Chapter 2: Chapter 2: Genes and Genes and

Medical Medical GeneticsGenetics

IntroductionIntroduction

• So far, it’s been all about the cell. So far, it’s been all about the cell.

• Last lecture we examined how cells divide. Last lecture we examined how cells divide. A process which facilitates life. A process which facilitates life.

• This time we’ll consider genetic and there This time we’ll consider genetic and there impact on how we “look,” and what impact on how we “look,” and what potential pitfalls may occur when cell potential pitfalls may occur when cell division and replication don’t “give us what division and replication don’t “give us what we wanted.”we wanted.”

IntroductionIntroduction

• As always we have several new terms.As always we have several new terms.• GenotypeGenotype: genes of the individual. Your : genes of the individual. Your

genetic makeup, if you will.genetic makeup, if you will.• PhenotypePhenotype: What you look like, or more : What you look like, or more

precisely, what physical manifestations are precisely, what physical manifestations are observed.observed.

• AlleleAllele: Alternate forms of a gene which are : Alternate forms of a gene which are located in the same position on a pair of located in the same position on a pair of chromosomes.chromosomes.

Fig. 02-01Fig. 02-01• Three paths: Three paths: Two results??Two results??

• Not really!Not really!

• Again, new terms…Again, new terms…

• Dominant allele Dominant allele (capital letter)(capital letter)

• Recessive AlleleRecessive Allele (lower case letter)(lower case letter)

• As you can see during As you can see during meiotic cell division, alleles meiotic cell division, alleles are isolated within each are isolated within each gamete.gamete.

• How do alleles (genes) get mixed How do alleles (genes) get mixed up?? up??

• Rem: Meiosis? Crossing over?? Rem: Meiosis? Crossing over??

• Alleles are spread around in an Alleles are spread around in an amazing way during sexual amazing way during sexual reproduction.reproduction.

• As we will see, this leads us to As we will see, this leads us to some interesting results which don’t some interesting results which don’t always seem logical.always seem logical.

• Alleles on autosomesAlleles on autosomes

• Basic Mendelian Genetics Basic Mendelian Genetics and the Punnett Squareand the Punnett Square

• If we “cross multiply,” it If we “cross multiply,” it becomes evident that not becomes evident that not all genotypes result in the all genotypes result in the same phenotype. same phenotype.

• The example at the left is a The example at the left is a single-trait cross.single-trait cross.

• Notice the expected Notice the expected phenotypic ratiophenotypic ratio (3:1). (3:1). This will become important This will become important later.later.

• What happens if we What happens if we mix it up a bit??mix it up a bit??

• Same method, differentSame method, different results.results.

• Now the phenotypicNow the phenotypic ratio is 50:50 (Better oddsratio is 50:50 (Better odds than you’d get in Vegas!)than you’d get in Vegas!)

• Okay, Emiril, let’s…Okay, Emiril, let’s… ““Kick it up a notch!!”Kick it up a notch!!”

• Dihybrid crossesDihybrid crosses (and beyond) are (and beyond) are handled similarly.handled similarly.

• You must be carefulYou must be careful to transfer all yourto transfer all your alleles though!!alleles though!!

• Once this is done, Once this is done, figuring out what figuring out what you have can beyou have can be challenging.challenging.

Genetic DisordersGenetic Disorders

• Most of the time, genes are expressed and Most of the time, genes are expressed and individual live quiet lives. individual live quiet lives.

• Sometimes two alleles get together that Sometimes two alleles get together that shouldn’t! shouldn’t!

• Many are Many are autosomal dominantautosomal dominant and and expressed with alarming frequency. expressed with alarming frequency.

• In the case of autosomal dominance, even In the case of autosomal dominance, even heterozygotes express the phentype. heterozygotes express the phentype.

Genetic Disorders:Genetic Disorders: Autosomal Dominant Autosomal Dominant

DiseaseDisease Occurence Occurence SymptomsSymptoms

Neurofibromatosis Neurofibromatosis 1/3500 1/3500 Dark SpotsDark Spots

Huntington Huntington ?? ?? NeuopathyNeuopathy

Polycystic KidneyPolycystic Kidney Nephretic cysts/ Nephretic cysts/ hypertensionhypertension

renal failure renal failure

Genetic Disorders: Genetic Disorders: Autosomal RecessiveAutosomal Recessive

DiseaseDisease OccurenceOccurence SymptomsSymptoms

Cystic FibrosisCystic Fibrosis 1/25001/2500 bronchial mucusbronchial mucus

clogged pancreatic clogged pancreatic ductsducts

PhenylketonuriaPhenylketonuria 1/50001/5000 high urinary high urinary phenylalanine phenylalanine concentrationconcentration

Tay-SachsTay-Sachs ???? Blindness, paralysis, Blindness, paralysis, deathdeath

Polygenic Inheritance:Polygenic Inheritance:

• Height, skin color, etc. Height, skin color, etc.

• The absolute reason for The absolute reason for expression of some traitsexpression of some traits such as a persons heightsuch as a persons height can be nebulous. can be nebulous.

• Many alleles dictate the Many alleles dictate the height of an individual, so height of an individual, so just because your dad just because your dad was 7’ doesn’t mean you was 7’ doesn’t mean you will be (though it doesn’t will be (though it doesn’t hurt either).hurt either).

Some traits share an additional genetic component, Some traits share an additional genetic component, despite differing environmental factorsdespite differing environmental factors

• Multiple Allelic TraitsMultiple Allelic Traits

• Blood types are codedBlood types are coded for by three different for by three different alleles. alleles.

• Codominance exists Codominance exists because A and B are because A and B are dominant over O.dominant over O.

• Incomplete dominanceIncomplete dominance

• Paul Mitchell aside,Paul Mitchell aside, hair curls, or lack of hair curls, or lack of them are expressed them are expressed as a mixture of straightas a mixture of straight vs. curly locks. vs. curly locks.

• What can result is a What can result is a continuum of hair curls,continuum of hair curls, waves and looks.waves and looks.

• Incomplete dominance is also observed in Incomplete dominance is also observed in Sickle-cell Disease. Here homozygotes (HH) are normal, Sickle-cell Disease. Here homozygotes (HH) are normal, heterozygotes (Hhheterozygotes (Hhss)are carriers, and recessives (h)are carriers, and recessives (hsshhss) ) have the disease. Oddly enough, carriers are superior inhave the disease. Oddly enough, carriers are superior in certain environments due to Malaria resistance conferred certain environments due to Malaria resistance conferred by the Sickle-cell trait.by the Sickle-cell trait.

• Sex-linked traitsSex-linked traits

• Occur when alleles are Occur when alleles are located on sex chromosomes.located on sex chromosomes.

• Sex-linked traits can be harsh.Sex-linked traits can be harsh.Ex.: Muscular distrophy &Ex.: Muscular distrophy & HemophiliaHemophilia

Fig. 02-14Fig. 02-14Muscular DistrophyMuscular Distrophy

Fig. 02-15Fig. 02-15Sex-Influence TraitsSex-Influence Traits

Fig. 2DFig. 2D

Fig. 2CFig. 2C