The following pedigree illustrates the inheritance of...

The following pedigree illustrates the inheritance of Nance-Horan syndrome, a rare genetic condition in which affected people have cataracts and abnormally shaped teeth. a. On the basis of this pedigree, what do you think is the most likely mode of inheritance for Nance-Horan syndrome? b. If couple III-7 and III-8 have another child, what is the probability that the child will have Nance-Horan syndrome? c. If III-2 and III-7 were to mate, what is the probability that one of their children would have Nance-Horan syndrome?

The following pedigree illustrates the inheritance of Nance-Horan syndrome, a rare genetic condition in which affected people have cataracts and abnormally shaped teeth.
a. On the basis of this pedigree, what do you think is the most likely mode of inheritance for Nance-Horan syndrome?
b. If couple III-7 and III-8 have another child, what is the probability that the child will have Nance-Horan syndrome?
c. If III-2 and III-7 were to mate, what is the probability that one of their children would have Nance-Horan syndrome?

Key Concepts

Genetic Risk Calculation

Genetic risk calculation involves applying principles of probability and Mendelian inheritance to predict the chance of offspring inheriting a genetic trait or disorder. This process often uses Punnett squares to map out possible genotypes from parental crosses. It accounts for the mode of inheritance, such as autosomal or X-linked patterns, and helps in counseling families by estimating the probability of occurrence of a trait in future generations.

X-linked Inheritance

X-linked inheritance refers to the transmission of genes located on the X chromosome. Since males have one X and one Y chromosome and females have two X chromosomes, genetic conditions on the X chromosome can manifest differently between sexes. In X-linked dominant conditions, a single copy of the mutated gene can cause the disorder in both males and females, albeit sometimes with varying severity, whereas in X-linked recessive conditions, typically only males are affected or severely affected, with females often being carriers.

Pedigree Analysis

Pedigree analysis involves studying family trees to determine the pattern of inheritance of a trait or disorder. It uses symbols to represent individuals and relationships, allowing one to trace the distribution of genetic disorders across generations. This method helps in identifying affected individuals, potential carriers, and the mode of transmission, which is essential for genetic counseling and risk assessment.

Transcript

00:01 To answer this question, we must complete pedigree analysis.

00:04 So first off, if we're looking at the mating between individuals seven and eight in the third generation, we find that both individuals are unaffected.

00:16 So they do not display the nance -huran syndrome.

00:21 However, they produce two out of three males that do display that syndrome.

00:29 So what does this mean? well, it means that we find one of those parents to be a carrier to the syndrome.

00:41 Although they don't display the syndrome themselves, they will be a carrier.

00:46 Well, now the question is, is this going to be, like, who is the carrier? well, if the carrier was the male, then we would find that this would be a y -linked disorder.

01:07 So it would be a y -linked trait.

01:10 However, we know that this is not true because all of our affected individuals are males.

01:24 So therefore, we can look at the pedigree as a whole and determine that, well, this must be an x -length disorder.

01:36 And specifically, if we look at generation 3 and look at individual 2, individual 2 is a male represented by a square, and the individual is affected, but produces a son in generation 4, individual number 1, that is unaffected.

01:53 This means that this must be an x -linked disorder.

01:59 It cannot be a y -length disorder because it's a were to be a y -linked disorder, then it would be for individual 1 in generation 4, they would definitely be affected.

02:09 So all that being said, we have now figured out that for choice a, for our first part, we know that this is a sex -linked disorder, and we also know that it is going to be, x -linked specifically.

02:31 So transferred through the x chromosome.

02:34 And finally, we know that this is going to be recessive.

02:37 And how do we know that? well, because simply having one allele for the disorder does not guarantee that the individual will display.

02:52 And we can find this in, once again, the cross between an individual's seven and eight in generation three.

02:59 So the mother is going to carry this, she will be a carrier for the disease, but she does not display it herself.

03:09 This means that it must be recessive.

03:11 Now, if we look at part b, we find that a cross between individuals seven and eight, right? we're looking specifically for the probability that the child will have nance herhand syndrome in a cross between individuals seven and eight...

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