Genetics and mutation

Unit 12 Genetics and mutation

Key unit competence: Explain the role of genes in inheritance and how genetic disorders arise, and describe the types, causes and effects of mutations in organisms.

Key ideas

• Key terms: a gene is a length of DNA coding for a polypeptide; its position on a chromosome is the locus. Alleles are different forms of a gene. Genotype: the alleles an organism has (e.g. Tt); phenotype: its observable features. Homozygous (TT, tt), heterozygous (Tt). A dominant allele is expressed in the heterozygote; a recessive allele only when homozygous. F₁ and F₂ are first and second filial generations. A test cross crosses an unknown with a homozygous recessive.
• Mendel's monohybrid cross: tall (TT) × dwarf (tt) peas gave all tall F₁ (Tt); F₁ self-crossed gave F₂ 3 tall : 1 dwarf (genotypes 1 TT : 2 Tt : 1 tt). Mendel's first law (segregation): the two alleles of a gene separate into different gametes during meiosis, so each gamete has one allele.
• Non-Mendelian monohybrid inheritance: codominance (both alleles expressed, e.g. roan cattle from red × white; AB blood group); incomplete dominance (blended intermediate, e.g. pink four-o'clock flowers, F₂ 1 red : 2 pink : 1 white); multiple alleles (ABO blood group: Iᴬ, Iᴮ codominant, i recessive); lethal alleles (yellow mice: 2 : 1 ratio).
• Dihybrid cross: round yellow (RRYY) × wrinkled green (rryy) gave all round yellow F₁ (RrYy); F₂ 9 : 3 : 3 : 1. Mendel's second law (independent assortment): alleles of different genes on different chromosomes are sorted into gametes independently, so RrYy makes RY, Ry, rY, ry in equal numbers. Test cross RrYy × rryy gives 1 : 1 : 1 : 1.
• Linkage: genes on the same chromosome tend to be inherited together and do not assort independently. Without crossing-over, a dihybrid test cross gives only parental types (1 : 1). Crossing-over in prophase I produces recombinants. Crossover value (COV) = recombinants ÷ total offspring × 100%. COV is proportional to the distance between genes: 1% = 1 map unit; used to make chromosome maps.
• Sex determination in humans: females XX, males XY; the father's sperm decides the sex, giving a 1 : 1 ratio. Autosomal recessive disorders: albinism (no melanin), sickle cell anaemia (HbS; heterozygotes have sickle cell trait and some protection against malaria), cystic fibrosis. Autosomal dominant: Huntington's disease, polydactyly, achondroplasia.
• Sex-linked genes are on the X chromosome (no matching locus on Y). Recessive X-linked disorders such as haemophilia and red-green colour blindness are more common in males, who need only one recessive allele (XʰY). Females can be carriers (XᴴXʰ). A carrier mother and normal father: half the sons affected, half the daughters carriers. Fathers cannot pass X-linked alleles to sons. Pedigree charts trace inheritance (squares male, circles female, shaded affected).
• A mutation is a change in the amount or structure of DNA. It is the source of new alleles and genetic variation for evolution; most are neutral or harmful, a few beneficial. Mutations in body cells are not inherited; in gametes they can be passed on. Causes (mutagens): chance errors in replication, ionising radiation (X-rays, gamma rays), UV light, chemicals (benzene, tar in tobacco smoke, mustard gas), some viruses.
• Gene (point) mutations: substitution (one base replaced; may be silent, missense — e.g. sickle cell: GAG → GTG, glutamic acid → valine — or nonsense, creating a stop codon); addition (insertion) and deletion cause a frameshift, changing every codon after the mutation; inversion. Chromosome mutations: deletion, duplication, inversion and translocation of parts; changes in number: aneuploidy (one extra or missing chromosome by non-disjunction in meiosis) and polyploidy (whole extra sets, common in crops like wheat).
• Chromosomal disorders: Down syndrome (trisomy 21, 47 chromosomes, more likely with older mothers), Klinefelter syndrome (XXY male, small testes, infertile), Turner syndrome (XO female, short, infertile). Gene disorders: sickle cell anaemia, albinism, haemophilia, cystic fibrosis. Genetic counselling helps families understand risks.

Common misconceptions

• "A dominant allele is always the most common." No: dominance refers to expression in the heterozygote, not frequency.
• "A 3 : 1 ratio means exactly 3 out of every 4 children." No: it is a probability; small families may differ.
• "Colour blind fathers pass it to their sons." No: sons receive the Y from the father; the X comes from the mother.
• "All mutations are harmful." No: many are neutral and some are beneficial, providing variation for evolution.

