Genetic Variation
Visão Geral
Put three generations of one family in a photograph and the same nose turns up twice, a chin skips a generation entirely, and somebody has eyes that match nobody else in the frame. None of that is coincidence and none of it is magic. Every one of those resemblances travelled from parent to child inside a single cell, carried by the same kind of instruction that works in a pea plant, a fruit fly and you.
This lesson takes you from the chromosome out to the characteristic you can actually see. You will learn what an allele is and why two individuals that look identical can carry different ones, why a characteristic can vanish for a generation and then return, why the sperm rather than the egg settles whether a child is a boy or a girl, and how to set out a genetic cross so cleanly that you can predict the offspring before they exist. By the end you will be building Punnett squares and reading family trees the way a geneticist does.
Objetivos
- Differences in the characteristics of individuals of the same kind may be due to differences in: the genes they have inherited (genetic causes); the conditions in which they have developed (environmental causes); a combination of genetic and environmental causes.
- The information that results in plants and animals having similar characteristics to their parents is carried by genes, which are passed on in the sex cells (gametes) from which the offspring develop.
- The nucleus of a cell contains chromosomes. Chromosomes carry genes that control the characteristics of the body. Chromosomes are normally found in pairs.
- In human body cells, one of the 23 pairs of chromosomes carries the genes that determine sex. In females the sex chromosomes are the same (XX); in males the sex chromosomes are different (XY).
- Different genes control the development of different characteristics of an organism. Some characteristics are controlled by a single gene. Each gene may have different forms called alleles. Students should understand that genes operate at a molecular level to develop characteristics that can be seen.
- If both chromosomes in a pair contain the same allele of a gene, the individual is homozygous for that gene. If the chromosomes in a pair contain different alleles of a gene, the individual is heterozygous for that gene.
- An allele that controls the development of a characteristic when it is present on only one of the chromosomes is called a dominant allele. An allele that controls the development of a characteristic only if the dominant allele is not present is called a recessive allele. Students should be familiar with principles used by Mendel in investigating monohybrid inheritance in peas. They should understand that Mendel’s work preceded the work by other scientists which linked Mendel’s ‘inherited factors’ with chromosomes. Students should be able to construct genetic diagrams of monohybrid crosses and to predict the outcomes of monohybrid crosses. They should be able to use the terms homozygous, heterozygous, phenotype and genotype. Students should understand that genetic diagrams are biological models which can be used to predict the outcomes of crosses. Students should be able to interpret genetic diagrams, including family trees.
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- What is an allele? A. A pair of chromosomes in a body cell B. An alternative form of a gene C. A sex cell produced by a reproductive organ D. A characteristic that can be observed Answer: B
- Which pair of sex chromosomes is found in the body cells of a human male? A. XX B. XY C. YY D. X only Answer: B
- An individual carries two different alleles of a gene. Which term describes this individual? A. Homozygous dominant B. Homozygous recessive C. Heterozygous D. Phenotypic Answer: C
- In mice, black fur (B) is dominant to brown fur (b). Two heterozygous black mice are crossed. What is the expected ratio of black to brown offspring? A. 1 black : 1 brown B. 2 black : 1 brown C. 3 black : 1 brown D. 4 black : 0 brown Answer: C
- A plant with the genotype Tt is crossed with a plant with the genotype tt. What percentage of the offspring is expected to be homozygous recessive? A. 0% B. 25% C. 50% D. 75% Answer: C
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