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.

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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.

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Example of Incomplete DominanceEx

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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

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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.

The Pinstripe is a pattern and color mutation.

The Pinstripe is a co-dominant trait in Ball Pythons.

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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.

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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.

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Graphic of cell surface antigensEx

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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.

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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.

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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? _____

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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.

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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.

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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.

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Inheritance of HemophiliaEx

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Sex-linked Genotypes

• To write genotypes, write the XY and XX chromosomes.

• Give the X chromosome a superscript representing the allele that it has.

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Sex-linked Genotypes

• 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?

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