9. Inheritance

Question 1

Distinguish between the terms gene and allele

Answer 1

A gene is a unit of inheritance that occupies a specific locus on a DNA molecule. It is a short and specific sequence of nucleotides that codes for a particular polypeptide, which fold to form a protein.

• It can be passed on from parent to offspring via chromosomes of the parents’ gametes.

An allele is an alternative form of a gene which occupies the same location (locus) on a pair of homologous chromosomes.

• Alleles of a gene impart a particular characteristic.

Question 2

Dominant Allele

Definition

Answer 2

A dominant allele is an allele that can always be expressed in the phenotype, when it occurs in the homozygous or heterozygous condition, e.g., TT or Tt.

A capital letter is always used to represent a dominant allele. (e.g. T or G)

Question 3

Recessive Allele

Definition

Answer 3

A recessive allele is an allele that is only expressed in the phenotype, when it occurs in the homozygous condition, e.g. tt.

A small letter is always used to present a recessive allele. (e.g. t or T)

Note: It is incorrect to describe an allele as recessive without reference to the allele it is recessive to. It should be: “The allele for the white eye is recessive to the allele for the red eye.”

Question 4

Homozygous Dominant

Definition

Answer 4

Organisms having two identical (dominant) alleles of a particular gene (e.g. TT) have a homozygous dominant genotype.

Two identical homozygous individuals that breed together will be pure breeding.

Question 5

Homozygous Recessive

Definition

Answer 5

Organisms having two identical (recessive) alleles of a particular gene (e.g. gg) have a homozygous recessive geneotype.

Two identical homozygous individuals that breed together will be pure breeding.

Question 6

Heterozygous

Definition

Answer 6

Organisms having two different alleles of a particular gene (e.g. Tt or Gg) have a heterozygous genotype.

If one heterozygous individual is involved in breeding, it will be impure breeding.

Question 7

Genotype

Definition

Answer 7

Genotype is the genetic makeup of an organism.

Question 8

Phenotype

Definition

Answer 8

Phenotype is an observable trait or physical appearance of an organism.

The homozygous dominant and heterozygous genotypes give the same phenotype.
The homozygous recessive individual will have a contrasting phenotype.

Question 9

Codominance

Definition

Answer 9

Codominance occurs when both alleles have an equal effect on the phenotype of the offspring.

In a heterozygote (organism with a heterozygous genotype), both alleles are expressed in the phenotype.

Question 10

Exaplin how genotype is linked to phenotype

Answer 10

Genotype is the genetic makeup of an organism, its specific sequence of DNA.
Phenotype is an observable trait or physical appearance of an organism.
While the genotype is the genetic information, the phenotype is the expression of this information in a tangible form.

Question 11

Explain why the observed ratios for the results of crosses often differ from the expectated ratios, especially when there are small number of progeny (offspring)

Answer 11

If a smaller number of organisms are used, the observed ratio for the offspring may not be so accurate. Statistically, ratios are often inaccurate when the sample size is small.

Since the ratio figures are based on chance and probabilities, the actual progeny (offspring) would be unlikely to match the expected number precisely.

Question 12

Multiple Alleles

Answer 12

If a gene exists in more than two alleles, it is said to have multiple alleles.

Question 13

Alleles that Determine Blood Groups in Humans

Answer 13

• I^A represents the allele for the production of type A antigen.
• I^B represents the allele for the production of type B antigen.
• I^O represents the allele that produces neither antigen.

Question 14

Genotypes for the Phenotypes of Blood Groups in Humans

Answer 14

Blood group A: I^AI^A or I^AI^O (I^A is dominant over I^O)
Blood group B: I^BI^B or I^BI^O (I^B is dominant over I^O)
Blood group AB: I^AI^B (I^A and I^B are codominant)
Blood group O: I^OI^O

Question 15

Sex Chromosomes and Autosomes

What they are

Answer 15

The sex chromosomes are the chromosomes which determine the sex of the organism.

• Not always homologous
• XX: Female
• XY: Male

Autosomes are the other homologous chromosomes.

• Always homologous

Question 16

Sex Chromosomes in Gametes

Female & Male

Answer 16

During gamete production, the female gametes contain an X chromosome each.
However, males produce two types of sperms, one containing X chromosome, the other containing Y chromosome.

