Phenylketonuria commonly develops in individuals who a. have PKU in their urine. b. are homozygo

Phenylketonuria commonly develops in individuals who a. have...

Key Concepts

Autosomal Recessive Inheritance

Certain genetic disorders, including this disorder, follow an autosomal recessive pattern, which means that an individual must inherit two copies of a mutated gene (one from each parent) to express the disease. This inheritance pattern is crucial for understanding who is affected by the disorder and how it is passed through families.

Enzyme Deficiency in Amino Acid Metabolism

This disorder is characterized by a deficiency of the enzyme responsible for converting one amino acid into another. This enzyme deficiency leads to the accumulation of specific substrates that can be toxic, highlighting the importance of understanding metabolic pathways and the role of enzymes in processing amino acids.

Transcript

00:01 In these questions, we're told that a couple both are carriers for the allele that causes the disorder, phenoketaluria or pqa, which is a protein digestion issue.

00:24 And so we're told that the parents are both heterozygous -poias.

00:28 So they carry the recessive allele, but they don't display it because, in order to display the disease or to actually have the disease you have to be homozygous recessive.

00:39 So let's say that the parents are n for normal and they have a little n for the pk u allele and both are heterocylus and in little and so they have three children and were asked about the probability that their children will have pqu in various combinations.

01:11 So first i want to do the punnit square for this particular couple so that we can solidify in our minds what the probability of each genotype and phenotype is.

01:29 So we go across the top and combine the gametes of the humanoid.

01:46 And so what we end up with is one homozygous dominant to two heterozygous states to one homozygous recessive.

02:05 And to be clear, both the homozygous dominant and the heterosigis phenotype or genotype will have a normal phenotype.

02:18 So this down here is the genotypic ratio, but the phenotypic ratio is actually three normal to one effective.

02:39 So that's the phenotypic ratio.

02:49 So when we're asked about whether or not the children will be affected, we need to keep in mind actually the phenotypic ratio rather than the genotypic ratio.

03:00 Because only the homozygous recessive will actually be effective.

03:06 Okay.

03:09 So first we're asked, what is the probability that all three children are normal? if they have three children, that all three children are normal, so that are unaffected.

03:23 I'm just going to say normal.

03:25 Okay.

03:27 So that means we would have three children.

03:36 That are either an homozygous dominant or heterozygous.

03:44 And they would all have to be this.

03:48 All right.

03:48 So i've left a blank here because you can either have a dominant allele here or a recessive illegal here.

03:55 It doesn't matter which.

03:56 And for all three of these children, let's use the rules of pop probability.

04:01 This is a one, or this is a three, sorry.

04:07 This is a three and four probability so that because of this ratio here they have three times the opportunity to be normal rather than have the disease so this is the probability for any single child to have a normal genotype or phenotype so so what is the probability for this to occur simultaneously? we multiply that in the rules of probability, any event, the likelihood that any event will occur simultaneously with another can be predicted by multiplying the individual odds.

04:59 So here we multiply across.

05:01 This is 27 and this is 64.

05:06 So there's a 27 and 64 chance for all three of them.

05:10 Their children will be normal.

05:13 So then the next question is, what is the probability that one or more of their children will have the disease or will be affected? so that one or more of three children are affected.

05:46 So just to be clear, this is a...

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