Table of contents

1. Introduction to Genetics51m
2. Mendel's Laws of Inheritance3h 37m
3. Extensions to Mendelian Inheritance2h 41m
4. Genetic Mapping and Linkage2h 28m
5. Genetics of Bacteria and Viruses1h 21m
6. Chromosomal Variation1h 48m
7. DNA and Chromosome Structure56m
8. DNA Replication1h 10m
9. Mitosis and Meiosis1h 34m
10. Transcription1h 0m
11. Translation58m
12. Gene Regulation in Prokaryotes1h 19m
13. Gene Regulation in Eukaryotes44m
14. Genetic Control of Development44m
15. Genomes and Genomics1h 50m
16. Transposable Elements47m
17. Mutation, Repair, and Recombination1h 6m
18. Molecular Genetic Tools19m
- Genetic Cloning
  9m
- Methods for Analyzing DNA
  9m
19. Cancer Genetics29m
- Overview of Cancer
  21m
- Cancer Mutations
  7m
20. Quantitative Genetics1h 26m
21. Population Genetics50m
- Hardy Weinberg
  19m
- Allelic Frequency Changes
  31m
22. Evolutionary Genetics29m

2. Mendel's Laws of Inheritance

Probability and Genetics

Genetics

2. Mendel's Laws of Inheritance

Probability and Genetics

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1:53 minutes

Problem A.5

Textbook Question

A woman, S.R., had a maternal grandfather with hemophilia A (OMIM 306700), an X-linked recessive condition that reduces blood clotting. S.R.'s maternal grandmother and paternal grandparents are free of the condition, as are her partner, his parents, and his grandparents. S.R. has no siblings. She wants to know the chance that a son of hers will have the condition. What is that probability?

Verified step by step guidance

Identify the inheritance pattern: Hemophilia A is an X-linked recessive disorder, meaning the gene causing the condition is located on the X chromosome, and males (XY) are affected if they inherit the affected X chromosome, while females (XX) are typically carriers if they have one affected X chromosome.

Determine the genotype of S.R.'s maternal grandfather: Since he had hemophilia A, he must have had the affected X chromosome (Xᴴ) and a Y chromosome (XᴴY), where Xᴴ represents the affected X chromosome.

Analyze S.R.'s mother’s genotype: Because S.R.'s maternal grandfather was affected and the maternal grandmother was unaffected, S.R.'s mother must be a carrier (XᴴX) — she inherited the affected X chromosome from her father and a normal X chromosome from her mother.

Calculate the probability that S.R. inherited the affected X chromosome from her mother: Since S.R.'s mother is a carrier (XᴴX), there is a 50% chance that S.R. inherited the affected X chromosome (Xᴴ) and a 50% chance she inherited the normal X chromosome (X).

Determine the probability that S.R.'s son will have hemophilia A: If S.R. is a carrier (XᴴX), each son has a 50% chance of inheriting the affected X chromosome (Xᴴ) and thus having hemophilia A. Multiply this by the probability that S.R. is a carrier (from step 4) to find the overall chance that her son will have the condition.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

X-linked Recessive Inheritance

X-linked recessive disorders are caused by mutations on the X chromosome. Males, having one X chromosome, are affected if they inherit the mutated gene, while females, with two X chromosomes, are typically carriers unless both X chromosomes carry the mutation. This pattern explains why males are more frequently affected.

Carrier Status and Transmission Risk

A female carrier of an X-linked recessive mutation has a 50% chance of passing the mutated gene to her children. Sons who inherit the mutation will be affected, while daughters who inherit it become carriers. Determining carrier status is essential to estimate the risk of affected offspring.

Pedigree Analysis and Probability Calculation

Analyzing family history helps identify carriers and affected individuals. In this case, the maternal grandfather had hemophilia, so the mother’s mother is likely a carrier. Using this information, one can calculate the probability that S.R. is a carrier and then the chance her son will inherit the condition.

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