Lecture 2.03 Exceptions To Mendels Laws

1) Cross the varieties 2) Genotype of the F1 3) Possible gametes of the F1 4) All possible ways the gametes can combine in the F2

Result: Phenotypic ratio is 9 Round/Yellow 3 Round/Green 3 Wrinkled/Yellow 1 Wrinkled/Green

Mendel’’ s Second Law Mendel The Law of Independent Assortment Distinct traits are passed from parent to offspring independently of each other This is why the product rule works for collections of Mendelian traits

Exceptions to Mendel’ Mendel’s First Law 1) Thalassemia An inherited disorder of red blood cells; victims incorrectly synthesize hemoglobin a) Thalassemia minor: minor: usually asymptomatic; sometimes slightly lowered hemoglobin with elevated RBC count. b) Thalassemia major: major : much more severe; onset of anemia in infancy or childhood leads to early death.

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Exceptions to Mendel’ Mendel’s First Law 2) Human ABO antigens Humans express proteins on the surface of their red blood cells; identification tags A

B

A B

Type A

Type B

Type AB

Type O

Exceptions to Mendel’ Mendel’s First Law 3) Duchenne Muscular Dystrophy Inherited muscle weakness and wasting disease; most common childhood dystrophy; progressive, usually leads to early death

Summary 1) Thalassemia – Incomplete dominance (blending of traits in the heterozygote; therefore, no dominance) 2) ABO blood groups – Co Co--dominance (both alleles expressed in the heterozygote; also no dominance) 3) DMD – Sex linked (gene exists on one sex chromosome but not the other. Specifically, DMD is X-linked linked— —it it’’s on the X but not the Y chromosome.)

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Exceptions to Mendel’ Mendel’s Second Law 1) Coat color in mice:

Exceptions to Mendel’ Mendel’s Second Law 1) Coat color in mice: In one experiment researchers crossed two black mice and obtained the following results

Approximately 9/16 were black Approximately 3/16 were brown (agouti) Approximately 4/16 were albino The fact that we get ratios of x/16 suggests we are dealing with two genes instead of one

Gene 1 (with alleles B or b) determines fur color if pigment is deposited in hair shafts. shafts. BB or Bb = black fur bb = brown fur

Gene 2 (with alleles C or c) determines if pigment is deposited in hair shafts.. shafts CC or Cc = pigmented cc = not pigmented

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Gene 1 (with alleles B or b) determines fur color if pigment is deposited in hair shafts. shafts. BB or Bb = black fur bb = brown fur

Gene 2 (with alleles C or c) determines if pigment is deposited in hair shafts.. shafts CC or Cc = pigmented cc = not pigmented

Gene 1 (with alleles B or b) determines fur color if pigment is deposited in hair shafts. shafts.

Epistasis

BB or Bb = black fur bb = brown fur

Gene 2 (with alleles C or c) determines if pigment is deposited in hair shafts.. shafts CC or Cc = pigmented cc = not pigmented

Example of another violation of Mendel’s laws Body color in fruitfruit- flies Gray body : BB or Bb Black body : bb

Wing length Normal : VgVg or Vgvg Vestigial : vgvg

What is the testtest-cross in this case? BbVgvg × bbvgvg

What are the expected results of this cross (with 2300 offspring)? offspring)? 575 Gray/Normal (BbVgvg ( BbVgvg)) : 575 Black/Normal (bbVgvg (bbVgvg)) : 575 Gray/vestigial (Bbvgvg (Bbvgvg)) : 575 Black/Vestigial (bbvgvg (bbvgvg))

Actual results – which law is violated, and how? 965 Gray/Normal (BbVgvg ( BbVgvg)) : 185 Black/Normal (bbVgvg (bbVgvg)) : 206 Gray/vestigial (Bbvgvg (Bbvgvg)) : 944 Black/Vestigial (bbvgvg (bbvgvg))

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Could Such Results Have Occurred by Chance? As always, we want an objective answer to the question-question --meaning, meaning, we need mathematics The problem is similar to this simpler one: Joe flips a coin 10 times, obtains 8 heads, 2 tails. Is the coin fair (meaning when flipped it has a 0.5 probability of landing on heads)

There is no way to be certain if the coin is fair or not BUT, there is a way to address the question rationally

Could Such Results Have Occurred by Chance? Basic Approach: 1) Assume, for the sake of argument, that the deviation between observed data and expectation from Mendel’ Mendel ’ s laws occurred by chance (i.e., assume that Mendel’ Mendel ’ s laws hold) Statisticians call this the null hypothesis 2) Calculate the probability that a deviation this large or larger occurred by chance Technically, calculate the proper statistic (in this case, calculate χ 2 ) and use its sampling distribution to calculate the required probability, called a p- value

3) If this probability is too low (usually < 0.05 or 1 in 20), then we reject the null hypothesis (conclude Mendel’ Mendel ’ s laws don’ don’ t hold). Otherwise, we accept the null hypothesis (conlclude that not enough evidence exists to reject the null)

Example ---Morgan Morgan’’s Drosophila results

1) Assume, for the sake of argument, that the deviation between observed data and expectation from Mendel’ Mendel ’ s laws occurred by chance (i.e., assume that Mendel’ Mendel ’ s laws hold)

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Example ---Morgan Morgan’’s Drosophila results

2) Calculate the probability that a deviation this large or larger occurred by chance

p = 2.2 × 10 −16

1 in 5 million billion

Example ---Morgan Morgan’’s Drosophila results

3) If this probability is too low (usually < 0.05 or 1 in 20), then we reject the null hypothesis (conclude Mendel’ Mendel ’ s laws don’ don’t hold).

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Recombination frequency – proportion of all offspring that show recombination

=

206 + 185 391 = = 0.17 206 + 185 + 944 + 965 2300

B and Vg genes are 17 cM apart

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