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Lecture 16

Lecture 16- Mendelian Traits in Humans & Human Pedigree Analysis

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University of Toronto St. George
Human Biology
Christian Campbell

Thursday, March 5, 2009 - Not Just Peas in a Pod. - First we see a picture we saw in the beginning of term, we can see that traits are inherited from one generation to the next and a great example was with Steven & Liv Tyler. The smile seems to be inherited, hair pattern as well so the bottom line is we know that traits are inherited in humans. - Are they ex hibi ti g the same mo de or pat terns of in heri tanc e as Men del observed in his peas? Of course they are, they are derived from mutations in the same way that the mutations that gave rise to the variants in Mendels peas were found. - So he provided but one example in the slide, it looks exactly like what we looked at on Tuesday and that is that we have for example, a dominant allele here in African black populations where what we find is that the dominant allele gives rise to a functional tyrosinase enzyme which converts tyrosine to melanin giving rise to the characteristic brown pigment so individuals that are heterozygous for functional tyrosinase allele and a recessive tyrosinase allele are still going to show that same melanin pigment whereas individuals that are homozygous for a loss of function allele of that tyrosinase are going to be unable to make that melanin pigment and we will see incidents of albinism or albinos in the population. - So it is very similar to what we were looking at with regards to production of flower pigment and here just pigments in humans. - The same sort of molecular mechanisms underpin what we see in terms of traits is the pattern of inheritance. We can find dominant & recessive traits in humans. - So here we have some. So dimples vs. roundso dominant vs. the recessive trait. The widows peak in males vs. the straight hairline is dominant vs. recessive. Freckles versus clear face, again freckles are dominant. Hair on the arms vs. no hair again dominant vs. recessive & we can see this then within families of individuals that they have mixtures of the dominant & recessive traits. - We can fin d this not only for visible traits but those that emerge in populations that are related to disease susceptibility. - He has provided a short shopp ing list of different diseases shown either in this instance right here recessive modes of inheritance vs. a dominant mode of inheritance. - He will come back to 2 of these so hell come back to hypercholesterolemia and Huntingtons Disease later in the term and probably also back to cystic fibrosis. Well see other examples of disease susceptibility loci showing again either dominant or recessive modes of inheritance. Well see some today in the pedigrees that were going to look at. - If were interested in the occurrence of some of these, again we can see they occur with varying degrees of frequency and we can consider why those frequencies are established in populations in his very last lecture. - If were interested in more of these traits, thou sands can be foundon the www.notesolution.comwebsite online for Mendelian inheritance in men which is a shopping list of all diseases found so far displaying simple Mendelian modes of inheritance, dominance versus recessive. - All of the traits described to us so far are what are known as autosomal traits they show so-called autosomal inheritance. What does that mean? Human autosomal traits are located on the non-sex chromosomes so chromosomes 1 through 22, not on the X and the Y. They may be inherited as either what are known as autosomal dominant or autosomal recessive traits and that is what he will focus on for the remainder of todays lecture: those traits and how theyre inherited in pedigrees. - Autosomal dominant trai ts sh ow a pat tern of in heri tanc e that weve se en already in peas. What we see there is that homozygous dominant & heterozygotes exhibit the affected phenotype & males & females are equally affected & they transmit the trait. - So we see that in a stereotypical Punnett square here which is a cross bw a homozygote & heterozygote & so we end up with the typical 1:1 segregation ratio of affected individuals vs. unaffected individuals, so individuals showing the traits vs. those not showing the trait. - What is important about these sort of traits and it will become more apparent why this is more important as we analyze pedigrees is that the affected phenotype does not skip generations. - Autosomal recessive traits by contrast of course are only apparent when the individual is homozygous recessive so only homozygous recessive individuals exhibit the affected phenotype. - Here for example, the example he pro vided is that males & females are equally affected & may transfer the trait as is the case with autosomal traits. Here we have 2 heterozygotes for curly hair which is the dominant trait & with their 4 progeny, there is a chance that those progeny are going to also have curly hair & a chanc e that their of fsprin g wi l lex hibi t straig ht hair, the recessive trait. - Now what is important with regards to this kind of trait and again in terms of interpreting pedigrees is that this kind of trait may skip a generation, that is you will not see the recessive trait in a given generation and then it willappear in the next generation. - So he has alluded to the fact that the way in which we track this of course is by looking at pedigrees so it is pedigrees that are used to study human genetics, to track traits from generation to generation. - Reasons for that are pretty simple: it is considered morally reprehensible to do breeding experiment with humans. What we have to use then to dissect traits in humans is a combination of pedigree analysis & of model organisms to better understand those traits and their inheritance so pedigrees then are orderly diagrams of families. - Note there is a typo in the handout, should be familys.
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