Genetics – SS3 Biology Lesson Note (First Term)

Genetics is the study of heredity, which is the transmission of characters from parents to offspring through genes carried on chromosomes.

Behavioural objectives

By the end of the lesson, students should be able to:

1. describe the location and structure of chromosomes, including the structure of DNA;
2. differentiate between autosomes and sex chromosomes;
3. explain how chromosomes transmit hereditary characters through meiosis and fertilisation;
4. describe sex-linked traits and other inherited conditions such as colour blindness, haemophilia and sickle cell anaemia;
5. state the two principles of probability in genetics and use the probability formula;
6. state applications of the principles of heredity in agriculture and medicine.

What are the key terms in genetics?

Key terms are the basic words used to describe heredity.

• Gene: a segment of DNA on a chromosome that controls a hereditary character.
• Gamete: a sex cell (egg or sperm) with one set of chromosomes.
• Zygote: the cell formed when an egg and a sperm fuse at fertilisation.
• Homozygous: having two identical genes for a character, e.g. AA or aa.
• Heterozygous (hybrid): having two different genes for a character, e.g. Aa.
• Diploid number (2n): the number of chromosomes in each body (somatic) cell.
• Haploid number (n): the number of chromosomes in a gamete, which is half the diploid number.

Where are chromosomes located and what is their structure?

Chromosomes are thread-like structures in the nucleus of eukaryotic cells that carry genes.

• Chromatin: the thread-like material in the nucleus from which chromosomes form.
• Homologous chromosomes: chromosomes occur in pairs of the same shape and size.
• Chromatids: each chromosome has two threads, joined at the centromere, when it is visible at cell division.
• Human number: each human body cell has 46 chromosomes, in 23 homologous pairs.
• Composition: each chromosome is made of protein units and a strand of DNA in a double helix.
• Genes: genes are segments of DNA arranged along the length of the chromosome.

What is the structure of DNA?

Deoxyribonucleic acid (DNA) is a very large molecule made of repeating units called nucleotides.

• Deoxyribose: the sugar part of each nucleotide.
• Phosphate: the second part of each nucleotide.
• Nitrogenous base: the third part, which is adenine, guanine, thymine or cytosine.
• Double helix: two chains twisted around each other.
• Complementary strands: one chain matches the other, because the bases pair in a fixed way.

What are sex chromosomes and autosomes?

Autosomes are the chromosomes that do not determine sex, while sex chromosomes are the pair that determines sex.

• Autosomes: 44 in humans (22 pairs), similar in shape and size in males and females.
• Sex chromosomes: the last pair, XX in human females and XY in human males.
• Birds, moths and butterflies: the female has two different sex chromosomes (written ZW) and the male has two alike (ZZ).
• Certain grasshoppers: the Y chromosome is absent, so the male is XO.

What are condensation and replication?

Condensation is the coiling and shortening of DNA into visible chromosomes, and replication is the copying of DNA.

• Condensation: before cell division, DNA coils tightly around protein bundles, so the chromosomes shorten and show under the light microscope.
• Replication: a DNA molecule makes an exact copy of itself.
• Importance: replication is the basis for passing hereditary material from parents to offspring.

Steps of DNA replication:

1. The double helix unwinds and the two strands separate.
2. Each strand acts as a pattern for a new strand.
3. Matching nucleotides join to each strand by base pairing.
4. Two identical DNA molecules result.

What is the role of chromosomes in the transmission of hereditary characters?

Chromosomes carry the genes, so they pass characters from parents to offspring.

• Genes: they express hereditary characters and sit on the chromosomes of body cells.
• Homologous pairs: the two chromosomes are alike in shape and size and carry genes for the same characters.
• Loci: genes for the same character, e.g. tallness and shortness, occupy identical positions on the pair.
• Genotype: the genes on a homologous pair decide whether an individual is homozygous or heterozygous for a character.

How do chromosomes transmit hereditary characters?

Chromosomes pass genes into gametes during meiosis, and the gametes combine at fertilisation.

1. The chromosomes pass the genes into the gametes during meiosis.
2. Homologous chromosomes separate into two daughter cells during the first stage of meiosis.
3. The two chromatids of each chromosome separate during the second stage of meiosis.
4. Each gamete therefore has one set of chromosomes, and so one copy of each gene.
5. During fertilisation, the gametes fuse to form a zygote.
6. The zygote receives two genes for the same character: one from the egg and one from the sperm.
7. If the two genes are the same, the offspring is homozygous. If they differ, it is heterozygous (hybrid).

Sex determination in humans follows the same steps:

• Egg: always carries an X chromosome.
• Sperm: carries either X or Y.
• XX zygote: a girl.
• XY zygote: a boy.

What are sex-linked traits?

Sex-linked traits are characters whose genes are carried on the X chromosome and inherited with it.

