Sexual vs Asexual Reproduction, Genetic Crosses & Inherited Disorders

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Sexual vs Asexual Reproduction

Sexual reproduction involves the fusion of male and female gametes (sperm and egg in animals; pollen and egg cells in flowering plants) formed by meiosis, mixing genetic information and creating variation. Asexual reproduction involves only one parent and cell division by mitosis; there is no fusion of gametes and offspring are genetically identical clones.

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Genotype, Phenotype, Homozygous & Heterozygous

An allele is an alternative version of a gene. Genotype is the combination of alleles an organism has (e.g., Bb). Phenotype is the observable physical characteristic expressed (e.g., brown eyes). Homozygous means having two identical alleles (BB or bb); heterozygous means having two different alleles (Bb).

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Dominant vs Recessive Alleles

A dominant allele (represented by a capital letter, e.g., B) is always expressed in the phenotype even if only one copy is present (BB or Bb). A recessive allele (lowercase letter, e.g., b) is only expressed if two copies are present (homozygous recessive, bb) and no dominant allele is present.

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Cystic Fibrosis (Recessive Inherited Disorder)

A genetic disorder of cell membranes caused by a recessive allele (f). Causes the body to produce lots of thick, sticky mucus in the air passages (lungs) and pancreas. An individual must inherit two recessive alleles (ff) to have the disorder; heterozygous individuals (Ff) are symptomless carriers.

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Polydactyly (Dominant Inherited Disorder)

An inherited condition in which a baby is born with extra fingers or toes, caused by a dominant allele (D). Because the allele is dominant, an individual only needs to inherit one copy of the faulty allele (Dd or DD) from one parent to have polydactyly; it is impossible to be an unaffected carrier.

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Embryo Screening & Gene Therapy Ethics

Amniocentesis, chorionic villus sampling (CVS), or pre-implantation genetic diagnosis (PGD during IVF) tests embryo DNA for genetic disorders. Advantages: reduces suffering and allows parents to prepare or terminate affected pregnancies. Disadvantages: invasive tests carry a small risk of miscarriage, high cost, and ethical concerns about 'designer babies' or prejudice against disabled individuals.

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1. Q1

   Two parents who do not have cystic fibrosis have a first child who is born with cystic fibrosis. Using a genetic cross (with F for the normal allele and f for the cystic fibrosis allele), explain how this happened and state the probability that their second child will also have cystic fibrosis. [5 marks]

   Check Mark Scheme

   Click “Check Mark Scheme” to compare your answer, 1. Cystic fibrosis is caused by a recessive allele (f). Since neither parent has the disorder, but they passed it to their child, both parents must be heterozygous carriers with genotype Ff [1 mark]. 2. Parental gametes: Both mother and father produce gametes containing either F or f [1 mark]. 3. Offspring genotypes from Punnett square (Ff x Ff): FF, Ff, Ff, ff [1 mark]. 4. The first child inherited one recessive allele (f) from each parent, giving the homozygous recessive genotype ff [1 mark]. 5. Every pregnancy is an independent event, so the probability that their second child will also be ff (have cystic fibrosis) is 25% (or 1/4 or 0.25) [1 mark].

2. Q2

   Compare the advantages of sexual and asexual reproduction for an organism such as the malarial parasite or a strawberry plant that can use both methods. [4 marks]

   Check Mark Scheme

   Click “Check Mark Scheme” to compare your answer, 1. Sexual reproduction mixes alleles from two parents via meiosis and fertilisation, producing genetic variation in the offspring [1 mark]. 2. Genetic variation creates a survival advantage if the environment changes or a disease/drug is introduced, as some individuals may possess beneficial alleles and survive by natural selection [1 mark]. 3. Asexual reproduction requires only one parent (no need to find a mate or pollinator), which saves time and energy [1 mark]. 4. When environmental conditions are already favourable, asexual reproduction allows the organism to produce large numbers of genetically identical, well-adapted offspring very rapidly [1 mark].

3. Q3

   In a family pedigree chart, a father and daughter both have Huntington's disease (or polydactyly), caused by a dominant allele (D), while the mother and son are unaffected. Determine the exact genotype of all four family members and explain your reasoning. [4 marks]

   Check Mark Scheme

   Click “Check Mark Scheme” to compare your answer, 1. The unaffected mother and unaffected son must both be homozygous recessive (dd), because if they had even one dominant D allele they would show the condition [1 mark]. 2. The affected daughter must have inherited one recessive allele (d) from her unaffected mother (dd) and one dominant allele (D) from her father, so her genotype is heterozygous (Dd) [1 mark]. 3. The affected father passed a recessive allele (d) to his unaffected son (dd) [1 mark]. 4. Therefore, the father cannot be DD; his genotype must also be heterozygous (Dd) [1 mark].

4. Q4

   Evaluate the use of embryo screening (such as Pre-implantation Genetic Diagnosis during IVF or amniocentesis) for couples with a family history of a severe genetic disorder. [4 marks]

   Check Mark Scheme

   Click “Check Mark Scheme” to compare your answer, 1. Advantage: Screening identifies whether an embryo has the faulty alleles, allowing only healthy embryos to be implanted during IVF, preventing a child from being born with a painful, life-limiting disorder [1 mark]. 2. Advantage: It reduces the emotional stress on families and saves long-term healthcare costs of treating lifelong genetic conditions [1 mark]. 3. Disadvantage: Invasive prenatal screening methods (like amniocentesis) carry a small medical risk of causing a miscarriage of a potentially healthy fetus [1 mark]. 4. Ethical concern: Screening during IVF results in embryos with faulty alleles being destroyed (or pregnancy termination), which some people oppose on religious/moral grounds or fear could lead to 'designer babies' and discrimination against disabled people [1 mark].

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