Genetics and gene technology

Unit 13 Genetics and gene technology

Key unit competence: Defend the role of genes in inheritance and evaluate the uses of gene technology.

Key ideas

• Genetics is the study of inheritance: how characteristics are passed from parents to offspring. A gene is a section of DNA on a chromosome that controls a characteristic. Different forms of a gene are alleles (e.g. T for tall, t for short).
• Key terms: genotype (the alleles an organism has, e.g. Tt); phenotype (the observable feature, e.g. tall); homozygous (two identical alleles: TT or tt); heterozygous (two different alleles: Tt); dominant allele (shows its effect in the heterozygote, capital letter); recessive allele (shows only when homozygous, small letter); locus (position of a gene on a chromosome).
• Monohybrid inheritance follows one characteristic. Mendel crossed pure-breeding tall (TT) and short (tt) pea plants: all F1 were tall (Tt). Selfing F1 (Tt × Tt) gave F₂ in the ratio 3 tall : 1 short (genotypes 1 TT : 2 Tt : 1 tt).
• A test cross (back cross) crosses an organism showing the dominant phenotype with a homozygous recessive: if all offspring show the dominant trait, the parent was homozygous; if half are recessive (1:1), it was heterozygous.
• Co-dominance: both alleles are expressed in the heterozygote. Example: blood group AB (alleles Iᴬ and Iᴮ both expressed); in some cattle, red × white gives roan (red and white hairs). Sickle cell: carriers (HbᴬHbˢ) make both normal and sickle haemoglobin.
• ABO blood groups are controlled by multiple alleles Iᴬ, Iᴮ (co-dominant) and Iᴼ (recessive): group A = IᴬIᴬ or IᴬIᴼ; B = IᴮIᴮ or IᴮIᴼ; AB = IᴬIᴮ; O = IᴼIᴼ.
• Sex determination: females are XX, males XY. The mother always passes an X; the father's sperm (X or Y) decides the sex of the child, giving a 1:1 ratio.
• Sex linkage: genes on the X chromosome (e.g. colour blindness, haemophilia) show more often in males, who have only one X. A carrier female (XᴴXʰ) has normal phenotype but can pass the allele to sons.
• Genetic engineering changes the DNA of an organism by transferring genes between organisms: the gene is cut out with restriction enzymes, inserted into a vector (often a bacterial plasmid) with ligase, and the plasmid is put into a host such as a bacterium, which then makes the product (e.g. human insulin).
• GMOs (genetically modified organisms): uses: human insulin and vaccines from bacteria, pest-resistant crops (Bt crops), drought-tolerant or vitamin-rich crops (Golden rice), disease diagnosis. Concerns: safety of food, spread of genes to wild plants, loss of biodiversity, cost and control of seeds, ethical issues; GMOs are regulated by law.

Common misconceptions

• "The dominant allele is always the most common one." No: dominant only means it masks the recessive allele in a heterozygote.
• "The mother decides the sex of the baby." No: the father's sperm carries X or Y and decides the sex.
• "A 3:1 ratio means a family of four children will have exactly three with the dominant trait." No: ratios are probabilities for each birth; real families can differ.
• "Recessive characters skip generations because genes disappear." No: the allele is present in carriers but hidden by the dominant allele.

Worked examples

Practice questions

1. Define the terms: (a) allele; (b) genotype; (c) phenotype; (d) recessive.

   Show answer

   (a) An alternative form of a gene. (b) The alleles an organism has. (c) The observable characteristic. (d) An allele expressed only when homozygous.

   Common mistake: Confusing genotype with phenotype.

2. Using a genetic diagram, show the cross between two heterozygous black guinea pigs (B = black, b = white).

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   Parents Bb × Bb; gametes B, b and B, b; offspring BB, Bb, Bb, bb; phenotype 3 black : 1 white.

   Common mistake: Writing gametes as Bb.

