L104_H8_Genetics_S2008-1

L104_H8_Genetics_S2008-1 - L104 Handout 8 Campbell et al....

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L104 Handout 8 Campbell et al. Chapter 9 Starr and McMillan Chapter 20 and 21 MENDELIAN GENETICS MENDEL'S FIRST LAW OF GENETICS Each organism contains two alleles for each trait and the alleles segregate during the formation of gametes. Each gamete then contains only one allele from each pair of alleles. When fertilization occurs, the new organism has two alleles for each trait, one from each parent. MONOHYBRID CROSSES In monohybrid crosses, the parents have different allelic pairs for the trait in question e.g. cc x CC Monohybrid crosses involve alleles that exhibit complete dominance: F1 progeny show only the dominant trait. In selfing the F1, both parental phenotypes reappear. Observed ratio is 3:1 i.e. 3 dominant: 1 recessive. E.g. Red (d) x White (r) pea flower. Or Widow's peak (d) x Continuous (r) hairline. (Review Punnett squares - a tool for determining the probable outcome of genetic crosses) Rules of probability apply to the inheritance of single gene traits. Thus if the genotypes of parents are known, it is possible to establish a potential child's chances for inheriting a particular genotype and thus for having a particular trait. 1
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MONOHYBRID TEST CROSS The results of a one-trait test cross can determine whether an individual who expresses the dominant phenotype is heterozygous or homozygous dominant. Perform Test Cross - Individual x homozygous recessive. If any of the offspring of the testcross expresses the recessive phenotype, the parent with the dominant phenotype is heterozygous. Cross gives two types of progeny in the ratio of 1:1. What happens when a cross is made between parents that differ? at two or more loci? See dihybrid cross - between 2 independent traits, E.g. spherical, yellow x dented, green. MENDEL'S SECOND LAW OF INHERITANCE Alleles of different genes assort independently of one another during gamete formation. Because of events during meiosis, either gene of a pair may end up in a particular gamete. A dihybrid cross can reveal evidence of independent assortment. This basic principle of inheritance states that a gene pair on one chromosome assorts into gametes independently of other gene pairs of other chromosomes. If both parents are heterozygous for the two genes in question, 16 genotypes and four phenotypes are possible. Following the principle of probability, these phenotypes have probabilities of 9/16, 3/16, 3/16 and 1/16 associated with them. The 4 phenotypes occur in a 9:3:3:1 ratio. 9AB:3Ab:3aB:1ab E.g. TWO TRAIT CROSS Yellow (d) Round (d) x Green (r) Wrinkled (r) Widow's peak (d) Short fingers (d) x Continuous (r), long fingers (r). 2
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VARIATIONS ON MENDEL'S THEMES (A) LINKAGE If significant deviations from 9:3:3:1 in dihybrid crosses are observed Linkage of genes is probably present. (B)
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This note was uploaded on 04/18/2008 for the course BIOL-L 104 taught by Professor Odle during the Spring '08 term at Indiana.

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L104_H8_Genetics_S2008-1 - L104 Handout 8 Campbell et al....

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