Today: Mendelian Genetics! Intro to Mitosis?

Gregor Mendel, The “Father” of Genetics?

Setting the Stage for Mendel Leading theory at the time is Blended Inheritance Mendel will need a good model organism! What makes a good model??

Mendel’s Technique: Studies peas- Typically Self- Fertilizing Multiple distinct CHARACTERS, with easy to identify TRAITS Several TRUE- BREEDING varieties available

What Mendel Observes, Part 1: What does this data suggest about “blended inheritance”?

What Mendel Observes, Part 2:

Mendel’s Hypothesis- Part 1 Different genes account for the variation in inherited characters

Mendel’s Hypothesis- Part 2 For each character, an organism inherits two alleles, one from each parent.

Mendel’s Hypothesis- Part 3 If the alleles are different, than one will control the organism’s appearance (the dominant allele) while the other will have no noticeable effect (the recessive allele)

Mendel’s Hypothesis- Part 4 The two alleles are separated during gamete production

Testing the Law of Segregation: The Punnett Square

Part 1: The Punnett Square for Mendel’s Experiments: What will the F1 Generation look like? The F2 Generation?

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Understanding the predicted results of a PUNNETT SQUARE, allows for a TESTCROSS What’s my phenotype? My genotype?

Part 2: Try a Test Cross! In dogs, there is an hereditary deafness caused by a recessive gene, “d.” A kennel owner has a male dog that she wants to use for breeding purposes if possible. The dog can hear, so the owner knows his genotype is either DD or Dd. If the dog’s genotype is Dd, the owner does not wish to use him for breeding so that the deafness gene will not be passed on. This can be tested by breeding the dog to a deaf female (dd). Draw the Punnett squares to illustrate these two possible crosses. In each case, what percentage/how many of the offspring would be expected to be hearing? deaf? How could you tell the genotype of this male dog?

Using Simple Mendelian Genetics Sickle Cell Disease

3A: Sickle Cell Disease Questions Two individuals who are heterozygous at the Sickle Cell locus have four children together. One of the children is affected with the disorder. Based on this information, is the sickle cell trait dominant or recessive?

3B: Sickle Cell Disease Questions If the affected offspring has a child with an unaffected individual (who does not carry the sickle allele), what is the probability that any given child will be unaffected? Be a carrier? Be affected?

An Aside: Unusual Gene Frequencies!? What do you notice? What does this suggest?

Mendelian Genetics- Example 4: Cystic Fibrosis is also an Autosomal Recessive Trait with Unusual Gene Frequencies A. If two carriers of the cystic fibrosis trait have children, what is the probability that their first child will be affected? B. If they eventually have three children, what is the probability that all three will be affected?

Calculating Probabilities

Dependent Assortment? Mendel’s Next Question: What happens in a dihybrid cross? What would the outcome look like if it’s dependent assortment??

What Mendel Sees: So is it dependent assortment?? You Try! Part 5.

Mendel’s Contributions Law #1: Segregation Law #2: Independent Assortment

Complication #1: (Mendel was lucky!) INCOMPLETE DOMINANCE Heterozygotes have a unique phenotype, between that of the homozygous dominant or recessive parents. Note: This is not blended inheritance ! Why?

Complication #1: (Mendel was lucky!) INCOMPLETE DOMINANCE

Another Exception: Codominance In codominance, both alleles affect the phenotype in separate, distinguishable ways. Example: Human blood groups M, N, and MN Group MN produce both antigens on the surface of blood cells

Another Exception: Codominance Example: Tay-Sachs disease- Heterozygous individuals produce both functional, and dysfunctional enzymes. organismal level = recessive, biological level = codominant. A section of the brain of a Tay Sachs child. The empty vacuoles are lysosomes that had been filled with glycolipid until extracted with alcohol in preparing the tissue.

Part 6: One Other Complication: Multiple Alleles and Codominance! Multiple Alleles: Suppose you’ve been asked to help a new mother identify the biological father of her child. She has Type A blood, and her new baby is Type B. Consider these three putative fathers: can any be the biological father? Why or why not? #1 (Type A) : Yes or No? #2 (Type B) : Yes or No? #3 (Type O) : Yes or No? #1 (Type A) : Yes or No? #2 (Type B) : Yes or No? #3 (Type O) : Yes or No?

Three Important Points about Dominant/Recessive Traits: 1.They range from complete dominance  incomplete dominance  codominance. (can be a subtle distinction!) 2.They reflect mechanisms through which specific alleles are expressed in the phenotype (i.e. this is not one allele subduing another at the DNA level) 3.They’re not related to the abundance of an allele within a population!

Further Complications: Pleiotropy Most genes have multiple phenotypic effects!

Further Complications: Pleiotropy No production of melanocytes during development causes: 1. White fur color and 2. Inability to transmit electrical signals to brain from hair cells in the ear.

More Complications: EPISTASIS Example: The “color gene”, C, allows pigment to be deposited in hair. When lacking, a mouse is albino, regardless of its genotype at the other locus.

Part 7: Epistasis and Lab Pups Black is dominant to Brown, so Heterozygotes (Bb) are black. The delivery gene is also dominant, so EE or Ee individuals both express their pigments. Only ee individuals are yellow. Coat color in labradors is determined by 2 genes, a pigment gene (B), and a pigment delivery gene (E).

If I cross a Brown Lab (bbEe) with a Black Lab (BbEe), can I expect any yellow puppies? If so, what proportion of the pups would I expect to be yellow? Part 7: Epistasis and Lab Pups

There’s more… Polygenic Inheritance This results in a broad norm of reaction

Many factors, both genetic and environmental, influence the phenotype. Other Issues: Environmental Effects on Phenotype