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.

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.

Example of Incomplete Dominance

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

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.

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.

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.

Graphic of cell surface antigens

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.

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.

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? _____

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.

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.

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.

Inheritance of Hemophilia

Sex-linked Genotypes

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

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

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?