Biology · Lesson 21

Applied genetics: DNA, sex determination, linkage, blood groups

JAMBWAECNECO

What the syllabus asks

JAMB: Variation and heredity: application of the principles of heredity

WAEC: Genetics: chromosomes and DNA (structure, base pairing, replication), sex determination, sex-linked characters (haemophilia, colour blindness), linkage and crossing over, blood groups (ABO, Rhesus) and sickle-cell inheritance, application of genetics (genetic counselling, breeding, genetic engineering, DNA fingerprinting)

NECO: Genetics: chromosomes and DNA (structure, base pairing, replication), sex determination, sex-linked characters (haemophilia, colour blindness), linkage and crossing over, blood groups (ABO, Rhesus) and sickle-cell inheritance, application of genetics (genetic counselling, breeding, genetic engineering, DNA fingerprinting)

What you will learn

Describe the structure of DNA and how it replicates.
Explain sex determination in humans using a genetic diagram.
Work out crosses for sex-linked traits.
Explain linkage and crossing over.
Solve ABO, Rhesus and sickle-cell inheritance problems.
Describe applications of genetics in medicine, agriculture and forensics.

Key points

DNA: double helix of nucleotides (phosphate, deoxyribose, base); A–T, G–C; replicates semi-conservatively.
Humans: 46 chromosomes = 22 pairs of autosomes + XX or XY.
The sperm decides sex; each child has a ½ chance of being a boy.
Sex-linked recessive traits (haemophilia, colour blindness) are on X; commoner in males; carrier mothers pass them to half their sons.
Linked genes on one chromosome travel together; crossing over at prophase I creates new combinations.
ABO: Iᴬ and Iᴮ codominant, i recessive; O universal donor, AB universal recipient.
Rh− mother with Rh+ baby risks haemolytic disease in later pregnancies; prevented by anti-D injection.
AS × AS: ¼ AA, ½ AS, ¼ SS; genotype testing and counselling; applications include GM crops, insulin, DNA fingerprinting.

Start here: Chromosomes, genes and DNA

Every cell nucleus contains chromosomes, thread-like structures that become visible during cell division. Human body cells have 46 chromosomes, in 23 pairs. One chromosome of each pair came from the mother and one from the father.

Each chromosome is one very long molecule of DNA (deoxyribonucleic acid) wrapped around proteins. Along the DNA are thousands of genes, each a section of DNA that carries the instructions for making one protein. Proteins (enzymes, haemoglobin, keratin, hormones like insulin) build and run the body, so genes determine traits.

Chromosome → contains DNA → divided into genes → each gene codes for a protein → proteins produce traits.

Of the 23 pairs, 22 pairs are autosomes (the same in males and females). The 23rd pair are the sex chromosomes: XX in females, XY in males.

Worked example 21.1

How many autosomes are in (a) a human skin cell, (b) a human sperm?

(a) Skin cell: 46 chromosomes, of which 2 are sex chromosomes, so 44 autosomes.
(b) Sperm is haploid: 23 chromosomes, including one sex chromosome, so 22 autosomes.

44; 22

⚠️ Common mistake: saying "a gene is a chromosome". A chromosome carries many genes.

Try it yourself. What is the diploid number of humans?

Show solution

46 (2n = 46).

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Practice questions

1. DNA stands for

deoxyribonucleic acid
dinitrogen acid
deoxyribose nucleotide amine
diribonucleic acid

Show answer

A: deoxyribonucleic acid
The genetic material.

2. The base that pairs with adenine in DNA is

guanine
cytosine
uracil
thymine

Show answer

D: thymine
A–T and G–C.

3. The shape of DNA is a

triple helix
circle only
double helix
single strand

Show answer

C: double helix
Watson and Crick.

4. In humans, the sex chromosomes of a male are

Show answer

B: XY
Females are XX.

5. The sex of a child is determined by

the sperm (X or Y)
the ovum
the mother's diet
the time of conception

Show answer

A: the sperm (X or Y)
Ova carry only X.

Key words

DNA: Deoxyribonucleic acid, the molecule carrying genetic information.
Nucleotide: A unit of DNA: phosphate, sugar and a nitrogenous base.
Gene: A section of DNA that codes for a particular protein or trait.
Allele: An alternative form of a gene.
Autosome: A chromosome that is not a sex chromosome.
Sex chromosomes: The X and Y chromosomes, which determine sex in humans.
Homogametic: Producing one kind of gamete for sex chromosomes (XX female in humans).
Heterogametic: Producing two kinds of gamete (XY male in humans).
Sex-linked: Carried on a sex chromosome (usually X).
Carrier: A heterozygous individual who carries a recessive allele without showing the trait.
Codominance: Both alleles are fully expressed in the heterozygote.
Linkage: The tendency of genes on the same chromosome to be inherited together.

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