Question 17

What is Sex Linkage & General Patterns of Sex-Linked Inheritance

Answer 17

Sex linkage refers to the carrying of the genes on the sex chromosomes.

X-linked recessive

• Usually more male than females are affected.
• Cross between affected male (X^bY) and normal female (X^BX^B) results in normal males and carrier females, making the trait skip generations in the pedigree.

X-linked dominant

• Cross between affected male (X^BY) and normal female (X^bX^b) results in all female offspring affected (X^BX^b) but no male offspring affected (X^bY).
• Approximately half of the offspring of affected females are affected, regardless of their sex.

Question 18

Variations

Definition

Answer 18

Variations is defined as the differences in traits between individuals of the same species.

Question 19

Causes of Variations

Answer 19

1. Genetic factors

• Crossing over at prophase I (in meiosis during cell division)
• Random or independent assortment of chromosomes at metaphase I (in meiosis during cell division)
• Random fertilisation of gametes (during sexual reproduction)
• Gene and chromosomal mutation (caused by mutagens)

2. Environmental factors

• Characteristics of an organism are mainly determined by genes.
• However, the expression of certain genes can be affected by specific environmental factors such as amount of sunlight, temperature, amount of available water, and amount of nutrients.

Question 20

Continuous Variation vs Discontinuous Variation

Answer 20

1. Continuous variation deals with a range of phenotypes while discontinuous variation deals with a few clear-cut phenotypes.
2. Continuous variation is greatly affected by the environmental conditions while discontinuous variation is relatively unaffected by the environmental conditions.
3. Continuous variation is controlled by many genes while discontinuous variation is controlled by one or a few genes.
4. In continuous variation, differences between individuals are quantitative. In discontinuous variation, differences are qualitative.
5. In continuous variation, there is a continuous range of intermediate values. In discontinuous variation, there are discrete groups and no intermediate forms.
6. Continuous variation is caused by genetic factors that affect many genes as well as environmental factors. Discontinuous variation is caused by genetic factors to one or a few genes.

Examples of continuous variation: height, body mass, skin colour, intelligence (IQ)
Examples of discontinuous variation: tongue rolling ability, blood group, single or double eyelid, albinism

Question 21

Evolution

Definition

Answer 21

Evolution is the change in the heritable characteristics of biological populations over successive generations.

Question 22

Purpose of Natural Selection

Answer 22

Nautral selection of small, inherited variations increase the individual’s ability to compete, survive, and reproduce.

Question 23

Natural Selection

Process

Answer 23

1. Variation within the population is caused by spontaneous mutation.
2. When there is a change in the environment or competition among varieties, only the fittest will survive.
3. Those with the genetic variation best adapted to the environment will be more likely to survive till maturity.
4. They will then have a higher chance of reproducing and passing down these desirable genes to the offspring.
5. Over time, their offspring increase in proportion in the population and so the proportion of favourable genes in the gene pool of the species increases.
6. Such preservation of favourable variations and elimination of unfavourable variations is known as natural selection.
7. When natural selection is repeated in every generation, more and more favourable genes are accumulated in the gene pool and the species will be better adapted to the environment.
8. Over a long period of time, it could lead to evolutionary changes and possibly the formation of a new species.

Question 24

Forces of Natural Selection

Answer 24

Environmentals factors act as forces of natural selection:

• For animals: amount of food or water, breeding space available
• For plants: availibility of mineral salts, light, and water

Also, as population increases in size and the individuals in the population compete for the limited resources available, there will also be a corresponding increase in the numbers of predators or pathogens.

Question 25

Why is sickle-cell anaemia common in areas affected by malaria (West Africa)?

Answer 25

• In individuals who are heterozygous for the sickle-cell allele, they have a small percentage of red blood cells which are sickle-shaped. Malaria parasites thrive in red blood cells of the bitten individual. However, they cannot survive in red blood cells that are sickle-shaped as the red blood cells are destroyed in the spleen relatively fast, thus destroying the parasites as well. This condition is known as heterozygous advantage.
• Hence, heterozygous individuals have a better chance of surviving and reproducing, and this selection ensures that the recessive allele persists in the population. This makes them more likely to survive malaria infection as compared to individuals with two copies of normal haemoglobin allele who have a greater risk of dying of malaria.