• Control: the genes are recessive.
• In males (XY): there is only one X, so one recessive gene shows in the male.
• In females (XX): two recessive genes are needed, so the trait is rarer in females.
• Carrier female: a woman with one recessive gene looks normal but can pass it to her children.
• Examples: colour blindness and haemophilia.

Colour blindness

• Meaning: a colour-blind person cannot tell near colours apart.
• Cause: a fault in the gene that controls the cone cells (light receptors for colour) in the retina.

Haemophilia

• Meaning: a disorder in which bleeding takes an abnormally long time to stop, because blood clots poorly.
• Danger: a small injury can cause serious loss of blood.
• Example: it appeared in the descendants of Queen Victoria of the British Royal Family.
• Origin: the gene probably arose as a mutation in Queen Victoria or one of her parents.

What are other inherited conditions?

Other inherited conditions are controlled by recessive genes but are not all carried on the X chromosome.

• Baldness: a recessive gene makes hair on the upper head fall out early, and it is more common in men.
• Albinism: the skin lacks the pigment melanin, so it is not coloured.
• Sickle cell anaemia: a recessive gene makes some red blood cells sickle-shaped.

How does sickle cell anaemia affect the body?

1. The affected person has abnormal haemoglobin in the red blood cells.
2. The abnormal haemoglobin carries oxygen poorly.
3. When oxygen concentration is low, the cells change into a sickle shape.
4. The sickled cells block small blood vessels and slow the flow of blood.
5. The affected body part gets less blood, oxygen and nutrients.
6. The person goes into crisis, with pains in the bones and joints.

Genotypes for sickle cell (A is normal, S is sickle):

What is probability in genetics?

Probability is the chance that an event will occur, expressed as a value from 0 to 1.

• Formula: probability = number of times an event occurs ÷ total number of trials.
• Mendel: his work on inheritance was based on probability.
• Value 0: the event cannot happen.
• Value 1: the event is certain.

What are the two principles of probability?

1. The result of one trial of a chance event does not affect the result of later trials of the same event.
2. The chance that two independent events will occur together is the product of their separate chances.

Example of the second principle: the chance of two heads in two coin tosses is ½ × ½ = ¼.

Worked example: two carriers of sickle cell (AS × AS)

• Offspring: one AA, two AS and one SS.
• Probability of SS (sickle cell anaemia): 1 out of 4, which is ¼.
• Albinism: two carriers (Aa × Aa) also have a ¼ chance of an albino child (aa).

How are the principles of heredity applied?

The principles of heredity are applied in agriculture to improve crops and animals, and in medicine to prevent and diagnose disease.

What are the applications in agriculture?

1. Controlled breeding: cross-fertilisation and self-fertilisation procedures.
2. Early maturing varieties: crops and animals that mature quickly.
3. Disease-resistant varieties: organisms that resist common diseases.
4. Adaptation: crops and animals that suit local climatic conditions.
5. Better products: improved quality and quantity of produce.

What are the applications in medicine?

1. Paternity: determining the father of a child.
2. Blood transfusion: matching the blood groups of donor and patient.
3. Diagnosis: identifying diseases, including inherited disorders.
4. Sex determination: knowing, and in some cases choosing, the sex of a baby.
5. Marriage counselling: advising couples to avoid genetic disorders, e.g. sickle cell anaemia.
6. Test tube babies: fertilisation outside the body for couples who cannot conceive naturally.

What is biotechnology?

Biotechnology is the use of living organisms, including the manipulation of DNA, for the benefit of humanity.

• Genetic engineering: the manipulation of DNA itself.
• Link to heredity: all the applications above show how biology is relevant to daily life.

Evaluation

1. What is a chromosome? Differentiate between autosomes and sex chromosomes.
2. Describe the structure of DNA.
3. Outline how chromosomes transmit hereditary characters from parents to offspring.
4. What is the probability of sickle cell anaemic children from parents with these genotypes?
   • (i) AA and AS, or HbAHbA and HbAHbS
   • (ii) AS and AS, or HbAHbS and HbAHbS
5. State five applications of genetics in medicine and agriculture.

Assignment

Read College Biology, chapter 18, pages 410 to 412, then answer these questions.

1. The basic hereditary unit is the …….. (a) cell (b) nerve (c) gene (d) nucleus
2. In man there are …….. autosomes and …….. sex chromosomes. (a) 46:2 (b) 44:2 (c) 44:1 (d) 45:1
3. The manipulation of DNA molecules for the benefit of humanity is known as (a) genetic (b) biotechnology (c) hereditary (d) bioremediation
4. A condition in which bleeding takes an abnormally long time to stop is known as (a) anaemia (b) sickle cell (c) haemophilia (d) albinism
5. What is the probability that a male and a female who are carriers of the albino gene will have an albino offspring? (a) ¼ (b) ½ (c) ¾ (d) 1
6. What are sex-linked traits? Give two examples.