Worked examples

Practice questions

1. Define gene, allele, genotype and phenotype.

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   Gene: DNA length coding for a polypeptide; allele: alternative form of a gene; genotype: alleles present; phenotype: observable characteristics.

   Common mistake: Mixing up genotype and phenotype.

2. State Mendel's laws of segregation and independent assortment.

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   Segregation: alleles of a gene separate into different gametes, each gamete receiving one. Independent assortment: alleles of different genes separate independently of each other during gamete formation.

   Common mistake: Saying genes on the same chromosome assort independently.

3. Using a genetic diagram, show the cross between two heterozygous tall pea plants.

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   Tt × Tt; gametes T, t and T, t; offspring TT, Tt, Tt, tt; 3 tall : 1 dwarf; genotypic ratio 1 : 2 : 1.

   Common mistake: Writing gametes as Tt.

4. Distinguish codominance from incomplete dominance with an example each.

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   Codominance: both alleles fully expressed, e.g. roan cattle, AB blood. Incomplete dominance: intermediate phenotype, e.g. pink four-o'clock flowers.

   Common mistake: Treating both as the same.

5. Explain how linkage changes the dihybrid test cross ratio.

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   Linked genes are inherited together; mostly parental gametes are produced; ratio approaches 1 : 1 parental types with few recombinants from crossing-over instead of 1 : 1 : 1 : 1.

   Common mistake: Forgetting crossing-over.

6. Explain why X-linked recessive disorders are more common in males.

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   Males have only one X chromosome; one recessive allele is expressed since Y has no matching allele; females need two recessive alleles and are usually carriers.

   Common mistake: Saying the gene is on the Y.

7. Distinguish gene mutations from chromosome mutations with examples.

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   Gene mutation: change in base sequence of one gene (substitution, deletion, insertion), e.g. sickle cell. Chromosome mutation: change in structure or number of chromosomes (deletion, duplication, inversion, translocation, aneuploidy, polyploidy), e.g. Down syndrome.

   Common mistake: Calling Down syndrome a gene mutation.

8. State four causes of mutations and two ways to reduce exposure.

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   Causes: replication errors, ionising radiation, UV light, chemicals such as tar and benzene, some viruses. Reduce: avoid smoking, protective clothing and sunscreen, limit unnecessary X-rays, safety in labs.

   Common mistake: Saying mutations are caused by exercise.

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 person with blood group AB has genotype:	a. IᴬIᴮ
   2. A mutation that shifts the reading frame is a:	b. UV light
   3. Which is a mutagen?	c. XY
   4. The sex chromosomes of a human male are:	d. deletion of one base

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   1 → a (IᴬIᴮ) · 2 → d (deletion of one base) · 3 → b (UV light) · 4 → c (XY)

   • Codominant alleles.
   • Every later codon changes.
   • It damages DNA.
   • The Y carries the SRY gene.

2. Drag and drop

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

   polyploid  ·  polyploidy  ·  test cross  ·  codominance
   1. Crossing an unknown with a homozygous recessive is a .
   2. Roan cattle show .
   3. An organism with whole extra sets of chromosomes is .
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   1: test cross · 2: codominance · 3: polyploid

   • Reveals the genotype.
   • Red and white hairs both show.
   • Common in crop plants.

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. Down syndrome is caused by:	a. crossing-over
   2. Haemophilia is more common in males because it is:	b. trisomy 21
   3. The F₂ ratio of a dihybrid cross is:	c. 9 : 3 : 3 : 1
   4. Recombinants are produced by:	d. X-linked recessive

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   1 → b (trisomy 21) · 2 → d (X-linked recessive) · 3 → c (9 : 3 : 3 : 1) · 4 → a (crossing-over)

   • 47 chromosomes.
   • Males have one X.
   • Independent assortment.
   • In prophase I.

4. Drag and drop

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

   10%  ·  non-disjunction  ·  1/2  ·  replication
   1. In a test cross 360 parentals and 40 recombinants appear. The crossover value is .
   2. Failure of chromosomes to separate in meiosis is .
   3. A carrier mother (XᴴXʰ) and normal father (XᴴY). The chance a son is haemophiliac is .
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   1: 10% · 2: non-disjunction · 3: 1/2

   • 40 ÷ 400 × 100.
   • It causes aneuploidy.
   • Sons get X from mother.

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