Lecture 11 - Single Gene Inheritance Flashcards
What is Mendel’s model of inheritance?
Each parent has two alleles for a trait that segregate during gamete formation (Law of Segregation).
Gametes carry only one allele for each trait.
Offspring inherit one allele from each parent, determining the phenotype.
What is a test for single-gene inheritance?
A 3:1 phenotypic ratio in F2 (genotype ratio 1:2:1) indicates single-gene inheritance.
How does a test cross determine an unknown genotype?
Cross with a homozygous recessive individual:
Heterozygous: 50% dominant, 50% recessive offspring.
Homozygous dominant: All offspring show dominant phenotype.
What are the four modes of single-gene inheritance?
Autosomal dominant.
Autosomal recessive.
Sex-linked dominant.
Sex-linked recessive.
What is the difference between diploid and haploid organisms?
Diploid: Two copies of each chromosome (e.g., somatic cells).
Haploid: One copy of each chromosome (e.g., gametes).
What are examples of diploid model organisms in genetics?
Saccharomyces cerevisiae (yeast).
Drosophila melanogaster (fruit fly).
Mus musculus (mouse).
What is sex-linked inheritance?
Traits determined by genes on the X or Y chromosome.
Example: Eye colour gene in Drosophila is X-linked.
How do X-linked recessive traits differ in males and females?
Males (XY): A single recessive allele results in the phenotype.
Females (XX): Both alleles must be recessive to show the phenotype.
What is a pedigree?
A family tree diagram showing inheritance of traits or disorders.
Helps identify patterns like autosomal or sex-linked inheritance.
How do you interpret a human pedigree?
Use symbols:
Squares for males, circles for females.
Shaded symbols indicate affected individuals.
Half-shaded indicate carriers (recessive traits).
What is autosomal dominant inheritance?
Affects both sexes equally.
Appears in every generation.
Only one copy of the dominant allele is needed to show the trait.
What is an example of an autosomal dominant disorder?
Marfan syndrome: Affects connective tissue, caused by mutations in the fibrillin gene on chromosome 15.
What is autosomal recessive inheritance?
Affects siblings, both sexes equally.
Requires two copies of the recessive allele to show the trait.
Often seen in consanguineous matings.
What is an example of an autosomal recessive disorder?
Phenylketonuria (PKU): Caused by a defect in the phenylalanine hydroxylase gene on chromosome 12.
What is X-linked dominant inheritance?
Affected fathers pass the trait to all daughters but not sons.
Affected mothers pass the trait to both sons and daughters.
What is X-linked recessive inheritance?
Affects mainly males.
Females are typically carriers unless homozygous.
Example: Haemophilia A.
What is a Punnett square?
A diagram to predict genotypes and phenotypes of offspring from parental crosses.
How are genes linked/unlinked written?
Linked genes: Written with a slash (e.g., AB/ab).
Unlinked genes: Written with a space or semicolon (e.g., A/a ; B/b).
What does a 1:1 phenotypic ratio in a test cross indicate?
The F1 individual is heterozygous.
What is the Law of Segregation?
Each parent contributes one allele for a trait, and alleles separate during gamete formation, ensuring offspring inherit one allele from each parent.
What is the significance of a 3:1 phenotypic ratio in F2 generation?
It suggests inheritance of a single gene with one dominant and one recessive allele.
What does the F2 phenotypic ratio indicate for X-linked recessive traits?
Male offspring are more likely to exhibit the recessive phenotype since they inherit a single X chromosome from the mother.
How can Mendelian inheritance be observed in haploid organisms?
Haploid organisms (e.g., yeast) have only one allele per gene, so phenotypes directly reflect the genotype without dominance effects.
What are key differences between autosomal and sex-linked inheritance?
Autosomal: Traits appear in both sexes equally.
Sex-linked: Traits often show sex-specific patterns (e.g., X-linked recessive traits are more common in males).
What is the inheritance pattern in consanguineous matings?
Autosomal recessive traits are more likely to appear due to increased chances of inheriting the same recessive allele from both parents.
How does X-linked inheritance explain male and female phenotypic differences?
Males (XY) are hemizygous for X-linked genes, so they express any allele present.
Females (XX) can be carriers or express the phenotype if homozygous.
How do you determine the genotype of linked genes in a test cross?
Analyze offspring ratios for linked (e.g., AB/ab) or unlinked genes (e.g., A/a ; B/b).
Linked genes tend to show fewer recombinant phenotypes.
Why do autosomal dominant traits appear in every generation?
Only one copy of the dominant allele is needed to express the trait, and affected individuals often pass it to offspring.
Why is pedigree analysis important in human genetics?
Helps identify inheritance patterns for single-gene traits and diagnose genetic disorders.
What are the key features of autosomal recessive pedigrees?
Often skips generations.
Affects both sexes equally.
Parents are usually carriers (heterozygous).
What is the purpose of calculating probabilities in genetics?
To predict the likelihood of inheriting specific genotypes and phenotypes.
Example: Tossing a coin analogy for allele segregation.
How do you differentiate between linked and unlinked genes in a genotype? (in writing)
Linked genes are separated by a slash (AB/ab).
Unlinked genes are separated by a semicolon or space (A/a ; B/b).
