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Genes and Inheritance

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(Credits to Mustafa Asif)



 Terms Definition
 ChromosomeA thread-like structure found in the nucleus on which genetic material is organised. Chromosomes carry the information for making new animal or plant bodies. This information is carried in the molecule, deoxyribonucleic acid (DNA)
 GeneA small segment of DNA in a chromosome where a piece of genetic information is stored. The place on the chromosome where the gene resides is called the gene locus. Each gene has a specific function
 Allele Different forms of the same gene. They occupy the same relative positions in a pair of homologous chromosomes
  • Dominant allele: a form of a gene that expressed itself and give the same phenotype in both the homozygous and heterozygous conditions
  • Recessive allele: a form of a gene that expressed itself only in the homozygous condition
Monohybrid inheritanceInheritance involving only one pair of contrasting characteristics
Discontinuous variationTraits that show limited variation in their phenotypes which are easily distinguishable. It is usually controlled by only one or a few genes. It is not affected by the environment. 
Continuous variationTraits with phenotypes ranging from one extreme to the other. It is brought about by combined (or additive) effects of many genes. It is affected by environmental conditions eg intelligence, height, skin colour
PhenotypeThe expressed trait in an individual eg outward appearances such as height 
GenotypeThe genetic combination in an individual 
HomozygousAn individual who has identical alleles for a particular trait 
HeterozygousAn individual who has unlike alleles for a particular trait 
Co-dominance    A condition when both alleles express themselves in the hybrid    
MutationThe sudden or spontaneous change in gene structure or a chromosome, or even the chromosome number, and may be inheritable 
MeiosisA form of nuclear division in which the daughter nuclei produced have half the number of chromosomes or hereditary materials as the parent nucleus
Artificial selectionA method used by man to produce plants and animals with desirable qualities 
Test crossThe crossing of an organism of unknown genotype with a homozygous recessive organism. This is carried out to determine the genotype of an organism.
Natural selectionA process which results in the best adapted organisms in a population surviving to reproduce and so pass on more of their genes to the next generation. 
Law of SegregationThe two factors in each pair separate or segregate during gamete formation and each gamete will contain only one factor. 


Discontinuous variationContinuous variation 
Deals with a few clear-cut phenotypes Deals with a spectrum of phenotype, ranging from one extreme to the other
controlled by a few genescontrolled by many genes 
genes do not show additive effectgenes show additive effect eg the more "dark" genes, the darker will be the colour
not modified by environmental changesmodified by environmental conditions eg greater exposure of the skin to sunlight will produce a darker colour

Cell Division

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1. There are 20 chromosomes in each leaf cell of a maize plant. What is the number of chromosomes in the male nucleus of a pollen grain of maize?
a. 5
b. 10
c. 20
d. 40

2. The list gives some of the stages involved in gamete and zygote formation.
1 Prophase I
2 Prophase II
3 Metaphase I
4 Fertilisation

During which stages do events occur that increase genetic variation in the zygote?
a. 1, 2, and 3
b. 1, 3, and 4
c. 2 and 3
d. 3 and 4

3. In which structure are the cells dividing by mitosis only?
a. ovary
b. root tip
c. stamen
d. testis

The process represents the basic changes during
a. mitosis
b. meiosis
c. development
d. fertilisation

5. The process takes place
a. only in animals that reproduce sexually
b. only in animals and plants that reproduce asexually
c. in all animals and plants that reproduce sexually
d. in all animals and in all plants

6. What is the diploid number of chromosomes in the above diagram?
a. 2
b. 4
c. 8
d. 16

7. Between which stages would "crossing over" most likely to take place?
a. A --> B
b. C --> D
c. D --> E
d. F --> G


MCQ Answers

1. b
2. b
3. b
4. b
5. c
6. b
7. a
8. b