Chapter 19 - Heredity

Question 1

What are alleles?

Answer 1

Alleles are different forms of the same gene that occupy the same relative positions (gene loci) on a pair of homologous chromosomes.

Question 2

Define Monohybrid inheritance.

Answer 2

It is the inheritance of 1 characteristic that has 2 contrasting forms.

Question 3

Define dominant allele.

Answer 3

It is an allele that expresses itself and gives the same phenotype in both homozygous and heterozygous conditions.

Question 4

Define recessive allele.

Answer 4

It is the allele that only expresses itself in the phenotype in the homozygous conditions.

Question 5

Define codominance and give an example.

Answer 5

It is a condition where both alleles express themselves equally in the phenotype. E.g. IA and IB alleles in blood group

Question 6

Define genotype.

Answer 6

It is the genetic make-up of an organism.

Question 7

Define phenotype.

Answer 7

It is the expressed trait in an organism. It is influenced by its genotype and the environment.

Question 8

How can genotype of an organism with dominant trait be tested for? What are the possible results?

Answer 8

Use a test cross. Cross the organism with a homozygous recessive organism. If the organism is homozygous dominant, all offspring should show the dominant trait. If the organism is heterozygous, half the number of offspring should show the dominant trait, and the remaining show the recessive trait.

Question 9

What is incomplete dominance? Give an example.

Answer 9

It is when 2 alleles both refuse to express themselves, and the intermediate trait is shown. For example, when a black and white organism mate, their offspring might be grey.

Question 10

What are autosomes?

Answer 10

Autosomes are chromosomes in cells other than the sex chromosome.

Question 11

What is the makeup of chromosomes of each cell?

Answer 11

22 pairs of autosomes and 1 pair of sex chromosomes.

Question 12

How is the gender of a baby determined?

Answer 12

Female gametes contain only the X chromosome, while male gametes contain either X or Y chromosome. There is an equal chance that the offspring could be male of female.
XX: female
XY: male

Question 13

What are multiple alleles?

Answer 13

If a gene for a particular trait exists in more than 2 alleles.

Question 14

What are 3 differences between continuous variation and discontinuous variation?

Answer 14

1) CV is controlled by the additive effect of many genes, while DV is controlled by one or a few genes
2) CV has a range of phenotypes while DV has clear-cut phenotypes with no intermediate forms
3) CV can be affected by environmental conditions while DV is not affected by environmental conditions.

Question 15

List 3 examples of continuous variation.

Answer 15

Skin color, height, weight

Question 16

List 3 examples of discontinuous variation.

Answer 16

Blood group, gender, eye color

Question 17

Define mutation.

Answer 17

It is a sudden random change in gene structure or chromosome number.

Question 18

Why do mutations occur?

Answer 18

Mutation occurs as a result of error during the replication if the gene or chromosome. Somatic (body cell) mutations cannot be inherited, but if mutation occurs in cells that give rise of gametes, it may be inherited by the next generation.

Question 19

What is the difference between chromosome and gene mutation?

Answer 19

Chromosome mutation is the change in structure of number of chromosomes, causing diseases like Down syndrome, while gene mutation is the change in structure of DNA, producing variation between individuals as it results in new alleles of genes. It causes diseases like albinism.

Question 20

Discuss about down syndrome.

Answer 20

It is a chromosomal number mutation. People with this syndrome has 47 chromosomes instead of 46, with the extra chromosome at chromosome number 21. It is causes by the defective stages of meiosis during anaphase I/anaphase II.

Question 21

Discuss about sickle-cell anemia.

Answer 21

It is a gene mutation. The gene that codes for normal haemoglobin is mutated to form abnormal haemoglobin. The abnormal haemoglobin causes red blood cells to be sickle shaped. They carry less oxygen around the body, and tend to clump together. Although homozygous recessive dies young, but heterozygous survives better in malaria prone areas.

Question 22

What are mutagenic agents?

Answer 22

Since the rate of spontaneous mutation is usually very low, mutagenic agents(mutagens) greatly increases the rate of mutation.

Question 23

List 2 examples of mutagens.

Answer 23

1) radiation (UV Light, alpha and beta radiation)

2) chemicals (mustard gas, formaldehyde)

Question 24

How does variation and competition lead to natural selection?(3)

Answer 24

1) mutation provides new alleles to the gene pool for natural selection to act on
2) genetic variation is important to help organisms adapt and survive in changing environments
3) competition arise due to differences in the same species, leading to differential survival and reproduction. nature selects varieties of organisms that are more resistant to diseases and better adapted to changes in the environment.

Question 25

Define natural selection.

Answer 25

It is a process in nature which results in the best adapted organisms in a population surviving to reproduce and pass on their genes to the next generation.

Question 26

define evolution.

Answer 26

It is the process by which present complex forms of living organisms have arisen from simpler ancestral forms.

Question 27

What are 5 environmental factors that act as forces of natural selection?

Answer 27

1) competition for food
2) competition for territory
3) climate
4) types of predators
5) availability of mates

Question 28

How does natural selection take place?(mechanism of evolution)

Answer 28

1) organisms reproduce rapidly as food supply is abundant
2) organisms migrate to different environments
3) spontaneous mutation takes place, resulting in variation in the organisms. Favourable traits will confer a selective advantage and such organisms will survive, reproduce and pass in their favourable genes to their offspring
4) these organisms become the predominant species in their environment. Offspring of survivors will inherit their advantageous characteristics and remain well adapted to environment.

Question 29

What are two kind of artificial selection?

Answer 29

1) selective breeding

2) hybridisation

Question 30

What is an example of selective breeding?

Answer 30

Take seeds with high oil content, grow them and take their offspring with the highest oil content and breed them again. The process is repeated for generations to select for the plants which produce seeds high in oil. Plants are inbred to retain positive qualities.

Question 31

Give an example of hybridisation.

Answer 31

Breed a sugarcane which has high oil content with a sugarcane which is resistant to diseases. This produces filial generations with both high sugar content and resistant to diseases and pests.

Question 32

What are 4 differences between natural and artificial selection?

Answer 32

1) NS is resulted from mutations in gene, while AS is resulted from manipulation by humans
2) NS is brought about by changes in environmental conditions, while AS is brought about by humans that select organisms with desired traits to reproduce
3) NS is a very slow process while AS is a relatively faster process
4) NS may be harmful or advantageous to man, while AS is always advantageous to man