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Biology - Inheritance

Grade 10IGCSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Inheritance is the transmission of genetic information from generation to generation via chromosomes made of DNADNA.

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A gene is a length of DNADNA that codes for a specific protein. An allele is an alternative form of a gene, often represented by letters such as AA or aa.

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Genotype is the genetic makeup of an organism in terms of the alleles present (e.g., TTTT, TtTt, or tttt).

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Phenotype is the observable features of an organism, determined by its genotype and its environment.

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Homozygous means having two identical alleles of a particular gene (e.g., TTTT or tttt). Pure-breeding involves homozygous individuals.

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Heterozygous means having two different alleles of a particular gene (e.g., TtTt). Heterozygous individuals do not breed true.

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A dominant allele is one that is expressed if it is present (TT); a recessive allele is only expressed when no dominant allele of the gene is present (tt).

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Codominance occurs when both alleles in a heterozygous organism contribute to the phenotype, such as in blood groups IAI^A and IBI^B, resulting in blood type ABAB.

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Sex is determined by the sex chromosomes: XXXX for females and XYXY for males. The YY chromosome is shorter than the XX chromosome.

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Continuous variation (e.g., height) is influenced by multiple genes and the environment, while discontinuous variation (e.g., blood groups) is caused by a single gene and has no intermediates.

📐Formulae

Monohybrid Phenotypic Ratio (Heterozygous Cross)=3:1\text{Monohybrid Phenotypic Ratio (Heterozygous Cross)} = 3:1

Monohybrid Genotypic Ratio (Heterozygous Cross)=1:2:1\text{Monohybrid Genotypic Ratio (Heterozygous Cross)} = 1:2:1

Test Cross Ratio (Heterozygous × Homozygous Recessive)=1:1\text{Test Cross Ratio (Heterozygous } \times \text{ Homozygous Recessive)} = 1:1

Probability of Sex Determination=12(XX):12(XY)\text{Probability of Sex Determination} = \frac{1}{2} (XX) : \frac{1}{2} (XY)

💡Examples

Problem 1:

In humans, the allele for brown eyes (BB) is dominant over the allele for blue eyes (bb). Predict the phenotypic and genotypic results of a cross between a heterozygous brown-eyed parent (BbBb) and a blue-eyed parent (bbbb).

Solution:

Genotypes: 50%50\% BbBb and 50%50\% bbbb. Phenotypes: 50%50\% Brown eyes and 50%50\% Blue eyes.

Explanation:

The heterozygous parent produces gametes with BB or bb alleles. The blue-eyed parent produces only bb gametes. Using a Punnett square, the offspring combinations are BbBb and bbbb in a 1:11:1 ratio.

Problem 2:

Show the inheritance of blood groups when one parent is heterozygous for Group A (IAIOI^A I^O) and the other is heterozygous for Group B (IBIOI^B I^O).

Solution:

Possible genotypes: IAIBI^A I^B, IAIOI^A I^O, IBIOI^B I^O, IOIOI^O I^O. Phenotypes: Blood groups ABAB, AA, BB, and OO in a 1:1:1:11:1:1:1 ratio.

Explanation:

Because IAI^A and IBI^B are codominant and IOI^O is recessive, all four blood groups are possible in the offspring of this cross.

Problem 3:

Explain why a male offspring always inherits his XX chromosome from his mother.

Solution:

The father provides the YY chromosome to make the offspring male (XYXY). Therefore, the XX chromosome must come from the mother (XXXX).

Explanation:

A male must have the genotype XYXY. Since the mother only has XX chromosomes to give, the male receives an XX from the mother and the YY from the father.