State the three types of point mutation that can occur during DNA replication.
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substitution; insertion; deletion
Mutations Inheritance And Genetic Screening
Explain why an insertion mutation often has a more severe effect on a protein than a substitution mutation.
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insertion causes a frame shift / shifts the reading frame; all codons after the insertion point are read differently / many amino acids change; substitution affects only one codon / one amino acid at most
Suggest two reasons why many mutations have no observable effect on the phenotype.
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genetic code is degenerate so the substituted codon may code for the same amino acid; mutation occurs in non-coding DNA / introns; changed amino acid may not affect protein shape or function
Define the terms genotype and phenotype.
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genotype is the combination of alleles an organism has / its genetic makeup; phenotype is the observable characteristics resulting from the genotype
Distinguish between codominance and dominance using an appropriate example.
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in dominance, only the dominant allele is expressed in the heterozygote / recessive allele is masked; in codominance, both alleles are expressed equally in the heterozygote; example such as ABO blood groups where Iᴬ and Iᴮ are both expressed in genotype Iᴬ Iᴮ
Explain why a heterozygote shows the dominant phenotype only if dominance is complete.
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dominant allele codes for a functional protein / produces the characteristic; recessive allele is masked and not expressed in the phenotype
A man heterozygous for cystic fibrosis (Ff) has a child with an unaffected woman (FF). Calculate the percentage chance of the child being a carrier.
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cross Ff × FF gives 1 FF : 1 Ff; 50% / one in two chance of being a carrier (Ff)
In a pedigree diagram, two unaffected parents produce an affected daughter. Explain what this tells you about the inheritance pattern.
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allele must be recessive / cannot be dominant because parents would show the trait; both parents must be heterozygous carriers (Aa); affected daughter is homozygous recessive (aa)
Suggest why a pedigree diagram alone may not always be enough to confirm whether an allele is dominant or recessive.
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family size may be too small / not enough individuals to observe expected ratios; affected individuals may not appear in every generation by chance; environmental factors may also influence the phenotype
Explain why red-green colour blindness is more common in males than in females.
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colour blindness allele is recessive and located on the X chromosome; males have only one X chromosome (XY) so a single recessive allele will be expressed; females need two recessive alleles (Xᵇ Xᵇ) to show the condition, which is rarer
A carrier woman (Xᴮ Xᵇ) has children with a colour-blind man (Xᵇ Y). Calculate the probability that a daughter will be colour-blind.
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cross Xᴮ Xᵇ × Xᵇ Y; daughters can be Xᴮ Xᵇ (carrier) or Xᵇ Xᵇ (colour-blind); 50% / 1 in 2 chance of a daughter being colour-blind
State why fathers cannot pass red-green colour blindness directly to their sons.
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sons inherit the Y chromosome from their father, not the X; colour blindness allele is on the X chromosome / not on Y, so cannot be transmitted from father to son
Describe the type of mutation that causes the most common form of cystic fibrosis.
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deletion mutation / three bases (one codon) deleted; one amino acid (phenylalanine) is missing from the CFTR protein
Explain why people with cystic fibrosis often suffer from malnutrition.
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faulty CFTR protein causes thick mucus to be produced; mucus blocks the pancreatic duct; digestive enzymes cannot reach the small intestine; food is not fully digested so nutrients (especially fats) are not absorbed properly
Discuss how a single gene mutation in the CFTR gene affects the gaseous exchange, digestive and reproductive systems in people with cystic fibrosis. Use information about CFTR function and your own knowledge to support your answer.
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CFTR normally transports chloride ions out of cells, with water following by osmosis to keep mucus thin; the CF mutation produces a faulty CFTR protein that cannot transport chloride properly; less water leaves the cells, producing thick sticky mucus on epithelial surfaces; in the gaseous exchange system, thick mucus blocks airways, traps pathogens and causes repeated lung infections, reducing gas exchange efficiency; in the digestive system, mucus blocks the pancreatic duct, preventing enzymes from reaching the small intestine, leading to poor digestion and malnutrition; in the reproductive system, mucus blocks the vas deferens in males causing infertility, and thick cervical mucus in females reduces sperm passage and fertility
Distinguish between amniocentesis and chorionic villus sampling.
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amniocentesis takes a sample of amniotic fluid containing fetal cells / CVS takes a sample from the chorionic villi of the placenta; amniocentesis is performed later (15–20 weeks) than CVS (10–13 weeks); CVS gives results earlier in pregnancy; CVS has a slightly higher miscarriage risk than amniocentesis
Explain how pre-implantation genetic diagnosis (PGD) reduces the ethical issues associated with prenatal testing.
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PGD tests embryos before implantation / before pregnancy is established; only unaffected embryos are implanted; this avoids the decision to terminate an established pregnancy
State two reasons why a couple might request genetic screening before having children.
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family history of a genetic disorder; both partners may be carriers / want to know risk to their child; one partner has an affected relative
Give two arguments in favour of genetic screening.
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allows parents to make informed reproductive decisions; identifies carriers / couples at risk; allows affected children to receive early treatment / preparation; reduces long-term healthcare costs / suffering
Discuss the ethical implications of pre-implantation genetic diagnosis (PGD).
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avoids termination of an established pregnancy / less ethically problematic than abortion for some; however, unused embryos are typically destroyed which raises concerns about life beginning at conception; some religious groups oppose embryo destruction; concern that PGD could lead to selection for non-medical traits / “designer babies”; PGD is also expensive so access may be unequal