Lect 3 - incomplete, codominance, multiple alleles, sex-linkage

WINTER Template 23 Genetic Crosses

• Simple Mendelian genetics depends on character traits which are determined by one gene (located on an autosome), for which there are two alleles, one completely dominant to the other. Pg 23

• Deviations from standard Mendelian ratios are observed when (1) Alleles are not completely dominant or recessive ; incomplete dominance or codominance (2) A particular gene has more than 2 alleles ; multiple allelic condition (3) A gene is carried on the sex chromosomes Sex-linked genes Pg 23

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• In the previous lectures, you learnt about the standard Mendelian ratios such as : 3 : 1 (monohybrid) 9 : 3 : 3 : 1 (dihybrid) when 2 heterozygotes cross ; 1 : 1 (monohybrid) 1 : 1 : 1 : 1 (dihybrid) when heterozygote cross with homozygous recessive individual in test cross) Standard Mendelian ratios will not be observed. Pg 23

WINTER Template 01What I need to know … • 2(n) Use genetic diagrams to solve problems in dihybrid crosses, including those involving incomplete dominance, codominance, multiple alleles and sex linkage

• Incomplete dominance • Codominance • Multiple alleles • Sex-linked genes

• The situation where the heterozygote (Aa) exhibits a phenotype which is intermediate between homozygous dominant (AA) and homozygous recessive (aa) forms.  For incomplete dominance, one allele of a gene may code for functional gene product while the other allele may code for non-functional or mutated gene product. Neither allele of an allelic pair is dominant or recessive with respect to each other, both alleles contribute equally to the phenotype of the heterozygote. Pg 24

 Thus when a cross between parents with two different phenotypes, some offspring with a “third” phenotype that is an intermediate of the parental traits would result. Pg 24

Snapdragon flower Pg 24