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Questions and Answers
What does high concordance in identical twins and low concordance in fraternal twins suggest about a trait?
What does high concordance in identical twins and low concordance in fraternal twins suggest about a trait?
How do alleles segregate between gametes during meiosis?
How do alleles segregate between gametes during meiosis?
Which of the following is NOT a method classical geneticists use?
Which of the following is NOT a method classical geneticists use?
Sex-linked genes deviate from standard Mendelian inheritance because:
Sex-linked genes deviate from standard Mendelian inheritance because:
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What influences the phenotypes of sex-linked traits?
What influences the phenotypes of sex-linked traits?
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What determines the male/female phenotype (sex) according to the module summary?
What determines the male/female phenotype (sex) according to the module summary?
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In X-Linked Dominant inheritance, how many copies of the disease allele are required for an individual to express the phenotype?
In X-Linked Dominant inheritance, how many copies of the disease allele are required for an individual to express the phenotype?
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How does consanguinity increase the risk of inheriting autosomal recessive diseases?
How does consanguinity increase the risk of inheriting autosomal recessive diseases?
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In X-Linked Recessive inheritance, what is the genotype of affected individuals?
In X-Linked Recessive inheritance, what is the genotype of affected individuals?
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What is a characteristic deviation from Mendel's rules for loci on the sex chromosomes?
What is a characteristic deviation from Mendel's rules for loci on the sex chromosomes?
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For X-Linked genes, what is a well-studied example mentioned in the text?
For X-Linked genes, what is a well-studied example mentioned in the text?
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In sex-linked traits, what causes most loci on the typical X chromosome to be absent on the Y chromosome during meiosis?
In sex-linked traits, what causes most loci on the typical X chromosome to be absent on the Y chromosome during meiosis?
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In X-linked dominant diseases, if a female is affected, what can be said about her sons?
In X-linked dominant diseases, if a female is affected, what can be said about her sons?
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Which of the following is an example of an X-linked recessive disease?
Which of the following is an example of an X-linked recessive disease?
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For a carrier female of an X-linked recessive disease, what is the chance that her daughters will inherit the disease allele?
For a carrier female of an X-linked recessive disease, what is the chance that her daughters will inherit the disease allele?
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In deviations from Mendel's rules, what can be said about phenotypes and genotypes?
In deviations from Mendel's rules, what can be said about phenotypes and genotypes?
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What is the likelihood that an affected male with an X-linked recessive disease will transmit the disease allele to his sons?
What is the likelihood that an affected male with an X-linked recessive disease will transmit the disease allele to his sons?
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Study Notes
Twin Studies
- High concordance in identical twins and low concordance in fraternal twins indicates a strong hereditary element for a trait.
- Equal concordance and discordance ratios between identical and fraternal twins suggest less hereditary influence and more environmental influence on a trait.
Classical Genetics
- A diploid can have up to two different alleles at a single locus.
- Alleles segregate equally between gametes during meiosis.
- Phenotype depends on alleles, their dominance relationships, and interactions with the environment and other factors.
Sex-Linked Genes
- Sex-linked genes are an exception to standard Mendelian inheritance.
- Phenotypes are influenced by the type of sex chromosome system and dosage compensation system found in the species.
- Male/female phenotype (sex) can be determined by chromosomes, genes, or the environment.
Autosomal Recessive
- Only one copy of a disease allele is necessary for an individual to be susceptible to expressing the phenotype.
- There is a 50% chance the offspring will inherit the disease allele.
- Affected individuals have a homozygous recessive genotype, while heterozygotes (carriers) have a wild-type allele that masks the mutant allele.
- Parents of an affected individual are not affected but are gene carriers.
- Marriage between relatives increases the risk of inheriting autosomal recessive diseases.
Pedigree Analysis
- A pedigree shows a family tree with respect to a given trait.
- Pedigree analysis reveals patterns of inheritance.
Sex-Linkage
- Loci on autosomes follow the normal Mendelian pattern of inheritance.
- Loci on sex chromosomes follow a sex-linked pattern of inheritance.
- The X chromosome has most loci absent from the Y chromosome, despite acting as a homologous pair during meiosis.
X-Linked Genes
- The white gene on the X chromosome of Drosophila melanogaster is a well-studied sex-linked gene.
- X-linked dominant diseases include Rett syndrome, Double-cortex syndrome, and Incontentia pigmenti type 1.
- X-linked recessive diseases include Duchenne muscular dystrophy, Colorblindness, Hemophilia A, and G-6-P-D.
X-Linked Recessive
- Two copies of a disease allele on the X chromosome are required for a female to be affected.
- Any male with one copy of an X-linked recessive disease allele is affected.
- Females are usually carriers because they only have one copy of the disease allele.
- For a carrier female, with each pregnancy, there is a 50% chance her sons will inherit the disease allele and a 50% chance her daughters will be carriers.
- Affected males transmit the disease allele to all of their daughters, who are then carriers, but to none of their sons.
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Description
Test your knowledge on autosomal recessive inheritance, where having only one copy of a disease allele leads to susceptibility. Understand the 50% chance of inheriting the disease allele and how affected individuals possess a homozygous recessive genotype.
