# Genetics: What is the probability that the third child will be affected

• Biology
• TytoAlba95
In summary, the probability of an unaffected daughter being born is 1/8 and the probability of an affected daughter being born is 1/4. The probability of a third child not being affected is 3/4.
TytoAlba95
Homework Statement
About 1% individuals in a population suffer from a genetic disorder. The cause was traced to such individuals being homozygous recessive for a single locus with two alleles. The elder of the two children of a family(where both the parents are normal) suffers from the disorder, while the younger one is normal. What is the probability that the third child will be a normal daughter?
(a) 49.5%
(b)12.5%
(c)37.5%
(d)25%
Relevant Equations
Ans: (c)
pg-363, sum-334

Attempt:

The disorder is autosomal recessive.
The probability = Probability of being a daughter x Probability of an affected child being born = ½×¼ = 1/8 x100 =12.5%

SanjuktaGhosh said:
Problem Statement: About 1% individuals in a population suffer from a genetic disorder. The cause was traced to such individuals being homozygous recessive for a single locus with two alleles. The elder of the two children of a family(where both the parents are normal) suffers from the disorder, while the younger one is normal. What is the probability that the third child will be a normal daughter?
(a) 49.5%
(b)12.5%
(c)37.5%
(d)25%
Relevant Equations: Ans: (c)

pg-363, sum-334

Attempt:

The disorder is autosomal recessive.
The probability = Probability of being a daughter x Probability of an affected child being born = ½×¼ = 1/8 x100 =12.5%

It looks like you've calculated the probability of an affected daughter.

PeroK said:
It looks like you've calculated the probability of an affected daughter.
P= ½×¾ x 100=37.5%

Thanks :)

I looked at this thread out of curiosity, and I find myself confused.

Since the first child has the disorder, and both parents do not, this means each parent carries the bad allele on just one of the two chromosomes which are home to the relevant gene. For a third child to not have the disorder, means the child must avoid receiving the bad allele from both parents. A child will get the bad allele from the mother half the time, and from the father half the time, and from both (1/2)*(1/2) = 1/4 the time. Thus the third child avoids inheriting the bad allele form both parents 1-(1/4) = 3/4 of the time. What really confuses me is that 75% is not listed among the four answer choices.

Buzz Bloom said:
I looked at this thread out of curiosity...
Thanks.
What really confuses me is that 75% is not listed among the four answer choices.

''What is the probability that the third child will be a normal daughter?''

Actually, the question is about a normal daughter, so its ½ × ¾.

Buzz Bloom and hutchphd
SanjuktaGhosh said:
Actually, the question is about a normal daughter, so its ½ × ¾.
Hi Sanjukta:

Thanks for the correction. One of these days I will regain my ability to read accurately.

Regards,
Buzz

TytoAlba95 and pinball1970
Buzz Bloom said:
Hi Sanjukta:

Thanks for the correction. One of these days I will regain my ability to read accurately.

Regards,
Buzz
Me too. I saw the answer and thought what?? Then I re-read the op.

## 1. What is the probability that the third child will be affected?

The probability that the third child will be affected depends on the genetic inheritance pattern of the particular trait or disorder. If the trait is inherited in a dominant manner, then the probability is 50% if one parent is affected and 75% if both parents are affected. If the trait is inherited in a recessive manner, then the probability is 25% if both parents are carriers and 6.25% if both parents are affected.

## 2. How is the probability of a child being affected determined?

The probability of a child being affected is determined by the genetic makeup of the parents. If both parents carry a copy of the gene for a particular trait or disorder, then there is a chance that their child will inherit the gene and be affected. The probability is higher if both parents are affected by the trait or disorder.

## 3. Can the probability of a child being affected be accurately predicted?

The probability of a child being affected can be estimated based on the genetic inheritance pattern and the genetic makeup of the parents. However, it is not possible to accurately predict the exact probability as there are other factors that can influence the expression of a gene, such as environmental factors and epigenetic changes.

## 4. Are there any factors that can increase or decrease the probability of a child being affected?

Yes, there are several factors that can increase or decrease the probability of a child being affected by a genetic disorder. These include the number of affected individuals in the family, the age of the parents, and the presence of other genetic or environmental factors that can influence the expression of the gene.

## 5. Can the probability of a child being affected change over time?

Yes, the probability of a child being affected can change over time as new genetic information is discovered and our understanding of genetic inheritance patterns improves. Additionally, the probability may change if there are changes in the environment or in the individual's genetic makeup that can influence the expression of the gene.

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