Can heredity follow different rules? Heredity does not always follow Mendel’s laws depending on the situation. Pleiotropy occurs when one gene affects multiple phenotypes. Polygenic inheritance occurs when multiple genes control the expression of a certain trait. Environmental factors also influence gene expression depending on the external and internal influences of the environment. Exceptions to Mendel This is the end!

What is incomplete dominance? Incomplete dominance occurs when heterozygous individuals express an intermediate phenotype rather than a dominant phenotype. For example, a cross between a plant that has red flowers and a plant that has white flowers produces offspring with pink flowers even though red is considered dominant. Exceptions to Mendel

Example of Incomplete Dominance Exceptions to Mendel

What are the expected phenotype and genotype numbers? What would the results be if you cross two plants with pink flowers? (complete the punnet square on next slide) What are the expected phenotype and genotype numbers? What percent of the offspring will have red flowers? ___% What about white flowers? ____% If the plants produced four offspring, how many of them would have pink flowers? ____ out of 4 Exceptions to Mendel

Punnet square

What is codominance? Codominance occurs when two alleles are not dominant over one another and are expressed simultaneously in a heterozygote. In an organism that is heterozygous for a codominant trait, both alleles are expressed. Exceptions to Mendel The Pinstripe is a pattern and color mutation. The Pinstripe is a co-dominant trait in Ball Pythons.

Human Blood Type An example of this is human blood type. Blood type is determined by the presence or lack of presence of certain proteins (antigens) on the surface of red blood cells. Exceptions to Mendel

CoDominance Both type A and type B blood are dominant over type O but they are not dominant over each other. Thus an individual with alleles for type A and type B blood expresses both antigens simultaneously and has type AB blood, which demonstrates codominance. Exceptions to Mendel

Graphic of cell surface antigens Exceptions to Mendel

How do we express alleles for blood type in genotypes? Type O blood is ii Type A blood is IAIA (homozygous) or Iai (heterozygous) Type B blood is IBIB (homozygous) or Ibi (heterozygous) Type AB blood is IAIB You can use these genotypes in Punnett squares. Exceptions to Mendel

Codominance Punnett Squares A male is heterozygous for type A blood. A female has type AB blood. If these two have offspring.. draw a Punnett square showing the expected genotype and phenotype results for blood type. Exceptions to Mendel

Codomiance Punnet Square What percentage of their offspring would have type O blood? _______ What percentage would have type A blood? ______ What is the probability that their child would have type B blood? _______ If these two parents have four offspring, how many of them would have type AB blood? ______ How many would have type O blood? __________ How many would be a carrier for type O blood? _____ Exceptions to Mendel

What are sex-linked traits? Sex-linked traits are traits that are controlled by genes located on sex chromosomes. For humans, sex-linked traits are located on the X-chromosome. Thus, males are more likely to get sex-linked disorders because they have only one copy of the X-chromosomes, from their mother. Exceptions to Mendel

Sex linked Disorders Some examples of sex-linked disorders are: color blindness (cannot see red or green) hemophilia (blood lacks clotting factors that help stop bleeding). These are recessive traits. Exceptions to Mendel

How do we show genotypes for sex-linked traits? For a recessive sex-linked disorder, females need two copies of the recessive allele on the X chromosome (from both the mother and father). Males only need one copy of the recessive allele on the X chromosome. Exceptions to Mendel

Inheritance of Hemophilia Exceptions to Mendel

Sex-linked Genotypes To write genotypes, write the XY and XX chromosomes. Give the X chromosome a superscript representing the allele that it has. Exceptions to Mendel

Capitalized superscripts are dominant while lower-case superscripts are recessive. For example, color blindness is represented by Xc. A color blind male would be XcY, while a normal male would be XCY. What would be the genotype of a female that is a carrier of color blindness? Sex-linked Genotypes Exceptions to Mendel