A young couple went to see a genetic counselor because each...

Key Concepts

Autosomal Recessive Inheritance

This concept refers to a mode of genetic transmission where an individual must inherit two copies of a mutated allele—one from each parent—to express the disease phenotype. Those inheriting only one mutated allele remain asymptomatic carriers. This basic principle is central to understanding the risk calculations in similar genetic problems.

Conditional Probability in Pedigree Analysis

When evaluating genetic risks based on family history, conditional probability is used to update the likelihood of an individual being a carrier given information such as the presence of affected siblings despite unaffected parents. This concept helps in recalculating risk estimates beyond the general population incidence.

Mendelian Inheritance and Punnett Square Analysis

Mendelian inheritance describes how alleles segregate from parents to offspring in predictable ratios. A Punnett square is a visual tool used to map these outcomes, allowing one to determine the probabilities of an offspring inheriting a particular genotype, such as being affected, a carrier, or unaffected.

Genetic Carrier Risk Assessment

This involves combining the probabilities that each parent is a carrier with the Mendelian ratios derived from a cross between carriers. By multiplying these probabilities, one can determine the overall chance that a child will inherit two copies of the mutant allele (be affected) or just one copy (become a carrier).

Transcript

00:01 In order to answer this question, let's talk about inheritance.

00:03 This question says, a young couple went to see a genetic counselor because each had a sibling with cystic fibrosis.

00:09 So this is the man and this is the woman.

00:14 And in this case, both of them have a sibling, suppose a woman or a man, okay? a brother and a sister.

00:27 Both of them have a sibling with the disease.

00:30 In this case, the sex doesn't matter because cystic fibrosis is an autosomal and a recessive condition.

00:36 So in this case, the homocylus dominant and the heterocygous are going to be normal people, while the homozygous recessive is for a person with cystic fibrosis.

00:45 So, in this case, both siblings have the condition.

00:52 It means both of them are homozygous recessive.

00:57 And it says, what is the probability that the female of this couple is a carrier? so remember that the person in here is one only from each parent.

01:02 In this case, this man has two -gessive -a.

01:09 It means one -guessive a came from the father and one -gessive -a came from the mother.

01:13 But as both parents are unaffected, then it means that the only possibility for them is to be heterocybo -s.

01:18 So both parents are heterocybo -s.

01:20 And the same in this case.

01:23 Both parents are heterocylus.

01:24 So they are asking you the probability for this woman here to be heterocygidot -s.

01:29 So let's make a cross between a heterocybo -s and not a heterocygilos.

01:34 Homocyglo -g heterocygloids.

01:36 Heterocyboes and homozygos, recessive.

01:39 Now, you have a total of four possibilities, right? but in this case, you're not going to consider that four possibilities, because you already know that this woman is unaffected.

01:47 And this diotype calls for an affected woman or an affected person in general.

01:52 So in this case, as this woman is, you already know that this woman is not affected, making it means that you're going to consider only these three possibilities.

01:59 And you want to know the probability that this woman is a cavil, it means heterozybos.

02:02 So out of three possibilities, one and two, as the chances for her to be heterozygous.

02:09 So the answer is two out of three that is the same as two -thirds.

02:12 This is the answer for question a.

02:14 Question b says, what are the chances that their child will have cysticabrosis? so again, both parents have two -thirds chances for being heterozybous.

02:24 Okay, because of what we have said right now here.

02:28 And also the same for the father because the father is particularly like a song from a similar couple that is here.

02:34 So the same is happening in both at both sides.

02:37 And now, once both of the parents are heterocycles, you have again this panel square.

02:41 And in this case, the child is not born yet.

02:44 So you don't know if a child is going to be affected or not.

02:46 So in this case, you're going to consider the four possibilities in this panel square...