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Genes – Basics

Genes – Basics. A gene is an inherited instruction consisting of a sequence of DNA. Genes control a variety of structures and functions. Human genetic material comprises an estimated 20,000 to 25,000 different genes. Genes are named after the function they control (see table p307).

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Genes – Basics

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  1. Genes – Basics • A gene is an inherited instruction consisting of a sequence of DNA. • Genes control a variety of structures and functions. • Human genetic material comprises an estimated 20,000 to 25,000 different genes. • Genes are named after the function they control (see table p307).

  2. Gene – Locations • Each gene is located on a specific chromosome. • Each gene has a specific position on a specific gene (LOCUS or LOCI). • Genes on the same chromosome are members of the same LINKAGE GROUP. • The number of linkage groups is equal to the HAPLOID number of chromosomes (HUMAN = 22 + X and Y)

  3. Different Genes • A gene that controls a function can occur in different forms = ALLELES. • One gene can have several alleles, characterised by the specific action. • In many cases there are only 2 alleles for a gene, but not always. • People have 2 copies of each gene located on homologous chromosomes, but the alleles may differ resulting in differences in structures and functions expressed. • A person with 2 copies of the same allele = Homozygous • ( Eg IAIA or AA or aa) • A person with 2 different alleles = Heterozygous. • (Eg IAIB or Aa)

  4. Activity • Answer questions from page 275 in Biozone.

  5. Genotype PP (homozygous) Pp (heterozygous) Pp (heterozygous) Phenotype Purple pp (homozygous) White Purple Purple Genotype and Phenotype • Genotype = Collection of genes or gene profile of an individual; the range of alleles the organism possesses. • Phenotype = Observable traits of an organism that arise because of the interaction between genes and the environment. • There can be different genotypes for a given phenotype due to dominance and recessiveness.

  6. Expression of Alleles • A phenotype for a heterozygous organism is identified and trait expressed is therefore dominant. • Dominant alleles are symbolised by capital letters and recessive alleles by lower case. • Complete Dominance - when the presence of the dominant allele always results in that trait in the phenotype. • Co-Dominance – when both traits from different alleles are both expressed in the phenotype. • Incomplete Dominance – When the phenotype of a heterozygote is an intermediate of traits for each allele.

  7. Activity • Complete the ‘Relationships and Dominance’ sheet.

  8. aaB BCC AaB BCC AAB BCC AaB bCc AAb bCC AAB Bcc AAB BCc AAB bCC AA aa AABb Aabb AaBb AAbb aaBb aabb Aa AABB AaBB aaBB AaBb Aa Aa AaBb X Parents X Parents Possibleoffspring Possibleoffspring Monohybrid cross Dihybrid cross AaBb Cc AaBb Cc X Parents Possible offspring (plus many more) Trihybrid cross Genetic Crosses • Crosses can be shown by separating the parental alleles (or allele combinations) and recombining them in the offspring: • Monohybrid cross: The inheritance of one gene (A) is studied. • Dihybrid cross: The inheritance of two genes (A and B) is studied. • Trihybrid cross: The inheritance of three genes (A , B, and C) is studied.

  9. Activity • Answer questions from pages 276 to 280 in Biozone.

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