BIO 122 Lecture Notes - Lecture 11: Dihybrid Cross, Quantitative Genetics, Genetic Linkage
30 May 2018
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Prior to Week 11 – Chapter 14 Mendel and the Gene
• Know
• What sex chromosomes are and that humans and many other (but not all!) organisms have an X and a Y.
o Biological female sex is XX and biological male sex is XY.
o That a cell can count X chromosomes and de-activate all but one (this is called X-inactivation).
• That sex-linked genes refer to genes found on one sex chromosome but not the other
• That linkage refers to two genes being physically on the same chromosome and that they are likely to be
transmitted together.
o That the frequency of recombinant offspring can help to estimate how far apart two genes are on a
given chromosome.
▪ I will not ask you to create a genetic linkage map on the final exam, but do recognize that
if two genes do not independently assort, they could be linked.
o That parental/non-recombinant and recombinant phenotypes refer to combinations of two
character traits that the parental generation exhibits, or does not exhibit, respectively.
▪ Recombinant
▪ Means crossing over occurred
• When a given gene has more than 2 phenotypes (ie, not just dominant and recessive), this is referred to as
polymorphic.
o If a heterozygote expresses more than one allele in its phenotype → co-dominance
▪ I.e. AB blood type
o If a heterozygote expresses a blending of two alleles → incomplete dominance
▪ I.e. pink phenotype from red and white parents
• Some single genes can be pleiotropic.
o Affect more than one trait
• Phenotype can be influenced by environmental conditions
o Temp can change pigment or growth
o phenylketonuria
• What a polygenic trait is
o Many genes influence one trait
• What a quantitative trait is and what a discreet trait is
o Quantitative → ranges
▪ Ie skin color, height
o Discreet → only 2 options
▪ Black or white
• That epistasis refers to the action of one gene on another
o The gene that determines if you will have a pigment or not influences the gene that determines the
pigment
• Understand
• How the respective genes for two characters will independently assort if they are on separate chromosomes.
o That if a dihybrid cross does not give a 9:3:3:1 ratio of offspring, there is something else to
explain this.
▪ If the ratio is some combination of these numbers (15:1 or 9:7), then that indicates
epistasis
▪ Meaning one gene influences the other
▪ If the ratio is not anything close to this ratio (or some combination of those numbers), the
genes are linked
▪ Located on the same chromosome
• That wild-type refers to the most common phenotype for a trait but does not dictate genotype. In other
words, the wild-type can be a recessive or a dominant phenotype.
• That linkage and sex-linkage refer to different and distinct phenomena
o Linkage
▪ Genes located on same chromosome
o Sex-linkage
▪ Genes located on sex chromosome
• That recombinant phenotypes are the result of crossing over in Meiosis
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