3. Explain how sex is determined in humans.

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   Females are XX and make eggs with X; males are XY and make sperm with X or Y in equal numbers; an X sperm gives a girl (XX), a Y sperm a boy (XY); ratio 1:1.

   Common mistake: Saying the mother decides.

4. Describe a test cross and explain its results.

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   Cross the dominant-phenotype organism with a homozygous recessive. All dominant offspring: parent homozygous dominant. Half dominant, half recessive: parent heterozygous.

   Common mistake: Crossing with another dominant organism.

5. Work out the possible blood groups of children of parents IᴬIᴼ and IᴮIᴼ.

   Show answer

   Gametes Iᴬ, Iᴼ and Iᴮ, Iᴼ; children IᴬIᴮ (AB), IᴬIᴼ (A), IᴮIᴼ (B), IᴼIᴼ (O): all four groups, ratio 1:1:1:1.

   Common mistake: Saying group O is impossible.

6. Explain why haemophilia is mainly seen in boys.

   Show answer

   The allele is recessive and on the X chromosome; boys have only one X, so one recessive allele causes the disease; girls need two, which is rare, and are usually carriers.

   Common mistake: Saying the allele is on the Y chromosome.

7. Outline the steps in producing human insulin by genetic engineering.

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   Cut out the insulin gene with restriction enzymes; cut a plasmid with the same enzyme; insert the gene and join with ligase; put the plasmid into bacteria; grow the bacteria in fermenters; extract and purify insulin.

   Common mistake: Leaving out the vector.

8. Give two benefits and two concerns of GMOs.

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   Benefits: higher yields, pest or drought resistance, more nutritious crops, medicines (insulin, vaccines). Concerns: possible health or allergy risks, genes spreading to wild plants, loss of biodiversity, high seed costs and control by companies.

   Common mistake: Giving only benefits.

Matching and drag-and-drop

1. Matching

   Match each item in column A with the correct answer in column B. Write the letter next to each number.

   Column A	Column B
   1. A woman with one normal and one colour-blind allele is a:	a. carrier
   2. A person with blood group O has the genotype:	b. 50%
   3. An organism with two identical alleles is:	c. homozygous
   4. The probability that a baby is a boy is:	d. IᴼIᴼ

   Show answer

   1 → a (carrier) · 2 → d (IᴼIᴼ) · 3 → c (homozygous) · 4 → b (50%)

   • She has normal vision.
   • Iᴼ is recessive.
   • Example: TT or tt.
   • X and Y sperm are equal in number.

2. Drag and drop

   Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

   phenotype  ·  genotype  ·  3 : 1  ·  XY
   1. The observable characteristic of an organism is its .
   2. Human males have the sex chromosomes .
   3. Crossing Tt × Tt gives a phenotype ratio of .
   Show answer

   1: phenotype · 2: XY · 3: 3 : 1

   • Example: tall or short.
   • Females are XX.
   • 3 dominant : 1 recessive.

3. Matching

   Match each item in column A with the correct answer in column B. Write the letter next to each number.

   Column A	Column B
   1. Haemophilia is an example of a:	a. alleles
   2. Different forms of the same gene are:	b. dominant
   3. A circular piece of bacterial DNA used as a vector is a:	c. plasmid
   4. An allele that is expressed even when only one copy is present is:	d. sex-linked disorder

   Show answer

   1 → d (sex-linked disorder) · 2 → a (alleles) · 3 → c (plasmid) · 4 → b (dominant)

   • The gene is on the X chromosome.
   • Example: T and t.
   • It carries the new gene into the host.
   • It masks the recessive allele.

4. Drag and drop

   Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

   sperm from the father  ·  mother's diet  ·  co-dominance  ·  gene
   1. A section of DNA that controls a characteristic is a .
   2. The sex of a child is determined by the .
   3. Red cattle crossed with white cattle give roan calves. This shows .
   Show answer

   1: gene · 2: sperm from the father · 3: co-dominance

   • Genes are on chromosomes.
   • Sperm carries X or Y.
   • Both colours are expressed.

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