body cell have?
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Two, one from each parent.
Check 19
How many alleles of a gene does a gamete have?
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One.
Check 20
What is a gamete?
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A sex cell, such as an egg or a sperm.
Check 21
Which sex chromosome do all eggs carry?
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X.
Check 22
What does a Punnett square show?
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The possible offspring from a genetic cross.
Check 23
Do two carriers always have a child with cystic fibrosis?
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No. Each child has a 25% chance.
Check 24
Name one concern about embryo screening.
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Embryos are destroyed, or it may lead to designer babies.
Check 25
Which parent decides the sex of a baby?
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The father, because the sperm carries X or Y.
Check 26
Can a small family differ from the 3 : 1 ratio?
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Yes. Each fertilisation is a chance event.
Check 27
What does a pedigree (family tree) show?
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How an inherited condition passes through a family.
Check 28
Two parents without a condition have an affected child. What does this show?
Show answer
The condition is recessive and both parents are carriers.
Keep going with Inheritance: genetic crosses, sex and inherited disorders
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