Monohybrid Inheritance
Overview
Why does a calf from two hornless parents sometimes grow horns? Why can two ordinary-looking cereal plants give some drought-tolerant offspring and some that wilt in the first dry spell? The answer lies in genes, the units of inheritance passed from parent to offspring. When a single gene controls one feature, we can actually predict the offspring and the ratios in which they appear, which is exactly what a farmer who is planning a breeding programme needs to know.
In this lesson you will learn the language of inheritance: chromosomes, genes and alleles, and what it means for an allele to be dominant or recessive. You will draw genetic diagrams, or Punnett squares, to predict the results of simple crosses and work out the 1:1 and 3:1 ratios that examiners ask about. Finally you will see why the difference between an animal's genotype and its phenotype matters so much when choosing which animals and plants to breed from.
Objectives
- (a) define the terms chromosome, gene, allele, homozygous, heterozygous, dominant and recessive.
- (b) calculate and predict the results of simple genetic crosses involving 1:1 and 3:1 ratios.
- (c) explain the meaning of the terms genotype and phenotype and assess their importance in animal and plant breeding.
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Lesson Evaluation
Congratulations on completing the lesson on Monohybrid Inheritance. Now that youve explored the key concepts and ideas, its time to put your knowledge to the test. This section offers a variety of practice questions designed to reinforce your understanding and help you gauge your grasp of the material.
You will encounter a mix of question types, including multiple-choice questions, short answer questions, and essay questions. Each question is thoughtfully crafted to assess different aspects of your knowledge and critical thinking skills.
Use this evaluation section as an opportunity to reinforce your understanding of the topic and to identify any areas where you may need additional study. Don't be discouraged by any challenges you encounter; instead, view them as opportunities for growth and improvement.
- Which term describes an organism that has two different alleles of a gene, such as Hh? A. Homozygous B. Heterozygous C. Dominant D. Recessive Answer: B
- The allele for no horns, H, is dominant over the allele for horns, h. What is the phenotype of an animal with the genotype Hh? A. Horns B. No horns C. Half-formed horns D. It cannot be predicted Answer: B
- Two heterozygous cattle (Hh) are crossed. What is the expected ratio of offspring with no horns to those with horns? A. 1:1 B. 1:2 C. 3:1 D. 1:0 Answer: C
- Which genotype is homozygous recessive? A. HH B. Hh C. hH D. hh Answer: D
- A drought-resistant cereal (D dominant) is crossed Dd x dd. What proportion of the offspring is expected to be drought resistant? A. None B. One quarter C. One half D. All Answer: C
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