Biology HS/Science Unit: 08 Lesson: 02 Non-Mendelian Genetics Patterns of inheritance that do not follow Mendel’s laws are referred to as non-Mendelian. Mendelian laws describe the inheritance patterns for traits control by one gene on chromosomes inside the nucleus. Some inherited traits do not follow this pattern or the dominant/recessive pattern we have seen so far in Mendelian genetics. Three non-Mendelian inheritance patterns are: incomplete dominance, co-dominance, and sex-linkage. Incomplete Dominance: There are two alleles for a gene, but neither is dominant or recessive. When both alleles are present (the heterozygous condition), the phenotype is a blend of the two alleles. This is seen in flower color in snapdragons. In these plants, one homozygous genotype produces red flowers, the other homozygous genotype produces white flowers, but the heterozygous genotype produces pink flowers. Sample Problem: The genotype that produces red flowers is RR. The genotype that produces white flowers is WW. Show the cross between a plant with red flowers and a plant with white flowers. What is the genotypic and phenotypic outcome of such a cross? Co-dominance and Multiple Allele Systems: Two alleles may be equally dominant, and both are expressed in the heterozygous condition. This is seen in human blood groups, where there are multiple alleles, A, B, and O. A and B alleles are co-dominant, and O is recessive. Thus, there are four possible phenotypes, or blood types: Type A (AA or AO genotypes), Type B (BB or BO genotypes), Type AB (AB genotypes), and Type O (OO genotype). Sample Problem: Show the cross between two people with Type AB blood. What is the genotypic and phenotypic outcome of such a cross? ©2012, TESCCC 11/05/12 page 1 of 4 Biology HS/Science Unit: 08 Lesson: 02 Sex-linkage: Traits that are controlled by genes located on the X and Y chromosomes (the sex chromosomes) are considered to be sex-linked traits. Alleles on the X chromosome do not have a corresponding allele on the Y chromosome. Thus, females, having two X chromosomes, have two alleles for the trait. Males, having an X and a Y chromosome, have only one allele for the trait. Redgreen color-blindness, Duchenne’s muscular dystrophy, and hemophilia are traits caused by recessive, sex-linked genes. When writing the genotype, write the sex of the individual and use a superscript for the sex-linked trait (female genotypes: XDXD, XDXd, XdX d, male genotypes: XDY, or XdY). Males tend to have sex-linked traits at a much higher rate than females because males only have one X chromosome and one Y chromosome. So, the presence of a single recessive allele will result in the recessive phenotype in males. Since females have two X chromosomes, the presence of a single recessive allele will result in the dominant phenotype, though these females are referred to as carriers because they carry the recessive allele. Sample Problem 1: Show the cross between a man and a woman. From which parent(s) do sons get their X chromosome(s)? From which parent(s) do females get their X chromosomes? Sample Problem 2: Show the cross between a hemophiliac man and a woman who is a carrier. What is the probability that they will have a child with hemophilia? ©2012, TESCCC 11/05/12 page 2 of 4 Biology HS/Science Unit: 08 Lesson: 02 Problems 1. Cross two pink snapdragons. What is the genotypic and phenotypic probability of such a cross? 2. Show a Punnett square to demonstrate how two individuals with Type A blood can have a child with Type O blood. 3. Show a cross between a person with Type AB blood and a person with Type O blood. What is the genotypic and phenotypic probability of this cross? ©2012, TESCCC 11/05/12 page 3 of 4 Biology HS/Science Unit: 08 Lesson: 02 4. In shorthorn cattle, hair color is governed by co-dominant alleles. Red alleles are co-dominant with white alleles, and heterozygotes have red and white hairs. They are called roans. Cross two roans. What is the genotypic and phenotypic probability of this cross? 5. Make a Punnett square to show how two people who do not have color-blindness can have a child who is color-blind. What is the probability that the color-blind child will be male? What is the probability that the color-blind child will be female? 6. Make a Punnett square to show the cross between a man who does not have Duchenne’s muscular dystrophy (MD) and a woman who does. What is the genotypic and phenotypic probability of such a cross? ©2012, TESCCC 11/05/12 page 4 of 4
