A rare recessive allele inherited in a Mendelian manner...

A rare recessive allele inherited in a Mendelian manner causes the disease cystic fibrosis. A phenotypically normal man whose father had cystic fibrosis marries a phenotypically normal woman from outside the family, and the couple consider having a child. a. Draw the pedigree as far as described. b. If the frequency in the population of heterozygotes for cystic fibrosis is 1 in $50,$ what is the chance that the couple's first child will have cystic fibrosis? c. If the first child does have cystic fibrosis, what is the probability that the second child will be normal?

A rare recessive allele inherited in a Mendelian manner causes the disease cystic fibrosis. A phenotypically normal man whose father had cystic fibrosis marries a phenotypically normal woman from outside the family, and the couple consider having a child.
a. Draw the pedigree as far as described.
b. If the frequency in the population of heterozygotes for cystic fibrosis is 1 in $50,$ what is the chance that the couple's first child will have cystic fibrosis?
c. If the first child does have cystic fibrosis, what is the probability that the second child will be normal?

Key Concepts

Conditional Probability in Genetics

Conditional probability in genetics deals with calculating the risk of an outcome (such as disease manifestation) given additional information (like family history or previous occurrences). It is key in scenarios where, for example, knowing that one child is affected changes the assessment of risk for subsequent children.

Independent Assortment and Statistical Independence

Independent assortment refers to the process by which alleles of different genes segregate independently of one another during gamete formation. In the context of sequential births or multiple offspring from the same parents, the genetic outcomes for each child are statistically independent, meaning the genotype of one child does not affect the probability for another.

Hardy-Weinberg Equilibrium and Carrier Frequency

The Hardy-Weinberg equilibrium provides a framework for estimating allele and genotype frequencies within a population under certain ideal conditions. It is used to calculate the frequency of heterozygous (carrier) individuals from the known prevalence of a recessive disease, thereby helping in determining the likelihood of disease occurrence in offspring.

Autosomal Recessive Inheritance

In autosomal recessive inheritance, a disease manifests only when an individual inherits two copies of a mutant allele, one from each parent. Carriers, who have only one copy of the mutant allele, are typically unaffected but can pass the allele to their children. Understanding this pattern is critical for assessing the risks in families with a history of diseases like cystic fibrosis.

Pedigree Analysis

Pedigree analysis involves charting a family tree to track the inheritance of specific traits or diseases. It helps in identifying carriers, affected individuals, and the pattern of inheritance across generations. This method is essential for visual representation and risk assessment in genetic counseling.

Mendelian Inheritance

Mendelian inheritance refers to the basic principles of genetic transmission discovered by Gregor Mendel, which include the segregation of alleles and independent assortment. It explains how traits regulated by single genes are passed from parents to offspring in predictable ratios, forming the foundation for understanding genetic diseases such as cystic fibrosis.

Transcript

00:01 About cystic fibrosis.

00:02 And so cystic fibrosis is an autosomal and it's recessive.

00:07 So if you have a big c allele, you're normal.

00:11 And if you have the little c .o.

00:13 See allele, that's the cystic fibrosis allele.

00:17 And to actually inherit the genetic disease because cystic fibrosis has a recessive allele, you need to be little c, little c.

00:26 That's how you get the disease.

00:28 All right.

00:29 So we're told about a man whose father.

00:32 So this is the man's father, has cystic fibrosis.

00:37 So we don't know anything about the mother, but we know that the man himself is phenotypically normal.

00:44 So that means he has to have at least one big c along the little c he got from his father.

00:51 So there's his mother.

00:54 She could be big c, big c, or she could be big c, little c.

00:59 We don't know, but we know that she had to have gotten given him a big c all.

01:03 So this man and the woman over here who's also phenotypically normal, and i'll put a little dash there for just a moment, are contemplating having a child, and the child would go there.

01:19 So this is the pedigree that's laid out for this question.

01:23 And then it says to us in part b, it says, what if the population as a whole, there's a one in 50 chance that a person has that is heterozygot has a little c allele.

01:42 What's the probability that this child here would have cystic fibrosis? there's two things that we need to think about.

01:49 One is we don't know anything about the cystic fibrosis status of this woman other than that she doesn't have it.

01:58 So she could be a carrier or she could not be a carrier...

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