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Yellow and Blue Make…? Objective : To understand other inheritance patterns Bell work: A man with straight hair marries a woman with curly hair. They have a child with wavy hair. How do you explain this? Some traits are not determined by simple dominant-recessive relationships but instead blend together. Let’s learn about it! incomplete dominance, Codominance , Multiple alleles & polygenic inheritance

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A Little Review… Mendel found in his experiments that the different traits could be: Dominant OR Recessive Tall Plant vs. Short Plant Meaning that when there are two alleles present in the HETEROZYGOUS state, the DOMINANT trait tends to COVER up the RECESSIVE trait. Mendelian genetics is GREAT, but how do you explain how you have green eyes when mom has brown and dad has blue? What about your brother’s wavy hair when everyone in your family has curly hair, except mom, who has straight hair? Let’s find out… TT, Tt tt

Homozygous cross:

Homozygous cross

Heterozygous cross:

Heterozygous cross

I. What is incomplete dominance?:

For example, when a red snapdragon is crossed with a white plant, all the offspring flowers are pink because both the red and white alleles are expressed. I. What is incomplete dominance? Sometimes two different alleles are neither fully dominant or recessive to each other. In heterozygous individuals, this creates a phenotype that is an intermediate mix of the other two. This is called INCOMPLETE DOMINANCE .

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Incomplete Dominance: when dominant & recessive are combined and result in a blending of the traits In Incomplete dominance, neither the dominant or recessive is shown, but instead they blend together to create an entirely different phenotype. Let’s say a black lab and a white lab mate, creating a litter of pups that are all brown . This is an example of incomplete dominance , where the heterozygous trait results in a blending of the two phenotypes. ?

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Incomplete Dominance Let’s complete a Punnett Square to see how all of this works: B B W W B B B B W W W W BW = brown Because of Incomplete Dominance, all of the offspring will be brown.

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Incomplete Dominance What if we cross our brown dog with another brown dog? B W B W B B W W B B W W Here we see a variety of traits, where the homozygous genotypes result in the black and white phenotypes again… BW

II. What is Codominance? :

Genotype Phenotype II. What is Codominance? Alleles are CODOMINANT if they are both expressed in the phenotype. They can be represented by two capital letters superscript to the letter representing the gene. For example, flower color in snapdragons Antirrhinum majus . C R = red flowers C W = white flowers C R C R homozygous red flowers C R C W heterozygous pink flowers C W C W homozygous white flowers

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Codominance : when dominant & recessive traits are combined & result in both traits being expressed In this example, let’s use two peonies, one that is white , the other peach . When crossed together, the HETEROZYGOUS trait results in the expression of both phenotypes, a white & peach peony. ?

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Codominance Let’s complete a Punnett Square to see how all of this works: P P W W P P P P W W W W PW = peach and white Codominance results in a white and peach phenotype expression in all of the offspring

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III. Multiple Alleles: when there are more than two alleles for a specific trait Remember that parents only have two alleles they can pass on to their offspring. However, when 4 or more phenotypes exist in a population, then there must be several different alleles (more than 2) to choose from. The classic example we use in biology is Blood Type.

ABO blood group:

Genotype Phenotype ABO blood group Some genes have MULTIPLE ALLELES (i.e. more than two), but only two can be present in an individual. For example, the ABO blood group gene (immunoglobulin) in humans. I A produces antigen A on the surface of red blood cells I B produces antigen B on the surface of red blood cells I O produces no antigen. A and B are codominant and o is recessive to both. I A I A and I A I O blood group A I B I B and I B I O blood group B I O I O blood group O I A I B blood group AB

Codominance in humans:

Codominance in humans

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IV. Polygenic Inheritance: when a trait is controlled by more than one (Poly=many) gene Sometimes, more than one gene may control the expression of a trait or characteristic. Eye, hair and skin color, as well as body shape and height are all examples of polygenic inheritance that occurs in humans. Eye color is controlled by three different genes- This results in 6 alleles that control what color your eyes are, from light blue to dark brown, depending on how those alleles are combined.

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Polygenic Inheritance Since six different alleles may control eye color, the combination of alleles from each gene may look something like this (for our purposes, we are simplifying this cross using Aa , Bb and Cc as our alleles): Remember that you only get one of each chromosome from each parent, so you may get a dominant or recessive trait from each gene. Let’s pretend that two people mate with the genotypes AaBbCc x AaBbCc What sort of genotype combinations can results from this breeding pair? A a C c

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Polygenic Inheritance When we cross AaBbCc x AaBbCc , the results of the general population look like a bell curve, with lots of variation within the offspring genotypes and phenotypes. ABC ABc AbC aBC abC aBc Abc abc ABC AABBCC AABBCc AABbCC AaBBCC AaBbCC AaBBCc AABbCc AaBbCc ABc AABBCc AABBcc AABbCc AaBBCc AaBbCc AaBBcc AABbcc AaBbcc AbC AABbCC AABbCc AAbbCC AaBbCC AabbCC AaBbCc AAbbCc AabbCc aBC AaBBCC AaBBCc AaBbCC aaBBCc aaBbCC aaBBCc AaBbCc aaBbCc abC AaBbCC AaBbCc AabbCC aaBbCC aabbCC aaBbCc AabbCc aabbCc aBc AaBBCc AaBBcc AaBbCc aaBBCc aaBbCc aaBBcc AaBbcc aaBbcc Abc AABbCc AABbcc AAbbCc AaBbCc AabbCc AaBbcc AAbbcc Aabbcc abc AaBbCc AaBbcc AabbCc aaBbCc aabbCc aaBbcc Aabbcc aabbcc Depending on the number of DOMINANT genes you inherit, you can have very dark eyes ( SIX DOMINANTS ) or very light eyes ( SIX RECESSIVES ). Variations in between would result in intermediate hues. This concept applies to height, sizes, shapes, and colors of many other traits as well! This example of Polygenic Inheritance is a challenging one, but it demonstrates how this type of inheritance can result in so many phenotypes within a population. Keep mind, however, that environmental factors such as diet and other conditions can affect whether these traits are fully expressed. For example, you may not reach your full height if you are malnourished.

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