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, written in a chemical code that is the same in a pea plant, a fruit fly and you.
This lesson takes you from the chromosome down to the base sequence and back 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, and how to read and finish a genetic cross so cleanly that you can predict the offspring before they exist. It is also the one biology topic on this course where the Core Tier and Extension Tier papers ask for genuinely different things, so the lesson marks that boundary precisely as it goes.
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. Extension Tier 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. Core Tier students should be able to interpret genetic diagrams of monohybrid inheritance and sex inheritance, but will not be expected to construct genetic diagrams or use the terms homozygous, heterozygous, phenotype of 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.
- Chromosomes are made up of large molecules of DNA. DNA contains the coded information that determines inherited characteristics.
- A gene is a small section of DNA. Each gene codes for a particular combination of amino acids, to make a specific protein.
- DNA is made of very long strands, twisted to form a double helix, which contain four different compounds, called bases. Students are not expected to know the names of the four bases or how complementary pairs of bases enable DNA replication to take place.
- A sequence of three bases is the code for a particular amino acid. The order of bases controls the order in which amino acids are assembled to produce a particular protein.
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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 protein made by a gene D. A group of three bases in DNA 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
- How many bases in DNA are the code for one amino acid? A. One B. Two C. Three D. Four Answer: C
- In fruit flies, normal wings (N) are dominant to small wings (n). Two flies that each carry the alleles Nn are crossed. What is the expected ratio of normal wings to small wings in the offspring? A. 1 normal : 1 small B. 2 normal : 1 small C. 3 normal : 1 small D. 4 normal : 0 small Answer: C
- A pea plant carrying the alleles Tt is crossed with a pea plant carrying the alleles tt. What percentage of the offspring is expected to show the recessive characteristic? A. 0% B. 25% C. 50% D. 75% Answer: C
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