Variation – SS3 Biology Lesson Note (Second Term)

Variation is the difference between individuals of the same species, including differences between parents and offspring and among the offspring.

Behavioural objectives

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

1. define variation and the main genetic terms, such as gene, allele, genotype, phenotype, homozygous and heterozygous;
2. state the causes of hereditary variation;
3. list transmittable characters in plants and animals and explain how characters are transmitted through gametes;
4. describe Mendel’s monohybrid and dihybrid experiments and state his two laws of inheritance;
5. explain incomplete dominance and codominance, using examples;
6. explain how sex is determined in human beings.

What is variation?

Variation is the difference in features between living things of the same species, such as height, skin colour and blood group.

There are two types of variation: continuous and discontinuous.

• Continuous variation: a full range of values between two extremes, as in height, body weight and skin colour.
• Discontinuous variation: clear-cut groups with no in-between forms, as in ABO blood group, tongue rolling and ability to taste PTC.

Variation also has two sources:

• Hereditary variation: caused by genes and passed from parents to offspring.
• Environmental variation: caused by factors such as food, light, climate and disease, and not passed on through genes.

What are the key genetic terms?

Genetic terms are the words used to describe heredity and variation.

• Genetics: the scientific study of heredity and variation in all living things. The word was coined by William Bateson (1906).
• Heredity (inheritance): the transmission and expression of characters from parents to offspring. It accounts for the usual resemblance between a parent and its offspring.
• Variation: the differences between parents and offspring and among the offspring.
• Genes: the basic units of inheritance, located in chromosomes, which pass characters from parents to offspring. Johannsen named them genes in 1909.
• Chromosomes: rod-like or thread-like bodies in the nucleus of a cell that carry the genes.
• Characters: inheritable features of an organism, e.g. height, complexion and colour.
• Gamete: a mature sex cell that takes part in sexual reproduction. Examples are pollen grains and ovules in plants, and sperms and ova in humans. A gamete is usually haploid.
• Zygote: the single cell formed when a male gamete and a female gamete unite. It is diploid.
• Allelomorphs (alleles): pairs of genes at the same position on a pair of chromosomes. They control contrasting characters.
• Locus: the site or position of a gene on a chromosome.
• Genotype: the genetic makeup of an individual, inherited from both parents. It includes dominant and recessive genes.
• Phenotype: all the observable features of an organism, including physical, physiological and behavioural traits, e.g. height and skin colour.
• Dominant trait: the trait expressed in an offspring when individuals with contrasting characters are crossed. A dominant gene controls it, e.g. T in Tt.
• Recessive trait: a trait that is masked in the presence of a dominant character. A recessive gene controls it, e.g. t in Tt.
• Homozygous: having two identical alleles for a character at a locus, e.g. TT or tt.
• Heterozygous: having two different alleles for a character at a locus, e.g. Tt.
• Filial generation: the offspring of parents. The generations are written F₁, F₂, F₃ and so on, each giving rise to the next.
• Hybrid: an offspring from a cross between parents that are genetically different but of the same species.
• Hybridisation: the crossing of organisms with contrasting characters. It may be monohybrid (one pair of contrasting characters) or dihybrid (two pairs).
• Haploid: having one set of chromosomes, as in a gamete (23 in humans). It is written n.
• Diploid: having two sets of chromosomes, as in a body cell (46 in humans). It is written 2n.
• Mutation: a change in the genetic makeup of an organism that produces a new inheritable characteristic.

What is hereditary variation and what causes it?

Hereditary variation is the difference among individuals that can be passed from parents to their offspring.

No two offspring inherit the same set of characters from their parents, except identical twins. Hereditary variation arises from these causes:

1. Genetic reshuffling in meiosis: segregation and independent assortment of chromosomes give gametes with different gene combinations.
2. Crossing over: in prophase of meiosis, homologous chromosomes come together, break and rejoin at points called chiasmata. Genetic material is exchanged, so the offspring differ.
3. Random fertilisation: any male gamete may fuse with any female gamete, which gives many possible combinations.
4. Mutation: a change in genes or chromosomes that creates a new inheritable character.

Which characters can be transmitted?

Only characters controlled by genes can be transmitted from parents to offspring.

Transmittable characters in animals

• Body features: body stature, height, and the shape or size of the head, nose and ear.
• Colouring: skin colour, hair colour and eye colour.
• Other traits: intelligence, the characteristic voice, baldness and tongue rolling.
• Blood and fingerprints: blood group and fingerprints.
• Inherited conditions: sickle cell anaemia, haemophilia and colour blindness.
• Taste: ability to taste phenylthiocarbamide (PTC).

Transmittable characters in plants

• Size and shape: height, weight, and the shape of the plant, fruit and leaf.
• Taste and food content: fruit taste and the food stored in the fruit or seed.
• Colour: colour of the leaf, flower, fruit or seed.
• Resistance: resistance to disease, pests and wind.
• Other features: leaf texture and life span.

How are characters transmitted?

Characters controlled by genes are transmitted from parents to offspring through gametes.

1. Diploid parents (2n) make gametes by meiosis in their reproductive organs.
2. Each gamete, the sperm cell or the ovum, is haploid (n). It has one copy of each gene from a homologous pair.
3. At fertilisation, the sperm and the ovum fuse to form a zygote.
4. The zygote is diploid (2n). It has two copies of each gene, one donated by each gamete.

ZYGOTE (2n)

What was Mendel’s work in genetics?

Gregor Mendel (1866) studied inheritance by crossing garden pea plants (Pisum sativum) and counting the offspring.

Why did Mendel use the garden pea?

