Genes are the basic unit of heredity. Any of two or more alternative forms of a gene is called an allele. It can be possible to refer to these alleles using letters as symbols. For example: call the allele that gives grey fur G and the allele that gives charcoal fur g.

In normal cases, the dominant allele completely masks the expression of recessive characters. Hence, in heterozygous condition, the characters of only the dominant allele expressed. But in certain cases, neither a pair of alleles is completely dominant or completely recessive. Instead of one of them completely hiding the effect of the other in a heterozygote, they both have an expression on the phenotype. This is called codominance. Example: In ABO blood grouping, I A I B condition cause AB blood group.

Explain the difference between DNA and RNA.

Feature DNA RNA Structure DNA is a double-stranded molecule consisting of a long chain of nucleotides that have adenine, thymine, cytosine and guanine as bases. RNA is a single-stranded helix consisting of adenine, uracil, cytosine and guanine as the bases. Feature DNA RNA Type of sugar Deoxyribose sugar is present in DNA that is derived from the sugar ribose by the loss of an oxygen atom. Ribose sugar is present in RNA.

E M B I B E

Cambridge IGCSE® Biology Coursebook Third Edition>Chapter 18 - Inheritance>Questions>Q 18.3

EASY

Upper Secondary: IGCSE

IMPORTANT

Earn 100

What are alleles?

Important Questions on Inheritance

MEDIUM

Upper Secondary: IGCSE

IMPORTANT

Cambridge IGCSE® Biology Coursebook Third Edition>Chapter 18 - Inheritance>Questions>Q 18.4

The allele for the brown eye is dominant to the allele for blue eyes. Write down suitable symbols for these alleles.

MEDIUM

Upper Secondary: IGCSE

IMPORTANT

Cambridge IGCSE® Biology Coursebook Third Edition>Chapter 18 - Inheritance>Questions>Q 18.4

The allele for the brown eye is dominant to the allele for blue eyes. What is the phenotype of a person who is heterozygous for this character?

EASY

Upper Secondary: IGCSE

IMPORTANT

Cambridge IGCSE® Biology Coursebook Third Edition>Chapter 18 - Inheritance>Questions>Q 18.5

What is codominance?

HARD

Upper Secondary: IGCSE

IMPORTANT

Cambridge IGCSE® Biology Coursebook Third Edition>Chapter 18 - Inheritance>Questions>Q 18.6

Alleles of the gene for the ABO blood group antigens in humans show unusual characteristics. What are these?

HARD

Upper Secondary: IGCSE

IMPORTANT

Cambridge IGCSE® Biology Coursebook Third Edition>Chapter 18 - Inheritance>Questions>Q 18.7

The following figure shows three possible genotypes for the blood group. Write down all the other possible genotypes and the phenotypes that is associated with each one.

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