Biology · Lesson 20

Genetics: Mendel in Nigeria

JAMBWAECNECO

What the syllabus asks

JAMB: Heredity and variation (Mendel's laws, chromosomes, genes, sex determination, sex-linked characters, blood groups, sickle-cell, application of genetics)

WAEC: Genetics, variation and evolution

NECO: Genetics, variation and evolution

What you will learn

Explain heredity in everyday terms and use the basic vocabulary of genetics correctly.
Describe Mendel's experiments and state the law of segregation.
Draw genetic diagrams and Punnett squares and predict ratios and probabilities.
Use a test cross to find an unknown genotype.
Explain ABO blood groups (codominance), sex determination and sex-linked inheritance.
Explain sickle-cell inheritance and the value of genetic counselling.
Distinguish continuous from discontinuous variation and describe applications of genetics.

Key points

A gene is a stretch of DNA; alleles are its alternative forms.
Genotype = alleles (TT, Tt, tt); phenotype = observable trait.
Law of segregation: allele pairs separate at gamete formation.
Tt × Tt gives 3 : 1 (phenotype) and 1 : 2 : 1 (genotype); Tt × tt gives 1 : 1.
Test cross: unknown × homozygous recessive.
ABO blood groups: Iᴬ and Iᴮ are codominant, and both are dominant to i.
XX female and XY male; the father's sperm decides the sex.
Sex-linked recessive conditions (haemophilia, colour blindness) are commoner in males.
AS × AS gives a ¼ chance of SS in every pregnancy.
Continuous (many genes, bell curve) vs discontinuous (distinct classes) variation.

Start here: Why do children look like their parents?

Look around at a family gathering. A girl may have exactly her mother's dimples; a boy may be tall like his father but have his grandmother's eyes. Some features skip a generation and appear again in grandchildren. How does this happen? The science that answers this is called genetics.

Definition. Heredity is the passing of characteristics (traits) from parents to their offspring. Genetics is the study of heredity and variation.

Think of each cell as holding a big recipe book for building a person. The book is written in DNA and divided into chapters called chromosomes. Each recipe in the book, the instruction for one feature such as blood group or the ability to roll your tongue, is called a gene.

Here is the important part. You received two copies of the recipe book: one from your mother (in her egg) and one from your father (in his sperm). So for each gene you have two copies, which sit at the same position (locus) on a matching pair of chromosomes.

Fig. 20.1: A pair of matching (homologous) chromosomes. One copy of the gene came from each parent.

The two copies are not always identical. For the "height" gene in pea plants, one version gives tall plants and another gives dwarf plants. These different versions of the same gene are called alleles.

Definition. An allele is one of the alternative forms of a gene. We use a capital letter for the dominant allele (T = tall) and the same letter in lower case for the recessive allele (t = dwarf).

⚠️ Common mistake: saying "T and t are two genes". They are two alleles of one gene.

Try it yourself. How many alleles of each gene does a normal human body cell contain, and why?

Show solution

Two: one on each chromosome of the pair, one inherited from each parent.

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Practice questions

1. Two parents with genotype AS have children. What fraction of their children is expected to have genotype SS?

Show answer

A: 1/4
AS × AS → 1 AA : 2 AS : 1 SS, so 1/4.

2. A cross between a pure tall (TT) and a pure dwarf (tt) plant gives F1 offspring that are

all dwarf
3 tall : 1 dwarf
1 tall : 1 dwarf
all tall

Show answer

D: all tall
All F1 are Tt, and T is dominant, so all are tall.

3. A man of blood group AB and a woman of blood group O cannot have a child with blood group

B
A or B
AB
A

Show answer

C: AB
AB × OO gives AO or BO children only — groups A or B, never AB or O.

4. Which of these is an example of continuous variation?

Sex
Height
Blood group
Tongue rolling

Show answer

B: Height
Height shows a range of values; the others fall into distinct groups.

5. The basic unit of heredity is the

gene
chromosome
nucleus
cell

Show answer

A: gene
A gene is a section of DNA.

Key words

Heredity: The passing of characteristics from parents to offspring.
Gene: A section of DNA on a chromosome that controls a particular characteristic.
Allele: An alternative form of a gene (e.g. T or t).
Dominant: An allele that shows its effect even when only one copy is present.
Recessive: An allele that shows its effect only when two copies are present.
Genotype: The pair of alleles an organism has (e.g. Tt).
Phenotype: The observable characteristic (e.g. tall).
Homozygous: Having two identical alleles (TT or tt).
Heterozygous: Having two different alleles (Tt).
F₁, F₂: First and second filial (offspring) generations.
Test cross: Crossing an organism with a homozygous recessive to find its genotype.
Codominance: Both alleles are fully expressed in the heterozygote (e.g. blood group AB).

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