1. Self-pollinating: peas pollinate themselves, so pure lines are easy to keep.
2. Short lifespan: several generations can be grown quickly.
3. Many contrasting characters: round or wrinkled seeds, tall or short stems, seed, pod and flower colour, pod texture and others.

Mendel used two methods: monohybrid inheritance and dihybrid inheritance.

What is monohybrid inheritance?

Monohybrid inheritance is the inheritance of one pair of contrasting characters, e.g. tall and short plants.

Mendel followed these steps:

1. He grew tall plants for several generations and got only tall offspring. Short plants gave only short offspring.
2. He cross-pollinated: pollen grains (male gametes) of the tall plant were placed on the stigma (female part) of the short plant.
3. He sowed the seeds and got all tall plants. He called this the first filial generation (F₁).
4. He crossed the F₁ plants, collected their seeds and sowed them. He got tall and short plants in the ratio 3:1. He called this the second filial generation (F₂).

The cross in symbols

• Parents’ genotypes: TT × tt (tall plant × short plant).
• Gametes: T and t.

• F₁: Tt, Tt, Tt, Tt (all tall).
• Self-pollinating: Tt × Tt.
• Gametes: T, t and T, t.

• F₂: TT, Tt, Tt, tt.
• Genotypic ratio (GR): 1:2:1.
• Phenotypic ratio (PR): 3:1.

What is Mendel’s first law of inheritance?

Mendel’s first law, the law of segregation, states that paired alleles separate from each other when homologous chromosomes separate during meiosis.

• Result in gametes: each gamete receives only one of the two alleles.
• Where it shows: the effect of segregation is seen in the F₂ generation.
• Basis: the law rests on the principle of complete dominance.

What is dihybrid inheritance?

Dihybrid inheritance is the inheritance of two pairs of contrasting characters at the same time.

Mendel crossed plants with round yellow seeds and plants with wrinkled green seeds. Seed shape (round or wrinkled) and seed colour (yellow or green) are the two pairs of characters.

• Parents’ genotypes: RRYY × rryy (round yellow seed × wrinkled green seed).
• Gametes: RY and ry.

• F₁: RrYy, RrYy, RrYy, RrYy (all round yellow).
• Self-pollinating: RrYy × RrYy.
• Gametes from each parent: RY, rY, Ry, ry.

The F₂ phenotypic ratio is 9:3:3:1.

What is Mendel’s second law of inheritance?

Mendel’s second law, the law of independent assortment, states that alleles of genes on different chromosomes assort independently during meiosis.

How is the genotype of a dominant phenotype determined?

The genotype of a dominant phenotype is found by a test cross or a back cross.

A dominant phenotype may have the genotype RR or Rr, so its genotype cannot be known by sight.

• Test cross: crossing the organism with a homozygous recessive organism (rr).
• Back cross: crossing an F₁ offspring with one of its parents, usually the homozygous recessive parent.

• All offspring show the dominant trait: the organism is homozygous dominant (RR).
• Offspring show dominant and recessive traits in the ratio 1:1: the organism is heterozygous (Rr).

What is the principle of incomplete dominance?

Incomplete dominance is the interaction of two contrasting alleles to give a heterozygous phenotype that differs from both homozygous phenotypes.

• Examples: Mirabilis jalapa (the four o’clock plant) and the Andalusian fowl.
• Difference from Mendel’s work: neither allele fully masks the other, unlike complete dominance.

The cross in symbols

• Parents’ genotypes: RR × rr.
• Phenotypes: red flower × white flower.
• Gametes: R and r.

• F₁: Rr, Rr, Rr, Rr (all pink flowers).
• Self-pollinating: Rr × Rr.
• Gametes: R, r and R, r.

• F₂: RR (red), Rr (pink), Rr (pink), rr (white).
• Ratios: PR = GR = 1:2:1.

What is codominance?

Codominance is the full expression of both alleles in a heterozygous individual, so neither modifies the effect of the other.

The human ABO blood group is an example.

• Allele I^A: adds antigen A to the surface of red blood cells, giving blood group A.
• Allele I^B: adds antigen B to the surface of red blood cells, giving blood group B.
• Genotype I^AI^B: both antigens are added, so the person has blood group AB.
• Allele I^O: recessive. I^A and I^B are codominant.

What are multiple alleles?

Multiple alleles are three or more alleles of one gene found in a population, e.g. I^A, I^B and I^O in the ABO blood group.

How do the three kinds of dominance differ?

How is sex determined in human beings?

Sex in humans is decided by the sex chromosomes: females are XX and males are XY.

• Chromosome number: each body cell has 23 pairs (46) of chromosomes, made up of 22 pairs of autosomes and one pair of sex chromosomes.
• Male: each body cell has X and Y, so each sperm carries either X or Y.
• Female: each body cell has two X chromosomes, so every egg carries one X.
• Fertilisation: the egg meets an X-carrying or Y-carrying sperm by chance, so a boy or a girl has an equal chance.

• Parents: XY × XX.

• Gametes: X, Y and X, X.
• Offspring: XX, XX, XY, XY (50:50).
• Ratios: GR = PR = 1:1.

Evaluation

1. Define hereditary, variation, alleles and locus. Differentiate between genotype and phenotype.
2. State Mendel’s first and second laws of inheritance.
3. State the F₂ phenotypic ratios of monohybrid and dihybrid inheritance.
4. Use the ABO blood group to explain codominance.
5. Show the cross between red and white four o’clock flowers up to the F₂ generation.

Assignment

Read College Biology, chapter 18, pages 409 to 428. Then show the cross between a group A parent (I^AI^O) and a group B parent (I^BI^O). State the genotypes and phenotypes of the offspring.